Page 198 - Shale Shakers Drilling Fluid Systems
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CENTRIFUGAL PUMPS 181
within the pump. Obviously, if fluid is pumped where h = Head, in ft of liquid
from an external source through a2x3ora5x 6 p = Pressure, lb/in. 2
pump, a greater pressure loss will occur through sp gr = Specific gravity of liquid
the smaller diameter pump.
A centrifugal pump acts as a standpipe or wa- Pressure read on a gauge attached to a column
ter tower that is constantly filled to the same level. or tank of fluid is an indication of the pressure
The flow rate from such a standpipe depends on created by the static head of liquid in the tank.
the discharge piping connected to it. For example, Centrifugal pumps generate a constant head when
if a standpipe is filled with fluid to a height of 100 the flow rate is low. The velocity of the impeller
feet, the flow rate of fluid from the standpipe tip is independent of the fluid density. It gener-
would be much smaller through a 4-inch-diameter ates a constant head because the fluid leaves the
pipe than through a 6-inch-diameter pipe. impeller tip with a constant velocity. In pump ap-
If the discharge piping is connected at the very plications, a centrifugal pump can conceptually be
bottom of the standpipe, the example would be considered a constant height stand pipe filled with
analogous to a centrifugal pump that generates fluid. The pressure at the bottom of the stand pipe
100 feet of head. depends on the density, or specific gravity, of the
fluid. The head depends only on the height of
the liquid in the stand pipe. However, regardless of
Discharge Head the source, any pressure can be converted into terms
of equivalent head using the previous equation.
The term "head" is sometimes confused with The head, or pressure energy, converted into
the term "pressure." In a piping system, head re- kinetic energy is termed "velocity head." This is
fers to the height of a liquid in a long, open-ended not a loss of energy, but a change in energy form.
pipe connected to the point at which the head, or If the velocity were cut in half, half of the kinetic
pressure, is measured. energy converts back to pressure energy, and the
A centrifugal pump, with a certain size impel- gauge reading increases accordingly.
ler and rotation speed, pumps fluid to a specific Head lost, or converted due to friction, is termed
height and maintains the fluid heigth with no fur- "friction head." Hydraulically this is a true loss be-
ther flow. The pressure at the bottom of the col- cause it is not converted back to pressure energy.
umn of fluid is a function of the height and den- The relationship between liquid flowing past the
sity of the fluid. For example, if a well is drilled gauge and its kinetic or velocity head is deter-
to 2,000 feet, the head at the bottom of the well mined by the equation:
is 2,000 feet. The pressure at the bottom of the
well depends on the mud weight (or density) of
the fluid in the well. The pressure may be deter-
mined using the following equation:
Pressure = 0.052 (MW) (Depth) where h v = Velocity head, ft of liquid
or V = Velocity, ft/sec 2
g = Gravitational acceleration, 32.2 ft/sec
Pressure = 0.052 (MW) (Head)
where Pressure = Hydrostatic pressure, psig
MW = Mud weight, ppg Suction Lift
Head or Depth = True vertical depth of the
well, ft Liquid is forced into the pump's suction when
a partial vacuum, or a pressure reduction, is cre-
If the drilling fluid in the 2,000-foot well weighs ated from the fluid discharge out of the centrifu-
10.5 ppg, the bottom-hole pressure would be 1090 gal pump housing by the impeller. Atmospheric
psig and the head would still be 2,000 feet. If the pressure, or line pressure, causes fluid to flow into
mud weight was 15 ppg, the bottom-hole pressure the pump because of the pressure reduction within
would be 1560 psig and, again, the head would be the pump. A positive head on the suction side of
2,000 feet. the centrifugal pump is desirable to prevent the
The pressure at the bottom of the well can also partial vacuum from becoming too low. In recip-
be calculated with the equation: rocating pumps, this pressure reduction is created
by the piston, in rotary pumps by the gear action,
and in centrifugal pumps by the liquid forced out
of the pump discharge.
