1. CENTRIFUGAL PUMPS:-
DESIGN & PERFORMANCE
Ir. N. Jayaseelan
2012

2. What is a centrifugal pump?
Basic components:
Volute, casing or body
Diffuser
Impeller
Driver (motor)

3. CENTRIFUGAL PUMP BASICS
Centrifugal force
Tending to fly from the center
Thrown from the center
Centrifugal pump impeller does just that
Water drawn into impeller eye, accelerated down the blades and thrown against pump casing that directs flow out of discharge
Centrifugal action is demonstrated well when a dog shakes … the skin rotates around the body and everyone is aware of the results. Liquid is flung from the animal with some force.
Same effect is evident if you observe a party-goer with a drink in one hand. Someone taps them on the shoulder and they turn around too fast. Their drink is shared by the people around them.
Each is a result of centrifugal action ... 2

6. Centrifugal Pumps Very simple design
Two main parts are the impeller and the diffuser
Impellers
Bronze
polycarbonate
cast iron
stainless steel
Diffuser
Cast iron

11. Large, Line Mounted Pump
Close coupled
18 sizes
To 2600 gpm
To 410 ft TDH
To 60 HP
To 8”x8”
Special Purpose Motor

12. Multi-stage pumps Multi-stage, diffuser design
8 sizes, stainless steel
to 600 gpm
to 1200 ft
to 75 hp, 3500 rpm
many options for suction and discharge nozzles

13. Multi-stage pump
“Diffusers”

14. Vertical Turbine Submersible 48 Models 5 - 14 bowls 40 - 2000 gpm
feet head
Lineshaft
88 Models
5-20” bowls
4 Styles
,000 gpm
feet head
Centrifugal Pumps

15. Double Suction Pump 37 sizes To 12500 gpm To 840 ft TDH To 1000 HP
To 14”x18”
General Purpose Motor

16. Typical Split Case Pump- Section View
Centrifugal Pumps

17. End Suction Pump 26 sizes To 4400 gpm To 520 ft TDH
To 150 HP To 8”x10”
General Purpose Motor

18. CENTRIFUGAL PUMP BASICS
Liquid into
impeller eye from
pump suction
Impeller Rotation
As liquid enters the pump impeller eye, the blades accelerate the liquid along the convex surfaces of the impeller blades, or vanes. The liquid is thrown from the impeller blade tips into the casing.
The casing is designed to accept the flow efficiently and channel the liquid to the casing discharge port, converting the liquid velocity to pressure energy.
The pump discharge pressure or pump head is determined ONLY by the impeller tip speed. Just as a sling shot with long strings can throw a stone further than a sling shot with short strings, if rotated at the same speed (a la David and Goliath); the larger the impeller diameter, the higher the pump head. The only other way to increase head is to increase the impeller tip speed by increasing pump speed (rpm)
You will not obtain more head (or more flow) by installing a larger motor (At the same speed) 3

19. Multistage impellers

20. Impellers

21. CENTRIFUGAL PUMP BASICS
Centrifugal Pump Curve
If a pump were sized for 231 feet shut off head, the impeller tip speed would be sufficient to raise (Throw) the water to a 231 feet height. There would be no flow at that height and the height would never increase. The discharge gauge on the pump would read 100 psig, for water. The pump is reacting similar to a discharge valve being closed while running. This pump would develop 100 psig and no more. If the pump continued to run, the pressure would not increase but the energy imparted into the water by the impeller would be wasted and the liquid would rapidly heat up to a temperature greater perhaps than the seal and pump is designed for. Catastrophic effects have resulted in pumps running at no flow conditions.
If a graph were drawn with flow (gpm, for example) on the “X” axis and column of water head (In feet) on the “Y” axis. A point may be marked at 231 feet head at 0 gpm.
If the pipe were now cut at a lower height, say 150 feet, and the rate the water overflowed was measured, perhaps by weighing the overflow, another point at that measured flow could be marked at 150 feet head on the “Y” axis.
The pipe could be cut at several descending heights, the overflowing volumes measured, and the points made and joined together to form a pump performance curve.
This is how pump manufactures assemble pump curves. Not by cutting pipe, obviously, but by getting similar effects by utilizing approved flow measuring and pressure reading devices and by taking exact power readings with torque shafts or with calibrated motors to establish pump efficiency. 4

22. CENTRIFUGAL PUMP BASICS
Centrifugal Pump Curve
Impellers are trimmed from full size and the pumping unit is retested at each size, until finally the fully assembled pump performance curves are completed. Pump impellers may be trimmed to any diameter in the range, but the normal tests and display is indicated in 1/2” to 1” trim increments, depending on the pump size.
A vacuum may be drawn on the suction tank in the pump test assembly, enabling the suction capabilities or NPSH requirements to be established. (More information on NPSH is available in the another Pump Basics presentation on “Cavitation”)
All the tested information, including flow and head curves, efficiency information, NPSH requirements and power requirements shown at standard available motor sizes, may be assembled into a published pump performance curve, as indicated on this slide.
A typical pump curve shows flow along the bottom (“x” axis), head in linear measure on the vertical (“y”) axis. A user friendly curve will incorporate the hp lines and the efficiency lines directly on the performance curves, as on this slide and in Armstrong’s catalog. It is easier to obtain the whole picture in this manner, at a glance.
Note that the single NPSH curve, shown on published performance curves, is for the design (Maximum) impeller only. NPSH requirements will change as the impeller is trimmed. (May increase) 5

