Page 250 - Shale Shakers Drilling Fluid Systems

232 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
content will increase the plastic viscosity,
which needs to be as low as possible. Bento-
nite is the ideal clay because only a small
amount is necessary to build yield point or
control filtration. The capability of hydrated
bentonite to disperse is much greater than
drilled solids.
41. The surface system should include a trip tank.
Trip tanks are needed to ensure the wellbore
receives the correct amount of fluid as the
drill string is pulled. For example, after 10 bbl
volume of steel is removed during a trip, the
liquid level in the wellbore should drop. This
might result in an influx of formation fluid at
the bottom of the hole. The trip tank continu-
ously supplies fluid to the bell nipple to keep
the well full of drilling fluid. If only 3 bbl of
drilling fluid are needed to fill the hole after
10 bbl of steel are removed, the additional
fluid must be entering the wellbore from the for-
mations. A blowout is imminent. The pipe is run
back to the bottom and the situation corrected.
CENTRIFUGAL PUMPS
1. Select a pump to handle the highest anticipated
flow. Select an impeller size to provide sufficient
discharge head to overcome friction in the lines,
lift the fluid as required, and have sufficient head installed in the tank to interrupt the formation
remaining to operate the equipment being fed.
Initially this guideline suggested that the pump
flange size be selected to provide the highest
anticipated flow, even though the flange size
has nothing to do with the flow rate. Most
pump curves are listed in terms of the flange
sizes. The size of the pump impeller housing
increases as the flange size increases. An im-
peller rotating at a constant speed will create
a constant head independent of the size of the
housing or the flanges. An impeller that fits
inside a 2 x 3, 3 x 4, 4 x 5, or 5 x 6 pump
will produce the same head in each pump if
it is rotated at the same speed. Because the
housing of a pump with a 2-inch and 3-inch
flange is smaller, the internal friction at a high
flow rate will be greater than a 5 x 6 pump.
This means that the capacity of the various
pump sizes will be indicated by their flange
sizes. The committee decided to only indicate
that the pump should be selected to handle
the highest anticipated flow rate, instead of
indicating that the flange size is commonly
used to specify pump size.
2. Install the centrifugal pump with a flooded suc-
tion which is sumped so that sufficient sub-
mergence is available to prevent vortexing or
air-locking. Foot valves are not needed or rec-
ommended with flooded suctions.
A small influx of air into the suction of a cen-
trifugal pump can create cavitation problems
and diminished flow. As the air enters the
chamber with the impeller, it tends to concen-
trate in the center of the impeller because of
the centripetal acceleration of the drilling fluid.
The liquid continues to move through the pump.
The air does not always continue to the im-
peller tip, but tends to remain in the center
of the impeller. This bubble of air forms a
barrier for the incoming fluid, which dimin-
ishes the flow rate into the pump. The air also
experiences a significant decrease in pres-
sure—possibly even below atmospheric pres-
sure. This causes implosions of vapor bubbles
that can remove metal from the impeller. The
pump will sound as if it is pumping gravel. If
it continues in this mode for a long period of
time, the impeller will be severely damaged.
Flooded suctions tend to eliminate most of
the air influx problems but sometimes a small
vortex will form in the mud tank. These small
vortexes can entrain a significant amount of
air. Increasing agitation in the tanks may pre-
vent a coherent cylinder of air from reaching
the suction line. Alternately, a plate can be
of a vortex.
In some cases, a centrifugal pump is placed
on the ground above a pond or buried tank.
Foot valves are needed if the centrifugal pump
is operated above the liquid level of the suc-
tion tank. Foot valves are check valves that
prevent the suction line from draining when
the pump is turned off. Care must be taken to
eliminate any air leaks in the suction line
because the absolute pressure will be below
atmospheric pressure. The pump and suction
line should be filled with fluid before the pump
motor is started. Centrifugal pumps do not
move air very well.
A centrifugal pump suction can only lift fluid
a certain height above a liquid level. These
heights are determined by observing the NPSH
(negative pressure suction head) values listed on
the centrifugal pump curves. If the NPSH is ex-
ceeded, cavitation can destroy the impeller.
3. Install a removable screen over the suction to
keep out large solids and trash. It can be made
out of half-inch expanded metal and should