Fluid Transfer Pumps

Nov. 15, 2002
Fluid-handling or transfer devices are not basically concerned with the modulation or transfer of power, but only with the movement of fluid.

Fluid-handling or transfer devices are not basically concerned with the modulation or transfer of power, but only with the movement of fluid. Two major types of fluid-transfer pumps are positive-displacement (either bulk-handling or metering pumps) and nonpositive-displacement (centrifugal).

Plunger or piston pumps are one type of commonly used positive-displacement pump. They usually consist of one or more pistons that draw fluid through an inlet check valve and expel it through an outlet valve. Fluid volume delivered depends on plunger diameter and stroke length; diameter cannot be varied in a given pump, so stroke length is made adjustable. Most plunger pumps must be stopped for stroke adjustment, but a few offer the option of in-service adjustment. Outlet pressures delivered by plunger pumps are as high as 50,000 psi for some lab units. Maximum pressures for industrial pumps usually range from 5,000 to 30,000 psi. Maximum flow is as high as 26 gpm for traditional plunger pumps and much higher for multipiston units.

Circumferential-piston pumps use counterrotating rotors driven by external timing gears. They are self-priming and have high suction lift capability. With capacities up to 450 gpm, the pumps are often used for shear-sensitive fluids, or those with entrained particles or gases.

Diaphragm and bellows pumps are used when pump leakage or process-fluid contamination cannot be tolerated. They offer the freedom from external leakage of a peristaltic pump, yet permit higher pressures and easy flow adjustment. Of course, they tend to cost more than peristaltic pumps for the same flow delivered. Generally, diaphragm pumps are built like a plunger unit, except that a bellows or diaphragm is fitted to the end of the plunger shaft. This configuration, while providing a positive seal, stresses the diaphragm because of unequal loading from the plunger. To equalize diaphragm loading, some pumps are built so the plunger never contacts the diaphragm; instead, the plunger pressurizes a small volume of hydraulic fluid as it moves, and the fluid displaces the diaphragm. Diaphragm pumps of this type can deliver outlet pressures to 5,000 psi.

Gear pumps, often used in fluid-power applications, perform equally as well as fluid-handling pumps. The gears can be arranged as a pair of similarly sized gears, as three stacked gears, as separated internal gears, or as gerotors. Displacement of gear pumps is fixed, and cannot be varied during operation.

Lobed pumps resemble gear pumps. Motion of the rotors creates an expanding cavity on the inlet side, a constant-volume cavity that carries fluid to the outlet side, and a contracting cavity that forces fluid out. In some models, rotors are driven by external timing gears to avoid rotor contact in the fluid stream. Lobed pumps have relatively large displacement, so they are often used for shear-sensitive fluids, as well as fluids with entrained gases or particles.

Flexible-vane pumps are similar to sliding-vane hydraulic pumps, but they substitute flexible elastomeric vanes for rigid vanes. These relatively inexpensive pumps pass medium solids and are easy to maintain in the field. Flexible-impeller pumps usually operate at discharge pressures of 20 to 30 psi. High-pressure blades with thicker cross sections boost operating pressure to about 60 psi. However, high-pressure impellers fatigue more rapidly because higher stresses develop when flexing. Operating temperature is limited to about 180°F.

Nutating pumps have a disc, held between two plates, that wobbles without rotating and creates line contact with both plates. As the contact lines pass the inlet port, liquid is pulled into the cavities between the disc and plates. The fluid, then, is swept through the pump to the discharge port (much as a squeegee wipes water from a window) where it is released under pressure. A so-called bridge separates inlet flow from outlet flow.

Nutating action provides a number of operating benefits. The pumps are self-priming from 6 in. when dry and 21 ft when wet. Also, because pump parts do not rotate, a bellows seal can be used on the shaft in place of a mechanical face or lip seal, providing longer seal life. Finally, the disc excursion flattens to relieve excessive pressure, bypassing flow and preventing damage to the pump parts. This feature eliminates the need for a separate pressure relief valve in the system.

Peristaltic pumps consist of a flexible tube that is progressively compressed by a series of rollers. As the rollers move along the tube, they force fluid through it. A chief advantage of these pumps is freedom from external leakage. Fluid is contained within the tube, and can leak only if the tube ruptures. Peristaltic pumps are simple and quite inexpensive for the flow rates they provide.

Displacement is determined by tube size, so delivery rate can only be changed during operation by varying pump speed. However, some models have an adjustable track height, so flow rate can be changed by stopping the pump, changing tubes, adjusting track height, and restarting the pump.

Two-roller pumps do a relatively poor job of drawing liquid into the pump, and often require gravity feed or a pressurized intake. However, three-roller pumps are said to pull a vacuum of 28 in. Hg when equipped with tubing that has a hardness of 60 Shore A.

