Different types of pumps & pumping System

1. Prem Baboo
Sr. Manager (Prod)
National Fertilizers Ltd. India
F.I.E., Institution of Engineers (India)
Technical Advisor & an Expert for
www.ureaknowhow.com

2. Pump Type
• Pumps can be classified according to their basic
principle as:-
Pumps
Dynamic Displacement
Centrifugal Rotary ReciprocatingSpecial
Effect
When different design can be used the centrifugal
pump is generally the most economical followed by
rotary & reciprocating pump.

5. Why prefer centrifugal pump
• Although the positive displacement is more
efficient than centrifugal pump, the benefit
of higher efficiency tends to be offset by
increased maintenance cost.
• That is the reason for incorporating
Ammonia feed pump of reciprocating type
with centrifugal type in recent Urea plant .

6. A Brief introduction to centrifugal pump
Mechanism:
Rotation of the impeller
imparts energy to the
liquid causing it to exit the
impeller’s vane at a
greater velocity than it
possessed when entered.
The liquid that exits the
impeller is collected in the
casing (volute/diffuser)
where its velocity is
converted to pressure.

7. A Brief introduction to centrifugal pump
•Do you know: Centrifugal pump was developed in the
late 1600’s. Its wide spread use, however has occurred
only in last seventy five years.
Theory:
•In operation, a centrifugal pump “slings” liquid out of the
impeller via centrifugal force.
•The flow and head developed in centrifugal pumps
depend on peripheral velocity of its impeller.
V ²=2gh

8. Performance Parameter of Pump
• Head: The quantity used to express the energy
content of the liquid per unit weight of the liquid.
Head is expressed in m of liquid.
• Capacity/Flow: Discharge delivered by the pump in
a unit time. It is expressed in m³/h or Lps or gpm.
• Hydraulic power: Theoretical power delivered to the
liquid by pump. [Mass flow(kg/s)*g (9.81m/s²)*H(m)] Watt
• BHP: Power delivered to the Pump shaft. [Hydraulic
Power/Pump efficiency]
• Pump Efficiency: The ratio of energy delivered by
the pump to the energy supplied to the pump shaft.
[Hydraulic power/BHP]
• NPSH: Net positive suction head. [m]

9. NPSH [Net positive suction head]
• NPSH: The value by which the pressure in the pump suction
exceeds the liquid vapour pressure. It is expressed as head of
liquid. NPSH is an analysis of energy condition on the
suction side of the pump to determine if the liquid will
vaporize at lowest pressure point in the pump.
• The value of NPSH needed at pump suction to avoid
cavitation in the pump is known as NPSHR.
• NPSHR is a characteristic of pump design. As the liquid
passes from the pump suction to the eye of impeller the
velocity increases and pressure decreases. There are also
pressure loss due to shock and turbulence as liquid strike
the impeller.
• A lower speed pump requires lower NPSH.
• A double suction pump requires 2/3rd
as much NPSH as
compared to similar rated single suction pump.
• NPSH required increases with increase in flow.

10. NPSH Calculation
NPSH: Hs (m of fluid) = -ΔP (suction piping) + (v1²-
vs²)/g+ (z1-zs+H1)

11. Cavitation:
Cavitation begins as the
formation of vapor
bubbles at the impeller
eye due to low pressure.
The bubbles form at the
position of lowest
pressure at the pump
inlet (see Figure 1),
which is just prior to the
fluid being acted upon
by the impeller vanes,
they are then rapidly
compressed.

13. Cavitation:
The compression of
the vapor bubbles
produces a small
shock wave that
impacts the impeller
surface and pits away
at the metal creating
over time large eroded
areas and subsequent
failure. The sound of
cavitation is very
characteristic and
resembles the sound
of gravel in a concrete
mixer.

14. Vapor pressure and cavitation
•There are two ways to
boil a liquid. One way is to
increase the temperature
while keeping the pressure
constant until the
temperature is high
enough to produce vapor
bubbles.
•The other way to boil a
liquid is to lower the
pressure. If you keep the
temperature constant and
lower the pressure the
liquid will also boil.

