Page 252 - Shale Shakers Drilling Fluid Systems
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234 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
installing baffles to break mixer vortices. Prop- diameter impeller. On a temporary basis, a
erly sized, baffled, and agitated compartments discharge valve can be partially closed. Even
will not vortex unless the drilling fluid level be- a large centrifugal pump is not damaged if
comes extremely low. only 10 to 20 gpm is discharged from the
pump. In fact, the lower flow rates will require
Centrifugal pumps can not pump aerated fluid. less horsepower to the motor than pumping
The air tends to gravitate toward the center fluid at a much higher flowrate.
of the impeller while the liquid moves toward
the outside. This creates an air bubble at the
center of the impeller. When the air bubble 9. Make sure the impeller rotation is correct.
becomes as large as the suction line diameter,
fluid will no longer enter the pump. This is Centrifugal pumps will pump fluid even if run-
ning in reverse. The head produced by the
called "air-locked." pump will be lower than it should be. The
Only a small cylinder of air vortexing into
the pump is sufficient to prevent the pump pressrue gauge installed between the pump
from moving liquid. Since the air accumulates and the first valve will assist with this diag-
nosis. Usually, switching two wires in the lead-
over a period of time, a small vortex the size in panel box will correct the rotation.
of a pencil is sufficient to eventually shut
down a 6 x 8 pump.
Baffles are inexpensive and easily installed 10. Startup procedure for an electric motor-driven
in an empty tank. Any vertical surface that centrifugal pump with a valve on the discharge
disrupts the swirling motion of the fluid in a side between the pump and the equipment be-
compartment is usually sufficient to destroy a ing operated, is to start the pump with the valve
vortex. Rig pump efficiency can decrease from just slightly open. Once the pump is up to speed,
99% to 85% efficiency if the drilling fluid con- open the valves slowly to full open. This ap-
tent rises to 6% volume. Air in the drilling fluid proach will reduce the startup load on the elec-
may be calculated by measuring the pressurized tric motor and will reduce the shock loading on
and unpressurized mud weight. equipment such as pressure gauges and hydro-
cyclones. An alternate startup procedure is to
Do not restrict the flow to the suction side of completely close the valve before startup and
the pump. Starving the pump suction causes then open the valve slowly immediately after
cavitation and this will rapidly damage the pump. startup to prevent overheating and possible dam-
age to the pump seals.
When a pump begins cavitating, small vacuum
bubbles adjacent to the impeller surface start An electric motor-driven centrifugal pump will
imploding. The pump sounds as if it is pump- immediately try to produce a constant head
ing gravel. The implosions quickly remove when it is turned on. If the pump is pumping
metal from the surface to the impeller blade. into an empty line, the flowrate is enormous.
In a very short period of time, holes will ap- Very high flowrates requires very high currents
pear in the metal. to the electric motor. Circuit breakers can stop
Starving the suction will decrease the out- the pump and avoid motor burn-out. Lower
put head. If the head, or pressure, produced horsepower is required if the pump is started
by the pump is too high, change to a smaller with the discharge valve closed.
