Fundamental Chemistry and Hydraulics - Mathematics and Statistics for the Environment - page 375

within the pump. Similarly, not all of the power of the electric current driving the motor (motor
horsepower) will be used to drive the pump; some of the current is used to overcome friction within
the motor, and some current is lost in the conversion of electrical energy to mechanical power.
Note: Depending on size and type, pumps are usually 50 to 85% efficient, and motors are usually
80 to 95% efficient. The efficiency of a particular motor or pump is given in the manufac-
turer's technical manual accompanying the unit.
A pump's brake horsepower equals its hydraulic horsepower divided by the pump's efficiency.
Thus, the brake horsepower formula becomes

Flow (gpm)head (ft) pecificgravity
39

× ×

BHP = (14.29)

60 × efficiency

or

Flow (gpm)pressure (psig)
1714

×

BHP = (14.30)

× efficien cy

■ EXAMPLE 14. 34
Problem: Calculate the BHP requirements for a pump handling salt water and having a flow of 600
gpm with 40-psi pressure differential. The specific gravity of saltwater at 68°F equals 1.03. The
pump efficiency is 85%.

Solution: Convert the pressure differential to total differential head (TDH) = 40 × 2.31/1.03 = 90
ft (rounded).

600 ××

×

90 103 .

BHP = = 16.5 HP (rounded)

3960 085 .

600 ×

×

40

B HP = = 16.5 HP (rounded)

1714 085 .

Note: Horsepower requirements vary with flow. Generally, if the flow is greater, the horsepower
required to move the water would be greater.

When the motor, brake, and hydraulic horsepower are known and the efficiency is unknown, a
calculation to determine motor or pump efficiency must be done. Equation 14.31 is used to deter-
mine percent efficiency:

HP output
HP input

Percentefficiency = ×100 (14.31)

From Equation 14.31, the specific equations to be used for motor, pump, and overall efficiency
equations are

BHP
MHP

Percentmotor efficiency = ×100 (14.32)

WHP
BHP

Percent pumpefficiency = ×100 (14.33)

WHP
MHP

Percent overall efficiency = ×100 (14.34)