Keep lobes in mind
Understand why and when to consider these positive-displacement pumps.
Shaft sealing
Lobe pumps are not available in seal-less designs. All require two shaft seals; some types with bearings external to the liquid need four. Industrial lobe pumps offer at least as many sealing solutions as other rotary or centrifugal pumps —including packed glands, cartridge-style single or double mechanical or triple lip seals (Figure 4), and component-type single or double mechanical seals (Figure 5). API seal plans are available. So, if a specific brand or type of seal works well in your problem applications, it often can be applied to an industrial lobe pump. For sanitary lobe pumps in industrial applications, seal options include packing, O-ring seals, lip seals and component-type single and double mechanical seals, but standard cartridge seals are not an option.
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| Figure 4. This pump features cartridge-style triple lip seals and bushings on both sides of rotors. |
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| Figure 5. These pumps employ component-style double mechanical seals with seal flush. |
Making a choice
The pump selection process involves too many variables to comprehensively discuss here. However, it is possible to give some general guidelines both for the initial step of choosing between PD and kinetic pumping principles, and then, if PD is picked, for selecting among lobe and other PD designs.
PD versus kinetic. Select PD over kinetic (used by centrifugal pumps) if:
• liquid viscosity exceeds 100 cSt (at which point centrifugal efficiency drops considerably);
• metering liquid into a process;
• reversible direction of flow is required;
• multiple viscosities may be handled by the same pump;
• suction lift may be necessary (such as with top-of-tank mounting); or
• liquid is shear sensitive.
Consider PD pumps, to achieve higher energy efficiency and lower cost of operation, for situations in which centrifugals are least efficient:
• moderately viscous liquids (from 25 cSt to 100 cSt);
• flow rates less than 50 gpm; or
• considerable potential variation in required flow rates (centrifugal operation very far from either side of the best efficiency point results in high cost of operation and can cause excessive wear, depending upon pump design).
Lobe vs. other PD options. If a PD pump is favored, consider lobe pumps when the application involves one or more of the following:
• shear sensitive materials;
• slightly to very viscous materials;
• solids present in the liquid;
• medium pressure (to 400 psi);
• steam cleaning or solvent flushing between batches;
• repeatable metering;
• reversal of flow direction; or
• the possibility of dry running.
The primary liquid-characteristic drivers in selecting lobe pumps are shear sensitivity (dilatant or thixotropic fluids), entrained solids or non-lubricating liquids. The primary process-design drivers favoring lobe pumps are the need for frequent cleaning (especially between batches), metering high volumes into a process, or where the pump flow must be reversed to strip the discharge line (Figure 6). Some lobe pumps are designed to be mounted with ports in either a horizontal or vertical orientation and, when mounted vertically, can be self-draining to minimize loss of costly liquids and further shorten the cleaning cycle.
Operator preference often is a key factor in selection. Many operators simply prefer all metal pumps (lobe, gear or screw) over pumps with elastomeric members (progressive cavity, diaphragm or peristaltic), because rubber parts potentially wear, shed particles and require replacement.
How PD pumps compare
|
Decision Criteria |
Lobe |
Progressive Cavity |
Air-Operated Diaphragm |
Timed Two-Screw |
Gear |
|
Low shear (speed dependent) |
Excellent |
Excellent |
Excellent |
Excellent |
Excellent |
|
Efficiency |
Excellent |
Excellent |
Poor |
Excellent |
Excellent |
|
Reversible MagazineSister Web Sites |
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