A reciprocating pump is a positive-acting, displacement pump, which creates flow by displacing liquid from a cylinder or cavity with a moving member, or piston. Each chamber, or cylinder, is filled and emptied by the mechanical motion of a piston that alternately draws-in and then expels liquid. Available horsepower and the strength of the pump's structural parts determine pressure capabilities. Volume, or capacity, delivered per stroke by a reciprocating pump is constant regardless of pressure. The flow rate varies with changes in piston speed, the diameter of the cylinder in which the piston moves, and the stroke length of the piston. Most reciprocating pumps use multiple cylinders (i.e., duplex, triplex, etc.) to regulate the pulsating flow generated by the reciprocating motion. They are used primarily on drilling rigs as mud and cement pumps. Centrifugal or rotary pumps, except for special applications, have replaced small reciprocating pumps, although they are still used where their variable-speed, stroke, and piston cylinder combinations are important considerations. High torque and decreasing efficiencies where solid abrasives are present in the fluid are major disadvantages of this type of pump.
A rotary pump is simple in design, has few moving parts, and, like a reciprocating pump, also uses positive displacement. A rotary pump consists primarily of two meshed cams, or gears (the idler and the drive gear), in a tight-fitting casing. The drive gear is connected to the power supply and its rotation drives the idler gear. Liquid fills the spaces between the gear teeth and, as they rotate, the liquid is literally "squeezed" out the discharge. As the gear teeth then separate, a partial vacuum is created causing liquid to continuously fill the pump chamber from the suction side.
Rotary pumps including gear, screw, deformed vane, sliding vane, axial-piston, and cam type are used when discharge pressures of 500 to 1000 psi or greater are needed. A rotary pump produces a continuous flow regardless of line backpressure and should not be used to pump erosive slurries. Because of close clearances and metal-to-metal contact by the gear teeth, rotary pumps work best when handling solids-free liquids with adequate lubricating qualities. These pumps are particularly adept at pumping liquids with high viscosity or low-vapor pressures. They can be used to move small capacities at medium discharge heads and where high suction lift is required. Applications include oils or viscous materials such as soaps, molasses, tars, and paints.
Diaphragm pumps are classed also as positive displacement pumps. The diaphragm acts as a limited displacement piston. Pumping action is obtained when the diaphragm is forced into reciprocating motion by compressed air, mechanical linkage, or from a pulsating external source. Their construction avoids contact between the liquid being pumped and motive force (motor, shaft, air pressure), largely eliminating leakage and corrosion or erosion of moving parts.
A jet pump lifts fluid by a partial vacuum created by a motive stream that is provided by another centrifugal or positive displacement pump. The motive stream is driven through a nozzle into a venturi tube. The partial vacuum, formed as the fluid enters the venturi tube, creates a suction. This vacuum draws additional fluid into the output stream.
The venturi tube is a "nozzle in reverse." A nozzle speeds the flow velocity of the fluid, thereby converting pressure into velocity energy, which decreases the ambient pressure around the nozzle discharge. A venturi tube slows the flow, converting kinetic energy back into a pressure head.
These pumps are used to pump large particles, such as shale cuttings, and are frequently used as mud hoppers. Many types of vacuum de-gassers also use this type of pump to remove mud from their vessels.
The Type II pump, or progressive cavity pump, is based on the Moineau principle. This principle defines the geometric fit between the rotation element (rotor) and the stationary element (stator) of the pump. The rotor has the shape of a single helix and is normally made of metallic materials. The stator is formed as a double helix and is normally made of an elastomer. The interference (compression) fit between the rotor and the stator creates a series of sealed chambers called "cavities." The rotor, turning eccentrically within the stator, produces the pumping action. Material enters the cavity formed at the inlet and progresses within that cavity to the outlet. The result is a positive, nonpulsing flow that is directly proportional to the pump's speed. These pumps are available in multiple stages (single-stage, two-stage, three-stage etc.); the more stages a pump contains, the more pressure that is produced.
Submersible pumps are special vertical centrifugal pumps designed to operate with the entire assembly (both pump and motor) submerged in a fluid. They are used to pump subsurface irrigation water or crude oil from wells. The submersible pump is a multi-stage (several or many impellers) centrifugal pump, directly coupled to a submerged electric motor. The impellers are rotated by a single shaft, each sitting in a bowl so that the flow from one impeller can be directed through a diffuser to the next impeller. The three parts—impeller, bowl, and diffuser—are known as a "stage." The impeller and diffuser widths largely determine the capacity of a multi-stage submersible pump. The diameter, number, and rotational speed of the impellers determine the amount of pressure developed. The more impellers (stages) rotating at higher speeds, the greater the pressure. Submersible pumps are much more efficient than jet pumps.
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