Page 204 - Shale Shakers Drilling Fluid Systems

CENTRIFUGAL PUMPS 187

Two characteristics of centrifugal pumps can be Example #2
observed from these curves:
• Capacity (Q) varies in direct proportion to im-
peller diameter.
1. The head produced is limited to that devel- • Head (h, in feet) varies in direct proportion
oped at shut-off (no flow). Thus, even if there to the square of the impeller diameter.
were a total blockage in the discharge line, • Horsepower (hp) varies in direct proportion
the pressure on the equipment between the
pump and the blockage would never exceed to the cube of the impeller diameter.
the shut-off head of the pump.
2. Horsepower decreases as the pump is throttled A pump operating at 650 gpm and 1750 rpm with
back to shut-off. Once again, even if there an 11.5-inch impeller, delivers 75 feet of head and
were total blockage in the discharge line, the requires 35 horsepower. What are the ratings with
motor driving the pump would not overload. a 14-inch impeller?

Centrifugal pumps are flexible, which allows a
casing or volute to be used with various impeller
diameters that are reduced from the maximum.
Therefore, a composite curve has evolved that
details the pump's performance when outfitted
to operate at a given speed with various impel-
ler diameters.

Example #1

• Capacity (Q) varies in direct proportion to ro-
tating speed.
• Head (h, in feet) varies in direct proportion
with the square of the rotating speed.
• Horsepower (hp) varies in direct proportion
with the cube of the rotating speed.

At 1750 rpm a centrifugal pump delivers 300 gpm
and 50 feet of head, requiring 5 horsepower. What
are the ratings at 1450 rpm?
HEAD LOSSES THROUGH PIPE

The piping system may consist of suction and
discharge tanks, valves, gauges, elbows, tees, pip-
ing, process or treating equipment, and the pump
itself. As fluid flows through a piping system, fric-
tion forces within the fluid and between the fluid
and the pipe walls decrease the pressure within
the pipe. This pressure loss may be calculated
from Darcy's Equation:

where h = Head loss, ft
f = Friction factor
L = Pipe length, ft
v = Velocity, ft/sec
g = Acceleration of gravity, ft/sec 2
d = Internal diameter of the pipe, in.