CHAPTER 7

ROTATING EQUIPMENT

CHE503 FLUID FLOW

OUTLINES

Introduction

Types of Pumps

Performance of Pumps

Types of Compressor

Performance of Compressor

INTRODUCTION-PUMPING OF FLUID

Mechanical pump is usually employed to pumping of

liquids or gases from one vessel to another or through long

pipes.

Eg:

a) The pumping of liquids such as sulphuric acid or

petroleum products from bulk store to process buildings.

b) The pumping of fluids round reaction units and through

heat exchangers.

The energy required will depends on;

a) The height through which the fluid is raised

b) The pressure required at delivery point

c) The length and diameter of the pipe,

d) The rate of flow

e) The physical properties of the fluid, particularly its

viscosity and density.

PUMPING EQUIPMENT FOR LIQUIDS

The liquids used in the chemical industries differ

considerably in physical and chemical properties, and

it has been necessary to develop a wide variety of pumping

equipment.

There are two broad classification of pumps :

1) Positive displacement pump

- Reciprocating piston

- Rotary gear

2) Centrifugal Pump.

Factors that influence the choice of pump for a particular

operation:

(1) The quantity of liquid to be handled affects the size of the

pump and its desirable to use a number of pumps in parallel.

(2) The head against which the liquid is to be pumped

difference in pressure, the vertical height of the

downstream and upstream reservoirs and by the frictional

losses which occur in the delivery line. The suitability of a

centrifugal pump and the number of stages required will

largely be determined by this factor.

(3) The nature of the liquid to be pumped the viscosity largely

determines the friction losses and hence the power required. The

corrosive nature will determine the material of construction both

for the pump and the packing. With suspensions, the clearances

in the pump must be large compared with the size of the particles.

PUMPING EQUIPMENT FOR LIQUIDS

(4) The nature of the power supply. If the pump is to be driven by

an electric motor or internal combustion engine, a high-

speed centrifugal or rotary pump will be preferred as it can

be coupled directly to the motor. Simple reciprocating pumps can

be connected to steam or gas engines.

(5) If the pump is used only intermittently, corrosion problems

are more likely than with continuous working.

The cost and mechanical efficiency of the pump must always be

considered, and it maybe advantageous to select a cheap pump

and pay higher replacement or maintenance costs rather than to

install a very expensive pump of high efficiency.

PUMPING EQUIPMENT FOR LIQUIDS

TYPES OF PUMPS

A. POSITIVE DISPLACEMENT PUMPS

Reciprocating Pumps

1. PISTON PUMP Consists of a cylinder with a

reciprocating piston connected to

a rod which passes through a gland

at the end of the cylinder .

The liquid enters from the suction

line through a suction valve and is

discharged through a delivery valve.

Delivery will rise from zero as the

piston begins to move forward to a

max, and then deliver will gradually

fall off to zero.

There will be an interval during the

return stroke and deliver will

remain zero

Single Acting Piston Pump

DOUBLE ACTING PISTON PUMP

The delivery will be similar in the forward &

return strokes.

Constant delivery can be achived.

2. THE PLUNGER OR RAM PUMP

Same principle as the piston type but differs in that the

gland is at one end of the cylinder making its

replacement easier than with the standard piston type.

The sealing of piston and ram pumps has been much

improved but, because of the nature of the fluids frequently

used, care in selecting and maintaining the seal is very

important.

The piston or ram pump may be used for injections of

small quantities of inhibitors to polymerisation units

or of corrosion inhibitors to high pressure systems, and also

for boiler feed water applications.

3. THE DIAPHRAGM PUMP

The diaphragm pump has beendeveloped for handling corrosiveliquids and those containingsuspensions of abrasive solids.

It is in two sections separated by adiaphragm of rubber, leather, or plasticsmaterial. In one section a plunger orpiston operates in a cylinder in which acorrosive fluid is displaced.

The movement of the fluid is transmittedby means of the flexible diaphragm to theliquid to be pumped. The only movingparts of the pump that are in contactwith the liquid are the valves, and thesecan be specially designed to handle thematerial.

