Page 213 - Shale Shakers Drilling Fluid Systems
P. 213
CENTRIFUGAL PUMPS 195
transmits the torque to the reading dial, is cali- mance. In drilling operations, NPSH problems can
brated so that the reading is measured directly in be minimized by using short, straight suction lines
centipoise. These viscometers are useful in mea- below the liquid level in the mud pits.
suring the viscosity of non-Newtonian fluids. By As fluid enters the suction, there is a friction
varying the rotating speed, or shear rate, it can pressure loss through both the flange and nozzle.
determine whether the fluid is plastic, pseudo- Another pressure reduction occurs, as well as tur-
plastic, dilatant, or thixotropic. bulent flow losses, when the liquid is turned to
Rotating viscometers yield the most reliable flow radially along the impeller. Any prerotation
readings in respect to the friction forces encountered of the fluid changes the inlet angle and results in
in a centrifugal pump. When testing solids-liquid more turbulence. It is best to have at least 10 pipe
slurries (such as a drilling fluid) solids tend to clog diameters of straight pipe upstream of any pump
the orifices or capillaries of other viscometers. suction. "Stacks" of flanged elbows should be
Many experiments have been conducted at- avoided. Once the liquid is accelerated inside the
tempting to determine a correlation for pump per- impeller, pressure begins to increase toward the
formance with viscous liquids. Unfortunately, no full discharge head.
acceptable general analytical method has been The pipe size must be large enough to effi-
developed because the shear stress-shear rate ciently carry the fluid volume. Otherwise, the high
properties of fluids vary too much. Manufacturers velocity will increase the friction losses. Fluid
take the position that if exacting viscous perfor- velocity should be above 5 ft/sec to minimize
mance data are needed, performance tests should solids settling and below 10 ft/sec to reduce
be performed using the liquid in question. How- friction. If a larger pump replaces a smaller
ever, this presumes a liquid of standard composi- one, consideration must be given to increasing
tion, density, and flow properties, which is cer- pipe diameters.
tainly not the case with drilling fluids. Importantly, piping configuration must be such
that liquid is properly led into the pump. Again,
the ideal piping should have a minimum of 10 pipe
Errosion-Corrosion diameters of straight pipe between the pump suc-
tion and the tank. It is preferred that the suction
Erosion-corrosion often involves high velocities tank be elevated above the pump.
and/or solids suspended in the fluid. Ideally, pump Piping should be in one plane. If an elbow bolts
construction materials should possess corrosion directly into the pump's suction, the fluid tends to
resistance to the media being pumped, strength, follow the long radius, creating a void—or even
ductility, and a high degree of hardness. Unfortu- reverse flow—along the shorter radius. This starves
nately, few materials combine all these features. part of the impeller. Multi-elbow configurations
Cavitation is a special form of erosion-corrosion ("stacks") are to be avoided, especially if used in sev-
resulting from the collapse or implosion of gas eral planes causing many changes in flow directions.
bubbles against the metal surface in high-pressure If branching is necessary, a "Y" fitting is pref-
regions. The stresses created by cavitation are erable to a tee header. Turbulence occurs in the
high enough to actually remove metal from the tee header perpendicular to the feed flow direc-
surface and destroy any corrosion-resistant, pas- tion. If pump suction is close to a tee, turbulence
sive layers. The same qualities for the construc- can enter the impeller. In general, however, only
tion materials mentioned above provide some one suction line should be connected to a centrifu-
resistance to cavitation. In most cases adequate gal pump. Multiple manifolding to provide "flexibil-
suction pressure and reduced pressure drop in ity" usually leads to severe problems in drilling
suction lines eliminates cavitation problems. fluid systems.
This emphasizes the need for proper pump and If a reducer is used, it should be an eccentric
suction piping designs so that friction losses reducer mounted to eliminate areas where voids
are minimized. or air pockets can form.
Free-falling discharge into the suction pit, which
entrains air into the suction fluid, should be avoided.
installation Design Considerations Suction intake should not be close to the tank
inlet line and should be surrounded with sufficient
The majority of hydraulic problems encountered fluid so that fluid moving to the intake remains
in centrifugal pump piping systems originate in the uniform and without turbulence. The suction lines
suction piping. Providing the best possible suction must be deep enough below tank level to avoid
piping layout is important, especially in minimiz- whirlpools, which allow entrained air to funnel
ing NPSH problems and optimizing pump perfor- into the pump intake.
