Pumps and Compressors

Quiz• Write down the steps to be followed for

accomplishing a pipeline project.

• In two-axis graph, draw approximated curve to

indicate the relationships between friction loss vs

flow rate for a horizontal pipeline in laminar flow

regime and turbulent flow regime

Law of conservation of energy

• Energy is defined as the ability to do work

• Energy can neither be created nor destroyed;

it can only be changed from one form to

another.

• Energy in pipeline is expressed in form of

pressure or pressure head

• Examples of such energy in pipelines: kinetic

energy and potential energy.

Law of conservation of energy

Energy in pipelines• Pipelines pocess internal energy in both static

(non-flowing) and dynamic (flowing)conditions.

• In static condition the total energy at anypoint is the internal static pressure head pluselevation

• In dynamic condition there is exerted energyand dissipated energy

• The exerted energy is from pumps orcompressors and the dissipated energy is dueto flow (pressure losses)

What is Pump?

A pump is a device that moves fluids (liquids or gases), or sometimes

slurries, by mechanical action.

Centrifugal pump is a kinetic pump

Schematic Diagram of Positive Displacement

Pump

http://www.youtube.com/watch?v=kKpESDDJQso

Advantages

ƒ

Versatile

Compact Design

High-Viscosity Performance

Continuous Flow Regardless of Differential Pressure

Ability to Handle High Differential Pressure

Disadvantages

This form of transferring an emotional response

Advantages and disadvantages of Positive

Displacement Pump

Parameter Centrifugal Pumps Reciprocating Pumps Rotary Pumps

Optimum Flow and Pressure

Applications

Medium/High Capacity,

Low/Medium Pressure

Low Capacity,

High Pressure

Low/Medium Capacity,

Low/Medium Pressure

Maximum Flow Rate 100,000 GPM 10,000 GPM 10,000 GPM

Low Flow Rate Capability No Yes Yes

Maximum Pressure 6,000 PSI 100,000 PSI 4,000 PSI

Requires Relief Valve No Yes Yes

Smooth or Pulsating Flow Smooth Pulsating Smooth

Variable or Constant Flow Variable Constant Constant

Self-priming No Yes Yes

Space Considerations Requires Less Space Requires More Space Requires Less Space

Costs Lower Initial

Lower Maintenance

Higher Power

Higher Initial

Higher Maintenance

Lower Power

Lower Initial

Lower Maintenance

Lower Power

Fluid Handling Suitable for a wide range including

clean, clear, non-abrasive fluids to

fluids with abrasive, high-solid

content.

Not suitable for high viscosity fluids

Lower tolerance for entrained gases

Suitable for clean, clear, non-

abrasive fluids. Specially-fitted

pumps suitable for abrasive-

slurry service.

Suitable for high viscosity fluids

Higher tolerance for entrained

gases

Requires clean, clear, non-

abrasive fluid due to close

tolerances

Optimum performance with

high viscosity fluids

Higher tolerance for entrained

gases

Pump Performance Trade offs

Centrifugal Pumps

• Centrifugal pumps are a type of pumps

use centrifugal force to develop pressure

• Centrifugal pumps can handle variable

head and flow rates

• Centrifugal pumps can handle

multiproducts and other liquids over wide

ranges of fluid properties

Centrifugal Pumps

Flow is a volume measure to establish pump capacity per unit of time, usually as

GPM

Head is a pressure measure represented by how high the pump can lift a column

of liquid, usually in feet.

Point 1Point 2

Important terms for Centrifugal Pump

Centrifugal pumps operation• The sequence of the operation of centrifugal

pump is as follows:

1• Mechanical rotation of the shaft by prime mover

2• Rotation of impeller inside casing

3

• Flow of liquid into the impeller from the suction pipe

4• Liquid is accelerated by impeller rotation

5• Conversion of velocity energy into pressure

Centrifugal pumps characteristics

Below are some characteristics ofcentrifugal pumps:

• Most common and preferred for application to pipelines

• Have minimal pulsation

• Capable of efficient performance over a wide range ofpressures and flow rates

• Discharge pressure is a function of liquid density

• Cheaper than other pumps

• High reliable

• Can be used with viscosity up to 300 cp with high efficiency

• Can be multistaged for higher pressure

Centrifugal pumps performance curves

Performance curves of an individual centrifugal

pump provides many information of the pumps

such as its model, size, rated speed, impeller type

and available diameters, pumps specific speed,

and net positive suction head required (NPSHR).

