1. PUMPS
The 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.

2. Background1/2 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

3. Background2/2 pumps move liquids. compressors add energy to gasses.
Pump and compressor are a device which converts
mechanical power into fluid power
Turbine converts fluid power into mechanical power.
Mechanical power is usually obtained by shaft
rotation
Pumps and fans do not appreciably affect the density of the fluids that they move and thus incompressible flow theory is applicable.

4. What do they look like? P.D. Pump Centrifugal Pump Axial flow Pump
Jet fluid Pump

5. Classification of Pumps
There are two basic types of pumps:
positive-displacement and
dynamic or momentum change pumps.
There are several billion of each type in use in the world today.
1- Positive-displacement pumps
PDP forces the fluid along by volume changes. A cavity opens, and the fluid is admitted through an inlet. The cavity then closes, and the fluid is squeezed through an outlet.

6. How P.D. Pumps Work?

7. A brief classification of PDP designs is as follows:
A. Reciprocating
1. Piston or plunger
2. Diaphragm
B. Rotary
1. Single rotor
a. Sliding vane
b. Flexible tube or lining
c. Screw
d. Peristaltic (wave contraction)
2. Multiple rotors
a. Gear
b. Lobe
d. Circumferential piston

8. A Schematic design of positive-displacement pumps:
(a) reciprocating
piston or plunger,
(b) external
gear pump,
(c) double-screw
pump,
(d) sliding vane,
(e) three-lobe pump,
(f) double
circumferential
piston,
(g) flexible-tube
squeegee.

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. Positive Displacement Pump
Works on the principle of letting fluid flow into a cavity from a low-pressure source, trapping the fluid, and forcing it out to a high-pressure receiver by decreasing the volume of the cavity
Simplest pump that can be found anywhere from liquid soap dispensers, to automobile fuel injectors, to the human heart.

11. Performance Curve of P.D. Pumps
“Constant-volumetric
flow rate device”
At fixed motor speed
“Must always have
some kind of safety
valve to relieve pressure if the discharge line is
suddenly blocked”

12. 2- Dynamic pumps
simply add momentum to the fluid by means of fast-moving blades or vanes or certain special designs. There is no closed volume: The fluid increases momentum while moving through open passages and then converts its high velocity to a pressure increase by exiting into a diffuser section.
Dynamic pumps can be classified as follows:
A. Rotary
1. Centrifugal or radial exit flow
2. Axial flow
3. Mixed flow (between radial and axial)
B. Special designs
1. Jet pump or ejector
2. Electromagnetic pumps for liquid metals
3. Fluid-actuated: gas-lift or hydraulic-ram

13. Comparisons Between the Two types1/3
A dynamic pump can provide very high flow rates (up to 300,000 gal/min) but usually with moderate pressure rises (a few atms).
In contrast, a PDP can operate up to very high pressures (300
atm) but typically produces low flow rates (100gal/min).
Dynamic pumps, also generally need priming; i.e., if they are
filled with gas, they cannot suck up a liquid from below into their
inlet. The PDP, on the other hand, is self-priming for most
applications.
At constant shaft rotation speed, the PDP produces nearly
constant flow rate and virtually unlimited pressure rise,
PDP has little effect of viscosity, but dynamic pumps are
ineffective in handling high-viscosity liquids

14. Comparisons Between the Two types2/3
The relative performance (P versus Q) is quite different for the
two types
The flow rate of a PDP cannot be varied except by changing the
displacement or the speed
The dynamic pump, provides a continuous constant-speed variation of performance, from near-maximum P at zero flow (shutoff conditions) to zero P at maximum flow rate.
High-viscosity fluids sharply degrade the performance of a dynamic pump.

15. Comparisons Between the Two types3/3
Comparison of performance curves of typical dynamic
and positive-displacement pumps at constant speed.

16. 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.
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.

17. Centrifugal Pump
Based on the concept of raising the pressure of a liquid indirectly by increasing the kinetic energy via the centrifugal action of the impeller and converting this kinetic energy to fluid work
Used predominantly for high-flow applications, less expensive, less complex thereby minimizing maintenance
Must be pre-charged with liquid or else it won’t pump at start-up. Positive displacement pumps don’t have this limitation.

18. Centrifugal Pumps
Impeller

19. How Centrifugal Pumps Work?
Fluid enters the eye of the impeller in an axial direction and
passes radially across the impeller acquiring a large tangential
velocity. The kinetic energy is converted to pressure
Radial-flow fluid machine

20. Kinetic Energy to Pressure (or Head)
b is the width of the impeller

21. Centrifugal Pump Performance Curve

22. Pump Curves
Pumps from manufacturers are typically rated by how much fluid work that can be achieved as a function of fluid flow
Fluid work in the pump curves are typically expressed in head form
Potential work decreases with increasing flow due to increased losses incurred at higher flow velocities
h is the pump head that equal to the change in pressure head flow between point 1, the eye, and point 2, the exit, as Z1=Z2=0 & V1=V2 around pump

23. System Curve
In a particular system, a centrifugal pump can only operate at one point on the ∆h against Q curve and that is the point where the pump ∆ h against Q curve intersects with the system ∆ h against Q curve as shown.
The intersection point should be near to the best efficiency point of the pump. The point of intersection is called duty point.
The system total head at a particular liquid flow rate.
Represent pump
Where (s) for suction side & (d) for discharge side

24. System Curve

25. System Curve (Cont’d)
Energy equation:

26. System Curve (Cont’d)

27. duty point

28. System Curve (Cont’d)

29. System Curve (Cont’d)

30. Centrifugal Pump Performance Curve

31. Centrifugal Pump Performance Curve

32. Pump Efficiency
The power delivered to the fluid simply equals the specific weight times the discharge times the net head change Pw=ρghQ (hp)
This is traditionally called the water horsepower.
The power required to drive the pump is the brake horsepower
bhp=ωT (hp)
where ω is the shaft angular velocity and T the shaft torque.
If there were no losses, Pw and bhp brake horsepower would be equal, but of course Pw is actually less, and the efficiency of the pump is defined as
The chief aim of the pump designer is to make ŋ as high as possible over as broad a range of discharge Q as possible.
The volumetric efficiency is
where QL is the loss of fluid due to leakage in the impeller-casing clearances.
(1 hp = 550 ft lbf/s = 746 W)
Power in

33. Example A pump delivers gasoline at 20°C and 12 m3/h. At the inlet,
p1 = 100 kPa, z1 = 1 m, and V1 = 2 m/s. At the exit p2 = 500 kPa, z2 = 4 m, and V2 = 3 m/s. How much power is required if the motor efficiency is 75%?
There is loss due to pipe length (hL)

34. Centrifugal Pump Performance 1/2
The operating characteristics of a pump are shown by plotting the: -total pump head ( ∆h),
-pump power (P),
-pump efficiency (ή) and
-(NPSH)R versus the flow rate (Q),
-rpm, impeller diameter and liquid viscosity
[Net Positive Suction Head ] Required to avoid cavitation
BEP is the best efficiency point which leads to the design flow rate (Q)

35. Centrifugal Pump Performance 2/2

36. Pump Map
For a given flow rate and pump head, the manufacturer’s pump map would indicate the operation of the pump at the physically available sizes
Impeller size
Pump power
Pump efficiency
NPSHR

37. Centrifugal Pump Cavitation
A centrifugal pump increases the fluid pressure by first imparting (provide) angular momentum (or kinetic energy) to the fluid, which is converted into pressure in the diffuser section.
Hence, the fluid velocity in and around the impeller is much higher than that either entering or leaving the pump, and the pressure is the lowest where the velocity is highest.
The minimum pressure at which a pump will operate properly must be above the vapor pressure of the fluid; otherwise the fluid will vaporize (or ‘‘boil’’), a condition known as cavitation.
The higher the temperature the higher the vapor pressure.

38. Below.

39. Net Positive Suction Head- Required (NPSHR)

40. NPSH – Net Positive Suction Head
In centrifugal pumps, the fluid must be brought up to the rotational speed of the impeller blades.
Increasing the fluid velocity would result in a decrease in pressure
This can cause boiling of the fluid or cavitation around the eye of the impeller.
To prevent this, there must be elevation of the fluid before the pump (ie pressure at suction side must be larger than v.p).
This height is known as the net positive suction head (NPSH).

41. System Curve (Cont’d)
(NPSHR)

42. Net Positive Suction Head
NPSH : is the difference between the absolute pressure head at the pump inlet and the absolute vapor pressure head of the liquid being pumped suction discharge side side Pa Pvap
NPSH: is the head required at the pump inlet to keep the liquid from cavitating or boiling.
The pump inlet or suction side is the low-pressure point where cavitation will first occur.

43. Net Positive Suction Head
If the pump inlet is placed at a height Zi above a reservoir whose free surface is at pressure pa, we can use Bernoulli’s equation to rewrite NPSH as
NPSH must be positive and larger than [NPSH]R
where hf i is the friction head loss between the reservoir and the pump inlet. Knowing pa and
hf i, we can set the pump at a height Zi that will keep the right-hand side greater than the “required’’ NPSH plotted in pump map
hfi Zi Pa

44. Example
The 32-in pump of Figure below is to pump 24,000 gal/min of water at 1170 r/min from a reservoir whose surface is at 14.7 lbf/in2 absolute. If head loss from reservoir to pump inlet is 6 ft, where should the pump inlet be placed to avoid cavitation for water at
(a) 60°F, Pv = 0.26 lbf/in2 absolute, SG 1.0 and γ = 62.4 lbf/ft3
(b) 200°F, Pv = lbf/in2 absolute, SG

46. Pumps in Series or Parallel