23. CENTRIFUGAL PUMP BASICS
Specific Gravity
WATER
1.0 sp gr
GASOLINE
0.8 sp gr
BRINE
1.2 sp gr
hp = gpm*hd*sp gr*100
3960*Pump Eff
Pump head (ft) = psig*2.31
sp gr
231 ft
120 psig
80 psig
100 psig
As the liquid is, literally, thrown from the pump to a specific height (Pump head in feet of water, as an example), the pump would throw any liquid to the same height. There is some sacrifice with viscous liquids, but as this is uncommon in hvac applications we’ll save that for another presentation.
For liquids of different density, or specific gravity, the pressure effects will be different for the same pump. The illustration on this slide shows the same pump pumping water, gasoline and brine (The 20% Calcium Chloride Soln mentioned in the previous slide. Typically used for freezing water by circulating under the skating rink floor)
Compared to water: Gasoline is 80% as heavy (0.8 sp gr) and Brine is 120% the weight of water (1.2 sp gr). Sp Gr always compares the weight of the liquids to the weight of water at 60DegF (62.34 lbs/ft3)
The gauge reading for the gasoline will be 80% of the water reading; the brine 120%. The head, in feet, will be the same for all liquids.
This is important when selecting pumps for liquids other than water.
Power requirements vary directly with Sp Gr as:
Power=Flow*head*sp gr*100/constant*pump efficiency
(Constant is 3960 for flow and head expressed in gpm & ft as 1 hp =33,000 ft lbs. Head is already in feet, so we must get the gpm into lbs. Constant converts the ft lbs into hp by dividing the 33,000 by the weight of US gallon of water [8.33 lbs]. 33,000/8.33=3960)
Pump heads do not change UNLESS THE REQUIREMENT IS EXPRESSED IN WEIGHT PER UNIT AREA (psig or kPa). If pressure units are specified, the pump head, for pump selection, is divided by the sp gr, as a heavier liquid will produce more psig/foot and a lighter one less.
Pump Shut-off Head 231ft 7

24. CENTRIFUGAL PUMP BASICS
System Resistance
Similarly, resistance to flow in piping and fittings varies to the square of the flow. That is: Friction loss in valves, piping … in fact the hvac system as a whole, may be thought of as varying to the square of the flow change.
Example: If the flow through a system is increased by 25%. The head to force the design flow through the same system would increase by (1.25*1.25) 56%. If the original flow through the system was 100 gpm at 100 ft head. The new conditions would be 125 gpm at 156 ft head.
The power would be (1.25*1.25*1.25) 95% greater than the original power requirement.
The chart in this slide is borrowed from the 1981 ASHRAE Fundamentals handbook, because of the Imperial units. A more recent handbook may be referenced for metric units.
The ASHRAE general recommendations have been marked. Basically that hvac piping may be sized for 4 fps up to 2” diameter piping and sized on 4 feet friction loss per 100 feet of pipe for larger piping.
One may follow the 500 gpm line vertical and see it intersect the 4 ft/100 ft line at just below 5” diameter piping. 5” diameter piping is the general recommendation for 500 gpm and would carry an 8 fps velocity.
To prove the square law, one may follow the 1000 gpm up to the 5” diameter pipe. For the law to be proven, the friction loss needs to be a little below (1000/500)2*4=(2)2*4=4*4=16. QED! Very close!
Friction Loss for Water in Commercial Steel Pipe (Schedule 40) 10

25. CENTRIFUGAL PUMP BASICS
Balancing
Valve
3-Way
Control Valve
Isolating Valve
LOAD
LOAD
LOAD
BUILDING
HEIGHT
LOAD
As mentioned on the previous slide: A “closed” system would, typically, have only the expansion tank as the interface with air (Or flexible membrane, as in a diaphragm or bladder tank). In a closed system the height of the system has no bearing on the pump head. The system is filled from some external source and the pump is sized only to circulate the design flow against the resistance of the piping. There is little resistance at close to “0” flow (90% reduction in flow would need [0.10*0.10]1% of the original head) so the system resistance curve begins at “0” head for “0” flow. The liquid in the system will not flow due to gravity (Unless there is a temperature change).
The pump flow is based on the total flow of all the components in the system, added together. As the piping to each load branches off, the remainder of the flow must go to the remaining loads.
The head on the other hand, must be sized for the worst system branch only. The resistance through the common piping (Piping carrying flow for more than one load) takes care of the cumulative flow issues but only the worse dedicated load piping is added to this to get the pump head.
In the slide above, only the worst friction loss of the (4) load/valve combinations is added to the common piping losses to calculate the pump head.
EXPANSION
SOURCE 12

