2. Goals
• Describe how centrifugal and positive-displacement
pumps operate and common applications.
• Calculate system head requirements.
• Determine head, pump efficiency, and pump.
horsepower from a typical centrifugal pump curve.
• Define net positive suction head (NPSH) and
understand how it relates to cavitation.
• Compute NPSH required by a pump.
• Determine an appropriate pump (impeller diameter,
efficiency, etc.) for a given required head.
• Describe how to modify system to operate on the
appropriate pump curve.
3. Background
Fluid Moving Equipment
Fluids are moved through flow systems using pumps, fans,
blowers, and compressors. Such devices increase the
mechanical energy of the fluid. The additional energy can
be used to increase
• Velocity (flow rate)
• Pressure
• Elevation
4. Background
Pump, fan, blower, and compressor are terms
that do not have precise meaning. Generally
pumps move liquids while fans, blowers and
compressors add energy to gasses.
Pumps and fans do not appreciably affect the
density of the fluids that they move and thus
incompressible flow theory is applicable.
5. Centrifugal Pumps
Most common type of pumping machinery. There are many
types, sizes, and designs from various manufacturers who
also publish operating characteristics of each pump in the
form of performance (pump) curves. The device pictured on
the cover page is a centrifugal pump.
Pump curves describe head delivered, pump efficiency, and
net positive suction head (NPSH) for a properly operating
specific model pump.
Centrifugal pumps are generally used where high flow rates
and moderate head increases are required.
9. Positive Displacement Pumps
To move fluids positive displacement pumps admit a
fixed volume of liquid from the inlet into a chamber
and eject it into the discharge.
Positive displacement pumps are used when higher
head increases are required. Generally they do not
increase velocity.
10. Pump Specification
Recall Mechanical Energy Balance
W
(
ˆ = ∆ αV
2
) + g∆z + ∆p + 4 f L V
+ ∑ Ki
2
N •m
2 ρ
D 2 kg
Wˆ = ( +
)
∆ α V 2 g∆z ∆p
+
L
+ 4 f + ∑ K i
V2 ft • lb f
2 gc gc ρ D 2 gc lbm
Both equations describe work that must be supplied to system
11. Pump Head
What happens if the MEB is multiplied through by g (gc/g)?
ˆ
=
(
W 1 ∆ αV 2 )
+ g∆z +
∆p L V
+ 4 f + ∑ K i
2
g g 2 ρ D 2
What are the units (SI)?
N • m s2 kg • m 3 s 2
2 = =m
kg m kg • s m
2 2
^
W/g has units of length and is known as the pump head
12. Example
2
3
1
Tank B
Tank A
Why do we choose point 2 rather than 3 for MEB?
What kind of valve to uses to control flow rate?
13. Example
2
3
1
Tank B
Tank A
Mechanical Energy Balance (in terms of head)
∆p L V
2
H = ∆z + + 1 + 4 f + ∑ K i
ρg D 2g
V 2
= H min + φ
2g
14. Head vs. Flow Rate
V 2
H = H min + φ
2g
Quadratic
In V or q
2
L V
1 + 4 f D + ∑ K i 2 g
g c ∆p
H min = ∆z +
ρg
15. System Response
2
3
1
Tank B
Tank A
What happens when flow control valve is closed?
• Resistance (f) increases
• Flow rate decreases
• Need more head to recover flow rate
16. System Response
Constant
Flow Response
Valve Closed Valve Open
Constant
Head Response
17. Pump Curves
Pump manufacturers supply performance
curves for each of their pumps. These are
normally referred to as ‘pump curves’. These
curve are generally developed using water as
the reference fluid.
The following can be read directly from a pump
curve:
• Head vs. flow rate information for any fluid
• Pump efficiency for any fluid
• Pump horsepower for system operating with water
18. Pump Performance Curves
Efficiency
Impeller NPSH
Diameter
Developed
Head
Horsepower
Flow Rate
http://capsicum.me.utexas.edu/ChE354/resources.html
19. Power Input
For fluids other than water:
Wˆ
P=m
η
ˆ m
W
g ft • lb f gal 1 ft 3 lbm
H
lb ∗ q
min ∗ 7.48 gal ∗ ρ ft 3
gc
P (hp ) = m
ft • lb f s
η ∗ 550 ∗ 60
s • hp min
20. Power Input
Easier Way
Pfluid ρ fluid
= = Sp. Gr. fluid
Pwater ρ water
Note: A less dense fluid requires less horsepower
22. Goulds Pump Curves
Manufacturers provide series of pumps to cover broad ranges of
capacities, heads, and suction and discharge piping diameters. Most
pumps can be equipped with different diameter impellers and can be
operated at different speeds to change capacities.
