Common Pumping Mistakes
by Jim Elsey
June 12, 2017

Rotors are typically 4140 tool or 316-SS austenitic steel that is chrome plated, but they can also have chrome oxide, carbide and ceramic coatings. The rotor will usually be solid metal, but some manufacturers also offer hollow steel as a weight (mass) reduction feature in larger rotors.

The pump portion of the unit may be able to pump the fluid, but the drive components may not be able to handle the required horsepower/torque. Consult with the OEM on the drive limits. Multiple PC pumps can be used in parallel for increased flow rates.

Tips for Reliable Operation & Reduced Maintenance

  • Stators will usually wear out first. Replace the rotor for every two stators replaced on a pump.
  • Used stators can often be reversed (flipped end for end) and reused.
  • Check the instruction manual for how to measure wear on the rotor and what the limits are. Check wear at the proper intervals and you can save the cost of a new rotor.
  • If rotor wear is caught before the chrome plating is removed they can be re-chromed.
  • Rotors worn past the chrome plating need to be replaced.
  • It is rare for the rotor to be bent.
  • Stators that are pitted, gouged or worn must be replaced.
  • Use a non-petroleum based lubricant when installing the rotors into the stators to preclude an elastomer attack and the resulting swelling. Castor oil is inexpensive and works great. Note: If the pump is food- or pharmaceutical-grade there may be restrictive and special requirements for lubricants, especially in contact with the fluid.
  • Do not over-torque the foot bolts around the stator as this can put the pump in a bind.
  • Flush product out of the pump while it is still in place to allow for easier and cleaner maintenance. Cover the suction and discharge flanges/opening to keep out foreign objects. Make notes of connections and orientations. Pictures are valuable tools in this case.

Stators are like red wine, you need to protect them in storage if you want to keep them in proper condition. Heat and humidity are an enemy of rubber stators. UV light and ozone are like kryptonite to the elastomer materials and will destroy the stators. To reduce exposure, place protective caps on the ends of the stators. If your spare stators are older than three to five years you can expect some deterioration in quality and consequently operational life.

Purchase your spare and replacement rotors and stators from the same manufacturer. Each manufacturer has different specifications, dimensions and tolerances that are ever so slight, but it will make a big difference when operating in the unit. A rotor from one source may not perform as well in a stator from a different source or vice versa.

For more Common Pumping Mistakes columns, click here.

Progressive Cavity Pumps

Advantages:

  • Good for solids handling and can also handle air entrained, multiphase and abrasive liquids
  • Excellent choice for high viscosity applications
  • Offer low net positive suction head required
  • Excellent self-primer pump
  • Do not vapor lock
  • Offer high accuracy as a metering pump
  • A given size/model of pump installation will handle a wide range of viscosities very well
  • Reversible direction/ bi-directionally. The pump can be operated in either direction, and the suction becomes the discharge and vice versa
  • Can be operated vertically
  • Quiet operation

Disadvantages

  • Due to the interference fit between the stator and rotor, a fluid film is required to lubricate the sliding (contacting) surfaces. Running the pump dry is by far the biggest reason these pumps fail.
  • Low speed also requires VFD (variable frequency drives) and or gear reducers that add to the cost of the installation. (These pumps are great at self-priming, but the rotor stator fit has to be lubricated in the process.)
  • Low speed also translates to low flow, which can be a disadvantage in many applications.
  • Flowrate and energy required to get the fluids to the pump will require technical review. With highly viscous fluids there is a pump speed above which the fluid will not flow fast enough into the pump. When this occurs, the volumetric efficiency of the pump is compromised. The manufacturer can assist in determining the speed limits for a given fluid and viscosity.
  • Possible high startup torques can require drivers (motors) to be larger than would otherwise be required. The running horsepower may only be half of the required startup horsepower.
  • Can only pump a limited distance. Generally, the distance to one foot of discharge piping to one PSI of pressure developed. For longer distances consult with your manufacturer.
  • Low viscosity fluids tend to slip by the rotor to stator fits and the pump will be less efficient.

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