Transportation & Metering of Fluids

Lecture # 8

Fluid Transportation

Fluids are transported through pipes or tubes:

Pipes Tubes

Heavy walled, large diameter, and

moderate length 20 ft to 40 ft

Thin walled and coils comes several

hundred feet long

Pipes can be threaded Tubing cannot

Rough surface Smooth surface

Pipes are joined by:

Screwed

Flanged

Welded fittings

Tubes are joined by:

Compression joints

Flare fittings

Soldered fittings

Pipes are made by:

Welding

Casting

Piercing a billet in a billet mil

These are extruded or cold drawn.

Sizing of pipes and tubes

Pipe sizes:

Pipes are specified by their “diameter” and “wall thickness”.

Steel pipes nominal diameter ranges from 1/8 to 30 in.

For pipes, more than 12 in diameter, nominal diameter is outside diameter.

Nominal value close to actual inside diameter for 3 to 12” pipe.

Appendix ‘5’ is for steel pipe sizes.

(IPS = Iron Pipe Size, NPS = Normal Pipe Size)

Thus, “2 in. nickel IPS pipe” means 2 in. nickel pipe having same dimensions as 2

in. steel pipe.

Wall thickness is represented by “Schedule #”.

10 schedule # are given as 10, 20, 30, 40, 60, 80, 100, 120, 140, 160.

For pipe, less than 8 in. diameter, only 40, 80, 120, & 160 are common.

Tube Sizes:

Tubes are sized by outside diameter.

Wall thickness is given by BWG (Birmingham Wire Gauge) number, ranges from

24 (very light) to 7 (very heavy).

Selection of Pipe sizesFor specific situation: the optimum size of of pipe depends on

Relative cost of investment

Power

Maintenance

Stocking pipe and fittings

In small installations rule of Thumb are sufficient.

• Low velocities should ordinarily favored for gravity flow from overhead tanks

• For large complex systems the cost of piping may be substantial and also computer

programs of optimizing pipe sizes are justified

Joints and FittingsMethod to join the tubes and pipes depends not only on the properties of fluid but also on the

thickness of wall.

Thick Walled tubular structures are joined by :

Screwed Fitting (higher schedule # pipe is required for threading……….because of difficulty

of threading and handling of large pipes they are rarely used in the field with pipe larger than

3in.)

Flanges

Welding

Thin-walled tubing get attached by:

Soldering

Flare or Compression fittings

Pipes made of brittle material ( like glass, carbon or cast iron) are joined by:

Flanges (Flanges are matching disks or rings of metal bolted together and compressing a

gasket between their faces. A flange with no opening used to close a pipe is called a blind

flange or blank flange)

Bell and Spigot Joints

Comparison of Joints and Fittings

For Larger steel pipe in process piping and high pressure services welding hasbecome the standard method.

Welding makes the stronger joint than screwed or flanges.

Welded joints are leak-proof whereas other types of joints are not.

Environmental protection Legislation considers flanges and screwed joints tobe the source of leakage and emission of Volatile matter.

The only drawback of welded joint is that it cannot be opened withoutdestroying it.

Allowance for Expansion

Pipes has to face varying temperature and pressure and such changes cause thepipe to expand or contract.

If the pipe is rigidly fixed to its support, it may tear loose, bend or even break.

In large lines, fixed supports are not used instead the pipes rests loosely onrollers or is hung from above by chains or roads.

For high temperature lines (for taking up expansion and to avoid the strain onthe valves and fitting) the bends, bellows, packed Expansion joints, and flexiblemetal hose are employed.

Leakage Prevention around Moving Parts

In Process Machinery sometimes one part has to move on another part withoutleakage like;

Packed Expansion Joints

Valve where the stem should be free to turn without allowing the fluid inthe valve to escape.

Shaft of Pump or Compressor

Agitator Shaft passes through the wall of pressure vessel

Common devices for minimizing the leakage while permitting relative motion areStuffing Box and Mecahnical seals.

Stuffing Box

Stuffing Box (Cont..)

Stuffing Box (Cont..)

Stuffing Box (Cont..)

Stuffing Box (Cont..)

Mechanical Seal

Mechanical Seal (Cont..)

Mechanical Seal (Cont..)

VALVES“A small obstruction can be placed in the path of the fluid that can be moved

about as desired inside the pipe with little or no leakage of the fluid from the

pipe to outside. That obstruction including its movement mechanism are boxed

in one unit which is called Valve”.

