Disaster recovery plans, procedures
In the wake of the natural disasters, maintenance professionals and motor repairers need creative solutions to speed the removal of moisture and contamination from thousands of swamped motors.
Natural events such as Hurricanes Katrina and Sandy occur with little or no warning. Besides immeasurable suffering, they often cause widespread flooding, leaving maintenance departments and motor repairers with the daunting task of cleaning muck and moisture from thousands of electric motors and generators. The shear volume of affected equipment means the process can take weeks, if not months, and requires special cleanup procedures for motors contaminated by saltwater, so it helps to have a recovery plan in place before disaster strikes.
Although the problems may be huge, you can get back in production as quickly as possible after a disaster by working collaboratively with service professionals and following a few tips that will make the cleanup more manageable. These include prioritizing motors and generators for repair or replacement, storing contaminated machines properly, and using special procedures to flush away saltwater contamination. Constructing temporary ovens on site or at the service center can also add capacity for drying the insulation systems of flooded motors.
Understanding the problem
The harm done to motors and generators by flooding extends beyond rusted shafts and contaminated bearings. Even brief intrusion of moisture can compromise the electrical insulation system, making the windings vulnerable to ground failures.
Saltwater flooding poses additional problems. Unless thoroughly flushed from the equipment before it dries, the residual salt will rust the steel laminations of the stator and rotor cores. It also may corrode the copper windings and aluminum or copper rotor cages. The result, predictably, will be subsequent motor failures after they are returned to service.
How to proceed
Begin by prioritizing motors by size and availability. Older, NEMA-frame motors are often good candidates for replacement with more energy efficient NEMA Premium models. The horsepower breakpoint will vary from plant to plant, depending on application, annual usage, energy costs, and other factors. But, considering the real possibility that your regular vendors may be backlogged with work, somewhere between 100 and 200 hp may be a reasonable place to draw the repair-replace line. By replacing those smaller motors with readily available energy-efficient models, you'll free up capacity for your service center to concentrate on the larger motors for which a replacement is not readily available.
Two ways to clean
Once you decide which motors to save, ask your service center to process those with open enclosures first. In cases of freshwater contamination, have them disassemble the motor and clean the stator windings and rotor with a pressure-washer. If the insulation resistance is acceptable after the windings have been thoroughly dried, the service center should apply a fresh coat of varnish and process the motor as usual (new bearings, balance the rotor, etc.). Windings that fail the insulation resistance test, after a thorough clean and bake cycle, should be rewound.
Saltwater contamination requires a more thorough cleaning process to reduce the possibility that salt residue will rust the laminations or corrode the windings. To accomplish this, have your service center clean the stator and rotor windings and insulation systems using the "Saltwater flush procedure" described below. For best results, the complete motors should be immersed in the freshwater tank before the saltwater dries, and left submerged until ready to dismantle.
For the same reason, do not disassemble contaminated TEFC or explosion-proof motors until there is room for them in the immersion tank. This will keep them full of water and prevent salt from drying on internal parts. If it will be a while before these motors can be cleaned, place them on their sides, with the lead openings up, and keep them filled with water.
Saltwater flush procedure
This procedure offers the best chance for removing saltwater from contaminated windings. As mentioned earlier, it works best if you do not allow the windings to dry first. The sooner the windings are immersed in the tank, the better the results.
The process is straightforward:
- Immerse stators and rotors in freshwater for 8 hours.
- Continuously agitate the water.
- Exchange water in the tank with freshwater at rate of at least 20 to 50 gallons per minute.
Tank construction. Select a dumpster or similar container that will hold enough water to completely immerse a number of stators and rotors and drill a drain hole of approximately 2 in. diameter near the top. Weld a pipe nipple to the drain hole and plumb to a suitable location as local ordinance permits.
Next, route a 3/4 in. or larger supply pipe into the top of the tank (roughly centered), down the inside wall, and across the length of the bottom. Cap the end of the pipe and then drill holes at a slight upward angle along both sides of pipe to serve as water jets. The hole size should be appropriate for the available water pressure, but no more than 1/8 in. in diameter. The more holes, the smaller they should be.
Flush procedure. Place the stators and rotors in the tank and fill it with freshwater. Process each batch for 8 hours, continuously exchanging the water in the tank at a rate of at least 20 to 50 gallons per minute. At the end of the cycle, remove and pressure-wash the stators and rotors, and then dry them thoroughly in a bake oven or temporary field oven.
Finally, test the insulation resistance to ground. If the test results are acceptable, have the service center apply a dip-and-bake varnish treatment before reassembling the motor. If the motor fails the insulation resistance test, bake it again and repeat the insulation test. Motors that fail the insulation resistance test a second time should be rewound. Per IEEE Std. 43-2010, the minimum resistance to ground is 5 megohms for random windings, or 100 megohms for form coil windings.
