Pumps & Systems, November 2007
Rotary positive displacement (PD) pumps are not well understood by many specifying engineers and users and are sometimes misapplied, incorrectly specified or simply not used where they could or should be. This article explores the areas of commonality between centrifugal and PD pumps and ten common PD pump misconceptions and the reality behind each.
There is considerable resident knowledge about centrifugal pumps, which comprise the majority of all pump installations. However, rotary positive displacement (PD) pumps are not well understood by many specifying engineers and users.
Rotary PD pumps account for 15 percent of all installations, with sales divided into many different pumping technologies. As a result, PD pumps are sometimes misapplied, incorrectly specified or simply not used where they could or should be. Though PD pumps move liquids through many different approaches, much commonality is shared in design and operation.
Centrifugal pumps move liquid in a much different fashion than do PD pumps, and their resulting performance differs as well. Centrifugal, or kinetic, pumps impart rotational energy to the liquid and convert it to potential energy (pressure) through the design of the volute. PD pumps, on the other hand, move liquids by transferring confined amounts of liquid (defined by pumping element geometry) from the inlet to the outlet of the pump.
The key here is that centrifugal pumps generate pressure and flow results, while PD pumps generate flow and pressure results. In other words, a PD pump generates just enough pressure to overcome system resistance created by the flow of liquid through it. Because of this, flow output from a centrifugal pump varies with differential pressure, whereas flow from a PD pump is essentially constant with varying differential pressure.
All of the many different types of PD pumps use geometry of the parts to expand and contract volumes of liquid. Volume expansion draws liquid into the pump and volume contraction moves liquid out of the pump. Though a number of geometries are involved, most rotary PD pumps share common design and operating characteristics:
- All try to displace the same amount of liquid with each rotation of the shaft.
- Flow is directly proportional to speed.
- All can pull liquid from below the pump or self prime.
- Most have close fitting internal parts.
- Most have one pumping element driving another (gears, rotor/vanes, etc.).
- All have a small amount of liquid that goes from discharge back to suction. This is called slip, and it varies with liquid viscosity and differential pressure.
- All require some form of overpressure protection.
Conventional wisdom says that a PD pump must be used over a centrifugal pump when one or more of the following application conditions exist:
- Liquid viscosity is too high (generally anything over 150-cps requires a PD pump)
- Constant flow is needed over varying differential pressure
- Suction lift or self priming ability is needed
Although these criteria are all sound, PD pumps may still not be used, or there may be rationale to use a PD pump when none of these conditions exist. At this juncture, many of the common misconceptions on PD pumps come into play. Let's explore ten of them.
1. PD Pumps Are Not Well Suited for Thin Liquids
Ask almost any pump user what type of pumps to use for a thin liquid application and the response is generally a centrifugal pump. This is often the right answer, but sometimes not. Some believe that PD pumps cannot be used on thin liquids at all due to their design characteristics. For example, how can a gear pump be used on a thin non-lubricating liquid when one gear drives the other?
The fact is most PD pumps - including gear pumps - can be used on thin liquids. Water is the most common thin liquid, and the internal gear pump was actually invented to handle it. Liquefied petroleum gas, refrigerants, solvents, fuel oils, gasoline and even liquid carbon dioxide are some of the other thin liquids handled very successfully with PD pumps. Selecting proper pump materials is important when moving a thin liquid, and most manufacturers offer many choices to handle the low lubricity and viscosity typically associated with thin liquids.
2. PD Pumps Do Not Need Overpressure Protection
A centrifugal pump that is dead headed, either accidentally or on purpose, can develop a predictable maximum shut off pressure. It will be above the normal operating pressure, but generally not much more.
A PD pump that is dead headed tries to displace the same amount of liquid for each revolution of the shaft. Because of this, pressure continues to increase until something breaks in the system, the pump is damaged or the driver runs out of power. None of these are safe or desirable conditions. To prevent this, having some form of overpressure protection either on the pump or in the system is important.