EPT07t06A Centrifugal Pumps
EPT07t06A Centrifugal Pumps
EPT07t06A Centrifugal Pumps
EPT 07-T-06A
Scope
This ExxonMobil Practice Tutorial (EPT) is part of a series, continuing with EPT 07-T-06B and EPT
07-T-06C. This EPT provides the project engineer with a basic understanding of centrifugal pump
selection. Refer to EPT 07-T-05A, EPT 07-T-05B and EPT 07-T-05C for information on
Reciprocating Pumps.
Version 0
EPT 07-T-06A Centrifugal Pumps October 1992 Draft
Table of Contents
Scope .................................................................................................................................. 1
Table of Figures.................................................................................................................. 3
1. References ................................................................................................................... 4
3. Performance Considerations.................................................................................... 20
Table of Figures
Figure 1: Tree Diagram for Kinetic Pumps .......................................................................6
Figure 5: Graphs Showing the Effects on Flow Rate and Head when the Pump Speed
Changes ............................................................................................................ 10
Figure 7: Typical System Curve Showing the Effects of Adding a Control Valve ...... 12
Figure 8: Head vs. Flow Rate Graph Showing the Effect of Installing Two Identical
Pumps in Parallel ............................................................................................. 13
Figure 9: Head vs. Flow Rate Graph Showing the Effect of Installing Two Identical
Pumps in Series ............................................................................................... 14
Figure 10: Head vs. Flow Rate Graph Showing the Differences between Installing
Two Pumps in Series or in Parallel Operation............................................... 15
Figure 11: Typical Pump Curve with a Steady Rising Head vs. Flow Rate
Characteristic ................................................................................................... 16
Figure 12: Typical Pump Curve with a Steep Rising Head vs. Flow Rate
Characteristic ................................................................................................... 17
Figure 13: Typical Pump Curve with a Flat Head vs. Flow Rate Characteristic .......... 18
Figure 14: Typical Pump Curve with a Drooping Head vs. Flow Rate Characteristic 19
Figure 15: Typical Centrifugal Pump Performance Curve (Courtesy Afton Pumps) .. 28
Figure 16: NPSH Reductions for Pumps Handling Hydrocarbon Liquids and High
Temperature Water (Courtesy Hydraulic Institute) ....................................... 30
1. References
The following publications form a part of this Tutorial. Unless otherwise specified herein, use the
latest edition.
2. General Considerations
Centrifugal pump selection shall be in accordance with requirements of this EPT, unless superceded
by more stringent local regulations.
Pumps serve many purposes in production facilities. The largest pumps are usually shipping or sales
pumps which increase the pressure of oil or condensate so that it can flow into a sales pipeline or be
loaded into tankers, barges, railroad cars, or trucks. Other large pumps are used with water injection
systems for disposing of produced water or for water flooding. Smaller pumps are used to pump
liquids from low to higher pressure vessels, to pump liquids from tanks at a low elevation to tanks at a
higher elevation, or to transfer liquids for further processing.
A facility's utility system often has many pumps, which may be used for firewater wash down and
utility water, heat medium, fuel oil or diesel, and hydraulic systems.
The project engineer shall be able to select the proper pump for each application, determine
horsepower requirements, design the piping system associated with the pump, and specify materials
and details of construction for bearings, seals, etc. On standard applications the project engineer may
allow the vendor to specify materials and construction details for the specified service conditions.
Even then, the project engineer shall be familiar with different alternatives so that he or she can better
evaluate proposals and alternate proposals of vendors.
• A kinetic pump adds energy continuously to increase the fluid's velocity within the
pump above the velocity in the discharge pipe. Passageways in the pump then reduce
the velocity until it matches that in the discharge pipe. From Bernoulli's Law, as the
velocity head of the fluid is reduced, the pressure head increases.
Therefore, in a kinetic pump the fluid's kinetic or velocity energy is first increased and
then converted to potential or pressure energy. Almost all kinetic pumps used in
production facilities are centrifugal pumps in which the kinetic energy is imparted to
the fluid by a rotating impeller that generates centrifugal force.
• A positive displacement pump decreases the volume containing the liquid until the
resulting liquid pressure equals the pressure in the discharge system. That is, the
liquid is compressed mechanically, causing a direct rise in potential energy. Most
positive displacement pumps are reciprocating pumps in which linear motion of a
piston in a cylinder causes the displacement. In rotary pumps, another common
positive displacement pump, a circular motion causes the displacement. Refer to the
tutorial EPT 07-T-05A on Reciprocating Pumps for further information.
Appendix D of EPT 07-T-06C shows some common pump types and their
typical uses.
Figure 5: Graphs Showing the Effects on Flow Rate and Head when the Pump
Speed Changes
• In most piping systems both the head and the flow rate vary because the system has its
own required pump head for a given flow rate. This can be seen in Figure 6. The
head required by the system, which is to be provided by the pump, is merely the
friction drop in the pipeline between points A and B, assuming the levels in both tanks
are identical. This is a function of flow rate; it can therefore be plotted as a "system
curve" on the pump head-flow rates curve. For this system, as the pump speed is
increased or decreased, a new equilibrium of head and flow rate is established by the
intersection of the system curve and the pump curve.
