Understanding the Enigma: Demystifying Stator Failure in Progressive Cavity P...

FredrikStavset

As an oilfield operations manager, ensuring continuous and reliable production is one of my top priorities. However, unplanned downtime due to equipment failures can wreak havoc on schedules and budgets. One recurring pain point has been unexpected damage to the statos in our progressive cavity pumps, more commonly known as rotor and stator pumps. While these specialized pumps are excellent for moving viscous fluids in challenging applications, their complex nature also makes diagnosing and preventing stator issues a mystery.
In this article, we aim to shed some light on the enigma of stator failure by exploring several of the more common causes that my team and I have uncovered during our troubleshooting over the years. My hope is that sharing our experiences will help others in the industry avoid similar surprises down the road. With a better understanding of potential stator vulnerabilities, proactive maintenance practices can be improved to maximize uptime and reduce repair costs for these crucial production components.

Fluid Incompatibility
One of the most frequent culprits we've found is using a rotor and stator pump to handle a fluid that is simply not compatible with its material construction. These pumps are often selected based primarily on their ability to transfer thick, sandy fluids vigorously. However, the chemical properties of certain fluids, such as their acidity, alkalinity or solvent characteristics, can potentially damage pump elastomers, metals and alloys over time.

Corrosion is a telling sign of fluid incompatibility. Pits, cracks and other surface defects indicate chemical attack of the stator that cannot be seen until failure occurs. While it may be impractical to switch to a different pump design, choosing construction materials resistant to the actual fluid properties handled can help avoid premature corrosion-induced stator damage.

Stator pumps are engineered for tough applications, but no pump is indestructible if exposed to an unsuitable process fluid. Careful material selection and periodic inspections for corrosion are important precautions to consider.

Excessive Start/Stop Cycling
Frequent starting and stopping of progressive cavity pumps, such as for intermittent batch transfers, can dramatically shorten stator life in some cases. Each startup and shutdown cycles the stator from zero to maximum rpm and subjects it to abrupt changes in fluid pressures, flow rates and mechanical forces. While designed to withstand such stresses, excessive cycling well above design parameters may lead to small cracks, tears or delaminations developing over time.

Continuous vibration during operation only serves to propagate any weak points that form. There are a few mitigating steps we've taken including inspecting statsors more closely if cycling exceeds recommended guidelines and utilizing variable speed controls to reduce pressure and flow surges during startup/shutdown transitions. Advanced monitoring of pump parameters also allows catching minor stator anomalies before they escalate to complete failure. With proactive adjustments, cycling stresses can be managed to optimize stator longevity.

Solids Settlement
Settling of abrasive solids in stagnant regions inside progressive cavity pumps, such as during extended downtime, poses risks. Sand and other grit that accumulate undissolved can slowly erode away at stator elastomers not adequately flushed of solids over time. While pumps are designed to handle some abrasives mixed in fluid passages, stationary solids concentrated in low flow areas act like sandpaper on pump internals.

To prevent this, our maintenance protocols involve thorough cleaning of pumps following procedures that effectively purge residuals. Filling empty pump casing with preservative fluid prior to lengthy outages helps displace solids and protect unguarded internals. Diligent cleaning coupled with regular inspections for signs of erosion in stagnant regions can preclude solids-induced stator wear leading to untimely failures down the road.

Suboptimal Operating Conditions
Deviations from standard operating conditions specified for rotor and stator pumps can threaten stator integrity as well. For instance, pumping fluids with viscosities far above or below design parameters subjects elastomers to higher than normal stresses. Pushing flow rates or pressures to extremes outside pump curves increases mechanical fatigue and thermal stresses on stator components.