In addition, many curves are included such as:

1. H-Q curves for different impellers

2. Flow rate versus power (P-Q) curves based on water

3. Flow rate versus efficiency (efficiency-Q) curves

Centrifugal pumps performance curves

Centrifugal pumps power and efficiency

Brake horsepower (BHP) is the actual power delivered to the

pump shaft expressed as:

Hydraulic horsepower is the liquid power developed by the pump

expressed as:

The pump efficiency is the ratio between the hydraulic

horsepower and brake horsepower

Centrifugal pumps affinity lawsPressures and flow rate of a centrifugal pump can be

changed by changing its speed or its impeller size.

For radial-impeller centrifugal pumps the pressure head,

flow rate, and power are changed following the

equations below:

In case of changing impeller diameter only:

In case of changing speed only:

Centrifugal pumps affinity laws

In case of changing both diameter and speed:

Centrifugal pumps H-Q curve and system

H-Q curveIf the system (pipeline) H-Q curve is changed the pump

H-Q curve will consequently change.

For example if the throttle valve at discharge side of the

pump is partially closed the system pressure will

consequently increase (throttle control) which turn in

changing the system Q-H curve.

If the speed of the pump is reduced from N1 to N2 or

the impeller diameter is reduced from D1 to D2 the

pump Q-H curve will change

Centrifugal pumps H-Q curve and system

H-Q curve

NP

SH

- m

Q (m /hr)

20

10

0 100 200

H (

m)

70

60

50

40

30

Pump Curve

NPSH

effi

cien

cy

300

3

400

6

70%

60%

50%

40%

420

Eff

icie

ncy

%

80%

General rules for sizing and selecting of

CentrifugalComponent General rule

Suction and

discharge

Suction is never smaller than discharge

The bigger discharge the higher flow rate

Impeller

diameter

The larger impeller diameter the higher discharge

pressure (the pressure head is proportional to the

square of impeller diameter)

The impeller diameter is limited by speed (1200 rpm

for 26” and 3600 rpm for 12”)

Speed Flow rate varies linearly, the head varies with

square, and the power varies with cubic of speed

Suction Determined by NPSHR

Centrifugal pumps cavitation

Cavitation is a phenomena that may lead to very severedamage of the centrifugal pump. Cavitation occurs due tothe following:

A local pressure drop causes an increase in velocity and henceacceleration

A liquid may convert to a vapor phase if the pressure falls below itsvapor pressure

The vapor occupies larger volume than liquid.

The effects of cavitation include:

Noise and vibration

Pump damage

Fall-off of the pump performance and efficiency

Net Positive Suction Head (NPSH)

NPSH is the total absolute suction pressure less the vapor

pressure of the pumped liquid.

It is the head required to push the liquid to the pump to

control cavitation.

Available and require NPSH

• ha is the absolute pressure head at the surface of the liquid supply

level

• hvp is the vapor pressure of the liquid at pumping temperature

• hst is the static pressure head at the pump inlet centerline

• hst is the suction losses including entrance losses and piping friction

Centrifugal Pump Characteristic Curves

• Pump manufacturers provide information on the performance of their pumps in the form of curves, commonly called pump characteristic curves (or simply pump curves).

• In pump curves the following information may be given:

• the discharge on the x-axis,

• the head on the left y-axis,

• the pump power input on the right y-axis,

• the pump efficiency as a percentage,

• the speed of the pump (rpm = revolutions/min).

• the NPSH of the pump.

Compressors

What is a Compressor?

◦ A mechanical device that increases the pressure of a gas by reducing its

volume.

◦ Similar to a pump – Increases the pressure on a fluid and transport it

through a pipe.

What is key difference between a Fluid and a Gas?

◦ Compressibility – a gas is compressible

What happens to gas volume as it is compressed?

◦ Decreases

What happens to the Temperature of the Gas as it is compressed?

◦ Increases

Compressors

Compressors are classified by how they work

Two Categories of Compressors

◦ Positive Displacement

◦ Dynamic

What is a Positive Displacement Compressor?

◦ A compressor that confines successive volumes of gas within a closed

space in which the pressure of the gas is increased as the volume of the

closed space is decreased.

Intermittent Flow

What is a Dynamic Compressor?

◦ A compressor using a rotating mechanism to add velocity and pressure

to gas.

Continuous Flow

Compressors

SURGING

AB

C

Design

Line

Net Flow line

Surging

Line

Surge - is the point at which the

compressor cannot add enough

energy to overcome the system

resistance.

This causes a rapid flow reversal (i.e.

surge). As a result, high vibration,

temperature increases, and rapid

changes in axial thrust can occur.

