What Are the Difference in Positive Displacement Pumps

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The world of Positive Displacement Pumps is very large and growing. As a manufacturer of custom process skids and pumping skids, I often get questions about the various types of pumps out there. We have to be familiar with a large variety of high-pressure pump technologies in order to design a system that will emulsify or homogenize fluids. High pressure for me is defined as anywhere from 300 psi to 10,000 psi. Once you get into this pressure realm, you are outside your typical material transfer pumps. A small sampling of low-pressure pumps would include:

  • Centrifugal pumps
  • Lobe pumps
  • Pneumatic Diaphragm pumps
  • Solenoid Piston pumps

This wikipedia article on pumps covers just about everything you’d need to know actually.  What isn’t really discussed in any detail, however, is practical experience that leads to choosing the right pump or how to design a system around such a pump.

High-Pressure PD Pumps

Once you start emerging into the world of high pressure above say 300 psi, your options start to become limited and things like fluid viscosity start to play a more significant role.

Rotary Gear Pumps

At 300 or 500 psi you can use Rotary Gear and pumps by manufacturers like Waukesha, Viking, Tuthill, Zenith and more. These pumps use 2 rotating gears set inside a housing with fairly tight tolerances between the gear outer edges and the housing inner wall. The tighter the tolerance between these surfaces, the more pressure the pump can theoretically handle. These pumps typically have a difficult time pumping water-like viscosities even though their pump curves are based on water. These pumps perform better with higher viscosity fluids because there’s less chance the fluid will slip outside that gap between the gear edge and inner housing wall.  The slip is fairly significant and something to think about.  With that slip comes a lot of noise.  The pumps hammer and bang when struggling against these high pressures and low viscosities.  If you are pumping water-like viscosities at these pressures, they might not be the best pump for you.  The nice thing about these pumps, however, is that they typically come in sanitary or hygienic designs good for pharma and personal care. That said, you can fabricate EHEDG sanitary homogenizer systems using these pumps:

Progressive Cavity Pumps

Progressive Cavity (PC) pumps are another option at this pressure region of 300-500 psi.  These pumps use a long spiral-shaped auger that rotates inside an elastomeric stator and moves fluid progressively along the auger.  Slip can occur in a similar manner at the space between the rotating auger and the stator.   The good news with these pumps is that there is no real gap between the rotating and stationary components because the metal rotor can essentially make contact with the elastomeric stator.  Now slip occurs due to compression of the elastomeric element.  These pumps handle water-like viscosities much better than the rotary gear type, but suffer flow losses due to intake efficiency drops when the viscosity is high.

Reciprocating Triplex Plunger Pumps

These pumps work in a completely different manner altogether by actually trapping material between a set of checking valves where it’s nearly impossible for fluid to migrate or slip back.  The problem here is that these pumps cannot handle higher viscosity fluids much above 3000-4000 cps (pancake batter) nor can they be used where 3A or EHEDG are required. However, they are easily cleanable, but do not bear those certificates.  Improvements can be made to handle higher viscosity by using a feed pump to pressurize the inlet.  These pumps can accomodate really high pressures now in the 10,000 psi range.  Cat Pumps manfactures such pumps and Sonic Corp has used them to in high-pressure Sonolator homogenizer systems that operate routinely at 5,000 psi to make fine emulsions.

There are other Reciprocating Plunger pump companies such as Lewa, SPX and more but these pumps are terrifically expensive per GPM as compared to Cat Pumps.

All told, it’s not as simple as Googling Positive Displacement Pumps to find what you want.  You’ll get 973,000 results ranging from storefronts for mini Jabsco vane pumps to obscure images of home-made pumping systems that look more like my kid’s go kart than a bone-fide system I’d want in my manufacturing plant.

With the internet’s over abundance of information, it’s time to seek an intimate connection with trusted partners.

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About the Author:

Rob Brakeman
Owner & Director