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Frequently Asked Questions

General

Does RPM affect torque?
No. RPM is determined by the flow rate. Torque is determined by the differential pressure
across the stator.

Can a power section with more lobes and more stages handle a higher differential pressure?
Yes. Differential pressure is affected by the number of cavities in the stator. When more
stages and/or more lobes are added, the number of cavities also increases; therefore more
differential pressure can be applied.

Does the finished profile change between chrome and carbide rotors?
No, the finished profile does not change between the chrome and carbide rotors on NEW
rotors. The PRE-COATED profile though IS different due to the different deposition methods of
the coating applications. On USED rotors that were originally chromed, with the chrome
stripped off and re-plated with carbide, you will find that the profiles will be different.

Why does a PDM cause vibration?
There are three main causes of vibration in a PDM motor. They are the mass, eccentricity and
nutation of the rotor. As the rotor spins inside the stator, these three factors determine the
amount of vibration that the power section generates. The higher the mass, the greater the
eccentricity and greater number of nutations the higher the vibration becomes.

Does vibration have an effect on the motor?
Vibration in the motor is an inherent action that is part of the nature of progressing cavity
motors. Excessive vibration has been known to damage other components of the motor
assembly and other tools in the drill string. Vibration can also be a source of excessive heat
in the elastomer and cause the elastomer to overload and fail

If the flow is doubled, does the fluid go through twice as fast?
Yes, velocity is a function of the Flow rate and the flow area. If the flow rate doubles the velocity through the stator also doubles.

As the interference increases, will the power section slow down because of a tighter fit?
No. The power section slows down because of leakage between the rotor and stator seal. If the interference increases, the seal becomes tighter and the rpm drop is reduced. Subsequently, the rpm doesn’t decrease, but remains relatively constant compared to the no load rpm of the power section.

Fit and Temperature Related

Does PV change their elastomers to compensate for high temperature applications?
PV offers an HSN type elastomer which is better suited to high temperature applications.
What is the maximum temperature for a stator?
Temperature is generally limited to 350° F (177°C). This is due to rubber compound limitations and thermal expansion.
Do the terms ‘standard’ and ’oversize’ refer to our group number?

Standard and Oversize are industry terms used to identify a stator and the size they are in reference to the interference fit to the rotor. PV does not use this nomenclature to determine sizing as PV designs the fits based on drilling and circulating temperature. PV categorizes fits by group numbers. For example, the Group 0 is designed for optimal fit at 138°F while a group 4 is designed for optimal fit at 240°F.

 

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