Valves are used in the piping networks to meet the following purposes;

To regulate the flow (i.e. to stop or slow down the flow)

Control the temperature, pressure, liquid level or other properties of fluid at

points remote from the valve itself

Unidirectional Flow under certain conditions of temperature and pressure

Terminology for Valve body Parts

CLASSIFICATION OF VALVES

• CLASSIFICATION BASED ON MECHANICAL MOTION

Linear motion valve

Rotary motion valve

Quarter turn valve

• CLASSIFICATION BASED ON VALUE SIZE

Small valves (NPS 2 and smaller)

Large valves (NPS 2 1/2 and larger)

• CLASSIFICATION BASED ON FUNCTION

Isolation

Gate valve, Ball valve, Butterfly valve, Diaphragm valve

Control (flow/pressure)

Globe valve, Ball valve, Butterfly valve, Diaphragm valve

Prevention of flow reversal

Check valve (swing, lift, piston, etc.)

Flow diversion

Ball valve, Plug valve, Angle valve (Three way, Four way, etc.)

ADVANTAGES & DISADVANTAGES OF GATE VALVE

• ADVANTAGES

Pressure drop through the valve is minimal.

Good shutoff characteristics.

Operation torque is smaller than those of globe valves.

• DISADVANTAGES

Cannot be throttled.

Not suitable for frequent switch-on/off operation.

Requires large space envelope for installation, operation and maintenance.

Repair or machining of valve seats in place is difficult.

Typical Usage of Gate Valve

Block valve for control valve

Pump suction valve

Pump discharge valve

Block valve for level controller & level gauge

Drain valve of equipment

Drain valve of process & utility line

First block valve of sampling nozzle

Block valve for safety valve

Block valve for equipment

Block valve for steam trap

By-pass valve for emergency shut-down valve

Flow control valve for large size gas & city water line

Valve symbols for PID (Piping and Instrumentation Diagram)

Turbo machine

A turbo machine is a device in which energy is transferred either to or from a continuously flowing fluid by the dynamic action of one or more moving blade rows

The word turbo is a Latin origin and implies that which spins or whirls around

Classification of Turbo-machinery

Major subdivisions

A. Power classifications (power is added or extracted from the fluid)

• Pumps are power addition machines and include liquid pumps, fans, blowers and compressors.

Fluids are water, fuels, air, steam, refrigerants.

• Turbines are power extraction devices and include windmills, water wheels, hydroelectric turbines, automotive engine turbochargers, gas turbines.

Fluids; gases, liquids, mixtures.

B. The manner in which the fluid moves through and around a machine

• Open flow

No casing or enclosure for the rotating devices

Examples: propeller is an open flow pumping device. Windmill is an open flow turbine

• Enclosed or encased flow devices

Classification of Turbo-machinery (Cont..)

C. Turbo-machines are further categorized according to the nature of the flow path through the passages of the rotor.

When the path of the through-flow is wholly or mainly parallel to the axis of rotation, the device is termed an axial flow turbo-machine.

When the path of the through-flow is wholly or mainly in a plane perpendicular to the rotation axis, the device is termed a radial flow turbo-machine.

Mixed flow turbo-machines are widely used.

The term mixed flow refers to the direction of the through-flow at rotor outlet when both radial and axial velocity components are present in significant amounts.

Classification of Turbo-machinery (Cont..)

Classification : Flow Path

D. Compressibility of the fluid

• Incompressible

The density is constant through the entire flow process; liquid pumps.

• Compressible; Gas flows: compressors, Fan and Blower

E. Impulse or reaction machines

• Impulse: pressure changes are absent in the flow through the rotor. In an impulse machine, all the pressure change take place in nozzles

Example: Pelton wheel

• Reaction: pressure changes in rotor are absent

Classification of Turbo-machinery (Cont..)

Terminology (Mechanical) of Fluid Moving Machinery

Terminology (Cont.)

Flow Dynamics in Fluid Moving Machine

Sectional view of Impeller

Sectional view of Impeller (cont.)

Sectional view of Impeller (cont.)

Pump Classification

Basic Definitions

Capacity: it is expressed in terms of volumetric flow rate.

Head: it is the height of fluid column equivalent to the total pressure differential (under

adiabatic conditions) measured immediately before and after device.

Suction Head: it’s a vertical distance

from pump center-line to liquid supply

line. The term suction lift would be used

when the pump is placed above the

liquid level.

Discharge Head: it’s a vertical distance

from pump Centre line to point of free

delivery of liquid.

Total Static Head: it is the vertical

distance between the discharge level and

supply level of liquid.

Velocity Head: it is vertical distance a

body would have to fall to acquire the

velocity V. it corresponds to the pressure

head that would cause that velocity.

