More processors in the chemical industry and elsewhere are opting to use rotary lobe pumps in applications formerly served by progressive cavity, diaphragm, peristaltic, screw and gear pumps. This trend is not surprising because lobe pumps offer benefits like low shear, reversible direction of flow, run-dry capability and easy cleaning between batches. Additionally, they have a proven track record in the food industry, where such pumps and their first cousins, circumferential piston pumps, have been widely used for half a century.
However, as with any pump, appropriate application requires knowledge of the advantages and disadvantages of the design. So, let’s examine factors to consider in selecting lobe pumps.
A lobe pump has two counter-rotating pumping elements (rotors), each with two or more lobes (Figure 1). Accordingly, there are two rotating shafts and two shaft seals. The drive shaft (connected to the driver) counter-rotates the driven shaft by means of oil- or grease-lubricated timing gears located outside of the pumping chamber.
|Figure 1. Non-touching lobes draw fluid into the pumping chamber and then force it out.|
Unlike a gear pump, the lobes never actually touch. This allows the rotors to be made of highly corrosion resistant materials like stainless steel and enables the pumping of non-lubricating liquids.
As the lobes separate on the suction side to create a void, fluid flows into the pumping chamber and is carried around the perimeter by each lobe’s fluid cavity. When the lobes re-converge, the cavity shrinks and the fluid is forced out the discharge port.
The rotors in lobe pumps take many forms (Figure 2), from two to six rounded lobes or two to three wings (sharp-edged, scimitar-shaped lobes). Each offers benefits and tradeoffs with respect to pump efficiency, solids handling, maintenance and fluid pulsation.
|Figure 2. Alternatives include bi-wing and four-lobe rotors, among others.|
As in any positive displacement (PD) pump, efficiency is largely affected by slip, which is the backflow of liquid past the rotors from the high pressure (discharge) side to the low pressure (suction side) of the pump. Excessive slip results in reduced efficiency and limits the pressure that the pump can develop. The outer edges of the lobes, using very close clearances to minimize slip, provide a dynamic seal as they rotate within the pumping chamber. When pumping high viscosity liquids, the slip is negligible regardless of the lobe form used. Bi-wing (scimitar) lobes have slightly larger sealing surfaces, so they are often recommended for low viscosity liquids to minimize slip.
Rotary lobe pumps can handle liquids containing most solids found in chemical processing applications. For many solids, bi-wing rotors are selected because their leading edges tend to scoop solids into the cavity, unlike multi-lobe designs that may trap solids at the lobe tip. Bi-wing fluid cavities also are slightly larger than tri-lobe or multi-lobe designs, allowing slightly larger solids to pass.
Lobe pumps must be precisely adjusted to prevent rotor contact. This is typically accomplished by shimming at the timing gears. Bi-wing rotor pumps, though, generally do not require such shimming, which simplifies maintenance and frequently leads to their selection unless process reasons dictate otherwise.
If pulsation must be minimized in a process, however, then multi-lobe rotors are preferred. All PD pumps, even rotary pumps, by nature exhibit some pulsation in their flow. Multi-lobe rotors display higher frequency of pulsation but reduced amplitude compared to bi-wing rotors. Pulsation is proportional to operating pressure. So for those applications requiring near-zero pulsation, select a pump with multi-lobe rotors and operate it at the lowest discharge pressure possible. Where higher pressures are necessary, a pulsation dampener may be warranted on critical applications.
Pressure capabilities depend upon the type of shaft support provided. An overhung load design is typical of sanitary pumps (Figure 3, left). To eliminate areas where material could be trapped and bacteria could grow, there are no bushings (sleeve bearings) in the liquid. The seals are located between the lobes and the nearest shaft support. This design is limited to about 200 psi (14 bar) due to the potential for shaft deflection and rotor contact, but also is the fastest to clean in place by flushing or steaming. Many sanitary pumps have front-loading seals that provide easy access for maintenance.
A typical industrial design features bushing-type shaft support on both sides of the lobes (Figure 3, right). It allows pressures to 400 psi (27 bar) or more by minimizing shaft deflection. An air gap between the pumping chamber and the gearbox helps to prevent cross-contamination between the liquid pumped and the gear lubricant in the event of a seal failure. Shaft slingers or labyrinth seals are usually offered on the gearbox to provide additional protection against water or chemical infiltration.
|Figure 3. Sanitary design (left) and industrial design (right) differ in shaft support provided and thus in pressure capabilities.|