CENTRIFUGAL PUMP

Vikram Sharma30th August 2016

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

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.

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.

ENERGY CONVERSION (CONT’D) Additional resistance created when the liquid

is decelerated (discharge nozzle), Velocity energy → Pressure energy

COMPONENTS IN CENTRIFUGAL PUMP Impeller

Imparts velocity to the liquid as the result from centrifugal force

COMPONENTS IN CENTRIFUGAL PUMP Casing

Provides a direction of liquid flow from the impeller

Converts Velocity Energy → Pressure Energy

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

COMPONENTS IN CENTRIFUGAL PUMP Stuffing box (b) Gland

Used in positioning and adjusting the packing pressure.

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.

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.

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.

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.

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

impellers.

COMPONENTS IN CENTRIFUGAL PUMP Bearings

Function to accurately locate shaft. Also to carry radial and thrust loads.

COMPONENTS IN CENTRIFUGAL PUMP Frame

To mount unit rigidly and support bearing.

COMPONENTS IN CENTRIFUGAL PUMP Coupling

It connects the pump to the driver

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.

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.

PUMP RATING CALCULATION Schematic diagram of a typical pump

scheme

PUMP RATING CALCULATION Pump Suction

Where:PS,MIN = Minimum suction pressure (barg)PSV = Pressure of the suction vessel (barg)SG = Specific gravity of the liquid at T and PS = Minimum liquid height from pump centerline (m)ΔPS = Pressure drop across the pump suction line (barg)

PUMP RATING CALCULATION NPSHA

Where:NPSHA = Net Positive Suction Head (m) PSV = Pressure of the suction vessel (bara)SG = Specific gravity of the liquid at T and PPVAP = Vapour pressure (bara)ΔPS = Pressure drop across the pump suction line (bara)

PUMP RATING CALCULATION Pump Discharge Pressure

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)

PUMP RATING CALCULATION Differential Height (DH)

Where:PD = Pump discharge pressure in bargPS,MIN = Minimum pump suction pressure in barg

PUMP RATING CALCULATION Pump Shut-off Pressure calculated by adding the suction vessel OP to

the shut-off pressure of the pump.

calculated by adding suction vessel DP to the OP of the pump

PUMP RATING CALCULATION Pump Shut-off Pressure (cont’d) calculated by adding the suction vessel DP to

the shut-off pressure of the pump

o The maximum value obtained from the above equations shall be the pump shut-off pressure. The constant K is typically 20%

PUMP RATING CALCULATION Power Estimation Hydraulic Power / Absorbed PowerDefined as the energy applied on the liquid being pumped to increase its velocity and pressure

Where:Phy,kW = Hydraulic power (kW)Q = Volumetric flowrate (m3/h)PD = Pump discharge pressure (barg)PS,MIN = Min. pump suction pressure (barg)

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

PUMP RATING CALCULATIONWhere: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.

PUMP RATING CALCULATION The developed head calculated is converted

to feet.

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

Where:PM,kW = Motor power (kW)ηM = motor efficiency (decimal format)

PUMP RATING CALCULATION Temp. rise due to pumping Temp. rise due to pump inefficiency

Where:H = Developed head (m)tR = Temperature rise (°C)CP = Specific heat at avg. temp. (J/kg·°C)

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: