Related Calculations. Use the techniques given here for any type of pump—
centrifugal, reciprocating, or rotary—handling any type of liquid—oil, water, chemicals,
etc. The methods given here are the work of Melvin Mann, as reported in Chemical Engi-
neering, and Peerless Pump Division of FMC Corp.NET POSITIVE SUCTION HEAD
FOR HOT-LIQUID PUMPS
What is the maximum capacity of a double-suction pump operating at 1750 r/min if it
handles 10O
0
F (37.8
0
C) water from a hot well having an absolute pressure of 2.0 in (50.8
mm) Hg if the pump centerline is 10 ft (30.5 m) below the hot-well liquid level and the
friction-head loss in the suction piping and fitting is 5 ft (1.52 m) of water?
Calculation Procedure:- Compute the net positive suction head on the pump
The net positive suction head hn on a pump when the liquid supply is above the pump in-
let = pressure on liquid surface + static suction head - friction-head loss in suction piping
and pump inlet - vapor pressure of the liquid, all expressed in ft absolute of liquid han-
dled. When the liquid supply is below the pump centerline—i.e., there is a static suction
lift—the vertical distance of the lift is subtracted from the pressure on the liquid surface
instead of added as in the above relation.
The density of 10O
0
F (37.8
0
C) water is 62.0 lb/ft
3
(992.6 kg/m
3
), computed as shown
in earlier calculation procedures in this handbook. The pressure on the liquid surface, in
absolute ft of liquid = (2.0 in Hg)(1.133)(62.4/62.0) = 2.24 ft (0.68 m). In this calculation,
1.133 - ft of 39.2^0 F (4^0 C) water = 1 in Hg; 62.4 = lb/ft^3 (999.0 kg/m^3 ) of 39.2^0 F (4^0 C)
water. The temperature of 39.2^0 F (4^0 C) is used because at this temperature water has its
maximum density. Thus, to convert in Hg to ft absolute of water, find the product of (in
Hg)(Ll 33)(water density at 39.2°F)/(water density at operating temperature). Express
both density values in the same unit, usually lb/ft^3.
The static suction head is a physical dimension that is measured in ft (m) of liquid at
the operating temperature. In this installation, hsh= 10 ft (3 m) absolute.
The friction-head loss is 5 ft (1.52 m) of water. When it is computed by using the
methods of earlier calculation procedures, this head loss is in ft (m) of water at maximum
density. To convert to ft absolute, multiply by the ratio of water densities at 39.2^0 F (4^0 C)
and the operating temperature, or (5)(62.4/62.0) = 5.03 ft (1.53 m).
The vapor pressure of water at 10O^0 F (37.8^0 C) is 0.949 lb/in^2 (abs) (6.5 kPa) from the
steam tables). Convert any vapor pressure to ft absolute by finding the result of [vapor
pressure, lb/in^2 (abs)] (144 in^2 /ft^2 )/liquid density at operating temperature, or
(0.949)(144)/62.0 = 2.204 ft (0.67 m) absolute.
With all the heads known, the net positive suction head is hn = 2.24 + 10 - 5.03 -
2.204 - 5.01 ft (1.53 m) absolute. - Determine the capacity of the pump
Use the Hydraulic Institute curve, Fig. 22, to determine the maximum capacity of the
pump. Enter at the left of Fig. 22 at a net positive suction head of 5.01 ft (1.53 m), and
project horizontally to the right until the 3500-r/min curve is intersected. At the top, read
the capacity as 278 gal/min (17.5 L/s).