16. Centrifugal pump: Characteristic curve
Discharge flow in gpm
BEP: Best efficiency point
BEP: The point on head - capacity curve that align with
highest point on the efficiency curve.
NPSH
NPSH
(M)
NPSH
(M)

17. Centrifugal Pump characteristic
• Capacity of the centrifugal pump decreases as
discharge pressure increases.
• It is important to select a centrifugal pump that is
design to do a particular job.
• The pump generate the same head of liquid
whatever be the density of liquid being pumped.
• Even a small improvement in pump efficiency
could yield very significant saving of electricity as
it is least efficient of the component that
comprises a pumping system.

19. Centrifugal pump performance curve

20. Performance curve for family of pumps

21. Performance curve for different impeller

22. Multiple speed performance curve

23. System Characteristic
• The objective of the pump is to transfer or circulate
the liquid.
• A pressure is needed to make the liquid flow at
required rate and overcome head losses in the
system. Losses are of two type- 1. Static 2. Friction
• Static head is simply difference of height of supply
and destination reservoir.
Flow
StaticHead
Static
head

24. System Characteristic-frictional head
• Friction head is the friction loss due to flow of liquid through
pipe, valves, fitting, equipments.
• Frictional losses are proportional to square of the flow rate.
• For given flow rate friction loss can be reduced by increasing
diameter of pipe, using improved fitting etc..
• A close loop circulating system without a surface open to
atmospheric pressure, would exhibit only frictional losses and
would have a system resistance curve as below.
Flow
FrictionHead

25. System Head vs flow- typical system
• Most system are a combination of frictional head and
static head however the ratio of two head may vary
from system to system.
Flow
FrictionHead
FrictionHead
Flow
Static Head
Friction Head
Friction Head
Static Head

26. Pump operating point
Head
Head vs Flow
System Curve
Operating
point
Flow
•The operating point will always be the intersection
point of System curve and Head-flow curve.

27. Pump Operating Point
• If the actual system curve is different in reality as
compared to calculated, the pump will operate at a flow
and head different to expected.
• An error in system curve calculation may lead to
selection of a centrifugal pump which will have efficiency
less than expected.
• Ideally, the operating point should correspond to the flow
rate at the pump’s Best Efficiency Point (BEP).
• In many applications, some margin in the pump capacity
may be needed to accommodate transient changes.
• However, it is generally desirable to limit over-sizing to
no more than 15-20%.
• Adding too much safety margin may lead to inefficient
pump selection in actual operation.

28. Over designed pump

29. Effect on system curve with throttling

30. Power Requirement for Pump [Mgh]
You can use any of the following formulas to
make your calculations (for water only):
Head (ft) X Capacity (gpm)
5308
Hydraulic
Power (kW)
=
Hydraulic
Power (kW)
Head (meter) X Capacity (m³/h)
360
Hydraulic
Power (kW)
Head (meter) X Capacity (lps)
100
=
=
For fluids other than water multiply with sp. Gravity of fluid to
calculated power

31. Estimation of energy loss in oversized pump
Back

32. Power calculations
Assume that we need to pump 68 m3/hr. to a
47 meter head with a pump that is 60%
efficient at that point.
Liquid Power = 68 x 47 / 360 = 8.9 KW
Shaft Power = 8.9 / 0.60 = 14.8 Kw

33. Using oversized pump
As shown in the drawing, we should be using impeller
"E" to do this, but we have an oversized pump so we are
using the larger impeller "A" with the pump discharge
valve throttled back to 68 cubic meters per hour, giving
us an actual head of 76 meters.
Now our hydraulic power will be =68 x 76 / 360
= 14.3 Kilowatts
and Pump input power =14.3 / 0.50 (efficiency)
= 28.6 Kilowatts
required to do this.

34. Loss in Energy
Subtracting the amount of kilowatts we should have
been using from the actual power used gives us
extra power used =28.6 -14.8
= 13.8 extra kilowatts [ being
used to pump against the throttled discharge valve]
Extra energy used =8760 hrs/yr x 13.8
= 120,880kw.
= Rs. 4,80,000/annum
In this example the extra cost of the electricity could
almost equal the cost of purchasing the pump.

35. Flow versus speed
If the speed of the impeller is increased
from N1 to N2 rpm, the flow rate will
increase from Q1to Q2 as per the given
formula:
Q1
Q2
=
N1
N2

36. Example: Affinity law
•The affinity law for a centrifugal pump with the
impeller diameter held constant and the speed
changed:
Flow:
Q1 / Q2 = N1 /N2
Solution:
100 / Q2 = 1750/3500
Q2 = 200 GPM
Suppose a pump delivers 100 m³/h at 1750 pump
rpm. What will be the capacity at 3500 rpm?

