Page 196 - Shale Shakers Drilling Fluid Systems

CENTRIFUGAL PUMPS 179

limited displacement piston. Pumping action is ob- bowl, and diffuser—are known as a "stage." The
tained when the diaphragm is forced into recip- impeller and diffuser widths largely determine
rocating motion by compressed air, mechanical the capacity of a multi-stage submersible pump.
linkage, or from a pulsating external source. Their The diameter, number, and rotational speed of the
construction avoids contact between the liquid be- impellers determine the amount of pressure devel-
ing pumped and motive force (motor, shaft, air pres- oped. The more impellers (stages) rotating at higher
sure), largely eliminating leakage and corrosion or speeds, the greater the pressure. Submersible pumps
erosion of moving parts. are much more efficient than jet pumps.
A jet pump lifts fluid by a partial vacuum cre-
ated by a motive stream that is provided by an-
other centrifugal or positive displacement pump. THE CENTRIFUGAL PUMP
The motive stream is driven through a nozzle into
a venturi tube. The partial vacuum, formed as the A centrifugal pump is a rotating vane, or im-
fluid enters the venturi tube, creates a suction. peller, within a chamber. This pump increases the
This vacuum draws additional fluid into the out- pressure of a liquid passing though it by increas-
put stream. ing the velocity. Liquid enters the pump at the
The venturi tube is a "nozzle in reverse." A suction flange and revolves along the rotating
nozzle speeds the flow velocity of the fluid, there- impeller vanes until thrown to the inner surface
by converting pressure into velocity energy, which of the chamber. Liquid flows from the tip of the
decreases the ambient pressure around the nozzle rotating impeller along the inner circumference
discharge. A venturi tube slows the flow, convert- through the pump discharge. If a long, open-ended
ing kinetic energy back into a pressure head. pipe is connected to the pump, the impeller causes
These pumps are used to pump large particles, fluid to move vertically to a height proportional to
such as shale cuttings, and are frequently used as the velocity of the fluid leaving the tip of the im-
mud hoppers. Many types of vacuum de-gassers peller. The height of the fluid is called the head
also use this type of pump to remove mud from created by the centrifugal pump.
their vessels. A centrifugal pump offers the following features:
The T/pe II pump, or progressive cavity pump,
is based on the Moineau principle. This principle • Simple construction
defines the geometric fit between the rotation el- • Low initial cost
ement (rotor) and the stationary element (stator) • Easy to maintain
of the pump. The rotor has the shape of a single • Small space requirements
helix and is normally made of metallic materials. • No close clearances
The stator is formed as a double helix and is nor- • No valves or reciprocating parts
mally made of an elastomer. The interference (com- • Impeller and shaft are the only moving parts
pression) fit between the rotor and the stator cre-
ates a series of sealed chambers called "cavities." Centrifugal pumps can be directly connected or
The rotor, turning eccentrically within the stator, belt driven to various power sources, motors, steam
produces the pumping action. Material enters the turbines, or engines. They produce a steady flow
cavity formed at the inlet and progresses within and have low start-up torque requirements. These
that cavity to the outlet. The result is a positive, pumps are used for a relatively wide range of flow
nonpulsing flow that is directly proportional to the rates and are suited to handle liquid slurries with
pump's speed. These pumps are available in mul- suspended solid matter. Centrifugal pumps are
tiple stages (single-stage, two-stage, three-stage well adapted to general water supply, irrigation,
etc.); the more stages a pump contains, the more and sewage service. They are used in mill, chemi-
pressure that is produced. cal and petrochemical process plants, mining op-
Submersible pumps are special vertical centrifu- erations, and drilling rigs.
gal pumps designed to operate with the entire Although centrifugal pumps seem simple in de-
assembly (both pump and motor) submerged in a sign, many incorrect concepts surround the basic
fluid. They are used to pump subsurface irrigation fundamentals about these pumps. For example,
water or crude oil from wells. The submersible many believe the flow rate from a centrifugal
pump is a multi-stage (several or many impellers) pump can be increased without changing the head
centrifugal pump, directly coupled to a submerged by installing a larger diameter impeller. This is a
electric motor. The impellers are rotated by a false assumption because the flow rate from a
single shaft, each sitting in a bowl so that the flow centrifugal pump depends on the piping system
from one impeller can be directed through a diffuser connected to the pump and not on the impeller.
to the next impeller. The three parts—impeller, The centrifugal pump is a constant head device. The