The decision to choose a positive displacement pump over a centrifugal is not always a clear one. They each have very different behaviors and if not familiar with them, it may be difficult to understand how a positive displacement pump could fit into your process.
Positive displacement pumps are a more efficient choice than centrifugal pumps in some situations. If any of the conditions or applications below are in your process, a positive displacement pump should probably be used.
1, HIGH VISCOSITYCentrifugals have issues pumping viscous liquids, becoming very inefficient at even modest levels. Positive displacement pumps, on the other hand, have no problem moving thick liquids.
2, LOW FLOWOftentimes, a centrifugal pump is run off its BEP when a lower flow is desired, much to the pump's detriment. If you read our previous post "How To Read a Centrifugal Pump Curve", you'll recall that running a centrifugal off its BEP can cause excessive energy consumption, damage to the pump, and overall poor performance.
A positive displacement pump, however, is well suited for these conditions, providing constant flow of fluid at a given pump speed.If you're trying to get the pressure or flow you need by operating a centrifugal pump off its best efficiency point (BEP), a positive displacement pump may be a better choice.
3, METERINGPositive displacement pumps are an ideal choice for metering applications. They deliver constant flow, allowing them to easily meet process requirements. Some common types of positive displacement pumps used for metering are:
Positive displacement pumps are excellent for applications that require high pressure, with some models producing over 1,000 psi (2,300 ft). Due to the positive displacement pump's design however, if kept in operation against a closed discharge valve, it will continue to build pressure until the line bursts, the pump is damaged, or both.
5, SHEAR SENSITIVE LIQUIDSCentrifugal pumps generally operate at higher speeds when compared to positive displacement pumps. the higher speeds can shear liquids, making centrifugal pumps a poor choice for liquids like tomato paste and latex paint. Positive displacement pumps operating at lower speeds can be more gentle on products, and are usually preferred in these types of applications.
Centrifugal pumps and positive displacement pumps behave very differently. Understanding their behaviors can help you be sure to choose the right one for your application.
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