37. Affinity law: Head vs Speed
H1
H2
=
N1²
N2²
The head developed (H) will be proportional to
the square of the pump speed, so that

38. Affinity law: Head vs Speed [Example]
H1
H2
=
N1²
N2²
Problem:
A pump is developing 30 meter head at 1750
rpm of pump, what will be the head at 3500 rpm
of pump.
Therefore 30/H2 = (1750/3500)²
⇒H2 = 30X(3500/1750)²
⇒H2 =120 meter

39. Affinity law: Power vs Speed
BHP1
BHP2
=
N13
N2³
The Power consumed (BHP) will be
proportional to the cube of the pump speed, so
that

40. Affinity law: Power vs Speed [Example]
BHP1
BHP2
=
N1
3
N2
3
Problem:
A pump is consuming 30 kW power at 2000 rpm
of pump, what will be the power at 4000 rpm of
pump.
Therefore 30/BHP2 = (2000/4000)³
⇒BHP2 = 30X(4000/2000)³
⇒BHP2 =240 kW

41. Effect of speed variation

42. The affinity law for a centrifugal pump-
for change in impeller diameter
•with the speed held constant and the impeller
diameter (D) changed:
Flow: Q1 / Q2 = D1 / D2
Example: 100 / Q2 = 80/60
Q2 = 75 GPM
Head: H1/H2 = (D1/D2)²
Example: 100 /H2 = (80 / 60)²
H2 = 56.25 Ft
Horsepower (BHP): BHP1 / BHP2 = (D1 / D2) ³
Example: 5/BHP2 = (80 / 60)³
BHP2 = 2.1

43. Effect of changing impeller diameter

44. Solution of over designed pump
•Reduce the speed / Trim the impeller
•Blue pump curve shows either of these option

45. Flow control strategies-by varying speed for
system with friction loss

46. Flow control strategies-by varying speed for
system with high static head

47. Flow control for permanent flow reduction
Flow
Head
Situation Before Impeller Trimming

48. Flow
Head
Situation After Impeller Trimming
Flow control for permanent flow reduction

49. Flow control strategies-Parallel
pump operation

50. Flow
Head
Flow control strategies-Parallel pump
operation

51. Flow control with control valve

52. Flow Control Strategies
• By-pass control: The pump runs continuously at
almost maximum load with a permanent bypass line
attached to outlet. When lower flow is required the
surplus liquid is passed.
– This system is even less efficient than throttling control.
• Start-stop control: This is a effective way to
minimize energy consumption where intermittent flow
are acceptable.
– e.g. Pumps in Raw water reservoir
– Pumps for sanitary water
– Pumps for fire water
** Note: Frequency of start/stop cycle should be
within the motor design criteria.

53. Variable speed drives
Flow
Head

54. Best practices in pumping system
• Ensure adequate NPSH at site of installation
• Ensure availability of basic instruments at pumps like
pressure gauges, flow meters.
• Operate pumps near best efficiency point.
• Modify pumping system and pumps losses to
minimize throttling.
• Adapt to wide load variation with variable speed drives
or sequenced control of multiple units.
• Stop running multiple pumps - add an auto-start for an
on-line spare or add a booster pump in a problem
area.
• Use booster pumps for small loads requiring higher
pressures.

55. Best practices in pumping system
• Increase fluid temperature differentials to reduce
pumping rates in case of heat exchangers.
• Repair seals and packing to minimize water loss by
dripping.
• Balance the system to minimize flows and reduce
pump power requirements.
• Use siphon effect to advantage: Avoid pumping head
with a free-fall return.
• Conduct water balance to minimise water
consumption.
• In multiple pump operations, judiciously combine the
operation of pumps and avoid throttling.

56. Best practices in pumping system
• Provide booster pump for few areas of higher
head.
• Replace old pumps by energy efficient pumps.
• In the case of over designed pump, provide
variable speed drive, or downsize / replace
impeller or replace with correct sized pump for
efficient operation.
• Optimize number of stages in multi-stage pump
in case of head margins.
• Reduce system resistance by pressure drop
assessment and pipe size optimization.

58. Specific speed