Click here for "Focus on Fundamentals" Part One, Part Two, Part Three and Part Five.
In the June 2008 issue, we turned our attention from centrifugal to positive displacement pumps. This month, we explore progressive cavity (PC) pumps, which belong to a class of rotary positive displacement pumps. They have smooth output flow and good self-priming ability. Capable of pumping both thick and thin fluids, they are successful pumping liquids with high solids and abrasives content.
These capabilities have made progressive cavity pumps a choice for many tough applications. They work well in the wastewater treatment industry, but they perform equally well at the "opposite end of the spectrum" (the food industry) due to their minimal impact on shear sensitive fluids, such as sauces, cream products and similar fluids.
Through the years, advances in pump design, electronic monitoring and materials of construction have improved PC pump energy efficiency, decreased maintenance requirements and allowed them to handle more severe application conditions.
Rene Moineau invented the PC pump in France in the 1930s. The pumping element1 is made from the rotor and stator elements. Normally, the rotor is made of steel and has the shape of a single helix (external shape). The stator is normally made from an elastomer and has the shape of a double helix (internal shape). The rotor is manufactured slightly larger than the stator so an interference fit exists when the rotor is inserted into the stator.
However, some designs actually have a clearance between these, referred to as a "single undersize rotor," "double-undersized" and even "triple-undersized." The designs with clearance between the rotating elements are rare (interference fits are probably common in 99 percent of applications). When designed with a clearance, it is limited to low differential pressures and relatively thick (viscous) product-otherwise a "slip" (loss of flow) would be substantial. As the rotor turns inside the stator, a cavity is formed between the two shapes and "progresses" (hence the name) axially from one end of the element to the other.
The PC pump is made of three major sections:
- The pumping element
- The suction housing
- The drive train
Some design enhancements could include features to make maintenance simpler and reliability better. A close-coupled, or so-called "block" design, results in a smaller pump package, less upfront cost and no drive alignment issues. Sealed pivot style universal joints (shown below) keep the joints lubricated.
Easy access to the mechanical seal is important to simplify seal service and reduce downtime with quick changeover. However, packings are still more accepted for sealing fluids in typical PC applications (as they rarely pump tough enough chemicals to require mechanical seals). When equipped with augers, PC pumps produce better NPSHR values with higher volumetric efficiencies and higher percent solids capabilities. Oversized open hopper inlets handle thicker liquids and eliminate bridging (for example, filter cake up to 55 percent solids can be handled by the auger-augmented PC pumps).
Improvements in the pumping element go beyond more traditional 1:2 geometry (single rotor lobe) to multi-lobe configurations, such as 2:3 (rotor/stator lobes) geometry. A multi-lobe design can increase flow per revolution and reduce initial pump cost. Equal wall stator doubles pressure capability per stage as compared to standard designs with constant stator outside wall thickness.
Tie rod construction makes the entire assembly much easier to service, as compared to more conventional threaded stator designs. Hollow cast rotors reduce inertia and result in lower vibrations. Coated chromed rotors resist wear and last longer.
A temperature probe, installed at the stator wall, can prevent rapid temperature rise and failure during dry running. In practice, few installations take advantage of this feature, since many maintenance departments tend to prefer simpler designs with fewer "gadgets."
PC pumps, like other pump types, have limitations. Typically, they are not useable in high temperature applications because of electrometric stators limitations. They require significant floor space and should not be run at speeds much higher than 300- to 400-rpm due to the unbalanced nature of the rotating element. However, when used correctly, the PC pump's benefits can be substantial. It will provide long-lasting service to any plant with proactive maintenance practices including vibrations trending programs, root cause analysis and similar equipment-caring techniques.
References
- Nelik, L., and Brennan, J., Progressing Cavity Pumps, Downhole Pumps and Mudmotors, Gulf (Pump Guides) Publishing, Houston, TX, 2005, ISBN 0-9765113-1-2
To learn more, register for an upcoming Pump School two day session.
Pumps & Systems, July 2008
