Basics of centrifugal. Topics covered are operating principles, energy conversion, components in centrifugal pump, the concept of NPSH, pump rating calculation and affinity laws

1. CENTRIFUGAL PUMP
Vikram Sharma
30th August 2016

2. TABLE OF CONTENTS
 Operating Principles
 Energy Conversion
 Components in Centrifugal Pump
 The concept of NPSH
 Pump Rating Calculation
 Affinity Laws

3. OPERATING PRINCIPLES
 Liquid enters the suction nozzle & later into the eye
of the impeller due to the rotation of the pump
impeller.
 Low pressure region “pulls” the liquid towards the
eye of the impeller.
 The rotation of the impeller radially pushes the
liquid → centrifugal acceleration.
 The centrifugal force & curved nature of the blade
pushes the liquid in the tangential and radial
direction.

4. ENERGY CONVERSION
 Convert velocity or kinetic energy into pressure
energy.
 The conversion of energy occur due to two main
parts of the pump → impeller and volute or diffuser.
 Impeller: Driver energy → kinetic energy
 Volute/Diffuser: Kinetic energy → Pressure energy.
 Resistance to flow → Kinetic energy of a liquid
coming out of an impeller is obstructed.
 Initial resistance created by pump casing, “catches”
the liquid & slows it down.

5. ENERGY CONVERSION (CONT’D)
 Additional resistance created when the liquid is
decelerated (discharge nozzle), Velocity energy →
Pressure energy

6. COMPONENTS IN CENTRIFUGAL PUMP
 Impeller
 Imparts velocity to the liquid as the result from
centrifugal force

7. COMPONENTS IN CENTRIFUGAL PUMP
 Casing
 Provides a direction of liquid flow from the impeller
 Converts Velocity Energy → Pressure Energy

8. COMPONENTS IN CENTRIFUGAL PUMP
 Stuffing box (a) Packing
 Means of throttling the leakage which would occur
at the point of entry of the shaft into the casing.
 Most common means of throttling the leakage
between the inside & outside of the casing

9. COMPONENTS IN CENTRIFUGAL PUMP
 Stuffing box (b) Gland
 Used in positioning and adjusting the packing
pressure.

10. COMPONENTS IN CENTRIFUGAL PUMP
 Stuffing box (c) Seal gage or Lantern ring
 Distribute sealing medium uniformly around the
portion of the shaft that passes through the stuffing
box.
 Essential when suction lift condition exist to seal
against in-leakage of air.

11. COMPONENTS IN CENTRIFUGAL PUMP
 Stuffing box (d) Mechanical seal
 It has one surface rotating with the shaft, one
surface is stationary face..
 Prevent the leakage of the liquid from the pump to
the external surroundings.
 Devices form the packing between rotor and
stationary parts of the pump.

12. COMPONENTS IN CENTRIFUGAL PUMP
 Shaft Sleeve
 Used as shaft protection where the shaft passes
through the staffing box.
 Usually used with packing, often not used if mech.
seals are employed.

13. COMPONENTS IN CENTRIFUGAL PUMP
 Wearing Rings
 Sacrificial components installed on the casing and
impeller to prevent liquid from recirculating back to
the suction from the discharge.
 Installed on the both the front and back of the
impeller.
 Typically used in closed-impeller.

14. COMPONENTS IN CENTRIFUGAL PUMP
 Wearing Plates
 Performs the same function as wearing rings.
 Typically used in open or semi-open impellers.

15. COMPONENTS IN CENTRIFUGAL PUMP
 Bearings
 Function to accurately locate shaft.
 Also to carry radial and thrust loads.

16. COMPONENTS IN CENTRIFUGAL PUMP
 Frame
 To mount unit rigidly and support bearing.

17. COMPONENTS IN CENTRIFUGAL PUMP
 Coupling
 It connects the pump to the driver

18. THE CONCEPT OF NPSH
 Cavitation
 Vapour Pressure is the pressure req. to boil a liquid
at a specific temperature.
 Can be avoided if the pressure of the liquid at all
points within the pump is above the atm. pressure.

19. THE CONCEPT OF NPSH
 Two NPSH parameters, i) available and ii) required.
 NPSHA: Difference between the pressure at the
suction of the pump & the saturation pressure of the
liquid being pumped.
 NPSHR: Min. net positive suction head req. to
avoid cavitation.
 NPSHA ≥ NPSHR
 General requirement: NPSHA is at least 2.0m of
liquid greater than the pump manufacturer requires
under the worst pump operating conditions.

20. PUMP RATING CALCULATION
 Schematic diagram of a typical pump scheme

21. PUMP RATING CALCULATION
 Pump Suction
𝑃𝑆,𝑀𝐼𝑁 = 𝑃𝑠𝑣 +
𝑆 × 𝑆𝐺
10.2
− ∆𝑃𝑆
∆𝑃𝑆 = ∆𝑃𝑠𝑢𝑐𝑡𝑖𝑜𝑛 𝑙𝑖𝑛𝑒 + ∆𝑃𝑓𝑖𝑙𝑡𝑒𝑟 + ∆𝑃𝑜𝑡ℎ𝑒𝑟
Where:
PS,MIN = Minimum suction pressure (barg)
PSV = Pressure of the suction vessel (barg)
SG = Specific gravity of the liquid at T and P
S = Minimum liquid height from pump centerline (m)
ΔPS = Pressure drop across the pump suction line (barg)

