PRODUCTION TECHNOLOGY: Keeping cantaminants out of stator key to long submersible motor life

Submersible motor use in offshore platform water pumping applications continues to grow. These motors have integrated the features that can tolerate extreme conditions used in oil field applications into a more conventional water pump type of motor.

In order to address the demands placed on submersible motors by offshore platforms, Franklin Electric has endeavored to understand the conditions that contribute to failure and has developed submersible motors to meet those conditions. Because these motors are placed under the ocean's surface in a relatively stable environment, they are immune to the weather and environmental factors that plague hollow shaft motors driving conventional line shaft turbines (LSTs). Submersible motors and pumps eliminate the drive shaft and bearing systems of LSTs, thus reducing the mechanical complexity and required maintenance. Submersibles also do not require structures to enclose them and do not produce surface noise.

Standard water well types of submersible motors are water (water/glycol) filled and rely on water as the internal lubrication for the motor. These motors are extremely reliable when applied within their design limits of temperature, hydraulic loading, and power requirements. They are designed for fairly constant operating pump flow and pressure and expect fairly constant pump thrust.

Unfortunately, these motors are often used in applications exceeding the design criteria resulting in failures, and the advantages of submersible motors are lost or quickly forgotten. Following are typical problems in offshore platform applications that result in the failure of motors.

Temperature

A very common problem affecting submersible motors is over-heating. Several causes for this are pumping hot water, overloading of the motor by the pump, loss of cooling flow past the motor, marine growth or scale buildup, and frequent motor starts and stops.

Submersible motors must cool themselves somehow. This is almost universally accomplished by transferring the motor's internally generated heat to the water that is flowing past the motor and into the pump. Most standard water well motors are designed to do this, but add little safety margin for the problems listed (safety margins add cost).

In submersible motors, the thrust bearing supports the weight of the water column being lifted by the pump and the thrust produced by the pump itself. In standard water well motors, this thrust bearing is a water lubricated "Kingsbury" type of bearing. A very thin film of water between the main elements of the thrust bearing provides lubrication between the two bearing surfaces. If the motor overheats for any reason, this water film can approach the boiling point. If it boils, the lubricating film is lost. At this point, the bearing surfaces come into contact with each other, and rapid heating takes place. Catastrophic failure of the thrust bearing is likely to occur.

Franklin is using an approved food grade mineral oil as the internal lubricant for their submersible motors. By doing this, the internal boiling point of the lubricant is raised far above that of water (250°C vs. 100°C for water). This provides protection of the motor bearing surfaces at very high temperatures and prevents the catastrophic bearing failure typical in overheated submersibles, even if the motor is overheated.

Stator failure is another problem that occurs when motors overheat. Water-filled submer-sibles use a PVC insulation, wet wound, which insulates the copper windings while immersed in water. This wire usually has a maximum usable temperature of between 70°C for standard motors to about 100°C for higher temperature motors. Once these temperatures are exceeded, the insulation system is damaged, and a winding turn-to-turn, winding phase-to-phase, or winding phase-to-ground fault becomes likely. Once these faults develop, failure of the motor is unavoidable.

With non-conducting mineral oil, the stator windings can be wound from a more conventional high temperature magnet wire and varnished. The stator is then left open to the bore of the motor and filled with mineral oil. The mineral oil acts to cool and provide dielectric insulation for the windings, which can operate at 200°C. This much higher allowable winding temperature of the motor serves two purposes:

• Allows the motor to run in high temperature applications (water up to 90°C).
• When used in cooler water, a large safety margin for unintended misapplication is gained.

The problems such as overloading, low cooling flow, scale or marine growth buildup, and rapid cycling are much less critical than with standard water well submersibles. The motor should perform well with marine growth or scale buildup. Any scale or coating tends to insulate the motor, thus keeping it from passing internally generated heat to the cooler water flowing past it. Again, because the motor is designed to run much hotter internally than standard water well motors, it can tolerate surface buildup, which insulates the motor. The reduced heat flow from high buildup is thus less of a problem.

Hydraulic loading

Another problematic area for submersibles in offshore applications is hydraulic shock loading or water hammer. Water hammer occurs when a rapidly moving column of water encounters an obstacle or suddenly changes velocity. The platform use of multiple pumps on a common supply manifold delivering water to several on/off applications, especially without pressure tanks, is a prime cause of water hammer. When a pump turns on or off, water hammer is generated. Additionally, when any kind of valving is actuated, water hammer can occur (fast-acting electromechanical valves can be the worst).

Check valves in the pump discharge string and at the surface plate are recommended by all manufactures to reduce water hammer. Unfortunately, check valves can be of the wrong type (swing-loaded vs. spring-loaded), can become corroded over time (spring corrosion is a common problem), or are simply not used.

When water hammer occurs, there is a sudden downthrust transmitted through the pump onto the thrust bearing of the motor. This rapid and generally repeated loading of the thrust bearing has been known to cause failures by overloading both the support structure and the lubricating film between the rotating and stationary elements of the bearing.

