Positive Disp Pumps

Category: IPCL Module No. Mechanical NC: Training Module IPCLDSMEC006

Prepared by: : Jitendra Purwar

Reviewed by: M Rajgopal

Approved by: A K Sood

Rev:00 Date:9/29/2005 Pages:1 of 94

Training Module

On

Positive Displacement Pumps






Training Module Objectives

This Training module is intended to help Engineers to understand the

fundamentals and applications of Positive Displacement (PD) Pumps in

Process industries .In this module , an effort has been made to provide ,much

needed source of information in the field of PD Pumps.

In this module user will learn about:

The Function of PD Pumps

Fundamentals & Operating Principles

Application

Selection criteria

Basic measures to Improve PD PumpS Efficiency.

Identify PD Pump Types

Identify Operating Characteristics of PD Pumps

Calculating PD Pump Power.

Evaluate Pump Performance

Controlling PD Pumps

Problem trouble shooting of PD Pumps






Confidentiality Statement

This training manual is prepared exclusively for the technical knowledge

enhancement of the personnel of Reliance Group of Industries.

No part of this document may be reproduced in any form , in an electronic

retrieval system or otherwise. The document must be returned when the

recipient has no further use of the same .The document or any part of the

document is not allowed to be taken out of the respective site or to be shared

with any person outside the Reliance Group.

Reliance Group of industries reserves the right to refuse access to the above

document on the grounds of Confidentiality.

Authorization for information disclosure is allowed with the written permission

of the respective site Engineering head.






Module Implementation Plan

Topic: Positive Displacement Pump Code No: IPCLDSMEC006 For: Date :12.10.2004 Rev:00 Site :IPCL-NC Sn Contents Author Resources Available

( Y/N) Learning Validation Trainer

1 Introduction JP Internet Y Self study ½ hr Quiz -

2 Terminology & Definitions JP Internet/ API 676 Y Self

study ½ hr Quiz -

3 Positive displacement Pumps Classification JP Internet Y

Self study

½ hr Quiz -

4 Fundamentals & Operating Principles JP Internet Y Self

study 3 hrs Quiz -

5 Performance Characteristics JP Internet Y Self

study 2 hrs Quiz

6 Controlling Positive Displacement Pump JP Internet. Y Self

study 2.1/2 hrs Quiz

Total ½ hrs

-






Sn Contents Author Resources Available

( Y/N) Learning Validation Trainer

7 Selection Guide Of Positive Displacement Pump JP Internet. Y Self

study 1 hr Quiz

-

8 Field Problems & Trouble shooting JP Internet. Y Self

study 1.1/2 hrs Quiz

-

9

Tips to Improve PD Pump Efficiency

JP

Internet / “Metering Pumps” by James P. Poynton

Y Self study ½ hr

Quiz

-

Category: IPCL Module No. Positive Displacement Pumps

NC: Training Module IPCLDSMEC007

Prepared by: Jitendra Purwar




Table of Contents

1.0 Introduction 8-11 2.0 Definitions & Terminology 12-18 3.0 Positive displacement Pumps Classification 19-21 4.0 Fundamentals & Operating Principles 22-61

4.1 Rotary Pump 4.1.1 Multiple Rotor 4.1.1.1 Gear 4.1.1.2 Lobe 4.1.1.3 Circumferential Piston 4.1.1.4 Screw 4.1.2 Single Rotor 4.1.2.1 Vane 4.1.2.2 Piston 4.1.2.3 Progressive Cavity 4.1.2.4 Screw 4.1.2.5 Peristaltic 4.2 Reciprocating Pump 4.2.1 Diaphragm 4.2.2 Piston Plunger 4.3 Function of Relief Valves in PD Pump 4.4 Calculating Friction Losses 4.4.1 Laminar Flow 4.4.2 Turbulent Flow







4.4.3 Reynolds Number

4.5 Slip 4.6 Volumetric Efficiency 4.7 Calculating Horsepower 4.8 Calculating Torque 4.9 Net Positive Inlet Pressure (NPIP) 4.10 Cavitation 4.11 Affinity Laws

5.0 Performance Characteristics 62-71 6.0 Controlling Positive Displacement Pump 72-82

6.1 Discharge Throttling 6.2 Suction Throttling 6.3 Recycle Control 6.4 Speed Control 6.5 Other Means Of Control 6.6 Machine Protection 6.7 Parallel Pump Installations 6.8 Series Pump Installations

7.0 Selection Guide Of PD Pumps 83-85 8.0 Field Problems & Trouble shooting 86-91 9.0 Tips to Improve PD Pumps Efficiency 92

10.0 Summary 93 11.0 References 94 12.0 Frequently Asked Questions (FAQs)







Chapter 1.0

Introduction to Positive Displacement Pumps







1.0 Introduction

Pumping devices have been an important way of moving fluids for thousands of

years. The Ancient Egyptians invented water wheels with buckets mounted on

them to move water for irrigation. In the 200's B.C. Ctesibius, a Greek inventor,

made a reciprocating pump for pumping water. At about the same time,

Archimedes, a Greek mathematician, invented a screw pump made of a screw

rotating in a cylinder (now known as an Archimedes screw). This type of pump

was used to drain and irrigate the Nile Valley. Times may have changed but

pumps still operate in the same basic way.

A Positive Displacement Pump has an expanding cavity on the suction side of

the pump and a decreasing cavity on the discharge side. Liquid is allowed to flow

into the pump as the cavity on the suction side expands and the liquid is forced

out of the discharge as the cavity collapses. This principle applies to all types of

Positive Displacement Pumps whether the pump is a rotary lobe, gear within a

gear, piston, diaphragm, screw, progressing cavity etc.






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A Positive Displacement Pump, unlike a Centrifugal Pump, will produce the same

flow at a given RPM no matter what the discharge pressure is. A Positive

Displacement Pump cannot be operated against a closed valve on the discharge

side of the pump, i.e. it does not have a shut-off head like a Centrifugal Pump

does. If a Positive Displacement Pump is allowed to operate against a closed

discharge valve it will continue to produce flow which will increase the pressure in

the discharge line until either the line bursts or the pump is severely damaged or

both.

In contrast Centrifugal Pumps rely on kinetic energy rather than mechanical

means to move liquid. Liquid enters the pump at the center of a rotating impeller

and gains energy as it moves to the outer diameter of the impeller. Liquid is

forced out of the pump by the energy it obtains from the rotating impeller.

Centrifugal pumps can transfer large volumes of liquid but efficiency and flow

decrease rapidly as pressure and/or viscosity increases. PD pumps allow a wider






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range of liquids, slurries and foams to be transported without product

degradation. Understanding where PD pumps can be used may lead to

opportunities to improve processes.

Comparisons Between Rotary and Centrifugal Pumps

Parameters Rotary Centrifugal

Max Viscosity , cSt 1,320,000 550

Max. Capacity ,M3/Hr 750 27,250

Pumping Efficiency E A

Energy Costs E A

Self-Priming Yes No

Flow Control E P

Life-Cycle Cost G G

Initial Cost A E

E = Excellent, G = Good, A = Average, P = Poor

*****End of Chapter 1.0****






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Chapter 2.0

Definitions & Terminology






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2.0 Definitions & Terminology 2.1 Fluids and liquids The term "fluid" covers liquids, gases, vapors and mixtures thereof. The word

"liquid" is used only to describe true liquids that are free of vapors and solids.

The word "fluid" is more general and is used to describe liquids that may contain,

or be mixed with, matter in other than the liquid phase.

2.2 Pumping chamber The pumping chamber is the space formed by the body and end plate(s), into

which fluid is drawn and from which fluid is discharged by the action of the

rotor(s).

2.3 Inlet or suction port

One or more openings in the pump through which the pumped fluid may enter

the pumping chamber.

2.4 Outlet or discharge port

One or more openings in the pump through which the pumped fluid may leave

the pumping chamber.

2.5 Body

The body is an external part which surrounds the periphery of the pumping

chamber and which also may form one end plate. It is sometimes called a casing

or a housing .






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2.6 End plate

An end plate is a part which closes an end of the body to form the pumping

chamber. One or more are used, depending on the construction of the pump. It is

sometimes called a head or cover .

2.7 Stator

The stationary parts of the pump which surround the pumping chamber.

2.8 Rotor

A rotor is a part which rotates in the pumping chamber. One or more are used

per pump. It is sometimes referred to by a specific name such as gear , screw ,

impeller , etc.

2.9 Bearing

A bearing is a part which supports or positions the shafts on which a rotor is

mounted. A bearing may be internal (wetted by the liquid being pumped) or

external and may be either a rolling element bearing (ball or roller) or fluid film

type (sleeve and journal).

2.10 Timing gear

A timing gear is a part used to transmit torque from one rotor shaft to another and

to maintain the proper angular relationship of the rotors. It may be outside the

pumping chamber and is sometimes called a pilot gear .






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2.11 Rotating assembly

The rotating assembly generally consists of all rotating parts essential to the

pumping action but also may include other parts specified by the manufacturer.