Figure 7: Typical System Curve Showing the Effects of Adding a Control Valve
Figure 8: Head vs. Flow Rate Graph Showing the Effect of Installing Two
Identical Pumps in Parallel
• Figure 9 shows the effect of installing two pumps in series. Curve A is
the head vs. flow rate curve for one pump. The combined curve for both
pumps, B, is constructed by doubling the head of Curve A at each value
of flow rate. The benefit of the additional pump can be seen by
inspecting the intersection of the system curves, C and D, with the pump
curves.
Figure 9: Head vs. Flow Rate Graph Showing the Effect of Installing Two
Identical Pumps in Series
• The choice of whether to add an additional pump in series or in parallel
is illustrated in Figure 10. If the system curve is shallow, more
throughput is obtained from parallel operation. If the system curve is
steep, more throughput can be obtained by series installation.
Figure 10: Head vs. Flow Rate Graph Showing the Differences between
Installing Two Pumps in Series or in Parallel Operation
Figure 11: Typical Pump Curve with a Steady Rising Head vs. Flow Rate
Characteristic
Figure 12: Typical Pump Curve with a Steep Rising Head vs. Flow Rate
Characteristic
Figure 13: Typical Pump Curve with a Flat Head vs. Flow Rate Characteristic
Figure 14: Typical Pump Curve with a Drooping Head vs. Flow Rate
Characteristic
In general, it is desirable to choose a pump that operates at its maximum
efficiency point or slightly to the left; however, this is not always
possible. Pumps are sold to operate over wide ranges, even at the
extreme ends of the rating curve. Normally, if the NPSH available is
sufficient to prevent cavitation, the pump shall operate satisfactorily.
2.5.1. Advantages
The advantages of centrifugal pumps are:
2.5.2. Disadvantages
Disadvantages of centrifugal pumps include:
3. Performance Considerations
3.1. Head
The term head is commonly used to represent the vertical height of a static column of
liquid; it corresponds to the energy contained in the liquid per unit mass. Head can also
be considered as the amount of work necessary to move a liquid from its original position
to the required delivery position. In this case, the term includes the extra work necessary
to overcome the resistance to flow.
1. Static pressure head represents the energy contained in the liquid due to
its pressure.
2. Potential head represents the energy contained in the liquid due to its
position measured by the vertical height above some plane of reference.
3. Velocity head represents the kinetic energy contained in the liquid due to
its velocity.
3.1.2. Equations
• Bernoulli's Law states that as a fluid flows from one point to another in a
piping system the total of potential, static and velocity head at the
upstream point (subscript 1) equals the total of the three heads at the
downstream point (subscript 2) plus the friction drop between points 1
and 2.
Equation 1
(HSH)1 + (HPH)1 + (HVH)1 = (HSH)2 + (HPH)2 + (HVH) + Hf
Where:
• The total fluid head required to pump a fluid between two points in a
piping system can be calculated by rearranging Equation 1 and including
the term for pump head.
Equation 2
Hp = (HSH + HPH + HVH)2 - (HSH + HPH + HVH)1 +Hf
Where:
• By substitution:
Equation 3
Metric:
(P2 - P1 ) + (V2 )
2
- V12
+ ( Z 2 - Z1 ) + H f
102
HP =
ρ 2g
Customary:
(P2 - P1 ) + (V2 )
2
- V12
+ ( Z 2 - Z1 ) + H f
144
HP =
ρ 2g
Where:
Equation 4
Metric:
Customary:
100 × 1.0
2.31
100 × 0.80
2.31
• The hydraulic power that shall be developed by the pump is given by:
Equation 5
Metric:
Hp ρ Q
HHP =
367,000
Customary:
Hp ρ Q
HHP =
550
Where:
Equation 6
Metric:
(SG ) q H P
HHP =
367.6
Customary:
(SG ) q H P
HHP =
3,960
Equation 7
Metric:
q∆P
HHP =
3,600
Customary:
q∆P
HHP =
1,714
Equation 8
Metric:
q∆P
HHP =
3,600
Customary:
Q′∆ P
HHP =
58,766
Where:
• The input power to the shaft of the pump is called the brake power; it is
given by:
Equation 9
HHP
BHP =
EM
Where:
• Cavitation occurs in a pump when the pressure of the liquid is reduced to a value
equal to or below its vapor pressure and small vapor bubbles or pockets begin to form.
As these vapor bubbles move along the impeller vanes to a higher pressure area, they
rapidly collapse.
