Page 199 - Shale Shakers Drilling Fluid Systems

182 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
One pound per square inch of water pressure
is equivalent to 2.31 feet of head in a water col-
umn. Atmospheric pressure equals 14.7 pounds
per square inch at mean sea level:
14.7 x 2.31 = 33.9 feet (one atmosphere of
pressure, on earth, is equivalent to 33.9 feet
of head of water)
This is the maximum theoretical suction lift a
pump should attain. There are, however, various
losses that reduce this theoretical lift including
losses through pistons, pump cams, stuffing box
leakage, and so forth. Losses in positive-action
pumps are less than losses in centrifugal pumps.
The maximum practical suction lift in positive
acting pumps—based on "water-like" fluid viscos-
ity—is 22 feet and 15 feet with centrifugal pumps.
A low-speed pump will operate safely with a
greater suction lift than one with a higher speed.
If the suction lift is very high (over 15 feet for wa- walls receive a tremendous shock from the rush
ter), slower pump impeller speeds and larger pumps
are necessary. Increased speeds without proper
suction conditions may cause serious problems
from cavitation, which can ruin a pump very
quickly. Head losses from fluid flowing through the
suction lines reduce the pressure, or head, at the
impeller. This produces the same effect as at-
tempting to lift fluid from a level below the pump.
Net Positive Suction Head
Net positive suction head (NPSH) is the pressure
at pumping temperature (expressed in feet of liq-
uid) available at the pump suction over and above
the vapor pressure of the liquid being pumped. In
most cases, NPSH is a combination of two param-
eters: fluid head and pressure existing in the suc-
tion line. The pressure in the suction line, if the
pump is above the liquid level in the suction tank,
may be less than atmospheric. Large friction losses
in the suction lines to a centrifugal pump may also
decrease the pressure at the pump suction to a
value less than atmospheric. Therefore, the net
positive suction head may be viewed as the ab-
solute pressure in the suction line at the pump.
NPSH conditions are checked for each pumping
application to determine whether or not the fluid
will vaporize inside the pump. Vaporization inside
a pump is referred to as "cavitation." Cavitation is and brines read from charts. The difference is the
usually evidenced by excessive noise and vibra-
tions and will sound like gravel being pumped
through it.
Pressure on a fluid is reduced as it moves from
the pump suction flange to the point where energy
is imparted by the impeller. The pressure reduction
must be compared to the liquid's vapor pressure, and
to the absolute pressure on the fluid as it enters the
pump, to determine if the fluid will vaporize.
The pressure reduction inside the pump is re-
ferred to as the "required NPSH." The difference
between the absolute pressure acting on the fluid
and its vapor pressure is called the "available
NPSH." When the available NPSH is greater than
the required NPSH the pump will not cavitate.
If the available NPSH is not greater than the
required NPSH, cavitation, as well as many other
serious problems can occur, such as a marked re-
duction in capacity, or even complete operational
failure. Additionally, excessive vibration can occur
when some sections of the impeller handle liquid
while other sections handle vapor. This will cause
pitting and erosion, which drastically reduces pump
life. Pitting and erosion result from the collapse of
vapor bubbles as they enter regions of higher pres-
sure. As the vapor bubbles collapse, the adjacent
of liquid into the cavity left by the collapsed bubble,
where small bits of metal actually flake off. Note
that the erosion does not occur at the point
of lowest pressure, but further upstream where
higher pressures cause bubble collapse. Pitting or
erosion often occurs on the impeller tips or volute,
but are resultant from cavitation caused by insuf-
ficient NPSH at, or closer to, the pump suction.
Energy expended to accelerate liquid into voids
left by collapsed bubbles causes a drop in the
developed head. Consider that there is a volume
increase of 50,000 times magnitude when water
vaporizes at standard temperature. Even a slight
amount of cavitation, therefore, will significantly
reduce capacity.
Pumps operating with insufficient NPSH will
often pump slugs or spurts of liquid. As the pump
is started, liquid accelerates in the suction flange
and impeller inlet "eye" until the pump reaches
operating capacity. With acceleration of the liquid,
friction losses increase and reduce the absolute
pressure until the liquid flashes into vapor. When
this occurs, pump capacities are reduced and flow
decreases, or ceases altogether. In this case, losses
are lower with the decreased flow, absolute pres-
sure is higher, liquid ceases to vaporize, and the
pump begins to transport fluid again.
The total suction head of the pump can be mea-
sured, and vapor pressures for common liquids
available NPSH.
The pump manufacturer determines the re-
quired NPSH for each pump by defined testing
procedures. NPSH requirements are published on
the standard composite performance curves for
each pump.