cent, or (50)(0.5) = 12.5 strokes/min. Hence, the pump speed will be 50.0 - 12.5 = 37.5
strokes/mm. Since the delivery is directly proportional to speed, the delivery of 125
0
F
(51.7
0
C) water = (37.5/5O)(IO) - 75 gal/min (4.7 L/s).
Related Calculations. Use this procedure for any type of reciprocating pump
handling liquids falling within the range of Table 1. Such liquids include oil, kerosene,
gasoline, brine, water, etc.
EFFECT OF VISCOSITYAND DISSOLVED
GAS ON ROTARY PUMPS
A rotary pump handles 8000-SSU liquid containing 5 percent entrained gas and 10 per-
cent dissolved gas at a 20-in (508-mm) Hg pump inlet vacuum. The pump is rated at 1000
gal/min (63.1 L/s) when handling gas-free liquids at viscosities less than 600 SSU. What
is the output of this pump without slip? With 10 percent slip?
Calculation Procedure:
- Compute the required speed reduction of the pump
When the liquid viscosity exceeds 600 SSU, many pump manufacturers recommend that
the speed of a rotary pump be reduced to permit operation without excessive noise or vi-
bration. The speed reduction usually recommended is shown in Table 2.
With this pump handling 8000-SSU liquid, a speed reduction of 40 percent is neces-
sary, as shown in Table 2. Since the capacity of a rotary pump varies directly with its
speed, the output of this pump when handling 8000-SSU liquid = (1000 gal/min) x (1.0 -
0.40) = 600 gal/min (37.9 L/s). - Compute the effect of gas on the pump output
Entrained or dissolved gas reduces the output or a rotary pump, as shown in Table 3. The
gas in the liquid expands when the inlet pressure of the pump is below atmospheric and
the gas occupies part of the pump chamber, reducing the liquid capacity.
TABLE 2. Rotary Pump Speed Reduction for Various
Liquid Viscosities
Speed reduction, percent
Liquid viscosity, SSU of rated pump speed
600 2
800 6
1,000 10
1,500 12
2,000 14
4,000 20
6,000 30
8,000 40
10,000 50
20,000 55
30,000 57
40,000 60