2.12 Relief valve

A relief valve is a mechanism designed to control or to limit pressure by the

opening of an auxiliary passage at a predetermined pressure.

A relief valve may be either integral with the body or end plate or attachable. It

may be adjustable through a predetermined range of pressures or have a fixed

setting. It may be designed to bypass the liquid internally from the pump outlet to

the pump inlet or externally through an auxiliary port. Bypass of liquid internally is

not recommended for continuous operation.

Terms commonly used in specifying performance are:

2.13 Cracking pressure

Sometimes called set pressure , start-to-discharge pressure , or popping

pressure --the pressure at which the valve just starts to open. This pressure

cannot be determined readily in a valve which bypasses the liquid within the

pump.

2.14 Full-flow bypass pressure

The pressure at which the full output of the pump flows through the valve and the

auxiliary passage.

2.15 Reseating pressure






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The pressure at which the valve is closed completely. This pressure is usually

below the cracking pressure and is difficult to measure accurately when the liquid

is bypassed within the pump.

2.16 Percent overpressure

Sometimes called percent accumulation or percent regulation --the difference

between full bypass pressure and cracking pressure, expressed as a percent of

cracking pressure.

2.17 Stuffing-box

A stuffing-box is a cylindrical cavity through which a shaft extends and in which

leakage at the shaft is controlled by means of packing and a gland or a

mechanical seal.

2.18 Gland

A gland is a part which may be adjusted to compress packing in a stuffing-box. It

is sometimes called a gland follower . A gland is also used to hold the stationary

element of a mechanical seal.

2.19 Packing

A pliable lubricated material used to provide a seal around that portion of the

shaft located in the stuffing-box (see Figure 3.28).

2.20 Lantern ring

A lantern ring is an annular ring located in a stuffing-box to provide space

between or adjacent to packing rings for the introduction of a lubricant or a






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barrier fluid, the circulation of a cooling medium, or the relief of pressure against

the packing. It is sometimes called a seal cage .

2.21 Seal chamber

A seal chamber is a cavity through which a shaft extends and in which leakage at

the shaft is controlled by means of a mechanical seal or a radial seal.

2.22 Mechanical seal

A mechanical seal is a device located in a seal chamber or stuffing-box and

consists of rotating and stationary elements with opposed seal faces. A rotating

element is fastened and sealed to the shaft. A stationary element is mounted and

sealed to the gland or body. At least one element is loaded in an axial direction,

so that the seal faces of the elements are maintained in close proximity to each

other at all times. Usually, the seal faces are flat, highly lapped surfaces on

materials selected for low friction and for resistance to corrosion by the fluids to

be pumped. Mechanical seals are sometimes called face type seals .

2.23 Radial seal

A radial seal is a device located in a seal chamber which seals on its outside

diameter through an interference fit with its mating bore and on the rotating shaft

with a flexible, radially loaded surface. Radial seals include: lip type seals, O

rings, V cups, U cups, etc., and may or may not be spring-loaded.

2.24 Direction of rotation

Drive shaft rotation is designated as "clockwise" (CW) or "counterclockwise"

(CCW) as determined when viewing the pump from the driver end.






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2.25 Jacketed pump

A jacketed pump is one in which the body and/or end plates incorporate

passageways through which steam, oil, water, or other fluid can be circulated to

control the temperature of the pump or the fluid in the pump.

****End of Chapter 2.0****






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Chapter 3.0

Positive displacement Pumps Classification






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3.0 Classification of Positive displacement Pumps The Hydraulic Institute Standards book differentiates PD pumps into rotary and

reciprocating pumps. Rotary pumps are defined as being: vane, piston, flexible

member, lobe, gear circumferential piston, or screw pumps. In all of the rotary

designs, the chamber is created progressively through rotation of the drive shaft.

There may be one or more chambers opened per revolution depending on the

design. The chambers are sealed off from suction by close clearance between

the rotor and the housing, or by close clearance between intermeshing rotors.

Rotation of the shaft moves the chamber along the bore or housing towards

discharge. The chamber is displaced to discharge by rotation. The release to

discharge progresses with rotation as the volume is expelled so that the flow is

typically pulsation free.

Reciprocating pumps are defined as being; steam, power, or controlled volume

pumps. In all reciprocating pumps, there are check valves on the suction and

discharge. Fluid flows through the suction valve and into the chamber as the

plunger, piston, or diaphragm recedes. At the end of the stroke, the chamber is at

its maximum size. The suction valve closes, the plunger moves forward into the

chamber, forcing the fluid out the discharge valve. The flow from each chamber

is a pulse flow. If the pump has several chambers, they are timed to have

sequential pulses to minimize the overall pulsation.






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Chapter 4.0

Fundamentals & Operating Principles






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4.0 Fundamentals & Operating Principles PD pumps come in many designs and operating ranges, but they all work on the

same principle. An increasing volume is opened to suction, filled, closed, moved

to discharge, and displaced. The delivered capacity is nearly constant throughout

the discharge pressure range. This constant capacity will intersect a system

curve at a defined point, allowing a high degree of system control.

4.1 Rotary Pump

By definition ,Rotary Positive Displacement (PD) pumps displace a known

quantity of liquid with each revolution of the pumping elements (i.e., gears,

lobes, screws, vanes). PD pumps displace liquid by creating a space between

the pumping elements and trapping liquid in the space. The rotation of the

pumping elements then reduces the size of the space and moves the liquid out of

the pump. Rotary PD pumps have very tight internal clearances which minimize

the amount of liquid that slips back from discharge to suction side of the pump.

Because of this, they are very efficient. PD pumps can handle fluids of all

viscosities up to 1,320,000 cSt capacities up to 1,150 M3/Hr , and pressures up

to 700 bar. Rotary pumps are self-priming and deliver a constant, smooth flow,

regardless of pressure variations(Differential pressure).

4.1.1 Multiple Rotor

4.1.1.1 Gear Pump

Internal Gear

Internal gear pumps carry fluid between the gear teeth from the inlet to outlet

ports. The outer gear (rotor) drives the inner or idler gear on a stationary pin.

The gears create voids as they come out of mesh and liquid flows into the






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cavities. As the gears come back into mesh, the volume is reduced and the

liquid is forced out of the discharge port. The crescent prevents liquid from

flowing backwards from the outlet to the inlet port.

Internal Gear Pump

1. Liquid enters the suction port between the rotor (large exterior gear) and idler

(small interior gear) teeth. The arrows indicate the direction of the pump and

liquid.

2. Liquid travels through the pump between the teeth of the "gear-within-a-gear"

principle. The crescent shape divides the liquid and acts as a seal between the

suction and discharge ports.

3. The pump head is now nearly flooded, just prior to forcing the liquid out of the

discharge port. Intermeshing gears of the idler and rotor form locked pockets for

the liquid which assures volume control.

4. Rotor and idler teeth mesh completely to form a seal equidistant from the

discharge and suction ports. This seal forces the liquid out of the discharge port.






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Advantages

• Only two moving parts.

• Only one stuffing box.

• Positive suction, nonpulsating discharge.

• Ideal for high-viscosity liquids.

• Constant and even discharge regardless of pressure conditions.

• Operates well in either direction.

• Can be made to operate with one direction of flow with either rotation.

• Low NPSH required.

• Single adjustable end clearance.

• Easy to maintain.

• Flexible design offers application customization.

Disadvantages

• Usually requires moderate speeds.

• Medium pressure limitations.

• One bearing runs in the product pumped.

• Overhung load on shaft bearing.

The rotor gear is driven by a shaft supported by journal or antifriction bearings.

The idler gear contains a journal bearing rotating on a stationary pin in the

pumped liquid. Depending on shaft sealing arrangements, the rotor shaft support

bearings may run in pumped liquid. This is an important consideration when

handling an abrasive liquid and can wear out a support bearing.

The speed of internal gear pumps is considered relatively slow compared to

centrifugal types. Speeds up to 1,150 rpm are considered common, although

some small designs operate up to 3,450 rpm. Because of their ability to operate

at low speeds, internal gear pumps are well suited for high-viscosity applications






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and where suction conditions call for a pump with minimal inlet pressure

requirements.

For each revolution of an internal gear pump, the gears have a fairly long time to

come out of mesh allowing the spaces between gear teeth to completely fill and

not cavitate. Internal gear pumps have successfully pumped liquids with

viscosities above 1,320,000 cSt / 6,000,000 SSU and very low viscosity liquids,

such as liquid propane and ammonia.

Internal gear pumps are made to close tolerances and are damaged when

pumping large solids. These pumps can handle small suspended particulate in

abrasive applications, but gradually wear and lose performance. Some

performance loss is restored by adjusting the pump end clearance. End

clearance is the closeness of the rotor gear to the head of the pump.

External Gear. External gear pumps also use gears which come in and out of

mesh. As the teeth come out of mesh, liquid flows into the pump and is carried

between the teeth and the casing to

the discharge side of the pump. The teeth come back into mesh and the liquid is

forced out the discharge port. External gear pumps rotate two identical gears

against each other. Both gears are on a shaft with bearings on either side of the

gears.