The squeeze-film bearing theory shows that the ability of the bearing to withstand this rapid transient load without the rotating and stationary faces coming into contact is raised by both increased film thickness and increased viscosity. Most standard water well motors use a fluid film type thrust bearing running made to operate with water as the lubricating fluid. The motors incorporate rugged oil-lubricated equalizing thrust bearings. As discussed above, this oil lubrication increases the ability of the bearing to resist a pressure pulsation by increasing both the viscosity and the film thickness over water-lubricated bearing values.

Often, it is the bearing support structure that suffers the most damage during water hammer. The motors should incorporate components that have been tested at loads much higher than they're rated, resulting in a bearing with a greater safety margin than in most standard water well motors. While not recommended for continual use at loads above rating, this heavy duty thrust bearing will tolerate overloads much better than standard water well motors. This is very advantageous as pumps wear and the hydraulic balancing of the impellers is lost (some pumps use hydraulically balanced impellers to decrease the downthrust). This can cause the thrust loads to rise above the motor design values. Standard water well motors can fail during these conditions.

Motor seals

The seals at the shaft of the motor keep salt water, sand, etc. from getting into the motor. Both water-filled and oil-filled motors need to do this in order to keep abrasives and seawater out of the internal bearings of the motors.

Standard water well duty submersible motors use a single mechanical seal. This seal can be of carbon/ceramic or silicon/carbide. Silicon carbide is considered a premium seal and is a harder seal than carbon ceramic. It is very useful in sandy applications.

The use of two mechanical shaft seals provides a redundant sealing system that assures a good seal even under extremely adverse abrasive conditions common to seafloors and other applications. A silicon carbide outer seal is used for its excellent abrasion resistance. A carbon ceramic inner seal is used as a backup to assure that no water enters the electrical and bearing section of the motor.

The space between the two seals is filled with the same mineral oil that is inside the motor, but it is kept separate in the Franklin special duty motor line, for example. Having two oil chambers keeps the seal system oil separate from the oil that fills the stator area, assuring integrity of the stator oil (keeping contaminants out of the stator area is a key to long motor life).

Voltage

Submersible motors, like any electric motor, require a good voltage supply at the motor terminals. Leading causes of submersible motor failure are under-voltage, over-voltage, and voltage spikes. Voltage problems on platforms are typically caused by:

(1) Sizing the drop cables (supply cables) too small

(2) Using generators that supply low or high voltage

(3) Voltage spikes, especially when changing generators.

Ohms law is always in effect in electrical circuits (V=I x Z). Voltage equals the current flowing in a circuit multiplied by the impedance (the resistance of a cable is usually the majority of the impedance). As smaller diameter drop cables are used (they are cheaper), the resistance increases (less copper equals higher resistance). As the resistance of the lead goes up, the voltage drop in the lead increases as well for a given operating current.

For a fixed supply voltage from a generator, as the voltage drop in the lead goes up, the terminal voltage at the motor must go down. As the voltage at the motor terminals falls, the motor must have more current to produce the same horsepower (which induction motors want to do). Eventually too much current is flowing for the size of winding wires used in the motor, and internal overheating begins to take place. This leads to failure of the motor.

Frequently, too, platform voltage is set higher than most submersible motors' rated voltage. Unfortunately, over-voltage also requires higher current supply as well, as both the efficiency and power factor are compromised. This results in higher temperatures and shorter motor life for most submersible motors.

Voltage spikes

A very serious problem for all induction motors is voltage spikes. These spikes are typically very short in duration and high in voltage. Lightning, other motors turning off, or generator switching can generate spikes. On platforms with multiple pumps and motors running, this can be a serious problem. Each time one of these motors is switched off, the stored inductive energy in the magnetic circuit of the motor is dumped back onto the power lines. This creates a very short, but very high voltage spike.

These spikes are often of a greater voltage level than the insulation system of the motor and will burn through the insulation. Once this occurs, the motor has a potential site for an internal short, and current can flow in a path that it was not designed for. This will typically raise the temperature of the windings near the shorted spot, and further burning will take place until a catastrophic failure occurs. If the voltage spike is large enough and has enough energy (lightning strike), it can completely burn a hole in the case of the motor.

External surge protection mounted near the motor starter is recommended to protect against severe voltage spike problems. This tends to "clip" the voltage spike before it can travel down the cables to reach the motor. These surge protectors do nothing until the voltage reaches a certain "critical" value. At that point, they begin to conduct current and continue to do so until the voltage falls below the critical value.

Heat is the worst enemy of motor life and accounts for most platform motor failures. Although seawater is relatively cool, motors should be designed to run in much higher ambient temperatures (either 60° C or 90° C). This gives them the ability to absorb the extra heat provided by platform operation and allows them to survive these conditions much better.