Friction Head: it is the pressure head

(in meters) required to overcome the

resistance to flow in pipes

NPSH (Net Positive Suction Head) and Cavitation

Net positive suction head is the term that is usually used to describe the absolute pressure of

a fluid at the inlet to a pump minus the vapor pressure of the liquid. The resultant value is

known as the Net Positive Suction Head.

The ‘Vapor pressure’ of a fluid is the pressure at which the fluid will boil at ambient

temperature. If the pressure within a fluid falls below the vapor pressure of the fluid, gas

bubbles will form within the fluid (local boiling of the fluid will occur). If a fluid which

contains gas bubbles is allowed to move through a pump, it is likely that the pump will

increase the pressure within the fluid so that the gas bubbles collapse. This will occur within

the pump and reduce the flow of delivered fluid. The collapse of the gas bubbles may cause

vibrations which could result in damage to the pipework system or the pump. This effect is

known as cavitation.

Formula for NPSH ------ see the text book

Pump Types

Positive Displacement Pump: Definite volume of the liquid is trapped in a chamber,

which is alternately filled from inlet and emptied at higher pressure through the

discharge.

Reciprocating type

Rotary type

Centrifugal pumps: these are the type of machines where mechanical energy of

liquid is increased by centrifugal action.

Double acting Piston Pump

Rotary Pumps: Discharge liquid by continuous scooping of liquid

from pump chamber due to rotation of one or more members within a

stationary casing

Gear Pump

Screw Pump ( A helical Screw rotor revolves in a fixed casing)

Diaphragm Pumps (A flexible diaphragm fabricated of metal, rubber, or plastic

material instead of piston or plunger, which reciprocate)

Lobe Pump (Pump delivers liquid by virtue of rotation of rotation of two, three or

four lobes in a stationary casing)

Vane pumps ( rotary pumps which operate on a principle of creating a vacuum inside a

pump due to rotation of rotor allowing the space to fill with a liquid and then forcing

the liquid out of pump under pressure by diminishing the volume)

Centrifugal pumps ( It consist of impeller rotating within a casing. Liquid enters the

pump near the center of the impeller and is thrown outward by virtue of centrifugal action)

Multistage Centrifugal Pumps (High energy Centrifugal pump can develop a head of

more than 650ft in single stage but generally when a head is greater than about 100ft is

needed two ore more impellers can be mounted in series on a single shaft.)

Axial Flow Pumps (Propeller Pumps)

Used for very high capacity and low head duties

Turbine Pumps (These have mixed flow impellers with the effect that the flow is

partly axial and partly centrifugal) usually casing submerged in the liquid to be pumped

Jet Pumps (It has no moving parts and works on the principle of momentum transfer of

one fluid to another fluid being pumped

Electromagnetic Pumps (works on the principle as induction motor)

A strong magnetic field is imposed perpendicularly to the liquid stream that carries

electric current this results in a driving force, mutually perpendicular to the magnetic

field and electric current, that causes the liquid to flow

Characteristic Curves

The Plots of Actual Head, Total power consumption, and efficiency versus volumetric

flowrate are called Characteristic curves

Head-Capacity Curve

The figure shows the head capacity relation. The

theoretical head-capacity relation is straight line

but the actual developed head is considerably less

and drops precipitously to zero as the rate

increases to certain value in any given pump.

The theoretical Zero head flow rate corresponds to

maximum flow the pump can deliver at any

condition.

The rated or optimum operating flowrate is of

course less than the zero value.

Reasons of Difference in Actual Head to theoretical Head

Circulatory Flow

Fluid Friction in the passage and channels of the pump

Shock losses from sudden changes in the direction of liquid leaving the impeller

Joining the stream of liquid traveling circumferentially around the casing

Important points

Friction is Highest at maximum flow rate

Shock losses are minimum at rated operating conditions of pump and becomes greater

as the flow rate is increased or decreased from rated value.

Power Curve

• Power Curve is drawn between Fluid power, total

power versus flow rate

• The difference of two powers (ideal power and fluid

power) represents the power lost.

• Power losses occur due to fluid friction and shock

losses those converts the mechanical energy into heat.

• Leakage (it is unavoidable reverse flow from

impeller discharge past the wearing ring to the

suction eye, and this reduces the volume of actual

discharge from pump per unit of power expended)

• Disk friction ( it is the friction between outer surface

of the impeller and the liquid in the space between

impeller and the inside of casing

• Bearing losses ( it constitute the power required to

overcome the mechanical friction in the bearing and

stuffing boxes or seals of the pump

Efficiency Curve

Pump Efficiency:

It is the ratio of Fluid power to the total power input.