For most service centers, the bake oven is the single biggest bottleneck. Even the largest oven will only hold a limited number of motors, and the drying time for each batch can take 12 hours or longer. Imagine the backlog after a disaster with hundreds of motors to process!
Of course, it's possible (but not very efficient) to dry windings by draping larger motors with tarps and applying external heat sources. Another way is to dry the windings is to energize them with a welder or other dc power source. The drawback here is that someone has to monitor the current and winding temperature and periodically move the welder leads to heat all three phases evenly. Welding machines also have a duty cycle that is significantly shorter than the two or three days it might take to dry out a large motor.
A better way to increase baking capacity is to build one or more temporary ovens that can dry motor and generator windings safely and efficiently. This approach is especially useful for drying large stators, which take a long time to heat to the required temperature, occupy the entire oven, and delay the processing of other motors. If necessary, temporary ovens can even be constructed on site. This can save the time and labor required to remove the motor from service, transport it, and later reinstall it.
Materials. Energy-shield (the hard-sided foam insulation that home builders install between the exterior frame and siding/brick) and aluminum duct tape are ideal for building temporary ovens–no matter what size or shape you might need. A stock item at most construction-supply super stores, energy-shield has a layer of aluminum foil on both sides and exceptionally good insulating value (R-29) for its thickness. The 4 ft x 8 ft sheets are lightweight and easy to cut with a safety knife. They're also reusable–as long as you store them where they won’t be damaged.
Oven construction. For motors with very large frames, box the motor by placing the foam boards directly on the frame, including the top. Seal the joints with aluminum duct-tape.
Placing the energy-shield directly on the frame also minimizes the volume of air that must be heated. This further reduces drying time because the insulation minimizes heat loss.
Heat sources. To heat the temporary oven, force air through it from an alternate heat source. If you use a torpedo heater, position it to blow hot air directly into the center of the bore. Energy calculations for oven design are complex. For this purpose, 100,000 BTU per 1,200 ft3 of oven volume will be adequate to heat the oven and contents within a reasonable time.
Temperature control. For an accurate record of winding temperature, directly monitor the motor’s RTDs, if it has them. If RTDs are not readily available, use HVAC instruments or candy thermometers to monitor temperature in each quadrant of the oven. The key is to keep the heat uniform within the motor, and not to exceed part temperatures of 250 F (121 C).
Because heat rises, it might seem reasonable to open exhaust ports at the top to let it out. But as those familiar with old-fashioned wood stoves can tell you, the best way to control oven temperature is to open or close dampers (exhaust ports) near all four corners on both sides.
To raise the temperature at one corner, for instance, open that damper farther. The increased flow of hot air through that area will raise the temperature. The ability to regulate temperature in this way greatly improves the drying process as compared with traditional methods such as a dc source or tarps.
How long to bake?
The bake cycle should be long enough to dry the windings completely. If it's too short, you'll need to repeat the process. If it's too long, you'll waste both time and energy. If the windings has RTDs, 6 to 8 hours at 200 F (93 C) should be sufficient. For windings not equipped with RTDs, here's a foolproof way to determine how long the bake cycle should be.
All you need are two lengths of RTD wire or similar small lead wire long enough to reach out of the oven and a DC voltmeter capable of reading millivolts. With the wet winding on the oven cart, attach one lead to the stator frame and the other to a winding lead. Finally, connect the free end of each lead to the DC voltmeter. You can be sure the windings are completely dry when the voltage on the millivolt scale reaches zero.
This procedure is one that many service centers use when they have large rush jobs to process. It often cuts hours from expected drying times, even for normal work. It also reduces the chance of damage that might result from excessive temperatures.
How it works. Like the setup, the principle behind this procedure is fairly simple. The iron frame and copper windings function as two plates of a crude battery. Electrolytic action across the wet insulation causes current to flow. As long as the cell is “wet”, it produces voltage. When the “cell” is dry, so is the insulation.
Note: This procedure works for everything except some form coil VPI insulation systems. Some of these windings are sealed so well that they may exclude moisture from the insulation, keeping the “wet cell” battery from developing.
Although no one could have been fully prepared a disaster like Hurricane Sandy, or the record snowfall that soon followed, the procedures outlined here can help speed recovery should catastrophe strike again. In the meantime, you can use them to prepare (or improve) your disaster recovery plan, and to foster plant-service center partnerships that maximize uptime.
Chuck Yung is a senior technical support specialist at the Electrical Apparatus Service Association, Inc. (EASA).