Positive Displacement Pumps

Positive Displacement Pumps
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The rotary positive displacement pump provides an approximate constant flow at fixed speed, despite changes in the back pressure. Rather than transferring quite large amounts of energy into the pumpage, as a centrifugal pump does, rotary positive displacement pumps move fluid directly by means of gears, lobes, vanes, flexible impellers, diaphragms, screws, rollers, pistons, or other mechanisms.  Reflecting the diversity of mechanisms used, the Hydraulic Institute Standards for Centrifugal, Rotary and Reciprocating Pumps, 14th Edition lists literally scores of designs for positive displacement pumps.

The flow of a centrifugal pump will change considerably with back pressure. Changing back pressure on rotary pumps will result in a minimal flow change. The pumps are typically designed with the finest tolerances possible to obtain the highest possible efficiency and suction capability. However, in some cases, it is necessary to increase the tolerances, for example, when the pumps must handle highly viscous liquids, liquids containing large particles or liquids of high temperature.

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Gear Pumps

In an external gear pump, two gears, both independently supported by shafts, mesh with each other. The term “external” refers to the fact that the teeth are cut on the outside — they project out from the gear. Internal gear pumps use one external gear rotating within a gear with teeth projecting inward. In internal gear pumps and in one design of external gear pumps, one gear is driven by the motor and directly contacts the second gear. Thus the first gear can drive the second gear directly. To allow larger tolerances, other designs use timing gears to drive the gears. The timing gears (also called pilot gears) keep the two gears in mesh without requiring direct contact. In any case, the operating principle is the same: the meshing creates a vacuum at the inlet of the pump and atmospheric pressure pushes the liquid into areas between the gear teeth. The rotation of the gears traps the pumpage in the cavity between the gear teeth and moves the pumpage from the suction to the discharge side of the pump. When the liquid passes the junction of the casing and the discharge port, it is released from the cavity. Tight clearances between the gears and the casing prevent recirculation of the pumpage back to the suction side, so the fluid follows the path of least resistance, i.e. out of the discharge port. The result: a continuous, repeatable flow — even when the discharge line is blocked. Without the possibility of recirculation, a block in the discharge line can cause pressure to rise until the pump or some other system component fails. The moral of the story: gear pumps require a pressure relief system. The relief valve may be integral to the pump or external.

The basic components of a typical gear pump are:

  • the casing, similar to the casing of a centrifugal pump, but for external gear pumps it is typically oval instead of round, to accommodate the two round gears
  • the gears, which may have straight (i.e., spur), curved (i.e., helical), or herringbone teeth
  • the shaft, which rotates the gear(s) and to which the gears are attached by a key or other device
  • the bearings, which support and position the shafts, and can be internal (i.e., lubricated by the pumpage) or external, and of roller design or fluid film design (e.g., sleeve or journal)
  • Wear plates are lubricating devices that fit between the gear ends and the housing and through which the shaft passes; designed to absorb any thrust movement of the gears and lubricate the region of contact between the gears and the casing
  • End covers, which close off each end of the case and support the bearings, shaft ends, wear plates (if applicable) and relief valve (if applicable)
  • Stuffing box is part of one end cover through which the motor shaft enters the pump; also holds the seals or packing
  • Seal which prevents pumpage from leaking out of the pump where the motor shaft enters the pump through an end cover; as with centrifugal pumps, the seal may be a mechanical seal, packing and a gland follower, or the sealless design which uses magnetic drive.

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Lobe Pumps

Similar to a gear pump, a lobe pump traps fluid between interlocking protrusions from the rotors, but a lobe pump uses only a few (one to five) large, smooth, rounded fingers (lobes) instead of the sharp teeth of a gear. Lobe pumps use external gears (i.e., no process contact) to drive the lobes. This design keeps the lobes close together but not contacting. Lobe pumps are well suited for highly shear-sensitive fluids. The smooth surfaces of the lobes also make lobe pumps a good choice for sanitary applications because the surfaces are easy to clean. Most sanitary lobe pumps are specially designed to facilitate disassembly and cleaning. Other sanitary pumps can be cleaned or steamed in place to meet increasingly stringent standards up to and including straight pipe cleanliness. For sanitary service, surfaces are finely polished to remove all possible cracks or crevices where pumpage could reside. Noncorrosive materials are also important for sanitary pumps. The 3A Food and Dairy Standards suggest design and define specifications for cleanability.

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Progressive Cavity Pumps

Progressing cavity (PC) pumps are unique among positive displacement pumps for their ability to pump a wide range of fluids. Clean, water-like liquids; delicate products such as maraschino cherries and viscous, solids-laden fluids are all appropriate applications for PC pumps. While PC pumps perform well in applications where other pumps do not, PC pumps should not be considered a pump of last resort. They can be valid choices even for less-demanding applications such as baby food, salad dressing or lubricating oils. By understanding the mechanical design of PC pumps, you will be able to match a pump to your pumpage.


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Flexible Impeller Pumps

Flexible impeller pumps can be used for soft solid, minimal shear to thixotropic fluids. The pump is self priming, meets 3A standard 02-09, is CIP able and easy to strip clean. These units are available in either tri-clamp or ASME threaded connections.

For additional information:
Jabsco Flexible Impeller Series

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