The curves provided are for a few variations of the Goulds model 3196
process pump. Each curve corresponds to a specific pump and a
specific RPM. Pump sizes are denoted with 3 numbers.
3x4-7
Discharge Suction Casing
Diameter Diameter Diameter
Inches Inches Inches
Note: Try to match process piping diameters
with the pump discharge and suction diameters.
23. Pump Selection
Goal is to find a pump whose curve matches the piping
system head vs. flow rate curve. We can superimpose the
previous head-flow rate curve on the manufacturers pump
curves.
To select a specific pump from a product line, find the pump
with the highest efficiency that does not require the use of
the largest impeller diameter. This will allow for future
production expansions.
Suppose that we have a process that requires a flow rate of
300 gpm and has a head requirement of 60 ft. at that flow
rate. Can a 3x4-10 model 3196 Goulds pumps be used?
24. Example
Impeller Diameter =
For Desired Q
Head =
How do can you force
the system to operate
on the pump curve?
25. Net Positive Suction Head (NPSH)
Associated with each H-Q location on the pump curve is a
quantity that can be read called NPSH.
An energy balance on the suction side of the fluid system
(point 1 to pump inlet) with pinlet set to the vapor pressure of
the fluid being pumped gives a quantity called NPSHA (net
positive suction head available).
g c p1 − pv L Vinlet
2
NPSHA = − 4 f + ∑ Ki + ( z1 − zinlet )
g ρ D 2
26. Net Positive Suction Head
The requirement is that:
NPSHA > NPSH
Otherwise (if NPSHA < NPSHpump), the pressure at the
pump inlet will drop to that of the vapor pressure of the
fluid being moved and the fluid will boil.
The resulting gas bubbles will collapse inside the pump as
the pressure rises again. These implosions occur at the
impeller and can lead to pump damage and decreased
efficiency.
Cavitation
27. NPSH
Do not use NPSH to size or select a pump unless all else
fails. Pump selection is governed by H vs. Q requirements
of system. When NPSHA is too small, it might be increased
by:
• Increasing source pressure (not usually feasible)
• Cooling liquid to reduce vapor pressure (not usually
feasible)
• Raise elevation of source reservoir
• Lower elevation of pump inlet
• Raise level of fluid in reservoir
28. If NPSHA Can’t Be Increased
If the pump must be modified to achieve proper NPSH:
• Larger slower-speed pump
• Double suction impeller
• Larger impeller eye
• Oversized pump with an inducer
29. Example
Flow = 600 gpm of benzene 60°F
2 P2 = 16.1 psia
Data for benzene: 5 ft
3
PVap = 7.74 psia P3 = 16 psia
ρ = 50.1 lbm /ft3
µ = 0.70 cP
150 ft
P1 = 16 psia globe valve (open)
1
5 ft
L = 300 ft, 5 inch Sch40
Use Goulds 3x4-10
L = 5 ft, 6 inch Sch40 @3560 RPM
30. Pump Selection from Many Choices of
Characteristic Curves
1. Examine pump curves to see which pumps operate
near peak efficiency at desired flow rate. This
suggests some possible pipe diameters.
2. Compute system head requirement for a few
diameters.
3. Compute V for some diameters. For water V in the
range of 1 – 10 ft/s is reasonable (see ahead).
4. Re-examine pump curves with computed head and
pipe diameters. This may give a couple of choices.
5. Pick pump with highest efficiency.
31. Selection of Pipe Size
Optimum pipe size depends mainly on the cost of the
pipe and fittings and the cost of energy needed for
pumping the fluids.
Cost of materials increase at a rate proportional to about
D1.5, while power costs for turbulent flow varies as D–4.8.
One can find correlations giving optimum pipe diameter
as a function of flow rate and fluid density, however the
optimum velocity is a better indicator as it is nearly
independent of flow rate.
32. Optimum Pipe Size
For turbulent flow of liquids in steel pipes larger than 1 in.
Vopt [=] ft s
0 .1
12 m
Vopt = 0.36 m[=] lbm s
ρ ρ[=] lbm ft 3
33. Remember
• Maximize pump efficiency
• Power input (hp) should be minimized if
possible
• Selected impeller diameter should not be
largest or smallest for given pump. If your
needs change switching impellers is an
economical solution
• NPSH required by the pump must be less
than NPSHA
34. Variable Speed Pumps
Advantage: Lower operating cost
Disadvantage: Higher capital cost
System head requirement
(no valve)
RPM1
RPM2
H (ft)
Pump curve
for Di
q (gpm)
q produced by pump
q* (desired) with no flow control
35. Affinity Laws
In some instances complete sets of pump curves
are not available. In this instance the pump
affinity laws allow the performance of a new
pump to be determined from that of a similar
model. This can be useful when modifying the
operating parameters of an existing pump.