4. THE METERING PUMP

Driven by constant speed electric motors used when a constant

and controlled rate of delivery of a liquid is required, and they will

maintain this constant rate irrespective of changes in the pressure

against which they operate.

The pumps are ;

-For low throughput and high-pressure applications, usually the

plunger (piston) is used

-for large volumes and lower pressures a diaphragm is used.

The rate of delivery is controlled by adjusting the stroke of the

piston element, and this can be done whilst the pump is in operation.

These pumps may be used for;

- the dosing of works effluents and water supplies

- the feeding of reactants, catalysts, or inhibitors to reactors at

controlled rates

- and although a simple method for controlling flowrate is provided, high

precision standards of construction are required.

B. ROTARY PUMP

1. GEAR PUMP AND LOBE PUMP

Gear and lobe pumps operate on the principle of using mechanical

means to transfer small elements or "packages" of fluid from the low

pressure (inlet) side to the high pressure (delivery) side.

General characteristics are similar with reciprocating piston

pumps, but the delivery is more even because the fluid stream is

broken down into so much smaller elements.

Capable of delivering to a high pressure, and the pumping rate is

approximately proportional to the speed of thSe pump and is not

greatly influenced by the pressure against which it is delivering.

The liquid is carried round in the spaces between consecutive gear teeth

and the outer casing of the pump, and the seal between the high and

low pressure sides of the pump is formed as the gears come into mesh

and the elements of fluid are squeezed out.

Gear pumps are extensively used for both high-viscosity

Newtonian liquids and non-Newtonian fluids.

GEAR PUMP LOBE PUMP

The lobe-pump is similar as gear pump, but the gear teeth are replaced by

two or three lobes and both axles are driven; it is therefore possible to

maintain a small clearance between the lobes, and wear is reduced.

GEAR PUMP AND LOBE PUMP

2. CAM PUMP

A rotating cam is mounted eccentrically in a cylindrical

casing and a very small clearance is maintained between

the outer edge of the cam and the casing. As the cam

rotates it expels liquid from the space ahead of it and

sucks in liquid behind it.

The characteristics again are similar to those of the gear

pump.

3. THE VANE PUMP

The rotor of the vane pump is mounted off centre in a

cylindrical casing (Figure 8.9). It carries rectangular vanes

in a series of slots arranged at intervals round the curved

surface of the rotor.

The vanes are thrown outwards by centrifugal action and

the fluid is carried in the spaces bounded by

adjacent vanes, the rotor, and the casing. Most of the

wear is on the vanes and these can readily be replaced.

3. THE VANE PUMP

The flexible vane pump

The pumps described above will not handle liquids

containing solid particles in suspension, and the

flexible vane pumps has been developed to overcome this

disadvantage.

In this case, the rotor (Figure 8.10) is an integral

elastomer moulding of a hub with flexible vanes

which rotates in a cylindrical casing containing a

crescent-shaped block, as in the case of the internal gear

pump.

4. The flow inducer or peristaltic pump

Consist of silicone rubber or other elastic tubing, typically

of 3 to 25 mm diameter, is compressed in stages by means of a

rotor.

The tubing is fitted to a curved track mounted concentrically

with a rotor carrying three rollers. As the rollers rotate, they

flatten the tube against the track at the points of contact.

These "flats" move the fluid by positive displacement, and the

flow can be precisely controlled by the speed of the motor.

These pumps have been

particularly useful for

biological fluids where

all forms of contact

must be avoided.

5. THE MONO PUMP

A special shaped helical metal rotor revolves eccentrically

within a double-helix, resilient rubber stator of twice the

pitch length of the metal rotor.

Gives a uniform flow and quiet in operation. It will pump

against high pressures.

The pump can handle corrosive and gritty liquids and

is extensively used for feeding slurries to filter presses. It

must never be ran dry. Its also can handle highly

viscosity liquid.

6. SCREW PUMPS

A highly viscous material is represented by the screw

extruder used in the polymer industry. Extruders in the

manufacture of simple and complex sections (rods,

tubes, headings, curtain rails, rainwater gutterings and a

multitude of other shapes).

