Normal head test by vendor was done for water at 20C.
Advantages of using Head--
Physical Properties Consideration
1) Increases Power consumed directly.
2) Max suction lift inversely.
Viscosity Pump efficiency decrease directly so Power required directly
Open or semi open impeller are better for highly viscose liquid.
Volatile liquid at boiling points require high NPSH.
Abrasive property of liquid or solid entrainment causes erosion and need specific MOC.
Corrosive liquid require specific MOC.
Centrifugal pump operation is most difficult when liquid handled contains solid particles.
Special attention required for selecting a centrifugal pump
Open Impeller for solids > 2%
Large cross section in Impeller & Volute
Min No. of Vanes
Inspection holes in tha casing & suction passage
Abrasion resistant MOC
Smooth corners & edges in lines
Stuffing boxes sealed with clear fluid
Fig Types of Impeller
Temperature of liquid Direct Impact on physical properties of liquid & Vapor Pressure and MOC.
Altitude P atm decreases with altitude & P atm has direct effect on NPSHa
Gas Dust Hazard if the surrounding atmosphere is hazardous/inflammable Flame proof & Dust proof MOC of Motor.
Stand by unit for vital application.
Selection of Pump Capacity & Head < 200 cSt < 25 m Upto 1 m 3 /h Peristaltic > 2% < 25% < 600 cSt 10500 m < 300 m 3 /h Positive Displacement > 2% < 5% Max 1050 m < 350 m 3 /h Rotary < 2% Upto 20% < 200 cSt Upto 105 m Upto 7500 m 3 /h Centrifugal % Gas Solid Viscosity Head Capacity Type
Flow Rate Design
Margins for rated/maximum capacity
PFD indicates normal flow rate without any margin & the Maximum flow is Considered for sizing of the pump with margin 30% Waste Heat Boiler pump 25% Boiler Feed water pump 0% Recirculation pump 3-5% Large cooling water pump 0-5% Transfer pumps 0% Intermittent pumps 20-25% Reflux pumps 10% Continuous process pumps Margin Service
Minimum flow rate ????
Pump centre line as datum for Hydraulic calculation
Pump centre Line from ground (estimated)
Minimum level in Suction & Maximum level in Discharge tank.
1.0 Above 200 0.9 100 200 0.7 0 100 Pump centre line above ground Flow Rate (m 3 /h)
Line Pressure Drop ?
Pressure Drop for Control Valve
The following criteria can be used for sizing the control valve
15~25% of the variable system drop is typically allowed.
On recycle and reflux pumps allow 1/3 of the variable system pressure with minimum of 0.7 bar.
For liquid system 0.7 bar
For system with large variable pressure drop ( >10 bar) ~15% of the variable pressure drop exclusive of control valve
Pressure Drop for Devices 0 Ultrasonic & electromagnetic Flow Meter 0.2 0.4 Corilolis Flow Meter 0.2 0.4 Vortex 0.02 0.05 Venturi Flow Meter 0.25 Orifice Flow meter 0.07 bar (continuous strainer) Y, T or Bucket type Strainer 1.0 bar Air cooler 0.35 0.5 bar per shell 0.7 bar per pass in tube side Shell & Tube type Heat Exchanger Press Drop (in bar) Devices in Flow Line
NPSHA = Suction Pressure Vapor Pressure
NPSHA should be 2 3 ft more than NPSHR.
It is the pressure enough to prevent formation of vapor bubbles due to vaporization or release of dissolved gases in the Impeller.
Pressure increases along the impeller on collapse of vapors Cavitation.
Cavitation Noise, Vibration, Drop in performance curve, high wear & tear loss.
NPSHA can be increased by
Raise the liquid level
Lower the pump
Reduce the friction losses in the suction line
Use a booster pump
Sub cool the liquid
NPSHR can be reduced by
Large impeller area
Inducers ahead of conventional pump at suction side