26. CENTRIFUGAL PUMP BASICS
“Open” System
Static
Suction Head
CONDENSER
Static Head
Example of “open” system is shown in this slide. An “open” system has more than one interface with air (Several expansion tanks may be installed in a closed system as this is the same as installing one large tank). This condenser water system has (2) interfaces with air:
Surface of tower reservoir and return piping outlet.
Friction loss in piping varies to the square of the flow change, so the system friction head varies as the square of the flow change.
One thing that does not vary with the flow change is the static height.
With no pump running the water in the system levels at the height of the tower reservoir level. So the static suction head (Height above pump) does not figure into the pump head calculations.
Only the difference in fill height (Reservoir level) and Outlet height (Entrance to tower) is added to system friction loss to calculate pump head.
In an open system, the pump fills the part of the system between the fill level and the final operating elevation. Assuming all (check) valving is ignored, the liquid in an open system will return to the fill level once the pump is stopped. The static difference in fill and operating levels is constant, regardless of the flow, so the system resistance curve starts at this point. 14

27. CENTRIFUGAL PUMP BASICS
Resistance Curve B
HEAD
System Head Curve
CAPACITY
Friction Losses
Total Static Head
An open system friction loss curve begins at the static height difference between the fill level (Liquid level when the system is first filled or the pump is stopped) and the highest operating level.
That is: The static height that must be overcome by the pump only. This height must be added to the friction loss head to determine the pump head and is constant. It does not vary with flow.
This could prove important to pump selections. Particularly parallel pump selections. See Parallel Pumping presentation for more parallel pumping selection information. 15

28. Pump Performance calculationS
Overall Efficiency = Hydraulic power ( P3) X 100/ Power input ( P1)
Pump efficiency. = Hydraulic power ( P3) X 100/ Power input to pump shaft ( P2)
Hydraulic Power ( P3) = Q X Total Head ( hd - hs ) X p X g / 1000
Q = discharge in m³/s

29. Pump Performance calculationS
p = density of fluid in kg/ m³
g = acceleration due to gravity ( m/s²)
P1 = X V X I X pf
P2 = P1 X eff.of motor.
P3 = P2 x eff. Of pump

30. Key ParameterS for determining efficiency
Flow
Head
Power

31. Flow Measurement Techniques
Ultrasonic flow measurement
Tank filling method
Installation of online flow meter

32. Determination of total head
Suction head
measured from pump inlet pressure gauge reading
Discharge head
This is taken from the pump discharge side gauge

33. Typical name plate details of pumpS
Make Beacon
hp/kW (3.7)
3 Pipe sizes S/D /50 mm
Head m
Capacity m3/s
Amps
Efficiency %

34. Flow control Strategies
Varying speed
Pumps in parallel
stop/start control
Flow control valve
By pass control valve
Trimming impeller
Use of VFDS

36. Energy conservation opportunities in pumping
Operate pump near best efficiency point.
Replace old pumps by energy efficient pumps
Reduce system resistance by pressure drop assessment and pipe size optimization.
Provide booster pump for few areas of higher head.

37. Energy conservation opportunities in pumping
Conduct water balance to minimize water consumption.
Ensure availability of instruments like pressure gauges, flow meters.
Repair seals and packing to minimize water loss.
Avoid valves in discharge side as far as possible.
Operate pumpset during non-peak hours.

38. Specific speed that is used to classify pumps
nq is the specific speed for a unit machine that is geometric similar to a machine with the head Hq = 1 m and flow rate Q = 1 m3/s

40. EXERCISE
Find the flow rate, head and power for a centrifugal pump that has increased its speed
Given data:
hh = 80 % P1 = 123 kW n1 = 1000 rpm H1 = 100 m
n2 = 1100 rpm Q1 = 1 m3/s

41. EXERCISE
Find the flow rate, head and power for a centrifugal pump impeller that has reduced its diameter
Given data:
hh = 80 % P1 = 123 kW D1 = 0,5 m H1 = 100 m
D2 = 0,45 m Q1 = 1 m3/s

42. Pump Impeller
Direction of
rotation
Vanes
Centrifugal Pumps

43. Typical single suction impeller
Centrifugal Pumps

44. Single suction impeller
Centrifugal Pumps

45. Impeller Types Open Semi-open Closed Single suction Double suction
Non-clogging
Axial flow
Mixed flow
Centrifugal Pumps

46. Impeller and volute Discharge Nozzle Cutwater Suction Eye
Arrows represent the
direction of water flow
Discharge
Nozzle
Cutwater
Suction Eye
Centrifugal Pumps

47. Impeller Profiles
Specific Speed

48. Generally: Maximum efficiency lies in the range:
2000<NS<3000
High head, low capacity pumps:
500<NS<1000
Low head, large capacity pumps:
NS>15000
Centrifugal Pumps

49. Comments? Questions? Observations?