SOCIETY
OF PETROLEUM ENGINEERS
Mid-Continent Section, Tulsa, Oklahoma
Abstracts courtesy of Ken Saveth, ksaveth@juno.com
"Designing PC Pumps for Abrasive Applications" - Majid Delpassand: Moyno Oilfield Products
The wear phenomenon occurring in PC pumps are dependent upon several things. Some of these things are:
Concentration of Abrasives
Particle Size & Hardness
Particle & Fluid Velocity
- average cavity velocity
- high speed (slip) velocity
Rotor/Stator Rolling Contact
The PC Pump’s wear life can be maximized through optimized pump design, proper materials of construction, & expert applications engineering. To understand what design criteria is to be looked at, it is important to know the basic components of flow through a PC pump. The equation to determine the flow through a PC pump is as follows:
Flow = 4 x e x D x Pitch x N
where: e = eccentricity of the rotor
D = diameter of the rotor
Pitch = pitch length of the stator
N = pump’s rotational speed
The ratio of D/e is one of the major things looked at in design optimization. It is the goal of pump design to minimize the pumps internal speed. At a D/e ratio of 4, this internal speed is minimized but it is not always possible to achieve this ratio. The pump pitch can also be selected so as to minimize the internal velocity. Pump pitch selection is a function of pump "lift" requirements. With the Pitch/D plotted against the pump’s internal velocity, the higher the D/e ratio, the higher the internal velocity.
To summarize the optimization process, the pump geometry can be optimized in order to reduce wear. Pumps are typically designed for "flow" & "depth" requirements.
As for pump materials of construction, the two components that are to be considered are the stator elastomer, & the rotor coating.
Stator elastomers are broken down into two categories;
Physical Properties
Wear Resistance
- Imbed vs. abrade
- RM103® Elastomer
The physical properties are made up of the various ingredients that make up the "recipe" for each elastomer compound. The amounts of each ingredient are determined by what duty the elastomer will be used for. By changing the amounts & types of compound ingredients, the elastomer’s physical properties are altered. It is a give-&-take as far as the properties are concerned. Sometimes to get one characteristic for the elastomer, another may suffer. When looking at wear resistance, it is always preferable to have the elastomer allow the abrasive particles temporarily imbed into the surface rather than abrade the surface. This then, deals with the elastomer’s resilience & hardness (often referred to as durometer). R&M’s RM103® attempts to maximize the physical properties as well as wear resistance for abrasive applications.
Rotor coatings are dependent upon:
Manufacturing Process
Wear Resistance
- MOYNO® Ultrashield® Coating
The focus here will mainly be on the rotor coating after manufacturing. A majority of rotors manufactured use Hard Chrome Plating (HCP) for abrasion resistance while still keeping a low
co-efficient of friction to minimize torque. HCP is very versatile but does have it’s limitations. For this reason, R&M has come up with their Ultrashield® coating.
Expert applications engineering is the last critical part of a successful application of a PC pump in an abrasive application. Proper pump selection is critical in every application, but when abrasives are concerned, an optimized design is the only way to assure that the pump’s internal velocity is minimized, the design rations are maximized, & the proper materials of construction are utilized. In pump selection, there are two main things to keep in mind:
Rotor / Stator Fit
Pump RPM & depth Capabilities
- select a pump that meets the flow & depth requirements while keeping the
lowest internal velocity.
Elastomer Selection
In order to verify the design factors, component fits, elastomers, & rotor coatings relative to abrasive applications, it is necessary to conduct laboratory or field tests. Laboratory wear tests are generally not reliable due to the fact that the acceleration of normal wear process often introduces new wear mechanisms. Field tests are more accurate however the data is often difficult to collect. In light of this, a field test was conducted at Wascana Winter to compare pump life between two elastomers in an abrasive application. The two R&M elastomers that were compared were Std. NBR & RM103® for ten wells. After completion of the test, it was seen that the RM103® showed longer life when compared to Std. NBR or Buna, had lower torques, fewer rod breaks, & had lower lifting costs due to lower power consumption.
In conclusion, it can be said that optimization of pump design is critical for best wear performance, the selection of the stator elastomer significantly influences the wear life of the pump, and expert application expertise is critical in dealing with wear problems.
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