Pumps & Systems, June 2008
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Part 4 and
May 2008 issues, we looked at centrifugal pumps. We now turn our attention to the Positive Displacement (PD) pump world, subdivided into pumps that operate on the linear motion principle (pistons, for example), or rotary motion, such as gear and lobe pumps. PD pumps move liquid differently than centrifugal pumps. Both transfer confined volumes of liquid from the inlet to the outlet port; centrifugals rely on imparting velocity to the liquid. Their performance characteristics are also different:
PD pumps always try to displace the same amount of liquid with each revolution of the pump shaft. If the discharge side of the pump is restricted or closed, pressure increases until something breaks, which is unsafe. For this reason, some form of overpressure protection is needed either on the pump or in the system-a pressure relief valve, torque sensing device or rupture disc. This is not true with centrifugal pumps because they have a maximum shutoff pressure generally not much higher than operational pressure.
The two basic types of gear pumps, internal and external, move liquid in much the same way: Gear teeth coming in- and out-of-mesh create flow through the pump. The internal gear pump has a larger gear encompassing a smaller one, with a crescent-shape piece taking-up eccentricity.
The external gear pump also has two gears, but the rolling contact is at pitch diameters external to each other.
In both internal and external gear pumps, gears move the liquid as well as transmit the torque (power) from the drive gear to the idler. A good lubricating film between meshing gears cushions the operation and extends pump life. For lower viscosity products, gears can be made from nonmetallic or other wear resistant materials to reduce wear from contact. Some wear is generally unavoidable.
Gear pumps are characterized by close internal clearances and tight gear meshing action. For this reason, gear pumps do not handle solids of any appreciable size. This application area is often handled by their "first cousins," lobe pumps.
Lobes pump product similarly to external gear pumps, but the lobes do not contact and can be made from stainless steel, which makes them excellent candidates for food applications. A set of timing gears is located behind the lobes, separated from the wet end by two mechanical seals. Timing of the rotors is important; if done incorrectly, the lobes will contact, wear and contaminate pumped product by wearing metallic lobes. This design allows passage of larger solids and has the added benefit of no metal-to-metal contact inside the pump, another reason they are often used for food applications or where no shear damage to the liquid or contamination can be tolerated.
Lobe designs vary, and could be one, two, three or more. Many factors determine a particular lobe design; solids size and liquid viscosity have the most effect. Newer designs are evolving, with special shredding disks to handle tough particulates, which rotary pumps do not traditionally handle very well.
Lobe pumps also have second cousins-rotary lobe blowers. Their operation principle is similar, with differences in the manner of sealing, clearances and compressibility aspects.
Rotary pump repair is relatively straightforward. Gear pumps with only two moving parts are particularly easy; the main emphasis is setting the proper internal clearances. Lobe pumps are a little more complex because of the external timing gears. They must be set in the proper position on reassembly, so lobe timing is correct. Otherwise, premature wear, or failure, can result.