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External Gear Pump

External gear pumps are similar in pumping action to internal gear pumps in that

two gears come into and out of mesh to produce flow. However, the external

gear pump uses two identical gears rotating against each other -- one gear is

driven by a motor and it in turn drives the other gear. Each gear is supported by a

shaft with bearings on both sides of the gear.

1. As the gears come out of mesh, they create expanding volume on the inlet

side of the pump. Liquid flows into the cavity and is trapped by the gear teeth as

they rotate.

2. Liquid travels around the interior of the casing in the pockets between the

teeth and the casing -- it does not pass between the gears.

3. Finally, the meshing of the gears forces liquid through the outlet port under

pressure.

Because the gears are supported on both sides, external gear pumps are quiet-

running and are routinely used for high-pressure applications such as hydraulic

applications. With no overhung bearing loads, the rotor shaft can't deflect and

cause premature wear.

Advantages

• High speed.






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• Medium pressure.

• No overhung bearing loads.

• Relatively quiet operation.

• Design accommodates wide variety of materials.

Disadvantages

• Four bushings in liquid area.

• No solids allowed.

• Fixed End Clearances

Because the gears are supported on both sides, external gear pumps are used

for high pressure applications such as hydraulics. Usually, small external gear

pumps operate at 1,750 or 3,450 rpm and larger versions operate at speeds up

to 640 rpm.

The design of external gear pumps allows them to be made to closer tolerances

than internal gear pumps. The pump is not very forgiving of particulate in the

pumped liquid. Since there are clearances at both ends of the gears, there is no

end clearance adjustment for wear. When an external gear pump wears, it must

be rebuilt or replaced.

External gear pumps handle viscous and watery-type liquids, but speed must be

properly set for thick liquids. Gear teeth come out of mesh a short time, and

viscous liquids need time to fill the spaces between gear teeth. As a result, pump

speed must be slowed down considerably when pumping viscous liquids.

The pump does not perform well under critical suction conditions. Volatile liquids

tend to vaporize locally as gear teeth spaces expand rapidly. When the viscosity

of pumped liquids rises, torque requirements also rise, and pump shaft strength






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may not be adequate. Pump manufacturers supply torque limit information when

it is a factor.

Reversing Gear Pump

This type of rotary gear pump moves liquid in the same direction regardless of

the direction the gears turn.

The valves (shown in yellow in this drawing) are forced open and closed by the

difference in pressure on the input and output sides.

In this drawing, fluid in the higher-pressure output side is shown in light blue

while fluid in the lower-pressure input side is dark blue.

Notice that when the green gear starts moving clockwise, the fluid in the outer

circular tubes changes pressure but stops flowing, and fluid flows more or less

straight up.

When the red gear is moving clockwise, fluid travels in a sideways 'S' shape

through the outer circular tubes.

The pump in above drawing reverses direction about every 8 seconds.






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4.1.1.2 Lobe Pump Fluid is carried between the rotor teeth and the pumping chamber. The rotor

surfaces create continuous sealing. Both gears are driven and are synchronized

by timing gears. Rotors include bi-wing, tri-lobe, and multi-lobe configurations.

How Lobe Pumps Work

Lobe pumps are similar to external gear pumps in operation in that fluid flows

around the interior of the casing. Unlike external gear pumps, however, the

lobes do not make contact. Lobe contact is prevented by external timing gears

located in the gearbox. Pump shaft support bearings are located in the gearbox,

and since the bearings are out of the pumped liquid, pressure is limited by

bearing location and shaft deflection.

1. As the lobes come out of mesh, they create expanding volume on the inlet

side of the pump. Liquid flows into the cavity and is trapped by the lobes as they

rotate.

2. Liquid travels around the interior of the casing in the pockets between the

lobes and the casing -- it does not pass between the lobes.

3. Finally, the meshing of the lobes forces liquid through the outlet port under

pressure.

Lobe pumps are frequently used in food applications because they handle solids

without damaging the product. Particle size pumped can be much larger in lobe

pumps than in other PD types. Since the lobes do not make contact, and

clearances are not as close as in other PD pumps, this design handles low

viscosity liquids with diminished performance. Loading characteristics are not as

good as other designs, and suction ability is low. High-viscosity liquids require






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reduced speeds to achieve satisfactory performance. Reductions of 25% of

rated speed and lower are common with high-viscosity liquids.

Lobe Pump

Lobe pumps are frequently used in food applications, because they handle solids

without damaging the pump. Particle size pumped can be much larger in lobe

pumps than in other PD types. Since the lobes do not make contact, and

clearances are not as close as in other PD pumps, this design handles low

viscosity liquids with diminished performance. Loading characterisics are not as

good as other designs, and suction ability is low. High-viscosity liquids require

considerably reduced speeds to achieve satisfactory performance. Reductions of

25% of rated speed and lower are common with high-viscosity liquids.

Lobe pumps are cleaned by circulating a fluid through them. Cleaning is

important when the product cannot remain in the pumps for sanitary reasons or

when products of different colors or properties are batched.






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Advantages

• Pass medium solids.

• No metal-to-metal contact.

• Superior CIP/SIP capabilities.

• Positive suction, nonpulsating discharge.

Disadvantages

• Requires timing gears.

• Requires two seals.

• Reduced lift with thin liquids.

4.1.1.3 Circumferential Piston

Fluid is carried from inlet to outlet in spaces between piston surfaces. Rotors

must be timed by separate means, and each rotor may have one or more piston

elements.

4.1.1.4 Screw Pump

Several different types of screw pumps exist as shown in following figures.. The

differences between the various types are the number of intermeshing screws

and the pitch of the screws. Screw pumps are used aboard ship to pump fuel and






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lube oil and to supply pressure to the hydraulic system. In the double-screw

pump, one rotor is driven by the drive shaft and the other by a set of timing

gears. In the triple-screw pump, a central rotor meshes with two idler rotors

In the screw pump, liquid is trapped and forced through the pump by the action of

rotating screws. As the rotor turns, the liquid flows in between the threads at the

outer end of each pair of screws. The threads carry the liquid along within the

housing to the center of the pump where it is discharged.

One of the disadvantages of screw-type positive displacement rotary pumps is

that they have performance characteristics sensitive to viscosity change.






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Most screw pumps are now equipped with mechanical seals. If the mechanical

seal fails, the stuffing box has the capability of accepting two rings of

conventional packing for emergency use.

Single screw pumps are commonly called progressive cavity pumps. They have

a rotor with external threads and a stator with internal threads. The rotor threads

are eccentric to the axis of rotation. Multiple screw pumps have multiple external

screw threads. These pumps may be timed or untimed.

Single Screw

The single screw pump is more commonly known as the Archimedean screw. It

is quite large; typical dimensions include a diameter of 12 inches or greater, and

a length up to about 50 feet. It is normally used as a water-raising pump with the

screw arranged at an angle of 30 degrees.

It can also be used for handling liquids containing solids in suspension with either

vertical lift or horizontal transport. The design of single screw pumps allows very

little fracturing of particles and little abrasion damage to the pump. One

disadvantage is the considerable bulk necessary to achieve high capacities since

rotational speeds are of the order of 30-60 rpm .

Intermeshing Screw Pump

The intermeshing screw pump is commonly called a rigid-screw pump. This type






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of pump is suitable for a wide range of sizes, and can be run at high speeds. The

larger screw pumps are used for bulk handling of oils and similar fluids. The

basic type is suitable for handling most clean fluids with low flow velocities and at

low heads.

Eccentric screw pump

The eccentric screw pump is versatile. It is capable of handling a variety of

liquids and products with high efficiency. It comprises of a rigid screw form rotor

rolling in a resilient internal helical stator of hard or soft rubber with a moderately

eccentric motion. It can handle viscous liquids, slurries, pastes, solids in

suspension, and delicate products. This is because of the low flow velocities

through the pump .

Capacity

The delivered capacity of any screw pump is the theoretical capacity minus the

internal leakage. In order to find the capacity of a screw pump the speed of the

pump must be known. The delivered capacity of any rotary screw pump can be

increased several different ways. The capacity can be increased by simply

increasing the speed, increasing the viscosity, or decreasing the differential

pressure. The capacity of the pump depends on several factors

Diameter of the screw

Speed of the screw

Number of flights mounted on the screw shaft






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Flights: Single, double, and triple flights are often used. Flights are also known

as helixes. With each increase in flights, there is a 20% increase in capacity.

Therefore, a single flight pump has a capacity that is 80% of a double flight

pump, which in turn has a capacity that is 80% of a triple flight capacity. The

three-flight pump can handle the most capacity in the least amount of space.

Advantages

Wide range of flows and pressures

Wide range of liquids and viscosities

Built-in variable capacity

High speed capability allowing freedom of driver selection

Low internal velocities

Self-priming with good suction characteristics






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High tolerance for entrained air and other gases

Minimum churning or foaming

Low mechanical vibration, pulsation-free flow, and quiet operation

Rugged, compact design -- easy to install and maintain

High tolerance to contamination in comparison with other rotary pumps

Disadvantages

Relatively high cost because of close tolerances and running clearances

Performance characteristics sensitive to viscosity change

High pressure capability requires long pumping elements (Fraser, et. al., 1986)

Applications

Utilities fuel oil service,

Industrial oil burners,

lubricating oil service,

Chemical processes,

Petroleum and crude oil industries

Power hydraulics

Listed below are some typical situations where a screw pump is used. Raw sewage lift stations: Can handle variety of raw sewage influent, are non-






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clogging, require little attention, are resistant to motor overloads, and are not

affected by running dry .

