chapter 6 pumps and compressor
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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