POWER AND EFFICIENCY OF CENTRIFUGAL PUMP

Power and efficiency of centrifugal pump
Brake Horse Power (BHP) and Water horsepower (WHP)

• The work performed by a pump is a function of the total head and the weight of the liquid pumped in a given time period.

Pump input or brake horsepower (BHP) is the actual horsepower delivered to the pump shaft.

Pump output or hydraulic or water horsepower (WHP) is the liquid horsepower delivered by the pump.

• These two terms are defined by the following formulas.

where

• Q = Capacity in gallons per minute (GPM)
• HT = Total differential head, ft
• Specific gravity= Specific gravity of liquid
• Efficiency = Pump efficiency, %

• The constant 3960 is obtained by dividing the number or foot-pounds for one horsepower(33,000) by the weight of one gallon of water (8.33 pounds). 

The brake horsepower or input to a pump is greater than the hydraulic horsepower or output due to the mechanical and hydraulic losses incurred in the pump.

• Therefore the pump efficiency is the ratio of these two values.

Best Efficiency Point (BEP)

• The Head, NPSHr, efficiency, and BHP all vary with flow rate, Q.

Best Efficiency Point (BEP) is the capacity at maximum impeller diameter at which the efficiency is highest.

Significance of BEP

• BEP as a measure of optimum energy conversion
• When sizing and selecting centrifugal pumps for a given application the pump efficiency at design should be taken into consideration.
• The efficiency of centrifugal pumps is stated as a percentage and represents a unit of measure describing the change of centrifugal force (expressed as the velocity of the fluid) into pressure energy.
• The B.E.P.(best efficiency point) is the area on the curve where the change of velocity energy into pressure energy at a given gallon per minute is optimum; in essence, the point where the pump is most efficient.
• The operation of a centrifugal pump should not be outside the furthest left or right efficiency curves published by the manufacturer.
• Performance in these areas induces premature bearing and mechanical seal failures due to shaft deflection, and an increase in temperature of the process fluid in the pump casing causing seizure of close tolerance parts and cavitation.
• BEP is an important parameter in that many parametric calculations such as specific speed, suction specific speed, hydrodynamic size, viscosity correction, head rise to shut-off, etc. are based on capacity at BEP.
• Many users prefer that pumps operate within 80%to 110% of BEP for optimum performance.

Specific Speed

Specific speed as a measure of the geometric similarity of pumps

• Specific speed (Ns) is a non-dimensional design index that identifies the geometric similarity of pumps.
• It is used to classify pump impellers as to their type and proportions.
• Pumps of the same Ns but of different size are considered to be geometrically similar,one pump being a size- factor of the other.

Specific speed Calculation

• The following formula is used to determine specific speed:

where

• Q = Capacity at best efficiency point(BEP) at maximum impeller diameter, GPM(gallons per meter)
• H = Head per stage at BEP at maximum impeller diameter, ft
• N = pump speed, RPM

• As per the above formula, it is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute flow against one-foot head.
• Specific speed should be thought of only as an index used to predict certain pump characteristics.
• The specific speed determines the general shape or class of the impellers.