Sewage plant lift stations: Used for sewage lifts up to 40 feet and have self-

regulating lift capacity (Normal lifts are 30 feet, while high lifts are 40 feet high.)

Return activated sludge: Little floc disintegration, non turbulent discharge into

effluent channel, low horsepower requirements, improved activated sludge

treatment.

Storm water pumping: Are ideal because of large capacity at low heads, no

prescreening necessary

Land Drainage: Used for flood control, can pump large volumes of water over

levees.

4.1.2 Single Rotor 4.1.2.1 Vane Pump

Despite the different configurations, most vane pumps operate under the same

general principle described below.

A slotted rotor or impeller is eccentrically supported in a cycloidal cam. The rotor

is located close to the wall of the cam so a crescent-shaped cavity is formed. The

rotor is sealed into the cam by two sideplates. Vanes or blades fit within the slots

of the impeller. As the impeller rotates (yellow arrow) and fluid enters the pump,

centrifugal force, hydraulic pressure, and/or pushrods push the vanes to the walls

of the housing. The tight seal among the vanes, rotor, cam, and side plate is the

key to the good suction characteristics common to the Vane pumping principle.






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Vane

Pump

The housing and cam force fluid into the pumping chamber through holes

in the cam (small red arrow on the bottom of the pump). Fluid enters the

pockets created by the vanes, rotor, cam, and sideplate.

3. As the impeller continues around, the vanes sweep the fluid to the

opposite side of the crescent where it is squeezed through discharge

holes of the cam as the vane approaches the point of the crescent (small

red arrow on the side of the pump). Fluid then exits the discharge port.

Vanes are the main sealing element between the suction and discharge

ports and are usually made of a nonmetallic composite material. Rotor

bushings run in the pumped liquid or are isolated by seals.






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Vane pumps usually operate at 1,000 rpm, but also run at 1,750 rpm. The

pumps work well with low-viscosity liquids that easily fill the cavities and provide

good suction characteristics. Speeds must be reduced

dramatically for high-viscosity applications to load the area underneath the

vanes. These applications require stronger-than-normal vane material.

Because there is no metal-to-metal contact, these pumps are frequently used

with low-viscosity non lubricating liquids such as propane or solvent. This type of

pump has better dry priming capability than other PD pumps. Vane pumps can

run dry, but are subject to vane wear.

Abrasive applications require the proper selection of vane material and seals.

Vane pumps have fixed end clearances on both sides of the rotor and vanes

similar to external gear pumps. Once wear occurs, this clearance cannot be

adjusted, but some manufacturers supply replaceable or reversible end plates.

Casing liners are a low-cost way of restoring pump performance as wear occurs.

Unlike lobe pumps, vane pumps cannot handle solids.

4.1.2.2 Piston Pumps The basic Piston Pump is very simple having just two valves and one stuffing

box. as shown in following figure.

In this example the reciprocating piston is driven back and forth by a rotating

mechanism.

This piston pump uses suction to raise water into the chamber. The lower valve

can be placed below water level.

The piston must be within about 25 feet of the water level, but the water can then

be raised quite high.






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Piston Pump

Radial Piston Pumps can produce a very smooth flow under extreme pressure.

Generally they are variable-displacement pumps.

In variable models, flow rate changes when the shaft holding the rotating pistons

is moved with relation to the casing (in different models either the shaft or the

casing moves.) Output can also be varied by changing the rotation speed.

Radial Piston Pump






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In this animation if the casing (shown in red) is moved to the left, the flow rate

would decrease to zero. If it is moved even further to the left the flow would

reverse.

Input in this animation is through the TOP two black holes near the center below

the "Pintle" (shown in yellow). Output is through the BOTTOM two black holes,

above the pintle. Higher pressure areas are indicated with a DARKER blue fluid

color.

The pistons are usually forced out by springs. They are forced back in, expelling

liquid, by the casing.

An odd number of pistons is always used to smooth the hydraulic balance. These

pumps revolve at speeds up to about 1200 RPM.

4.1.2.3 Progressive Cavity Pump

Progressive (or Progressing) Cavity pumps, a type of Single Screw pump, are

used for highly viscous liquids such as peanut butter or glue, and also for liquids

with significant amounts of solids such as cement or sand slurry.

Fuild proceeds from the entrance, at the top on the right side here, to the left.

The rotor revolves inside the stator.

The stator is a twisted cavity with an oval-shaped cross-section. It is usually

made of natural or synthetic rubber, steel, or plastic. The rotor is usually steel.

For a given diameter and shape of the rotor, doubling the number of stages (the

length) will double the output pressure.






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The area of the cross-section of the rotor determines the backpressure the pump

must withstand.

progressive cavity pump is a kind of pump which moves fluid by means of a

cavity which progresses along the body of the pump. As the cavity moves, fluid is

Progressive Cavity Pump

sucked in to fill the cavity, further rotation of the pump causes the fluid to flow

and be delivered from the pump.

The rotor of the pump is a steel helix which has been coated in a smooth hard

surface, normally chromium. The rotor fits inside a pump body or stator which

normally is a rubber lined steel tube. The rubber core of the stator has a helical

cavity, the wavelength of each helix in the rotor is exactly half that of the rotor.

Rotation of the rotor inside the stator causes the cavity to progress along the

pump thus inducing fluid flow. Compare to a Archimedes' screw.

Whilst progressive cavity pumps offer long life and reliable service, abrasive

fluids will significantly shorten the life of the stator. The interface between rotor

and stator is lubricated by the fluid being pumped, however if the pump is

allowed to 'run dry' rapid deteriotation of the stator results.

Applications

Small sewage pumping

Sewage sludge pumping

Slurry pumping






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4.1.2.3 Peristaltic Pump A peristaltic pump is a type of positive displacement pump used for pumping a

variety of specialized fluids. The fluid is contained in a flexible hose fitted inside a

circular pump casing. A rotor with a number of cams (also called 'shoes' or

'wipers') attached to the external circumference compresses the flexible tube. As

the rotor turns, the part of tube under compression rotates thus forcing the fluid to

be pumped to move through the tube. This process is called peristalsis and is

used in many biological systems such as the gastrointestinal tract. The pump

casing is typically filled with lubricant to prevent abrasion of the exterior of the

pump tube.

Peristaltic Pump

Since the only part of the pump in contact with the fluid being pumped is the

interior of the tube, it is easy to sterilise and clean the inside surfaces of the

pump. Furthermore, since there are no moving parts in contact with the fluid,

peristaltic pumps are inexpensive to manufacture.

Applications

Dialysis machines

Food manufacturing

Pharmaceutical production






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Sewage sludge

tabletop decorative fountains and waterwalls

4.2 Reciprocating Pump

The pumping action in any reciprocating pump is dependent upon the positive

displacement or the fluid pumped by a piston or plunger. The capacity of the

pump is, therefore, determined by the area of the piston and its rate of travel. In

order to obtain a practical machine, some method of reversing the direction of the

pistons is required. In the direct-acting steam pump, this is accomplished by the

steam valves and valve gear; in power pumps, this is accomplished by use of

crank and connecting rods.

The ability of the pump to produce pressure is dependent upon the ratio of total

steam force (steam pressure per unit area x area of Steam piston) to total liquid

force (pump head x area of liquid piston). In order that pumping may occur, it is

necessary that the steam force exceed the liquid force by an amount which

slightly exceeds the various mechanical and hydraulic losses encountered. The

basic principles for steam pump operation are shown below.






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Types Of Reciprocating Pumps

•Direct-acting

.Horizontal or vertical.

.Single or duplex. A single pump has one liquid piston or its equivalent

single or double-acting plunger; a duplex pump has two liquid pistons or their

equivalent single or double-acting plungers.

.Single or double-acting. A single-acting unit pumps on one direction of

piston travel only whereas double-acting units pump on both strokes. Direct-

acting steam pumps are usually double acting.

2. Direct-acting steam pumps are conventionally described by stating the steam

cylinder diameter, the liquid cylinder diameter, the length of stroke, horizontal or

vertical (H or V), single or duplex (S or D), and single or double-acting (SA or

DA). Thus a pump identified as 11 x 8 x 18

Usage In Maritime Service

•Auxiliary feed.

•Standby fuel oil service.

•Fuel oil transfer.

•Auxiliary circulating and condensate.

•Fire and bilge.

•Ballast.

•High pressure evaporator.

•Lubricating oil transfer.

•Cargo stripping.

•General service.






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Direct-acting steam reciprocating pumps are not obsolete. If the steam conditions

are not too severe in pressure, temperature, or superheat, they have many

features of simplicity, reliability, and economy of operation and maintenance that

still warrant serious consideration for many services.

Pumping Capacity

Simplex single acting pumps discharge the cylinder volume for each 2 strokes.

The forward stroke discharges the cylinder and the back stroke or reverse stroke

fills the cylinder.

Duplex double acting pumps use 2 double-acting cylinders in parallel, and pump

two cylinder volumes for each pump stroke.






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Duplex single acting pumps use 2 single-acting cylinders in parallel, and pump

one cylinder volume for each pump stroke.

Pump Capacity i= volume discharged in m3 per pump stroke multiplied by

strokes per minute.

To determine the volume of the cylinder, multiply the area of the circle by the

height of the cylinder.

Volume of a Cylinder is equal to:

= (area of the circle) * (height)

= (PI X r2) * (height)

4.2.1 Diaphragm-type reciprocating pump

The diaphragm pump is an offshoot of a plunger pump. Because of the risk that

contamination could travel between the plunger and the cylinder, the diaphragm

is safer for microbial processing .Type is indirect acting .This applet is crude but

shows how a reciprocating piston (plunger) pump works. The flywheel that

moves the plunger can attach the arm to the plunger at various points to change

the amplitude of the stroke and thus the pumping rate. The pumping rate can






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also be changed with a different rotational speed, but variable speed motors or

mechanical means of changing rpm are expensive.






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4.2.2 Plunger type Metering Pumps Metering pumps provide precision control of very low flow rates. Flow rates are

generally less than 1/2 gallon per minute. They are usually used to control

additives to the main flow stream. They are also called proportioning or

controlled-volume pumps. Metering pumps are available in either a diaphragm or

packed plunger style, and are designed for clean service and dirty liquid can

easily clog the valves and nozzle connections.

This is a picture of a plunger-type metering pump:

4.2 Function of Relief Valves in PD Pump

A relief valve on the discharge side of a Positive Displacement Pump is an

absolute must - no ifs, and / or buts about it!

Internal Relief Valves

Pump manufacturers normally have an option to supply an internal relief valve.

These relief valves will temporarily relieve the pressure on the discharge side of

a pump operating against a closed valve. They are normally not full ported, i.e.

cannot by pass all the flow produced by the pump. These internal relief valves

should be used for pump protection against a temporary closing of a valve.






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External Relief Valve

An external relief valve (RV) installed in the discharge line with a return line back

to the supply tank is highly recommended to provide complete protection

against an unexpected over pressure situation.

4.3 Calculating Friction Losses

Frictional losses are directly proportional to

Length of pipe

Pipe Diameter

Flow rate

Frictional losses due to flow in pipes occur in two principle modes:

Laminar & Turbulent

4.3.1 Laminar Flow

The fluid moves through the pipe in concentric layers with maximum velocity in

the center of the pipe, decreasing towards the walls where the fluid particles are

essentially standing still. A cross section of the velocity would appear as shown

below. There is very little mixing of the fluid across the pipe cross section.






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4.4.2 Turbulent Flow

In turbulent flow considerable mixing takes place and the velocity is nearly

the same across the pipe cross section as shown below. Turbulent flow is

more likely to occur in thinner liquids and is often characterized by higher

friction losses than would be expected.

4.4.3 Reynolds Number

A convenient number, called the Reynolds number, can be used for estimating

the transition between laminar and turbulent flow. The Reynolds number, a ratio

of flow rate to viscosity, can be computed by the relation:

R = Q (/) d (x) u (/) SG

Where

R = Reynolds Number

Q = Flow in GPM

d = Pipe Diameter in inches

u = Viscosity in poises






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SG = Specific Gravity

Laminar Flow exists if R > 63

Turbulent Flow exists if R > 63

Whether flow is under Laminar or Turbulent conditions becomes extremely

important when computing frictional losses in valves and fittings.

4.5 Slip

A major effect on positive displacement pump performance is the loss in flow due

to slip. The expanding cavity on the inlet side of a positive displacement pump

creates a low pressure area that asks to be filled with fluid. This cavity can be

filled with fluid from the inlet line in normal performance. However, if slip occurs,

the cavity will also be partly filled with fluid flowing back through the pump

clearances from the outlet side.

Pump performance is dependent on the slip which occurs in a pump.

Slip Increases:

Directly with Pressure

Directly with Clearance

Inversely with Viscosity






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4.6 Volumetric Efficiency

When a positive displacement pump is operating under a slip condition the pump

looses the ability to deliver the volume of fluid it is theoretically capable of

pumping.

Volumetric Efficiency = Actual Flow / Theoretical Flow

For a given pump and fluid ,the slip is proportional to the pressure differential

from outlet to inlet. If the pump had no slip the volume pumped would be directly

proportional to speed (RPM).

When slip occurs the Actual Flow is reduced.






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If a certain flow is required at a given pressure the speed (RPM) must be

increased.

At a given speed (RPM) and viscosity if the pressure is increased the slip will

increase and the actual flow will decrease.

For a given pressure differential if the viscosity increases the slip will decrease.






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4.7 Calculating Horsepower

The Horsepower required to operate a Positive Displacement Pump has

two factors

The Work Horsepower (WHP) - the actual work done:

WHP = Flow(GPM) (X) Pressure(PSI) (/) 1714

The Viscous Horsepower(VHP) - the power required to turn the rotors,

gears, etc. inside the viscous fluid. The Viscous Horsepower required is

determined by the pump design and speed and is supplied by the pump

manufacturer

HP = WHP + VHP

4.8 Calculating Torque

Torque(T) is a function of Horsepower(HP) and Speed(RPM)

T = HP (X) 5250 (/) RPM

The Torque required by a pump application must not exceed the Torque

limit of the pump shaft.






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4.9 Net Positive Inlet Pressure

The Net positive inlet pressure (NPIP) available is the absolute pressure above

fluid vapour pressure at the pump inlet and is determined as follows.

NPIP available = Pa + Pz – Pf - Pvp

Where:

Pa = Pressure at surface of liquid in kg/cm2(a)

Pz = Static head (+) or Static Lift (-) in kg/cm2(a)

Pf = Inlet line , valve & fitting friction losses at maximum viscosity in kg/cm2(a).

Pvp = Fluid vapor pressure or gas dissolution pressure in kg/cm2(a)

NPIP required is a function of pump type , speed and viscosity of fluid pumped.

NPIP available must always be greater than NPIP required to prevent occurrence

of cavitation.

4.10 Cavitation

Cavitation occurs when the pump suction is under a low pressure/high vacuum

condition where the liquid turns into a vapor at the inlet of the pump. This vapor is

carried over to the discharge side of the pump where it no longer sees vacuum

and is compressed back into a liquid by the discharge pressure. This imploding

action occurs violently and attacks the pump rotors, gears etc. Rotors, screws,

gears, etc. that have been operating under a suction cavitation condition have

large chunks of material removed from their faces causing premature failure of

the pump.






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4.11 Affinity Laws for Positive Displacement pump

The affinity laws accurately predict the affect of changing the speed of a

centrifugal or rotary pump, and they also do a fairly good job of predicting the

affect of changing the diameter of a centrifugal pump.

Rotary pumps are very different from centrifugal pumps:

• They do not have a best efficiency point (BEP).

• There is no impeller shape (specific speed) to consider.

• There is no system curve to match.

• Their capacity is a constant even if the head changes.

Take a look at the following two curves. The one on the left describes a

centrifugal pump curve with the curve shape determined by the specific speed

number of the impeller. The curve

on the right describes the curve we get with a typical rotary pump.






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H = Head in meters , Q = Capacity in M3/hr

•Capacity varies directly with the speed

•There is no direct change in head with a change in speed. The pump generates

whatever head or pressure necessary to pump the capacity.

•The Power varies directly with the speed.

•The NPIP required varies by the square of the speed

Rotary pumps are often used with high viscosity fluids. There is a set of affinity

laws for changes in viscosity, but unlike changes in speed the change in viscosity

does not give you a direct change in capacity, net positive suction head required

(NPSHR), or horsepower.

As an example: an increase in viscosity will increase the capacity because of

less slippage, but twice the viscosity does not give you twice the capacity.






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Since there are a variety of rotary pump designs operating over a wide range of

viscosities, simple statements about changes in operating performance are hard

to make, but the following relationships are generally true.

Here are the viscosity affinity laws for rotary pumps:

• Viscosity1>Viscosity2 = Q1 > Q2

• Viscosity1>Viscosity2 = Power1 > Power2

• Viscosity1>Viscosity2 = NPIPR1 > NPIPR2

• Viscosity1>Viscosity2 = No direct affect on differential pressure.







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Chapter 5.0

Performance Characteristics






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5.0 Characteristic Curves

PD pumps create flow, centrifugal pumps create pressure. In a PD pump, flow is

created by enclosing a volume at suction, moving it to discharge, and releasing it.

Pressure is created by the system's response to flow. If there was no connection

at the discharge flange, the flow would exit the pump at atmospheric pressure.

Centrifugal pumps create pressure by first imparting velocity to the fluid with the

impeller, then converting the velocity to pressure with the volute. If there was no

discharge flange connection, the flow would exit the pump with that developed

pressure.

The system requirements will determine the type of pump required in most, but

not all, cases. If a system calls for a pressurized network of piping with a

constant pressure at various flow rates, a centrifugal pump is the best option.

•An example of this type of system is municipal water. In this system, a PD

pump would be less efficient.

•Oil pipelines normally require a constant flow at various pressures. At a constant

flow rate, pipeline pressure will vary with changes in viscosity. Pipelines transport

different products with different viscosities and pressures. Product cooling will

increase viscosity and pressure. This is of concern if a pipeline is temporarily

shut down then restarted. A centrifugal may not be able to produce a high

enough pressure to clear the line. A PD pump will overcome the pressure and

restart flow.

•Fuel delivery systems require a constant fuel flow to control turbines or boilers.

Pressure may vary as nozzles become clogged or erode open, but flow

requirement remains constant. PD pumps supply a steady source of power.






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The quality and quantity of information on suction conditions will determine the

ultimate success or failure of any pump installation. The majority of pump

problems, both centrifugal and PD, start at the suction. There must be a

minimum amount of absolute pressure available to supply fluid to the pump

suction. PD pumps generally require less absolute pressure at Suction than centrifugal pumps.

Net Positive Inlet Pressure Required (NPIPR), at the pump suction flange, is the rating of total inlet losses within that pump at rated conditions.

These losses include the fluid friction loss along the internal suction path, the

change in elevation from the suction flange to the enclosed volume, the fluid

friction loss of entering the enclosed volume, and the acceleration to the velocity

of the enclosed volume.

For any given size, NPIPR will increase with increased viscosity or flow

(increased flow = increased speed).

Centrifugal pumps are usually not applied above viscosities of 4000 SSU, due to

the rapid loss of efficiency as viscosity increases. PD pumps maintain high

efficiencies throughout the viscosity range. Entrained gasses can be handled in

large quantities by most PD pump designs.

System Response The point at which the system curve and the pump performance curve intersect

is the condition of operation. Typically, a system curve will require higher

pressures for higher flow rates. Centrifugal pump performance has reduced flow

with increased pressure. The slopes are of the two curves are opposite. If the

slopes are both gradual, the angle of intersection will be small. Minor variations






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of resistance to flow (partially clogged strainer, valves, etc.) will shift the point of

intersection over a range of operation.

By looking at the performance curve you can see just how different they are. The

centrifugal Pump has varying flow depending on pressure or head, whereas the

PD pump has more or less constant flow regardless of pressure.

Another major difference between the pump types is the effect viscosity has

on the capacity of the pump. You will notice in the flow rate chart how the

centrifugal pump losses flow as the viscosity goes up but the PD pump actually

increases flow. This is because the higher viscosity liquids fill in the clearances of

the pump causing a higher volumetric efficiency. This chart shows only the effect

of viscosity on the pump flow. Remember, when there is a viscosity change there

is also greater line loss in the system. This means you will also have to calculate

the change in pump flow from the first chart for pressure changes.






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The pumps behave very differently when considering mechanical efficiency as

well. By looking at the efficiency chart you can see the impact of pressure

changes on the pumps efficiency. Changes in pressure have little effect on the

PD pump but a dramatic one on the centrifugal.

• The most obvious reason to use a PD pump is when you have a high

viscosity application. It is common knowledge that a centrifugal pump

becomes very inefficient at even modest viscosity. However, there are

many other reasons to select a PD pump over a centrifugal other than

high viscosity. In fact PD pumps are very commonly used on thin liquids

like ammonia and solvents.

• A simple rule of thumb for selecting a PD pump is: whenever you might be

operating a centrifugal pump at other than at the BEP. Of course the further away






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from the BEP, PD pump will be a better choice. This can typically happen at low

flow conditions, modest to high head conditions, or any type of elevated viscosity.

As you can see from the efficiency curve it takes more horsepower to operate a

centrifugal pump outside of its BEP.

This horsepower has a cost, the initial cost of the larger motor plus a higher life

cycle cost in energy consumed. Many times the PD pump will have a lower initial

cost as well as a lower operating cost.

• Another reason to use a PD pump would be if the application has variable

pressure conditions. A centrifugal pump will "walk" up and down the curve

which can cause process problems. A PD pump will give near constant

flow that makes it possible to match the flow to the process requirements.

The desire to have constant flow is the reason that a PD pump is the

pump of choice for metering applications.

• Obviously, if there is changing viscosity in the application the PD pump is

the best choice. As can be seen from the charts, viscosity has a major

impact on the centrifugals performance. Even a small change in viscosity,

like 200-400 SSU, has a large impact on the centrifugal.

• PD pumps generally can produce more pressure than centrifugals. This

will depend on the design of each pump but pressures of 250 psi (580

feet) are not unusual for a PD pump with some models going to over 1000

psi (2,300 feet). This is a significant difference between the two principles.

The capability for a PD pump to produce pressure is so great that some

type of system overpressure protection is required.

• Generally speaking pumps tend to shear liquids more as speed is

increased and the centrifugal is a high speed pump. This makes the PD

pump better able to handle shear sensitive liquids. Shear rates in PD






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pumps vary by design but they are generally low shear devices, especially

at low speeds. Internal gear pumps, for example, have been used to pump

very shear sensitive liquids. It is important to contact the manufacturer for

specific information on shear rates and application recommendations.

• By their nature, PD pumps create a vacuum on the suction side so they

are capable of creating a suction lift. The standard ANSI centrifugal does

not create a vacuum so it can not lift liquid into the suction port. There are

self-priming centrifugal designs that can lift liquid an average of 15 feet.

This corresponds to a vacuum of 13" hg. Wetted PD pumps (a pump that

is not full of liquid but with some liquid in it) can often reach vacuums of 25

to 28" hg. So a PD pump is the logical choice when there is a suction lift

required.

• As mentioned earlier, PD pumps tend to run at lower speeds than

centrifugals. This will have an impact on seal life, so PD seals tend to last

longer than seals in centrifugal pumps. In addition, to assure adequate

seal life a centrifugal will typically require one of the seal flush plans. A PD

pump, because of its lower shaft speed typically does not need an

external flush plan. Also, generally speaking, low speed mechanical

devices tend to operate longer than high speed mechanical devices.

• At certain combinations of flow and pressure centrifugals are inherently

inefficient, due to the design of the impeller and the short radius turn the

flow must make. These applications are generally under 100 GPM but

particularly under 50 GPM. A PD pump, by contrast, is very well suited for

low flow conditions. Centrifugals, by contrast, tend to do very well in high

flow conditions.






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Worked Example (Positive Displacement Pump)

A positive displacement type pump is used to transfer oil fuel from an

underground storage tank to the oil burners on a boiler plant system. At

maximum fire, the burners require a total of 700 L/h of oil at a minimum pressure

of 40 kPa (gauge). A pressure relief valve, set at the minimum required burner

pressure, allows the unused part of the oil to return to the storage tank when the

boiler operates at less than full-fire capacity. The burner is located 1 metre higher

than the pump and the oil level in the vented storage tank is 2 metres lower than

the pump. Measurements indicate a pump speed of 280 rpm, 700 L/h flow, 90

kPa total differential pressure, and 37.0 W power draw. The friction head for the

piping system is proportional to the square of the flow. The specific gravity of oil

is 0.9.

What is the overall efficiency of the existing pump operation? As part of an

energy conservation retrofit, new burners are being installed which at full-fire






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conditions, require only 600 L/h at a minimum pressure of 30 kPa. At what speed

should the existing pump now operate to suit these new conditions and what is

the new power requirement?

First, the problem should be simplified by converting all parameters to standard

units (i.e. m /h flow and equivalent metres water head). The existing flow is 0.7 3

m /h [700 /1,000] and the new flow will be 0.6 m /h. The elevation difference 3 3

between the oil source and destination is 3 metres [2 + 1]. With oil at * = 0.9, the

equivalent water column is 2.7 metres [3 x 0.9]. The 2.7 m elevation head across

the pump will not change with the new operation. The oil in the tank is at zero

gauge pressure (vented to atmosphere) and at the destination (burner), the

pressure is 4.1 metres head [40 / 9.81]. With the new burner, the relief valve

setting can be reduced to 30 kPa or 3.1 metres head [30 / 9.81]. Thus the

existing and new system pressure differential across the pump is 4.1 m and 3.1

m, respectively. The total pump head is currently at 9.2 metres head [90 / 9.81].

Existing Pump Efficiency

Existing pump efficiency is a ratio of theoretical power to actual power. The

theoretical power is:

The overall pump efficiency, based on a measured power of 37.0 W, is:






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New Pump Speed

New pump speed (previously 280 rpm) is proportional to the change in flow

from 700 L/h to 600 L/h.

New Power Requirement

The new power requirement depends on the new total head across the pump.

Total head (P ) is the sum of elevation head (P ), system pressure (P ) head t e s

and friction head (P ). The friction head is not known for the existing 70 L/h flow f

but can be determined as the difference between the other heads.

The new friction head (proportional to square of flow) at 600 L/h is:

The new total head across the pump will be:

The new power is proportional to the flow and the total head. Thus:







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Chapter 6.0

Controlling Positive Displacement Pump






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6.0 Controlling Positive Displacement Pump

The positive displacement pump is in some ways an even simpler device to

control than the centrifugal pump. It has the same function, namely to provide the

pressure necessary to move a liquid at the desired rate from point ‘A’ to point ‘B’

of the process. Below mentioned figure shows a 'generic' process with a positive

displacement pump connected to deliver liquid from A to B.






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There is a great variety of positive displacement pumps. They are divided into

two broad categories: Rotary and reciprocating. From the controls point of view,

however, they are all similar. Their characteristic curve is so simple that it is

rarely drawn. It is essentially a straight vertical line, as shown in previous Figure.

All are constant flow machines whose pressure rises to whatever value is

necessary to put out the flow appropriate to the pump speed. If the discharge is

blocked, the pressure will rise until something yields -- preferably a relief valve.

Close examination of the curve shows a slight counter clockwise rotation. This is

due to internal leakage.

For PD pumps the major cause of leakage is the small amount of reverse flow

that occurs before a check valve closes and possibly past the check valve after it

is closed. Leakage past the piston is negligible. Diaphragm operated PD pumps

have no cylinder to leak past. Rotary PD pumps, such as gear pumps or

progressing cavity pumps have internal clearances which permit a small reverse

flow, called "slip" or "blow by". There is another reason why the curve may rotate

to slightly lower flows at higher discharge pressures: The driver may slow down

as the load increases. None of these have a significant effect in curving the slope

of the characteristic enough that this slope can be used for control. For most

practical purposes the slope is vertical. The system curve of the process is also

shown in above Figure. Its intersection with the pump characteristic defines the

operating point.

As always, the process control engineer has the responsibility of matching the

capacity of a specific piece of equipment to the demands of the process at every

instant of time. Rarely does the actual system curve fall exactly on the one used

for design and selection. As with any two port device, there are three locations in

which a control valve can be placed: On the discharge, on the suction, and as a

recycle valve.






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6.1 Discharge Throttling

Discharge throttling does not work! Looking at the process from the point of view

of the pump, discharge throttling rotates the system curve counter clockwise so

that the modified system curve intersects the pump curve higher up. The

additional pressure is dropped through the valve so that the pressure and flow to

the process is (almost) exactly the same as before. The "almost" is due to small

increase in internal leakage that results in an equally small reduction in flow. An

increased wear rate and a shortening of the life of the machine are the only

results of this approach. If the pump is seen from the point of view of the process

so that the valve is considered part of the pump, the same result is obtained. To

obtain a modified pump characteristic curve, the pump curve must be rotated

clockwise around the intersection with the pressure axis. The problem is that this

hypothetical intersection is far off the top of the operating range. It is the point

where the pressure is so high that 100% internal leakage occurs.






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The machine would self-destruct from excess pressure if one were stubborn

enough to attempt to find this point. To cut a long story short, you can't control

a PD pump with discharge throttling.

6.2 Suction Throttling

Suction throttling has the same effect on the characteristic curve as discharge

throttling and doesn't work either. PD pumps have a Net Positive Suction Head

required (NPSHR) just as centrifugal pumps do. In fact their requirements are

even more stringent. Therefore restrictions and pressure drops in the suction

lines must be similarly avoided.

6.3 Recycle Control

Recycle control is the only means of using a valve to control a PD pump. The

valve is installed in a line teeing off from the discharge and leading back to the

source of the liquid, possibly a surge tank. It must be full open , of course. Below

mentioned Figure shows its effect on the characteristic curves. Viewing the

process from the point of view of the pump, its effect is to rotate the system curve

clockwise around its intersection with the pressure axis. Note that the little "tail"

at the bottom left of the modified system curve is due to the flow through the

recycle valve before the discharge check valve has opened. The flow through the

pump is essentially as before but the pressure to the process has been reduced.

Process flow will, of course, also be reduced by the amount flowing through the

recycle line.






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Viewing the pump from the process point of view gives a different perspective on

the same phenomenon. This time it is the pump curve that is rotated counter

clockwise around its intersection with the flow axis. This modified pump curve

gives the effect of greatly increased internal leakage. From the point of view of

the process, this is exactly what is happening. Note that I have not used the

same operating points in Discharge throttling curve as I did in Recycle control

curve. It is simply impossible to show any significant reduction in flow on a curve

representing the effects of discharge throttling.






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Recycle control is an efficient method of control for PD pumps. Since the flow

rate is essentially constant, the power requirement is roughly proportional to

discharge pressure. Since the effect of recycle is to drop the discharge pressure,

it results in significant reductions in power requirement. Nevertheless there is still

wasted power in proportion to discharge pressure times recycle flow.

Recycle valves experience rather severe service if the pressure drop is high.

Cavitation will destroy them if they are not appropriately selected. Two

approaches exist to deal with this problem

-The first solution is to drop the pressure in many small stages through the use of

many twists and turns in the valve trim.

-The second is to tolerate the resulting cavitation by shooting the liquid as a jet

through a small hole in the middle of a disk. The jet then blasts directly into the

discharge piping. The line diameter is often increased immediately downstream

of the valve and the wall thickness is also increased. In this way the jet cavities

down the middle of the pipe. It makes a terrific racket.

In either case it may be necessary to put a fixed restriction downstream of the

valve. It should be sized so that the ratio of the high to intermediate pressure is

the same as the ratio of intermediate to low pressure. Keep in mind that the

restriction will reduce the range of the valve by making it act like a quick opening

valve. This is because the restriction becomes the dominant factor in the line

once the valve is about half way open. From that point on, the valve has little

control.

Recycle lines for PD pumps should be run back to the suction vessel. This allows

any entrained bubbles to escape. If they do not, they can build up to the point

Where Pump Capacity Is Impaired. It May Even Vapor Lock.






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6.4 Speed Control

Speed control is an obvious method of controlling the flow rate of PD pumps

since flow is essentially proportional to speed. Pressure can also be controlled by

sliding up and down the system curve. Any point on the system curve can, in

theory, be reached. Most drivers, however, have low speed limits which limit the

turn down of the system.

Variable speed electric motors are somewhat modified versions of normal

motors. They require special provision for cooling and lubrication at low speed. In

addition, they require specialized electronic power supplies called "invertors".

These units provide power of the appropriate frequency and voltage. They are,

unfortunately, still quite expensive and do not have the reliability of control

valves. There is another reason why large variable speed electric drives are

seldom used with reciprocating pumps. The large inertia of the system means

that speed changes cannot be made quickly. If it is possible for a valve in the

process side to close suddenly, a variable speed electric cannot reduce speed






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fast enough to prevent a severe pressure rise. A recycle valve will be required to

protect the pump, as detailed below in the section on machine protection. A more

simple type of electronic control is frequently used for small chemical injection

pumps.

7.5 Other Means Of Control

The great variety of types of PD pumps results in a variety of specialized means

of flow control. A pneumatic actuator may be used to vary the geometry of the

crank arrangement of a reciprocating pump so that each cycle displaces a

greater or lesser amount of cylinder volume. Direct acting diaphragm pumps

driven by compressed air or some other gas can be controlled by regulating the

gas supply. There is also a technique known as "lost motion" whereby the crank

arrangement first compresses a spring or volume pocket before it begins to work

on the piston or diaphragm. These specialized methods are usually integral parts

of the equipment and the controls engineer simply connects a pneumatic or

milliamp signal to the appropriate input port. None of these methods changes the

essentially constant flow nature of the pump curve. (The flow is still "constant"

but at a different value.)

"Stroke Counting" is a method used when fixed amounts of liquid must be

injected at specific intervals such as in batch processes. An electronic device is

used to count the number of revolutions of a PD pump. After a sufficient number

has been counted, the pump is shut off. When this method is used for pH control,

the correct number of strokes can be calculated from a titration curve.

7.6 Machine Protection. The greatest danger to positive displacement

pumps is overpressure. The rigid, unyielding nature of the pump characteristic

means that overpressure is certain if the discharge is blocked. Many smaller(non

API)






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pumps, such as the gear pumps used to supply lube oil for larger equipment,

have integral relief valves to release pressure from the discharge back to the

suction. In the majority of cases, an external relief valve must be supplied by the

user. It must be connected as closely to the pump discharge as possible and

must not have any means of blocking either its inlet or its outlet. It should

discharge back to the pump supply. If, for any reason, the discharge is blocked

and the relief valve is not capable of relieving, the pressure will rise very rapidly

until something busts. It may be connecting rods, the check valves or even the

cylinder head. Don't count on the motor stalling because events unfold very

rapidly and the inertia of the system is sufficient to cause major damage. The

most likely point of failure is the bolting on the discharge flanges.

It is often advisable to install a high discharge pressure shutdown switch or

transmitter in addition to the relief valve.

7.7 Parallel Pump Operation

PD pumps are quite suitable for parallel operation. Since the discharge pressure

of each pump rises as necessary, all pumps will discharge into the common

header. A common recycle valve is sufficient for flow or pressure control.

