Progressive cavity pumps (PC pump), may be commonly referred to in industry as a mono-pump, screw pump, eccentric screw pump, or worm pump and all are positive displacement type pumps. The inventor of the progressive cavity pump is said to be Rene Moineau. Often these pumps may be referred to as a Mono pump, Moyno pump, Mohno pump or Moineau pump alllikely derivatives of Rene Moineau name. Moineau was designing a compressor for early jet engines in 1930 when he determined that this type of principle could work efficiently for pumping of fluids. Moinea teamed up with Robert Bienaime of Gevelot Group to found PCM Pompes in 1932 to develop the world’s first known progressive cavity pump.

The progressive cavity pump consists of two major components, a rotor and a stator. The rotor is helix shaped, machined from either stainless steel, hard coated carbon steel, or hard coated stainless steel. Depending on the type of process media, the stator can be composed of nitrile rubber, natural rubber, EPDM, or Viton. As the rotor turns within the stator, the fluid is driven to the surface within the spaces, or cavities, created between the rotor and the stator. Hence, the PC pump motivates the fluid from the suction to the discharge end by continuously shifting spaces (which are the “progressing cavities”) between the rotor and the stator.Since the rotor and stator have a small interference fit and low rotating speed, the internal shear rates are very low. It is due to this reason that the PC pump is highly recommended for highly viscous and/or shear sensitive liquids and sludge’s.

The most significant industries for PC pumpsare pulp & paper, waste water and oil and offshore. Mining and minerals, original equipment manufacturers, chemical industries and others as well may have a significant number of PC pumps installed. Common applications can be found in flocculant dosing in water & waste water treatment plants to transfer of slurries in the pulp & paper and waste water industry.The main applications in the pulp and paper industry are pumping clays, calcium carbonate, glues, starches, titanium dioxide, polymers and talc to name quite a few. In the food industry PC pumps are popular to pump various liquids such as molasses. In waste water PC pumps are used to pump various sludges and flocculants.

In general there are two types of stators that can be used in PC pumps. One that is a standard round stator and another that is Evenwall spiral technology. Evenwall designs are a relatively new development in progressive cavity pumps. Standard round stators have been in existence for the past 80 years. Even rotors with 2D and 3D geometry have been around for many years. However spiral stator technology is a modern advancement in PC pumps with only a fraction of the PC pump manufacturers offering evenwall technology.

Picture 1: Spiral stator (left) allows higher pressure per stage and 2/3-lobe elliptic rotor higher flow rate

Evenwall technology often offers higher efficiency and higher pressure and flow capabilities. Evenwall technology has a much thinner rubber lining in the stator. Standard round stator technology have much greater mass of rubber inside the stator. Since there is much more rubber in standard round stators the tolerances need to be much less than that of evenwall technology. This results in much less efficiency and wear caused in round stator designs. Round stators need to provide additional tolerances not found in evenwall technology for thermal expansion of the rubber in the stator during start-up and normal operation. Round stator technology suffers some of its most extreme wear during start-up conditions. For instance highly abrasive mediums will slip or leak backwards due to rubber having not yet heated up or expanded. This expansion of the thicker rubber in the stator is required to tighten the tolerances to allow more efficient pumping. During this time period the PC pump will suffer more wear or even damage. With evenwall technology the tolerances do not need to be as extreme. Thus the evenwall technology is much more efficient both at start up and during normal operation.

-Evenwall Spiral Stator technology vs conventional round stator

The Flowrox PC pump utilizes a unique even wall spiral stator technology which allows for a more rigid and tighter pumping unit. With the enhanced compression of rotor and stator friction, spillover and wearing is minimized so the PC pump can be used with the same rotor in higher temperatures. When compared to the conventional round stator design, the even wall spiral stator consist of a smaller interference between the rotor and the stator which results in a lower starting torque and a significantly higher efficiency.

-Higher efficiency & improved hydraulic performance

Fig 1.1 Relative flow (gpm) vs. counter pressure (Bar)

Fig 1.2 Efficiency (%) vs. counter pressure (Bar)

When comparing hydraulic performance of evenwall PC pump to the conventional PC pump, the results are higher pressure, lower back flow, and significantly higher efficiency. Figure 1.1 is a comparison of the relative flow (gpm) vs the counter pressure (bar) between the evenwall PC pump with the even wall spiral stator and the conventional PC pump with a conventional round stator. Fig 1.1 shows that the relative flow rate drops as the pump counter pressure increases. This trend reflects how the pump hydraulic efficiency drops due to counter pressure increase. When comparing the hydraulic efficiencies of both PC pumps at 6 bar, the conventional pump results in an efficiency of 81.6% while the evenwall PC pump has an efficiency of 93.6%. The results at a counter pressure of 10 bar are also similar, resulting in a higher pressure with lower backflow.

The total efficiency difference measured from pump power demand is shown in figure 1.2. The conventional pump’s maximum efficiency is 54 %, while the evenwall PC pump has a significantly higher maximum efficiency of 63 %.

-Longer service lifetime

Not only does the higher hydraulic efficiency reduce the amount of energy consumed, but italso facilitates a longer pump service life. The PC pump will have a longer service life due to the minimized back flow between the rotor and stator which can cause a high degree of wear.

-Patented double action mechanical shaft seal assembly

The most common failure in PC pumps is caused by problems in the shaft sealing. Some designs utilize patented shaft and seal allowing the mechanical shaft seal to be replaced quickly and without dismantling the entire pump or its extended shaft and coupling. In most applications where PC pumps do not have special designs for removal and replacement of the mechanical seals the entire pump needs to be removed from the pipeline and taken to a repair shop to have the pump mechanical seals replaced. This is often an 8 – 16 hour projects. Advanced designs which allow for in-line replacement of mechanical seals will cut this repair time to less than one hour. In critical processes and pumping applications these designs can offer significant advantages on pump availability. Approximately 25% of all PC pump repairs are related to mechanical seals.

One final design advancement is a very simple advancement but can come in very helpful in difficult piping arrangements. Some designs have manufactured their suction pump inlet flanges in a manner in which the suction inlet can be rotated in 270 degree rotation. So if the inlet piping is non-conventional and not directly vertical to the top of the suction side of the pump then the suction flange of the pump inlet can be rotated to mate up to this piping without having to redesign the piping arrangement to fit the limitations of the pump inlet.

Conclusion

There are many PC pump manufacturers and designs to choose from. Evenwall PC pump technology offers a higher efficiency, lower power consumption, and a longer service life time. When compared to conventional PC pumps with a round stator, evenwall PC pumps deliver higher pressure and flow rate with an expanded temperature range in a smaller foot print. The PC pumps are excellent self-primers and are great for highly viscous and shear sensitive process media because they produce minimal agitation to the fluid. Evenwall PC pumps require less energy, are easier to install, and require less maintenance resulting in very low Total Cost of Ownership (TCO). Flowrox chooses evenwall over all other designs because it simply works.

References

[1] Nelik, Lev, and Jim Brennan.Progressing Cavity Pumps, Downhole Pumps and Mudmotors. Houston, TX: Gulf Pub., 2005. Print.

[2] Ruutiainen, Risto . “Larox Flowsys Progressive Cavity Pump PC Pump”. November 2010.Flowrox, Inc. 28 March 2012