Page 214 - Shale Shakers Drilling Fluid Systems
P. 214
196 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
ANATOMY OF CENTRIFUGAL PUMPS a lubrication port in the stuffing box. If pump
media is corrosive, the packing is usually made
The typical centrifugal pump consists of the from asbestos, teflon, or graphite fibers.
power end, stuffing box, backing plate, impeller, A small amount of liquid must be allowed to
and fluid housing. seep through the packing to cool and lubricate the
shaft surface contacting the packing. This leakage
Power end. Centrifugal pump power ends consist rate is hard to control, and the usual tendency
of bearings, oil or grease seals, bearing housing(s), is to over-tighten the packing gland to stop, or
the shaft-bearing lubrication system, and the pump control, the leakage. The consequence of over-
shaft connecting the drive force motor and driven tightening can be rapid scouring of the shaft sur-
impeller. The power end assembly is usually made face. This creates an abrasive surface and makes it
of iron or steel. When venting of the bearing hous- difficult, if not impossible, to properly adjust pack-
ing is required, the vent must be designed to pre- ing compression. The main advantages of a packed
vent the entry of water. stuffing box are:
Stuffing box. The stuffing box is located in front • Relative inexpensive initial cost
of the power end behind the fluid housing. It uses • Slow sealing deterioration allows replace-
either packing or a mechanical seal to prevent ex- ment to be scheduled when pump is off-line
cessive liquid loss between the shaft and the stuff- • Simple to adjust and/or replace
ing box. There are many problems inherent with
either packing or mechanical seals, including: With simple water-based fluids, leakage is
best tolerated rather than controlled. Expen-
• Seal mounted on and/or around a rotating sive, corrosive, or toxic fluids present another
shaft, which may or may not be running con- problem, usually better approached with mechani-
centric, or may be worn cal shaft seals.
• Pressure differentials across the seal or pack-
ing may vary
• Abrasive solids may be entrained in the fluid Mechanical seals. Mechanical seals overcome
• Temperature may vary most of the shortcomings of rope packing. The
• Corrosion may occur disadvantages of these seals, however, are their
high cost and sudden seal failure, which can re-
In the stuffing box area, the wet shaft surface sult in inconvienent maintenance downtime.
must be hard enough to resist wearing. Centrifu- Construction of mechanical seals varies but the
gal pumps often have carbon steel shafts overlain basic components are similar. There are two mat-
with a replaceable sleeve, which can be made of ing seal rings, one stationary mounted inside the
ceramics, stainless steels, or even glass. Some power end housing or stuffing box, and one rotat-
sleeves are permanently bonded to the shaft, which ing with the shaft. The seal rings are held in con-
helps minimize deflection. "Plasma-spray" pro- tact with each other by compression springs. A flat
cesses apply ceramic and stainless steel. gasket, or O-ring, provides the seal between the sta-
tionary ring and packing gland. O-rings or V-seals
Shaft packing. Packing usually consists of a fi- are used to seal the rotating seal ring and shaft.
brous material woven into a rope, and then cut The pumped liquid will form a thin film between
and molded into packing rings with a rectangular the seal faces. This is the key to the success
cross-section. Packing materials include braided of the mechanical seal. If the liquid has ade-
asbestos, lead, graphite, teflon, and others. The quate lubricating properties, the seal will provide
rings must be sufficiently pliable to conform and fill long service.
the spaces between the stuffing box and shaft. Pres-
sure is then applied with the use of a packing gland. Rotating shafts. Bent or out of round pump
However, if the shaft has any radial movement shafts will erode large holes between the packing
caused by run out, deflection, whip, or loose bear- and shaft, allowing liquid to escape. Lack of static
ings, leakage will occur in the stuffing box. or dynamic balance in the impeller produces a dy-
Packing is usually impregnated with grease that namic bend in the shaft with the same result.
provides lubrication between the shaft and pack- Bent shafts also impair mechanical seal func-
ing rings. The grease also fills the pores between tioning as rotating members of the seal bend with
the packing fiber and helps distribute friction caused each revolution. If shaft deflection is greater than
by heat. Most packing lubricants break down above nominal, the flexible springs do not react suffi-
250°F. Additional grease can be introduced through ciently to keep the seal faces together.
