Practices That Have Been Used to Reduce Shutdown Time

The main tool used to shorten shutdown time is the turbine stress monitor. The stress monitor is used to determine the fastest shutdown possible, staying within the stress limits of the turbine casings and rotors. The stress monitor uses finite element analysis to determine the highest stressed component of the turbine and keeps the stresses at that location within their allowable limit. Stress monitoring is done by controlling the thermal ramp rates of the turbine during

Unit Shutdown

If there is no turbine stress monitor installed on the turbine, the manufacturer’s operating instructions must be used to shut the turbine down. These instructions give limits for reducing load when bringing the turbine off-line. Many of the manufacturers’ instructions are

conservative, as a result of the time during which they were written. Until the 1980s, the turbine manufacturers were not using finite element analysis routinely to design turbine components.

This means that there can be areas where the actual stresses referred to by operating instructions are below the allowable stresses for the turbine.

When bringing the turbine down for a planned maintenance inspection, the critical time is from full speed no load until the unit can be taken off turning gear. This period is usually about 48 hours as a consequence of the large mass of metal involved and the amount of insulation on the turbine casings. The length of time to cool the turbine until taking the unit off turning gear can be reduced by cooling the turbine rotors and casings while shedding load from the turbine. If the turbine is brought off-line while the casing temperatures are high, it will take much longer to get to the point where the unit can be taken off gear than if the casings were cooler coming off-line.

This cooling period can be reduced substantially by reducing the temperature and pressure of the steam during the shutdown process. Cooling of the casings is performed by opening the high-pressure steam control valves to wide open, lowering the main and reheat steam temperature and pressure, and letting the load decay. When the desired temperature is reached, the remaining load can be removed by closing the steam control valves. It is important to note the amount of

superheat in the steam to keep water out of the turbine casings. The turbine manufacturers give allowable limits for steam temperature and pressure for shutting the turbine down.

Some operations that can reduce the turbine casing temperature after the unit has been placed on turning gear are:

• Remove control valves from valve chests.

• Remove video inspection port covers.

• Remove reheat turbine crossover pipes.

• Remove reheat turbine crossover pipes.

• Open shell drains

• Ventilate the casings at these locations using forced air

Air cooling of the turbine casings can reduce the turbine cooling period, but it must be monitored to ensure that the turbine metal temperature ramp rates are not exceeded. Areas to monitor while air cooling the turbine are:

• Casing temperature

• Casing expansion

• Differential expansion

• Eccentricity

Unit Shutdown

Steam turbine manufacturers often include cooling curves that give the rate of cooling for the high-pressure section under normal operating conditions. These cooling curves should be used to determine the amount of time needed to cool the casings after the unit is tripped. The curves are generally found in the operations section of the turbine instruction manual.

3.6.1 Overspeed Trip Testing

The turbine overspeed trip test is necessary to ensure that the turbine will not reach a speed that will damage the turbine or generator in the event that the load is lost while the steam admission valves are open. The overspeed trip test allows the turbine to reach speeds up to 112% of rated speed on some units. Rotor speeds above 100% of rated speed create high stress and can result in damage to the rotating components during overspeed trip testing. The main requirement for an overspeed trip test is that the rotor bore temperature is above the fracture appearance transition temperature (FATT). Damage can occur if high stresses are imposed to rotors below the FATT.

Before a turbine overspeed trip test can be performed, all bore locations on the high temperature rotors must be at least 450°F (232.2°C). The cold end of the high temperature reheat rotors is the exhaust end where the steam enters the crossover; therefore, the crossover temperature must be at least 450°F (232.2°C) before overspeed trip testing the turbine.

The overspeed trip test should be performed during shutdown while the rotor bore temperatures are still high. This is done at full speed with no load on the turbine. After the testing is complete, the unit can be brought down for maintenance.

If the overspeed trip test is performed on a cold turbine, 25% load must be carried for a minimum of four hours to warm the high-temperature rotor bores to allow overspeed tests be performed.

One way to reduce the outage duration is to perform the overspeed trip test while the unit is down for maintenance. This test can be performed by removing the shaft extension where the overspeed trip is located and having the overspeed trip set under simulated conditions. This requires that the shaft extension be spun up on a special lathe and have the trip speed recorded.

This requires the shaft extension to be at the design operating temperature in an oil bath. The simulated overspeed trip test will keep the entire turbine train from undergoing the high stresses involved by performing an actual overspeed trip test, as well as saving time during a shutdown of the turbine.

3.6.2 Electrical Trips vs. Mechanical Trips

Electrical control systems have three or more speed pickups located in the front pedestal that measure the speed of the turbine rotor. By controlling the speed with electronic speed pickups, the electrical control systems do not require mechanical devices and are, therefore, more reliable and easier to maintain. The electrical control system can measure acceleration as well as speed, and electrical controls are used for overspeed protection of the turbine.

Unit Shutdown

Mechanical control systems on older steam turbines can be upgraded to electrical control

systems, and the controls can be more reliable. Electrical control systems can control speed, load, and overspeed much more accurately than mechanical control systems. Mechanical control systems have governors, speed relays, pre-emergency governors, gear drives, cup valves, bellows, and pilot valves. These components, with their connecting levers and linkages, require extensive maintenance. Electrical control systems have fewer moving parts and, consequently, require less maintenance. It is easier to simulate an overspeed trip with an electrical control system, which makes periodic overspeed testing of turbines much easier. Valve tests are also easier to perform with electrical control systems.

Although electrical control systems are better than mechanical control systems, they are expensive to retrofit onto older turbines. The addition of electrical controls to older turbines is often justified by more accurate speed/load control, lower maintenance costs, ease of testing, and better reliability.

3.6.3 Boiler/Reactor Feed Pump Turbine Controls

The feedwater systems in many nuclear plants use steam turbine-driven feedwater pumps, which are necessary to supply a large flow of feedwater at the required pressure with high reliability.

This type of pump driver in the feedwater cycle effectively uses the plant steam cycle to economically drive a large horsepower pumping operation. To establish good maintenance on the turbine/pump is important, considering that nuclear records have indicated that turbine-driven feedwater pumps are a high contributor to plant derates and forced outages. These turbines use one of three control systems: mechanical/hydraulic control (MHC), electrical/hydraulic control (EHC), and electronic digital control (EDC).

The EPRI report Feedwater Pump Turbine Controls and Oil System Maintenance Guide, 1003094, [12] is available to provide maintenance information for the feedwater pump turbine MHC oil systems. The guide is intended to assist nuclear power plant maintenance personnel in troubleshooting and maintaining the MHC system. It contains a reference for understanding the control philosophy, technical descriptions for the different elements within the control system, and also routine and preventive maintenance guidance to improve reliability.