Mechanical Maintenance Activities For Steam Turbines

Advertisement

MECHANICAL MAINTENANCE ACTIVITIES FOR STEAM TURBINES

Advertisement 

• Proper maintenance is essential to the continuous and efficient operation of the turbine. Items requiring maintenance should not be permitted to accumulate but rather should be taken care of as soon as the trouble arises. The exception, of course, is the general overhaul, which is scheduled in advance and during which careful inspection and examination are made of the complete machine. Breakdown or emergency maintenance is best avoided by using a preventive-maintenance schedule. 

• It should be remembered that timely replacement of parts and adjustment for wear may prevent a shutdown and save costly repairs. 

MAINTENANCE INSPECTION:

• An annual inspection of a turbine would consist of examination of blading and nozzles for wear, erosion and corrosion, cracks, and scale deposits. examination of main bearings and the thrust bearing for wear. inspection of clearance between stationary and moving parts and packing clearance and thorough inspection of the oil pumps and the various governor mechanisms. 

• Because turbines remain in service for longer periods of time, factors such as thermal stresses, chemical attack from steam impurities, erosion, and corrosion all have become real concerns to an operator. Operational situations such as process and load swings, plant upsets, and extended turbine shutdowns with accompanying corrosion damage and occasional boiler carry-over can affect turbine performance and its maintenance costs. 

• Maintenance and operating problems on large turbines that are found in electric utilities are often attributable to 

1. Operating the turbine beyond its design life 
2. Operating at continued outputs higher than design 
3. Increased cycling operation on the turbine

Advertisement 

WEEKLY INSPECTION:

• Inspect the turbine exterior. Keep control-line valves free from dirt and grease at the guides. Clean and oil the spindle of the throttle valve and connections between levers of the governing mechanism, using a light oil to prevent gumming repair all oil leaks. Check all vibration measurements. 

MONTHLY INSPECTION:

• Inspect the oil reservoir to determine if any sludge is accumulating or any water is getting into the oil. Check the oil strainer. Check the operation of the governor and automatic tripping devices and vacuum breakers. Observe the governor operation to determine how it acts and whether it will carry the maximum turbine load. If the turbine is operating at condensing, inspect the condensing equipment and check the vacuum to see if it is the best obtainable with the equipment and circulating-water temperatures. Check and verify instrumentation. 

YEARLY INSPECTION:

• Check turbine-generator alignment. Dismantle turbine and auxiliaries check for corrosion, dirt, scale, and fouled passages and for encrusted, corroded, and eroded blading. Check for erosion on casing and nozzles. Check all valves and seats. Check governor knife edges and bearing blocks and the emergency tripping device. Check vacuum pumps and ejectors. Check for air leaks.

Advertisement 

TESTING FOR STEAM TURBINES:

• A test should be made annually to check the spring calibration and to determine the speed above normal at which the oil-tripped emergency governor will operate. In operation, the emergency governor should be tested weekly. Testing the emergency governor should be done after the turbine has been placed back in service or approximately once a month. 

1). Attach a vibration tachometer to the turbine (or observe the permanent tachometer) so that it can be read by the person operating the throttle valve; the person at the other end of the turbine should be provided with a hand tachometer. 

2). Operate the throttle valve slowly to bring the turbine up to the required speed, where the governor takes over. 

3). Disconnect the governor beam from the vertical connecting linkage. Hold it firmly to prevent it from closing the controlling valve. Lift it slowly, permitting 

the control valve to open and the speed to increase slowly until the tripping point is reached on the emergency governor. Do not permit the speed to exceed the safe limit established. 

4). Take tachometer readings to check the speed at which the emergency governor operates. 

5). If and when the emergency governor acts, check to see that the throttle valve trips and speed decreases. If the emergency governor fails to operate, repeat all the previous check points to determine what the problem might be. 

Advertisement

6). Trip the emergency governor two or three times to make certain speed check is correct and all parts are in working order. 

7). Test the oil-tripped governor when shutting down the turbine by: 

a). Running the turbine without load, throttle valve wide open, and turbine under control of the operating governor. 

b). Opening the oil valve in the pipe supplying oil to the nozzle and checking that the governor operates instantly when the oil is applied and the throttle 

valve trips, at which time the speed should drop, indicating the throttle valve has closed. 

c). When the governor trips, shutting the oil valve and determining whether the governor returns to normal; then resetting the tripping mechanism. Never keep a turbine in service unless you are certain that the emergency devices are reliable. A complete record should be kept of the changes and adjustments made when tests are conducted. 

Advertisement