However, the shape of section produced in a given material

is dependent only on the profile of the hole through which

the fluid is pushed just before it cools and solidifies.

SCREW PUMP

C. THE CENTRIFUGAL PUMP

The centrifugal pump is by far the most widely used type

in the chemical and petroleum industries. It will pump

liquids with very wide-ranging properties and suspensions

with a high solids content including, for example, cement

slurries, and may be constructed from a very wide range of

corrosion resistant materials.

The whole pump casing may be constructed from plastics

such as polypropylene or it may be fitted with a corrosion

resistant lining. Because it operates at high speed, it

may be directly coupled to an electric motor and it

will give a high flowrate for its size.

The centrifugal pump

In this type of pump (Figure 8.19), the fluid is fed to the

centre of a rotating impeller and is thrown outward by

centrifugal action. As a result of the high speed of

rotation the liquid acquires a high kinetic energy and the

pressure difference between the suction and delivery sides

arises from the interconversion of kinetic and pressure

energy.

The impeller (Figure 8.20) consists of a series of curved

vanes so shaped that the flow within the pump is as

smooth as possible. The greater the number of vanes on

the impeller, the greater is the control over the

direction of motion of the liquid and hence the smaller are

the losses due to turbulence and circulation between the

vanes.

Mechanisms:

1. Fluid enters the centrifugal pump

to the center of the pump impeller.

2. The impeller is continuously rotate

at high speed pushes the liquid to the

pump casing.

3. The force created from the impeller

will increase the liquid pressure

at the casing of the pump.

4. When the pressure of the fluid is

higher from the outside environment,

the fluid will flow out through outlet

pipe at the casing.

THE ADVANTAGES OF THE CENTRIFUGAL PUMP

(1) It is simple in construction and can, therefore, be made in a wide

range of materials.

(2) There is a complete absence of valves.

(3) It operates at high speed (up to 100 Hz) and, therefore, can be

coupled directly to an electric motor. In general, the higher the speed

the smaller the pump and motor for a given duty.

(4) It gives a steady delivery.

(5) Maintenance costs are lower than for any other type of pump.

(6) No damage is done to the pump if the delivery line becomes

blocked, provided it is not ran in this condition for a prolonged

period.

(7) It is much smaller than other pumps of equal capacity. It can,

therefore, be made into a sealed unit with the driving motor, and

immersed in the suction tank,

(8) Liquids containing high proportions of suspended solids are

readily handled.

THE DISADVANTAGES OF THE CENTRIFUGAL PUMP

(1) The single-stage pump will not develop a high pressure.

Multistage pumps will develop greater heads but they are very

much more expensive and cannot readily be made in corrosion-

resistant material because of their greater complexity. It is

generally better to use very high speeds in order to reduce the

number of stages required.

(2) It operates at a high efficiency over only a limited range of

conditions: this applies especially to turbine pumps.

(3) It is not usually self-priming.

(4) If a non-return valve is not incorporated in the delivery or suction

line, the liquid will run back into the suction tank as soon as

the pump stops.

(5) Very viscous liquids cannot be handled efficiently.

CENTRIFUGAL PUMP

DISCUSSION (20 MIN)

IDENTIFY THE ADVANTAGES AND

DISADVANTAGES OF RECIPROCATING PUMP

AND CENTRIFUGAL PUMP.

ADVANTAGES OF CENTRIFUGAL OVER

RECIPROCATING PUMP

Simplest centrifugal pumps are cheaper than the

simplest reciprocating pumps

Centrifugal pumps deliver fluid at uniform

pressure without shocks and pulsations (steady

delivery).

Can directly connected to motor derive without

the use of gears or belts.

They can handle wide range of fluid(eg: with

large amounts of solids in suspension or varies in

viscosities).

Simple in design and small in size but high in

capacity.

ADVANTAGES OF RECIPROCATING OVER

CENTRIFUGAL PUMP

Can be designed for higher heads than

centrifugal pumps

Are not subjected to air binding (presence of air

released from water) and the suction may be

under a pressure less than atmospheric without

necessitating special devices for priming.

They operate at nearly constant efficiency over a

wide range of flowrates.