The efficiency rises rapidly with flow rate at low

rates , reaches to maximum in the region of rated

capacity, then falls as the flowrate approaches the

zero-head value

Pump Priming (A centrifugal pump trying to operate on air can neither draw a liquid

upward from an initially empty suction line nor force the liquid along a full discharge line. A

pump with air in its casing is air bound and can accomplish nothing until the air has been replaced

by a liquid)

The theoretical Head developed in a centrifugal pump depends on

Impeller speed

Radius of impeller

Velocity of fluid leaving impeller

(If all factors are constant then developed head would be same for all fluids of all

densities and is same for liquids and gases.)

Important point

The increase in pressure in the pump, however, is the product of developed head and

fluid density.

If the pump develops a head of 100ft and is full of water, the increase in pressure

is 100x62.3/144 = 43psi (2.9 atm).

If the pump full of air at ordinary density , the pressure increase is about 0.1psi

(0.007atm).

Positive displacement pumps can compress a gas to a required discharge

pressure and are not usually subject to air binding

Turbomachinery for compressible fluids

Following devices can be utilized to transport the compressible fluids;

Fan

It discharge large volume of gas ( usually air) into open spaces or large ducts

These are classed as low speed rotary machines

Generates pressure of order of a few inches of water

Density of fluid does not change appreciably (incompressible flow theory is

adequate to discuss the phenomena)

Blower

These are high speed rotary devices (either positive displacement or centrifugal)

develop a maximum pressure of about 2 atm

Density changes should incorporate in the analysis

Compressor

Discharge at pressure from 2 atm to thousands of atmospheres

Density changes should incorporate in the analysis

Fans

Large fans are usually centrifugal (operating principle is exactly same as centrifugal

pump)

Impeller blades are curved forward

Fan impellers are mounted inside light steel metal casing

Clearances are large and discharge heads low from 5 to 60in. (130 to 1500mm) H2O

In ventilating fan all the added energy is converted to into velocity energy and almost

none into pressure head.

Due to negligible change in density, equations relating to centrifugal pump are adequate

to use.

Fans are rated in Standard Cubic Feet

Volume in standard cubic feet is that measured at specified temperature and pressure

regardless of actual temperature and pressure.

Common standard temperature and pressure are 60°F and 30 in.Hg, corresponding

molal volume is 378 ft3/Ib-mol

Blowers

Positive displacement blower shown in figure.

Similar to gear pump except the design of “teeth” and clearance is only few thousands of an inch.

Relative position of impellers is maintained by heavy external gears

Single stage blower can discharge at 0.4 to 1atm gauge and 2 stage blower at 2 atm

Positive Displacement Blowers (cont.)

Centrifugal Blowers (cont.)

It resembles with centrifugal pump, except casing is narrower and diameter of casing

and discharge scroll are relatively larger than pump.

Operating speed is 3600 r/min or higher

Reason of high speed and larger diameters is that very high heads (measured in meters

and of low density fluid) are needed to generate moderate pressure ratios. So the

velocity in the vector diagram for centrifugal blower is approximately tenfold those of

centrifugal pump.

Centrifugal Blowers (cont.)Fan, blower and compressor\3-D-Blower-Animation.flv

Compressors

Centrifugal Compressor (Cont.)Fan, blower and compressor\CENTRIFUGAL-

COMPRESSORNPOSAVI.flv

These are multistage units consisting of series of impellers on a single shaft rotating at high

speed in a massive casing

These can work on enormous amount of air or process gas up to 200000 ft3/min at inlet to an

outlet pressure of 20atm.

Smaller capacity machines can deliver up to several hundred atmosphere

Interstage cooling is required on high pressure units

Axial flow compressors (cont.)Fan, blower and compressor\How-Axial-

Compressors-Worksflv-by-PRAVIN-TATHOD.flv

In these units rotor vanes propel the gas axially from one set of vanes directly to the

next.

Axial flow machines handle even larger volumes of gas 600000ft3/min, but at the

lower discharge pressure of 2 to 10 or 12atm

Interstage cooling is usually not required

Positive displacement Compressors (cont.)Fan, blower and

compressor\reciprocating-compressor-working-animation---maintenance.flv

These machines are operated in the same way as the reciprocating pumps

The important difference lies in the prevention of leakage and rise in temperature during

compression

Most compressors operating at discharge pressure above 3 atm are reciprocating positive

displacement machines

Reciprocating compressors are usually motor driven and nearly always double acting

Sometimes, high compression ratios are required which is achieved by providing interstage

cooling.

Equations for Blowers and Compressors

From Text Book

Vacuum Pumps

A compressor that takes suction at a pressure below atmospheric and discharges against

atmospheric pressure is called a vacuum pump.