Most turbo machines are designed to

easily withstand occasional surging.

These occurrences can damage

the rotor seals

rotor bearings

the compressor driver and

cycle operation.

http://www.youtube.com/watch?v=OT8Y0DeQ_cw

Pressu

reVolume flow

The major characteristics are:

Size

Starts about 500 hp.

1,000 hp increments to 20,000 hp.

Advantages

High horsepower per unit of space and weight.

Turbine drive easily adapted to waste-heat recovery for high fuel

efficiency.

Easily automated for remote operations.

Can be skid mounted, self-contained.

Low initial cost.

Low maintenance and operating cost.

High availability factor.

Large capacity available per unit.

Disadvantages

Lower compressor efficiency.

Limited flexibility for capacity.

Turbine drives have higher fuel rate than reciprocating units.

Large horsepower units mean that outage has large effect on process

or pipeline capabilities.

Centrifugal compressors

The major characteristics are:

Size

• Numerous sizes from 50 hp to 3000 hp.

• 2, 4, or 6 compressor cylinders are common.

Advantages

• Can be skid mounted.

Self-contained for easy installation and easily moved.

• Low cost compared to low-speed reciprocating units.

Easily piped for multistage compression.

• Size suitable for field gathering offshore and onshore.

• Flexible capacity limits.

• Low initial cost.

Disadvantages

• High-speed engines are not as fuel efficient as integral engines

(7,500 to 9,000 Btu/bhp-hr).

• Medium range compressor efficiency (higher than centrifugal; lower

than low-speed).

• Short life compared to low-speed.

• Higher maintenance cost than low-speed or centrifugal.

Reciprocating compressors

Compressor Station Schematic

Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to

gauge pressure plus atmospheric pressure.

Gauge pressure is zero-referenced against ambient air pressure, so it is equal to

absolute pressure minus atmospheric pressure. Negative signs are usually

omitted.

Absolute Pressure and Gauge Pressure

Pa = Pg + Patm

Compression Ratio

Problem

Consider Ps = 850 psig and Pd = 1430 psig

Problem 3- Characteristic curves

• Consider a pipeline with the following data:

Total length=1504 km, D=28”, viscosity=0.2Pa.s,

density=850kg/m3, flow rate=200000bbl/day. Draw

the characteristics curve within laminar and turbulent

ranges?

Solution

Using the exponential equation

Problem 3- Characteristic curves

1- calculate the kinetic viscosity (0.000236 m2/s)

2-convert the flow rate to SI unit (0.3686 m3/s) andcalculate velocity (0.931 m/s)

3- calculate Reynolds number (2809 turbulent)

4- calculate the critical velocity (0.696 m/s) and thecritical flow rate (0.275 m3/s)

5. Consider the pipeline is horizontal with negligiblesecondary losses. Use the characteristic equation.Substitute the values of β and m according to flowregime (laminar and smooth turbulent)

Problem 3- Characteristic curves

Results

Characteristic equations:

Laminar

Turbulent

QH f 5779

75.123330QH f

Problem 3- Characteristic curves

ResultsFlow rate m3/s Pressure loss m

0 0

0.1 578

0.12 693

0.15 866

0.18 1040

0.21 1214

0.24 1387

0.27 1560

0.3 2837

0.34 3531

0.37 4100

0.4 4690

0.43 5300

0.46 6000

0.5 6900

Laminar Flow

Turbulent Flow

Problem 3- Characteristic curves

Results: Characteristic Curve

Laminar

Turbulent

CW

• Redo the calculation considering uphill inclination

with 20⁰, viscosity=100 mpa.s.

• Draw three characteristic curves for pipelines with the

same data considering three diameters 20, 26, and

32”.

Problem 3- Operating Point

A pump station is used to operate the pipeline above.The pump station was operated at two flow rates, thepressure head at each flow rate is listed in thefollowing table:

Formulate the pump station performance equation in the form , draw the performance curve and determine the operating point?

Flow rate m3/s Pressure head m

0.05 10000

0.35 6000

2BQAH p

Problem 3- Operating Point

Procedure

1- Convert the flow rate units to m3/hr

2- form two equations using the flow rates and

corresponding pumping pressure from the table above.

3- determine the values of A and B in the pump performance

equation

4- use the same flow rates used to draw the pipeline

characteristics curve to draw the pump performance curve

on the same paper

5- the intersect point of the two curves is the operating pont

Problem 3- Operating PointResults

The pump performance equation200257.010083 QH p

CW

What would be the pipeline diameter if we want to

increase the operating point 10%.

5 minutes Q&A