PERFORMANCE

OF PUMPS

PERFORMANCE OF PUMP

Capacity of the pump/Volume flow rate;

Performance of the pump is characterize by net head,

H.

Water horsepower;

Brake horse power, bhp;

Efficiency

mQ

inout

zg

V

g

Pz

g

V

g

PH

22

22

shaft

horsepowerwater

shaft

horsepowerwater

pumpT

gQH

bhp

W

W

W

shaftshaft TW

gQHW horsepowerwater

PUMP PERFORMANCE CURVE

Free delivery Shut off

H=0 Q=0

Is achieve when there is no flow

restriction at the pump inlet/outlet-no

load to the pump

Is achieve when the outlet port of

pump is blocked off.

Q is very large, but H=0; the pump

efficiency is zero because the pump

did not do any useful work.

H is very large but Q =0, the pump

efficiency is zero

For given piping system, major

+minor losses, elevation changes,

etc., it caused the required net

head increase with the volume flow

rate.

Available net head of pumps

decreases with flow rate.

The pump efficiency reach the maximum value between

shut off condition and the free delivery condition. (Note as

H*, bhp* and V@Q *)

For steady condition, the pump can operate only along the

performance curve

Operating point of a piping system is established as the

volume flow rate where the system curve and the pump

performance curve intersect.

BEP should be close to operating point for best

efficiency.

In unfortunate situations the system curve and the pump

performance curve intersect at more than one operating

point.

It can happen when the system curve is almost flat & meet a

pump that has a dip on its net head performance curve.

This situation should be avoided because the system may hunt

for an operating point, leading to unsteady flow situation

Required net head, H required

This equation is evaluated from inlet(upstream) to

outlet (downstream).

totalLturbineupumprequired hhzzg

VV

g

PPhH ,12

2

11

2

2212, )(

2

Pump head delivered to the fluids

does 4 things:-

1. It increase it static pressure of the

fluid from P1 to P2

2. It increases the dynamic pressure of

the fluid from P1 to P2

3. It raises the elevation of the fluid

from P1 to P2

4. It overcomes irreversible head

losses in the piping system

availablerequired HH

Static, dynamic and elevation can be ve/+ve, but head

losses always +ve.

Thus at operating point:-

PUMP CAVITATIONS

When pumping liquids, it is possible for the local

pressure inside the pump to fall below the vapor

pressure of the liquid,

When , vapor filled bubbles called

cavitations bubbles appear. In other words, the liquid

boils locally.

After cavitations of bubbles are formed they are

transported through the pump region where the

pressure is higher, causing collapse of the bubble.

vPP

vPP

Repetition of bubble collapse leads to erosion of the

blade and causing blade failure.

This will cause noise, vibration, reduce efficiency and

damage to impeller blades.

To avoid cavitations , pressure of the pump should

above vapor pressure. vPP

CAVITATION

NPSH NPSH- net positive suction head, define as the

difference between the pump inlets stagnation pressure

head and the vapor pressure head.

Minimum NPSHrequired necessary to avoid cavitations

in the pump.

NPSHrequired increases with volume flow rate.

g

P

g

V

g

PNPSH v

inletpump

2

2

At the point whereby the NPSH and NPSH required

intersect, the maximum volumetric flowrate can be

estimated

To make sure there is no cavitations , actual NPSH

should be higher then NPSHrequired.

Value of NPSH varies not only with flow rate, but also

with temperature and type of the liquid being pump.

How to increase available NPSH?

Lower the pump/ raised the inlet reservoir level.

Use larger diameter of pipe.

Reroute the piping system such that fewer

minor losses (to decrease minor losses)

Shorten the length of the pipe upstream of the

pipe

Use smoother pipe (to decrease minor losses)

Use elbow with minor loss coefficient.

EXAMPLE 8.2

A centrifugal pump is required to circulate a liquid of density 800

kg/m3 and viscosity 0.5 x 10-3 Ns/m2 from the reboiler of a

distillation column through a vaporiser at the rate of 0.004 m3/s,

and to introduce the superheated vapour above the vapour space

in the reboiler which contains a 0.07 m depth of liquid. If smooth-

bore 25 mm diameter pipe is to be used, the pressure of vapour in

the reboiler is 1 kN/m2 and the Net Positive Suction Head

required by the pump is 2 m of liquid, what is the minimum

height required between the liquid level in the reboiler and the

pump?

