OVERSPEED SAFETY DEVICES

OVERSPEED SAFETY DEVICES

Mechanical overspeed trips depend upon the centrifugal forces developed by the engine and must be maintained in good working condition. A faulty overspeed device can endanger not only the engine but also the personnel. The engine could explode or fly apart because of the uncontrolled speed.
The engine instruction manual contains information as to the speed at which the overspeed device is designed to function. Most overspeed trips are adjustable. Before making any changes in the adjustment of the overspeed trip, you must determine the cause. If the engine did not trip out, was it for some reason other than the action of the element of the overspeed trip? You should first check the accuracy of the tachometer and then test the overspeed trip. Remember that all spring tension and linkage adjustments to an overspeed are critical. Instructions for these adjustments are found in the manufacturer’s instruction manual. You MUST follow these instructions!
Hydraulic overspeedtrips are extremely sensitive to dirt. Dirt or lacquer like deposits may cause the trip to bind internally. The speed-sensitive element and all parts of the linkage and mechanisms incorporated with the speed-sensitive element must be kept clean. When painting around the engine, you must avoid allowing paint to fall on joints, springs, pins, or other critical points in the linkage.
The overspeed trip will not function properly if parts arc bent, badly worn, improperly installed, or dirty, or if their motion is restricted by some other part of the engine. In some situations the driveshaft of the overspeed trip may be broken; this would prevent rotation of the flyweight and the overspeed trip. Insufficient oil in the hydraulic trip may be another source of trouble. You should maintain a proper oil level as specified by the instruction manual. The following are some general procedures you should follow to keep the overspeed safety devices in proper operation:
- Keep the overspeed trip and its linkage clean.
- Remove the source of binding.
- Replace faulty parts.
- Maintain a proper oil level in the hydraulic overspeed trip.
- Adjust the speed-sensitive element according to the instruction manual.
- If the trip has been damaged, replace it with a spare and completely rebuild or overhaul the damaged one according to the instruction manual.
Test overspeed trips and governor mechanisms once each quarter and after each major engine overhaul. To verify if the safety device is in proper working order, overspeed the engine. When you are making this test, use a tachometer to check the speed at which the overspeed mechanism will operate. These safety devices should operate at the speed specified in the engine instruction manual. If this information is not available, the following values should be used for the test:
l For large, slow-speed engines, the value is 107 percent maximum-rated speed.
l For high-speed engines, the value is 110 percent maximum-rated speed.
If there is any irregularity during testing, stop the test and check the overspeed safety device and correct the problem before continuing the test procedure.

SUMMARY

This chapter has presented several common facts in maintenance, repair, and overhaul of speed controlling devices. Maintenance personnel must secure the appropriate manufacturer’s instruction manual. No repair, maintenance, or overhaul of these precision pieces of equipment should be made until the appropriate manual is obtained. You must read, understand, and strictly follow the instructions from the manufacturer. Be sure to pay particular attention to any safety precautions given in these instructions.