22. PUMP RATING CALCULATION
 NPSHA
𝑁𝑃𝑆𝐻𝐴 =
𝑃𝑆𝑉 − 𝑃𝑉𝐴𝑃 × 10.2
𝑆𝐺
+ 𝑆 −
∆𝑃𝑆 × 10.2
𝑆𝐺
𝑁𝑃𝑆𝐻𝐴 ≥ 𝑁𝑃𝑆𝐻 𝑅
Where:
NPSHA = Net Positive Suction Head (m)
PSV = Pressure of the suction vessel (bara)
SG = Specific gravity of the liquid at T and P
PVAP = Vapour pressure (bara)
ΔPS = Pressure drop across the pump suction line
(bara)

23. PUMP RATING CALCULATION
 Pump Discharge Pressure
𝑃 𝐷 = 𝑃2 +
𝐻 × 𝑆𝐺
10.2
+ ∆𝑃 𝐷
∆𝑃 𝐷 = ∆𝑃𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒 𝑙𝑖𝑛𝑒 + ∆𝑃𝑓𝑖𝑡𝑡𝑖𝑛𝑔𝑠 + ∆𝑃𝑜𝑡ℎ𝑒𝑟
Where:
P2 = Max. OP of the receiving vessel or B.L (barg)
H = Liquid static height (HD,MAX – HPD) (m)
SG = Specific gravity of liquid at T and P
ΔPD = Pressure drop across the discharge line (barg)

24. PUMP RATING CALCULATION
 Differential Height (DH)
𝐷𝐻 =
𝑃 𝐷 − 𝑃𝑆,𝑀𝐼𝑁 × 10.2
𝑆𝐺
Where:
PD = Pump discharge pressure in barg
PS,MIN = Minimum pump suction pressure in barg

25. PUMP RATING CALCULATION
 Pump Shut-off Pressure
 calculated by adding the suction vessel OP to the
shut-off pressure of the pump.
𝑃𝑆𝑂 = 𝑃𝑆𝑉,𝑀𝐴𝑋 +
𝑆𝐺
10.2
× 𝑆 + 1 + 𝐾 𝐷𝐻
 calculated by adding suction vessel DP to the OP of
the pump
𝑃𝑆𝑂 = 𝑃𝑆𝑉,𝐷𝐸𝑆𝐼𝐺𝑁 +
𝑆𝐺
10.2
× 𝑆 + 𝐷𝐻

26. PUMP RATING CALCULATION
 Pump Shut-off Pressure (cont’d)
 calculated by adding the suction vessel DP to the
shut-off pressure of the pump
𝑃𝑆𝑂 = 𝑃𝑆𝑉,𝐷𝐸𝑆𝐼𝐺𝑁 +
𝑆𝐺
10.2
× 𝑆 + 1 + 𝐾 𝐷𝐻
o The maximum value obtained from the above
equations shall be the pump shut-off pressure. The
constant K is typically 20%

27. PUMP RATING CALCULATION
 Power Estimation
 Hydraulic Power / Absorbed Power
Defined as the energy applied on the liquid being
pumped to increase its velocity and pressure
𝑃ℎ𝑦,𝑘𝑊 =
𝑄 × 𝑃 𝐷 × 100 − 𝑃𝑆,𝑀𝐼𝑁 × 100
3600
Where:
Phy,kW = Hydraulic power (kW)
Q = Volumetric flowrate (m3/h)
PD = Pump discharge pressure (barg)
PS,MIN = Min. pump suction pressure (barg)

28. PUMP RATING CALCULATION
 Shaft Power
 Defined as the power supplied by the motor to the pump
shaft.
 Sum of the hydraulic power & power loss due to
inefficiencies seen in the power transmission from the
shaft to the liquid
𝑃𝑆,𝑘𝑊 =
𝑃ℎ𝑦,𝑘𝑊
𝜂 𝑝
𝜂 𝑃
= 80 − 0.2855 ∙ 𝐻 + 3.78 × 10−4 ∙ 𝐻 ∙ 𝑄
− 2.38 × 10−7
∙ 𝐻 ∙ 𝑄2
+ 5.39 × 10−4
∙ 𝐻2

29. PUMP RATING CALCULATION
Where:
PS,kW = Shaft Power (kW)
ηP = pump efficiency (decimal format)
H = Developed head (ft); Q = Liquid flowrate (GPM)
 The applicability of the ηP eq. is limited to 15.24-
91.44 m developed head and 22.7-227 m3/hr.
 The developed head above can be calculated using
the equation provided below.
𝐻 =
𝑃 𝐷 − 𝑃𝑆,𝑀𝐼𝑁 +
𝐻 𝑃,𝐷 − 𝐻𝑆,𝐷 × 𝑆𝐺
10.2
× 10.2
𝑆𝐺

30. PUMP RATING CALCULATION
 The developed head calculated is converted to feet.

31. PUMP RATING CALCULATION
 Motor Power
 Power consumed by the pump motor that rotates
the pump shaft.
 Combination of the shaft power & inefficiencies in
converting electric energy into kinetic energy
𝑃 𝑀,𝑘𝑊 =
𝑃𝑆,𝑘𝑊
𝜂 𝑀
Where:
PM,kW = Motor power (kW)
ηM = motor efficiency (decimal format)

32. PUMP RATING CALCULATION
 Temp. rise due to pumping
 Temp. rise due to pump inefficiency
𝑡 𝑟 =
9.8067 × 𝐻 ×
1
𝜂 𝑃
− 1
𝐶 𝑃
Where:
H = Developed head (m)
tR = Temperature rise (°C)
CP = Specific heat at avg. temp. (J/kg·°C)

33. AFFINITY LAWS
 Assumption made in arriving at the affinity laws is
that the two operating points that are compared are
at same efficiency.
 Pumps with fixed speed, the affinity laws become:
 Pumps with fixed diameter, the affinity laws
become: