Gene Vogel
EASA Pump & Vibration Specialist
“Submersible Pump Seal Fluid Guidelines,” the November 2023 Currents article, addressed options for seal oil in submersible pumps. In addition to seal oil, some submersibles have an oil filled stator cavity. The oil in the stator cavity very efficiently transfers heat from the stator winding and core, and especially from the rotor, to the pump casing where convection carries it to the pumpage in which the pump is submerged. While the specifications for seal oil are fairly flexible, stator cavity oil has more constraints.
A primary constraint on oil filling any stator is electrical conductivity. The ability of a material to resist current flow may be referred to as its dielectric strength. Obviously, we want oil in the stator to have high dielectric strength. Transformer oil is the product recommended for use in the vast majority of stators. The primary characteristic of transformer oil is its high dielectric strength, which is developed in the refining process by enhancing the naphthenic content of the oil (vs. paraffinic content) and eliminating moisture from the oil. In addition, some types of transformer oil (Type II) include anti-oxidation additives. These are helpful in transformers which breathe with heat cycles pulling in external air and oxygen. A third characteristic of transformer oil is low viscosity, typically in the range of 8-10 cSt (one centistoke equals one hundredth of a stoke) at 40°C. The low viscosity enhances thermodynamic circulation induced in transformers.
But the characteristics of submersible pump stators and transformers are different. Submersible pump stators don’t breathe; they are sealed and submerged, so oxidation is not a concern. And the dielectric strength requirements are much less on a low voltage stator than on a medium or high voltage transformer. So, these questions sometimes arise: Is transformer oil the best choice for submersible pump stators? and Could turbine oil be substituted? The answer is yes or no. Yes, there may be special circumstances where turbine oil could work; no, it is not a recommended practice.
If a turbine oil had the proper viscosity, it could certainly cool the stator and rotor. The most common turbine oil in service centers is ISO 32 (32 cSt), too viscous for use in stators. An additional concern for submersible pumps, especially those in sewage applications, is explosion proof ratings. Many sewage pumps have Factory Mutual (FM) explosion proof ratings. Since FM certifies the machine design, not specific machines, these pumps can be repaired in EASA service centers if the design is not changed. That often requires original equipment manufacturer (OEM) replacement parts and off-spec oil in the stator would be a design change. Underwriters Laboratories (UL) certified pumps have more stringent repair restrictions.
An additional concern is the compatibility of oil with the insulation and sealing components of the submersible pump. Electrical insulation vendors can provide compatibility information when rewinding a submersible pump stator. But those materials in an existing winding would not be known. The consequences of an oil leak into the pumpage should also be considered. Some applications require food grade cooling and lubricating fluids.
For a non-EX-rated pump, the proper viscosity turbine oil could be substituted for transformer oil in an emergency if a customer agreed. But it is not a practice EASA Technical Support would recommend. EASA’s recommendation is to use the oil specified by the machine manufacturer. Of course, members can contact EASA Technical Support for help with any specific situation.
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