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:
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:
Solenoid Actuated:

• 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 Pumps:
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.