The compression ratio used in the vacuum pumps is higher than in the compressors

Jet Ejectors

An important type of vacuum pump that does not use moving parts is the jet ejector.

Multistage ejectors can also be used to create more vacuum, as many as five stages are

used in industrial processing.

Jet ejectors needs very little attention and maintenance and are especially valuable with

corrosive gases that would damage mechanical pumps.

They are rarely used to produce absolute pressure below 1mmHg.

Flow Measuring Devices

It is essential to measure the amount of material for the control purposes

Selection of Meter depends on many factors:

The applicability of the instrument to the specific problem

Its installed cost and costs of operation

The range of flowrates it can accommodate (Its range ability)

The accuracy of the measurement

Flow meter Types

Few types of flowmeters measures the mass flowrate directly, but majority measures the

volumetric flowrate

Volumetric flow meters

1. Differential Head type

A. Orifice plates

B. Venturi meters

2. Differential Area type (Rotameters)

3. Electromagnetic flowmeter

4. Vortex flowmeter

5. Ultrasonic flowmeter

6. Turbine flowmeter

7. Positive displacement flowmeter

Mass flow meters

1. Coriolis Mass flowmeter

2. Thermal Mass flowmeters

Venturi meter

A venturi tube also measures flow rates by constricting fluids and measuring adifferential pressure drop.

In the upstream cone of the Venturimeter, velocity is increased, pressure isdecreased.

Pressure drop in the upstream cone is utilized to measure the rate of flow throughthe instrument

Basic Equations of Venturi meter

From Text book

Pressure Recovery

If the flow through venturi meter were frictionless, the pressure of the fluid leaving the

meter would be exactly equal to that of the fluid entering the meter and the presence of

the meter in the line would not cause a permanent loss in pressure .

In properly designed meter, the permanent loss is about 10% of the venturi differential

and approximately 90% of the differential is recovered.

Venturi meter (cont..)

Disadvantages

Highly expensive

Larger and heavier to handle.

Ratio of throat diameter to pipe diameter cannot be changed

Although t he Venturi meters can be applied to the measurement of gas, they are most

commonly used for liquids, especially water.

For a given meter and manometer system, the maximum measureable flow rate is

fixed. So if the flow range is changed the throat diameter is too large to give an

accurate flow rate or too small to accommodate the larger flow rate

The orifice meter meets these objections and to the venturi but at the price of larger

power consumption

Orifice meter (The reduction of cross-section of the flowing stream in passing

through the orifice increases the velocity head at the expense of pressure head, and

reduction in pressure between the taps is measured by manometer)

Orifice meter (Cont.)

V-Element Meters The segmental wedge element is a proprietary device designed for use in slurry,

corrosive, erosive, viscous, or high-temperature applications.

It is relatively expensive and is used mostly on difficult fluids, where the dramatic

savings in maintenance can justify the initial cost.

Flow Co-efficient is constant at low flow rates. The minimum Reynolds number is only

500, and the meter requires only five diameters of upstream straight pipe run.

The V-shaped restriction characterized by the H/D ratio, where H is the height of the

opening below the restriction and D is the diameter. The H/D ratio can be varied to

match the flow range and to produce the desired d/p.

Target meters

A sharp edge Disk is set at right angles to the direction of flow as shown below

Drag force exerted on the disk by the fluid is measured.

The flow rate is proportional to the square root of this force and to the fluid density.

Turbine Meters

Consists of a multi-bladed rotor mounted at right angles to the flow & suspended in the

fluid stream on a free-running bearing.

The diameter of the rotor is slightly less than the inside diameter of the flow metering

chamber.

Speed of rotation of rotor proportional to the volumetric flow rate.

Not usable in dirty streams or with corrosive materials. Subject to fouling by foreign

materials -fibers, tars etc.

Positive Displacement Meters

This meter repeatedly entraps the fluid into a known quantity and than passes it out.

The quantity of the fluid that has passed is based on the number of entrapments.

The volume flow rate can be calculated from the revolution rate of the mechanical

device.

Can be used in viscous liquid flow

Not suitable for fluids with suspended solids and for r low flow rate

ULTRASONIC FLOWMETERS

A pair (or pairs) of transducers, each having its own transmitter and receiver, are placed on

the pipe wall, one (set) on the upstream and the other (set) on the downstream.

The time for acoustic waves to travel from the upstream transducer to the downstream

transducer td is shorter than the time it requires for the same waves to travel from the

downstream to the upstream tu.

The larger the difference, the higher the flow velocity.

Only clean liquids and gases can be measured