Most pumps permit a maximum outlet pressure of only 50 psi or less. Two-roller pumps provide flows as high as 3.5 gpm, but three-roller pumps providing flows up to 40 gpm are available.

Centrifugal pumps are a practical choice for fairly constant, large flows of over 100 gpm at moderate pressures and low fluid viscosities. The first step in selecting a centrifugal pump is to determine application requirements: quantity of flow, pressure rise (or change in head) in ft of fluid, and other conditions such as high fluid viscosity or temperature.

After operating requirements have been set, specific speed, Ns, should be determined. Specific speed is a characteristic quantity used to describe a centrifugal pump, and is found from:

Ns = N * (Q^ / h ^) where N = impeller speed, rpm; Q = flow rate, gpm; and h = net positive suction head, ft. The result is usually expressed as a dimensionless number.

If the required specific speed falls between 500 and 15,000, a volute, diffuser, or propeller pump becomes the best selection. If specific speed is less than 500, a peripheral or even a positive-displacement pump should be considered. For a specific speed above 15,000, a parallel system of two or more pumps is usually necessary.

Within the limits of net positive suction head (NPSH) available, the pump with the highest specific speed is generally the best choice, because it operates at the highest rotational speed and is the smallest that can be used. (NPSH is the total of potential and kinetic energy heads in the fluid at the intake to a pump, minus fluid vapor pressure.)

Volute and diffuser pumps draw liquid into the impeller at its center and fling it outward by centrifugal force. The liquid leaves the impeller with higher pressure and velocity than when it entered. The velocity -- especially its tangential component -- is then partially transformed into additional pressure by the pump casing. The amount of energy transformed and efficiency of the transformation depend upon the shape of the casing.

In a volute pump, the impeller discharges the liquid into a "volute" -- a channel of gradually increasing area. In a diffuser pump, stationary diffuser blades in the casing around the outside circumference of the impeller blades are curved in the opposite direction from the blades. The diffuser has less slippage and higher pump efficiency than the volute, but the additional blades increase complexity and cost.

Most single-stage horizontal pumps are built with volute casings. Diffusers are usually used in mixed and axial-flow vertical pumps, and in multistage pumps.

Propeller and mixed-flow pumps are commonly used at very high flow rates and low heads (above 300 gpm and below 40 ft); in this range, they provide more efficient pumping in a smaller package than volute or diffuser types.

Propeller pumps operate like a boat propeller encased in a tube. Liquid is drawn into the pump, parallel to the axis of the impeller, and is pushed out with no change in the direction of flow. Propeller pumps are available for vertical or horizontal operation, with specific speeds from 10,000 to 15,000. Suction characteristics of propeller pumps are not good, so intakes must be located below (or only slightly above) the surface of the liquid being pumped.

Mixed-flow pumps can produce a larger range of heads than straight pumps. Because the rotors are similar to those in water turbines, this pump is often called a turbine pump. In a mixed-flow pump, the head is generated partly by propeller action and partly by centrifugal force in a volute casing. As with the propeller type, the mixed-flow pump can have only a single-section inlet. Thus, the mixed flow pump bridges the gap between the propeller and purely centrifugal types.

Peripheral pumps have circular, rotating impellers but provide characteristics similar to those of a positive-displacement pump. These low-volume, high-head pumps deliver 1 to 50 gpm and up to 500 ft of head discharge. They have excellent suction characteristics, drawing up to 28 ft of head. They are sometimes called turbine-vane, viscous-drag, or regenerative pumps. Peripheral pumps usually cost less than centrifugal or positive-displacement pumps, but often have a much shorter life.

Options often determine the usefulness and applicability of a centrifugal pump. Among the most important options are impeller type, number of stages, shaft positions, and suction intakes.

The impeller in a centrifugal pump can be open, semiopen, or closed. An open impeller consists of blades attached to a hub. A semiopen impeller has a circular plate, or inner shroud, on the inside of the blades. This may extend to the outer ends of the blades. The shroud supports the use of thinner blades. A closed impeller has an outer shroud (attached to the outer edge of the blades) as well as an inner one. Liquid is, thus, confined to the space between the shrouds. The confined passage reduces friction losses in the pump and thus increases efficiency.

Staging is used to increase head or flow of centrifugal pumps. A two-stage pump is essentially two pumps in series -- volume flow remains about the same, but output pressure is almost doubled. The impellers of a multistage pump are on the same shaft, and the housings are a single unit. Because liquid velocity can be kept high as it moves through the housing, a two-stage pump is more efficient than two single-stage pumps connected in series to produce the same head and volume.

Suction intake can also affect pump performance. An impeller can be designed for either single or double-suction operation. A double-suction impeller draws in liquid at both sides, so flow is almost twice that of a single-suction type for the same net positive suction head.

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