TYPES

OF COMPRESSOR

PUMPING EQUIPMENT FOR GASES

Essentially the same types of mechanical equipmentare used for handling gases and liquids, though thedetails of the construction are different in the two cases.

Over the normal range of operating pressures, the densityof a gas is considerably less than that of a liquid withthe result that higher speeds of operation can be employedand lighter valves fitted to the delivery and suctionlines.

Because of the lower viscosity of a gas there is a greatertendency for leakage to occur, and therefore gascompressors are designed with smaller clearancesbetween the moving parts.

Fans, Blowers and compressors are used to increasepressure and to cause the flow of air and other gases inducts and piping systems.

DIFFERENCES BETWEEN FANS, BLOWERS AND

COMPRESSORS

A fan is a gas pump with relatively low pressurerise and high flow rate. Common examples of fansare window fans, ceiling fans, fans incomputers and other electronics equipment,radiator fans in cars.

A blower is a gas pump with relatively moderate tohigh pressure rise and moderate to high flowrate. Common examples of blowers are leafblowers, hair dryers, air blowers in furnacesand automobile ventilation systems.

A compressor is a gas pump designed to deliver avery high pressure rise, typically at low tomoderate flow rates. Common examples ofcompressors are tire pumps, refrigerator and airconditioner compressors.

Fans and rotary compressors

Fans are used for the supply of gases at relatively low

pressures (

CENTRIFUGAL AND TURBOCOMPRESSORS

Multistage centrifugal compressors are mainly used for the

higher pressure ratios and particularly for the

requirements of high capacity chemical plants.

THE RECIPROCATING PISTON COMPRESSOR

Capable of developing very high pressures, such as the pressure

of 35 MN/m2 required in the production of polyethylene.

Compressors may be either single-stage, or multiple-stage where

very high pressures are required.

For a single stage two-cylinder unit, the cylinders are fitted with

jackets through which cooling water is circulated, and inter stage

coolers are provided on multistage compressors which may consist

of anything from 2 to 12 stages.

Cooling is essential to avoid the effects of excessively high

temperatures on the mechanical operation of the compressor, and

in order to reduce the power requirements.

PERFORMANCE

OF COMPRESSOR

In practice, it is not possible to expel the whole of the gas from the

cylinder at the end of the compression; the volume remaining in

the cylinder after the forward stroke of the piston is termed the

clearance volume. This clearance will have a significant effect on

the work done per cycle.

The volume displaced by the piston is termed the swept

volume.

Therefore the total volume of the cylinder is made up of the

clearance volume plus the swept volume.

The clearance c is defined as the ratio of the clearance volume to

the swept volume.

POWER REQUIRED FOR COMPRESSION

COMPRESSION OF GASES.

COMPRESSION OF GASES

TOTAL VOLUME

Where,

Vs= volume swept

c= clearance percentages/ratio

= isentropic ratio

TOTAL WORK DONE ON FLUID PER CYCLE

1

1

241 1

P

PccVVV s

1

1

2

1

1

21 11

1 P

Pcc

P

PVP s

Work of compressor in a compressor (Isentropic

condition)

Work of compressor in a compressor (Isothermal

condition)

Work of compressor in a compressor in n stages

11

1

1

211

P

PVP

11

1

1

211

n

P

PVnP

1

211 ln

P

PVP

COMPRESSION OF GASES

EXAMPLE 8.3

A single-acting air compressor supplies 0.1 m3/s of air

measured at, 273 K and 101.3 kN/m2 which is compressed to

380 kN/m2 from 101.3 kN/m2. If the suction temperature is

289 K, the stroke is 0.25 m, and the speed is 4.0 Hz, what is

the cylinder diameter?

Assuming the cylinder clearance is 4 per cent and

compression and re-expansion are isentropic (y = 1.4), what

are the theoretical power requirements for the compression?