Progressive Cavity Pump - RECOMMENDED:
As a general rule, VeloDyne recommends progressive cavity metering pumps for metering neat polymer.
- Progressive cavity pumps are more reliable than diaphragm pumps since they do not rely on check valves – a progressive cavity pump can more easily “extrude” the normally occurring polymer agglomerations found in liquid emulsion polymer where diaphragm pump check valves are easily plugged by these agglomerations.
- Polymer is an excellent lubricant which translates into a long pump life, providing the pump does not run dry.
- Packing as a shaft seal is strongly recommended over mechanical seals since mechanical seals have a tendency to fail when neat polymer is allowed to set up between the seal face - as good as polymer is as a lubricant when it’s in liquid form, it’s an excellent glue when it dries, causing premature mechanical seal failure.
- While progressive cavity pumps and associated controls are more expensive than diaphragm pumps, their superior reliability is worth the additional investment in most cases.
Gear pumps are preferred less over progressive cavity pumps:
- Gear pumps must be run at slower speeds to prevent shearing the polymer. Advanced Polymer Activation Technologies
- Because a gear pumps slip-to-speed ratio is greater than that of a progressive cavity pump the gear pump is less linear at lower speeds.
- Gear pumps are generally more expensive than progressive cavity pumps.
- The shaft seal sees the pump pressure resulting in more seal packing seal seepage than a progressive cavity pump and mechanical seals are not recommended for the above reason.
- Diaphragm pumps are less reliable due to their reliance on both an inlet and discharge check valve.
- Liquid emulsion polymer has normally occurring agglomerations. These agglomerations will collect at any pinch point in the polymer line, including diaphragm pump check valves.
- Further aggravating the problem is when a diaphragm pump stroke length is set at a low point, minimizing the displacement (opening) of the check valve.
- Due to their lack of reliability and need to frequently disassemble and reassemble the plugged check valves, polymer contact and polymer spillage can be a hazard, not to mention a job no operator likes to have.
- The only real benefit to a diaphragm pump is its cost when a solenoid actuated pump is used.
Mechanically and Hydraulically Actuated:
- The same weaknesses apply to mechanically and hydraulically actuated diaphragm pumps as those found in solenoid actuated diaphragm pumps.
- The additional disadvantage to these pumps is their relatively high cost, which can be significantly more than the more reliable progressive cavity pump.
Peristaltic metering pumps are excellent pumps in many applications, however are not ideal for metering neat polymer in most applications.
- Due to the relatively low flow rates required in most polymer applications and therefore the required small tube sizes in a peristaltic pump, the normally occurring polymer agglomerations may plug the hose causing loss of flow and premature failure.
- Hose technology has increased the life of peristaltic pump hoses, however, the higher the pressure the less life the tube has. Under no circumstance do we recommend a peristaltic pump that uses clear PVC tubing as this tubing as a very short life
- A leak detector should always be used – if the tube fails the pump will continue to pump polymer (onto the floor).
- Suction condition is very important in all metering pump applications, but even more so on peristaltic pumps due to the viscosity of neat polymer.
- Avoid using “throw-away” peristaltic lab pumps (the small clear-bodied/clear PVC tube pumps) as they are highly unreliable and the tube life is very short.
- A high quality peristaltic metering pump is costly, roughly two to three times as expensive than a progressive cavity pump.
For more information contact VeloDyne at 303-530-3298 or visit us online at www.velodynesystems.com.