Progressive cavity pumps are self-priming rotary positive displacement pumps with smooth output flow.
They are capable of pumping both thick and thin fluids, and do very well at pumping liquids with high solids and abrasive content. These capabilities have made the progressive cavity pump the pump of choice for many applications in the wastewater treatment industry.
In recent years, advances in pump design, electronic monitoring and materials of construction have allowed the progressive cavity pump to handle more severe application conditions, while at the same time improving the pump’s energy efficiency and decreasing maintenance requirements.
The purpose of this article in WWD’s Pump Source is to give an overview of the progressive cavity pump’s operating principle. The article will also review some of the new technical innovations that end users may be able to benefit from when it comes to selecting this type of pump for their wastewater applications.
Rene Moineau invented the progressive cavity pump in France in the 1930s. The pumping element is made from the rotor and stator elements.
Normally, the rotor is made of steel and is the shape of a single helix external shape. The stator is normally made from an elastometer and is the shape of a double helix internal shape. The rotor is manufactured to be slightly larger in size than the stator so that there is an interference fit when the rotor is inserted into the stator. As the rotor turns inside the stator, a cavity is formed between the two shapes and progresses axially from one end of the element to the other (Figure 1).
The progressive cavity pump is made of three major sections: the pumping element, the suction housing and the drive train (Figure 2).
In recent years, many improvements have been made in these areas to improve the pumps overall performance.
Improvements in drive train design
- A closed coupled or block design provides a smaller pump package, less up front cost and no drive alignment issues. It also eliminates the need for safety guards;
- Easy access to mechanical seals allow progressive cavity pump users to simplify their seal service, which reduces down time;
- Sealed pivot style universal joints offer fewer parts, while offering a longer life and easier maintenance. This setup also protects the drive shaft, the coupling rod and the rotor from damage; and
- The auger’s feed screw and coupling rods provide users with better NPSHR values, higher volumetric efficiencies and higher percentage solids capabilities.
Improvements in suction housings
- Oversized open hopper inlets allow the progressive cavity pump to handle thicker liquids;
- Special compression zones offer increased volumetric efficiency; and
- An oversized hopper with a double auger eliminates bridging and can handle filer cake up to 55% solids.
Improvements in pump elements (Figure 4)
- New 2x3 rotor/stator geometry allows for increased flow per revolution and reduces initial pump cost;
- Equal wall stators double pressure capability per stage;
- Tie rod construction provides users the ability to easily change out the stator with a standard socket set;
- Hollow cast rotors increase rotor and stator service life;
- A temperature probe protects against dry run; and
- Ductile coated chrome rotors extend the wear life of the rotor and it does not flake off like standard chrome coatings.