• The accompanying noise is the most easily discernible sign of cavitation. Aside from
impeller damage, cavitation normally results in reduced capacity due to the vapor
present in the pump. In addition, the head may be reduced and unstable and the power
consumption may be erratic. Vibration and mechanical damage such as bearing
failure also can occur from operating in cavitation. The only way to prevent the
undesirable effects of cavitation is to ensure that the NPSH Available (NPSHA) in the
system is greater than the NPSH Required (NPSHR) by the pump.
Equation 10
NPSH = HA - HVPA - HSH - HVH - Hf
Equation 11
NPSH = HA - HVPA - HSH - HVH - Hf
Where:
HA = The head on the surface of the liquid supply level. This will be
barometric pressure if suction is from an open tank or sump; or it will be the
pressure existing in a closed tank or pressure vessel, m (ft)
HVPA = The head corresponding to the vapor pressure of the liquid at the
temperature being pumped, m (ft)
HSH = Static height that the liquid supply level is above or below the pump
centerline or impeller eye, m (ft)
HVH = The velocity head in the pump suction piping minus the velocity head
in the suction supply, m (ft). The velocity head in the suction supply is
Hf = All suction line losses including entrance losses and friction losses
through pipe, valves and fittings, etc., m (ft)
HA - HVPA = 0
Figure 16: NPSH Reductions for Pumps Handling Hydrocarbon Liquids and
High Temperature Water (Courtesy Hydraulic Institute)
1. The NPSH reductions shown are based on laboratory test data at steady
state suction conditions and on the gas-free pure hydrocarbon liquids
shown; its application to other liquids shall be considered experimental
and is not recommended.
2. No NPSH reduction shall exceed 50 percent of the NPSH required for
cold water or 3 m (10 ft), whichever is smaller.
3. In the absence of test data demonstrating NPSH reductions greater than 3
m (10 ft), the chart has been limited to that extent, and extrapolation
beyond that point is not recommended.
4. Vapor pressure for the liquid is true vapor pressure at bubble point and
not Reid vapor pressure.
5. Do not use the chart for liquids having entrained air or other
noncondensable gases which may be released as the absolute pressure is
lowered at the entrance to the impeller, in which case additional NPSH
may be required for satisfactory operation.
6. In the use of the chart for high temperature liquids, particularly with
water, due consideration shall be given to the susceptibility of the suction
system to transient changes in temperature and absolute pressure. This
might require additional NPSH - far exceeding the reduction otherwise
permitted for steady state operation - to provide a margin of safety.
− However, this is greater than one half the cold water NPSHR. Thus,
the corrected value of the NPSHR is one half the cold water NPSHR
or 2.4 m (8 ft).
Equation 12
Metric:
51.65 R p (q ) 2
1
Ns =
(H )
3
4
p
Customary:
R p (q ) 2
1
Ns =
(H )
3
4
p
Where:
A pump's specific speed is always calculated at its point of maximum efficiency. It is not
a dimensionless number, so it is critical that the units used in calculating the specific speed
are known.
Equation 14
H 1 (N 1 )
2
=
H 2 ( N 2 )2
Equation 16
H 1 (D1 )
2
=
H 2 (D 2 )2
Equation 17
BHP1 (D1 )
3
=
BHP2 (D 2 )3
Where:
For example, if a pump with a 150 mm (6 in) impeller will deliver 22.7 m3/hr
(100 gpm) and 30 m (100 ft) of head at 1800 RPM using 14.9 kw (20 BHP),
when we increase the impeller diameter to 200 mm (8 in) at the same RPM:
Metric:
q2 =
200
(22.7 ) = 30.3 m 3 / hr
150
200 2
H2 = (30) = 53.3 m
150 2
200 3
BHP2 = (14.9) = 35.3 kW
150 3
Customary:
q2 =
8
(100) = 133 gpm
6
82
H2 = (100) = 177 ft
62
83
BHP2 = (20) = 47 BHP
63
3.6. Viscosity
Centrifugal pump performance is adversely affected by viscous liquids. A marked
increase in brake horsepower, a reduction in head and some reduction in capacity occur
with moderate and high viscosities.
The following equations can be used for estimating the viscous performance of a pump
when the water performance of the pump is known:
Equation 18
qvis = CQ x qw
Equation 19
Hvis = CH x Hw
Equation 20
Evis = CE x Ew
Equation 21
q vis × H vis × (SG )
BHPvis =
3960 × E vis
Where:
qvis = Viscous capacity, m3/hr (gpm)-the capacity when pumping a viscous liquid
BHPvis = Viscous brake horsepower-the horsepower required by the pump for the
viscous conditions, kW (HP)
Equation 22
q vis H vis SG
BHPvis =
367.7 E vis
The use and application of Figures 18 and 19 are subject to the following
limitations:
3.7. Troubleshooting
Often the engineer will receive frantic telephone calls describing pump problems in the
field or job site. A quick check of the troubleshooting chart (Table 1) will serve as a
starting point to solve problems and can save considerable time and effort in the office and
field.
Check Problem
Check Problem
Check Problem
Check Problem
Check Problem