Starting up a pump that is discharging into a header that is already pressurized

by other pumps may overload its driver. To prevent this it is necessary to have an

individual recycle valve on each pump. This may be a slow acting ball valve.

Starting the pump then becomes a simple timed sequence in which the valve is

first opened, then the pump is started, and finally the valve is closed again. The

pump should also be shut down in the same sequence. Remember that the ball

valve will be opening against the full discharge head and may need a large

actuator. In water service it is extremely important that the appropriate water

resistant grease is used.






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If variable speed pumps are used, the majority should be placed on fixed speed.

One pump is then selected for process control to take the swings in demand.

7.8 Series Pump Operation

PD pumps are not generally installed in series. Since series pumps must both

discharge an identical flow and both are discharging a "constant" flow, it is

extremely unlikely that the two can be matched without complex controls. It is

common, however, to have one or more parallel centrifugal pumps servings as

boosters to one or more parallel PD pumps. The centrifugal pumps serve to

provide the NPSH that the PD pumps require. The PD pumps in turn can provide

a very high discharge pressure.

The centrifugal boosters should have sufficient flow capacity to supply the

pulsating requirements to the PD pumps. This means the full peak flow, not the

average. If they need controls they should be on pressure control by way of a

recycle valve since there should be no interference in the suction to the PD

pumps.







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Chapter 7.0

Selection Guide Of PD Pumps






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7.0 Selection Guide Of Positive Displacement PumpS Before attempting to select a pump for a particular application, it is important to

understand the types of pumps available and how they operate. This course

covers the fundamentals of centrifugal and positive displacement pumps,

including the major components of a pump, the operating principle and the

different types of pumps available within each category. The course material

covers Module 3 - "Pumps" in the DOE Mechanical Science Fundamentals

Handbook, which is an excellent source of information in the Engineering

Sciences.

High-Viscosity Liquids. Internal gear pumps are often a good

choice.

Sanitary Liquids. Sanitary lobe pumps work well for most food,

beverage, pharmaceutical, and biotechnology applications

Solids. Pump selection depends on the particular application. For food-type

applications containing solids, begin searching sanitary lobe pumps. For slurries

and other industrial-type solids, start with internal gear pumps.

Corrosive Liquids. Proper selection of the right materials of

construction will have the greatest impact on pump performance. Composite

external gear and stainless steel internal gear pumps are good starting points.

Abrasive Liquids. A number of factors can combine to minimize the

effects of abrasion. Begin with internal gear pumps manufactured with hardened

steel parts.






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Toxic, Hazardous, or Hard-To-Seal Liquids. Preventing leaks is

critical for handling these fluids. Magnetically-driven or mechanically sealed

internal or external gear pumps offer a good starting point

Extreme Temperature Conditions. Internal gear pumps with jacketing

features offer excellent temperature control.

PD Pumps Selection Summary

Pump Abrasives Thin Liquids

Viscous Solids Dry Prime Diff. Pressure

Internal

Gear G G E P A G

External

Gear P G G P A E

Lobe G A E E A G

Vane P E A P G A

E = Excellent, G = Good, A = Average, P = Poor







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Chapter 8.0

Field Problems & Trouble shooting






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8.0 Field Problems & Trouble shooting

No liquid discharge.

• The pump is not primed. Prime it from the outlet side by keeping the outlet

air vent open until liquid comes out the vent.

• The rotating unit is turning in the wrong direction.

• Valves are closed or there is an obstruction in the inlet or outlet line.

Check that the flange gaskets have their center cut out.

• The end of the inlet pipe is not submerged. You can either increase the

length of the inlet pipe into the liquid level or raise the level in the tank.

• The foot valve is stuck.

• A strainer or filter is clogged.

• The net inlet pressure is too low.

• A bypass valve is open.

• There is an air leak some where in the inlet line. Air can come in through

gaskets or valves above the fluid line.

• The stuffing box is under negative pressure. Packing is allowing air to get

into the system. You should convert the packing to a mechanical seal

• The pump is worn. The critical clearances have increased.

• Something is broken. Check the shaft, coupling, internal parts, etc.

• There is no power to the pump.

The pump is putting out a low capacity.

• The pump's internal clearances have increased. Set the clearances right /

replace worn out parts.

• The net inlet pressure is too low; the pump is cavitating.

• A strainer or filter is partially clogged.

• The speed is too low. Check the voltage.






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• The tank vent is partially frozen shut.

• A bypass line is partially open.

• A relief valve is stuck / partially open.

• The inlet piping is damaged.

• A corrosion resistant liner has collapsed in the inlet piping.

• Air is leaking through the packing. Replace packing or go for Mechanical

seal.

The pump looses its prime after it has been running for a while.

• The liquid supply is exhausted. Check the tank level; sometimes the float

is stuck, giving an incorrect level reading.

• The liquid velocity has increased dramatically.

• The liquid is vaporizing at the pump inlet.

• A bypass line is heating the incoming fluid.

• An air leak has developed in the suction piping.

The pump is taking too much power

• The speed is too high.

• The liquid viscosity is higher than the design.

• The discharge pressure is higher than design.

• The packing has been over tightened. Adjust gland tightness or go for

Mechanical seal.

• A rotating element is binding. Misalignment could be the problem or

something is stuck in a close clearance and binding the rotating element.

Excessive noise and vibration.

• Relief valve chatter.

• Foundation or anchor bolts have come loose.






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• The pump and driver are misaligned.

• The piping is not supported properly.

• The liquid viscosity is too high. The pump is starving. Check the

temperature of the incoming liquid. Check to see if the supply tank heater

has failed.

Excessive noise or a loss of capacity is frequently caused by cavitation. Here is

how the NPSH required was determined initially:

With the pump initially operating with a 0 psig. inlet pressure and constant

differential pressure, temperature, speed and viscosity; a valve in the inlet line is

gradually closed until cavitation noise is clearly audible, there is a sudden drop

off in capacity or there is a 5% overall reduction in output flow. Cavitation occurs

with:

• A loss of suction pressure.

• An increase in fluid velocity.

• An increase in inlet temperature.

Cavitations problem

• A foot valve or any valve in the suction piping is sticking.

• Something is occasionally plugging up the suction piping. If the pump

suction is coming from a river, pond or the ocean, grass is a strong

possibility.

• A loose rag is another common cause.

• A collapsed pipe liner.

• A filter or strainer is gradually clogging up.

• The tank vent partially freezes in cold weather.

• The sun is heating the suction piping, raising the product temperature

close to its vapor point.






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• The level in the open suction tank decreases causing vortex problems that

allow air into the pump suction.

• Several pumps in the same sump are running, decreasing the level too

much.

• The suction tank float is stuck. It will sometimes show a higher level than

you really have.

• A discharge recirculation line, piped to the pump suction, opens and heats

the incoming liquid.

• Sometimes the suction lift is too high. The increase in pipe friction will

reduce the suction head.

• The vapor pressure of the product is very close to atmospheric pressure.

The pump cavitates every time it rains because of a drop in atmospheric

pressure.

• The tank is being heated to de-aerate the fluid. Sometimes it is being

heated too much.

• The process fluid specific gravity is changing. This can happen with a

change in product operating temperature or if a cleaner or solvent is being

flushed through the lines.

• The source tank is changing from a positive pressure to a vacuum due to

the process.

• A packed valve in the suction piping is at a negative pressure and air is

leaking in through the packing.

• The tank is being pumped dry.

• The inlet piping has been moved or altered in some way. Has a foot valve,

strainer, elbow, or some other type of hardware been installed in the

suction piping?

• Has a layer of hard water calcium or some other type of solid formed on

the inside of the suction piping reducing its inside diameter over some

period of time?






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Rapid pump wear

• There are abrasives in the liquid you are pumping causing erosion

problems. You may have to go to a larger pump running at a slower

speed.

• There is some corrosion in one or more of the pump elements.

• There is a lack of lubrication.

• You have a severe pipe strain problem. It could have been caused by

thermal growth of the hardware.

• Too much misalignment.

• The pump is running dry.

• When all else fails the best way to reduce NPSH required is to select a

larger pump and run it at a slower speed.







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9.0 Tips to Improve Pump Efficiency

•Shut down unnecessary pumps.

•Restore internal clearances if performance has changed.

•Control by Recycle or speed .

•Replace oversized pumps. •Use multiple pumps instead of one large one. •Use a small booster pump. •Change the speed of a pump for the most efficient match of horsepower requirements with output.






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10.0 Summary

In this module user have learned about:

Types of PD Pumps,

Pump Operating Principles

Pump Power Requirements

Multiple Pump Systems

Pump Cavitation

Trouble shooting

PD Pump Selection criteria

You should now be able to perform the following tasks

Identify Pump Types and Characteristics

Measure Operating Conditions

Calculate Power and Energy Consumption

Evaluate Pump Performance

Develop System Improvement Recommendations.






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11.0 References

1. Internet

2. API 676

3. Metering Pumps” by James P. Poynton

Documents

Positive Disp Pumps