Application Discussion
AD117
June 15, 2003

During startup, shutdown and load disturbances in a power plant, the boiler and steam turbine need to be isolated from one another. This is to protect the turbine from any water carry over, but also to protect additional plant equipment from large thermal transients. By isolating the boiler and steam turbine it is also possible to reduce fuel consumption during startup and shutdown. In the event of a load rejection, reloading times can also be improved through the turbine bypass system.

The turbine bypass system is designed to provide the quickest startup time by controlling both boiler pressure and temperature. Bypassing the steam around the turbine allows the steam to attain the desired qualities before being routed through the turbine. If the turbine bypass system were not used, the firing rate of the boiler would have to be reduced, which could lead to slagging and eventually tube failures. The bypass system allows the boiler to be fired at full capacitywithout introducing large thermal gradients in the thick walled components such as the boiler drum and any separators or flash tanks.

Figure 1 shows a conventional power cycle with single reheat capability. In order to protect the reheater tube banks from overheating, the high pressure steam is routed around the high pressure turbine into the cold reheat piping. The steam flows through the reheater tube banks and is then routed through the low pressure turbine bypass valve to the condenser. This type of arrangement ensures that the boiler can remain in operation during a load rejection, which keeps the safety valves from lifting, and keeps other equipment from tripping allowing immediate reloading of the turbine.

Figure 1: Single Reheat Turbine Bypass System

As the pressure in the cold reheat section of the boiler is designed for much lower pressures than the high pressure sections. The bypass valve must reduce the pressure to match that in the cold reheat piping. Temperature is another issue that must be addressed. The temperature of the high pressure steam must be reduced to match that in the cold reheat piping.

One of the main concerns in this application is the noise generated when the pressure is let down from the high pressure section (2000 – 3500 psig) to the cold reheat section (450 – 650 psig). The noise generated can lead to vibration that can damage the downstream equipment.

Another concern is temperature control. Since the temperature in the high pressure section is around 1000 degrees, the temperature must be dropped to match that typically seen in the cold reheat section (400 – 500 degrees). This protects the piping and equipment in the reheat section of the boiler and ensures that the steam temperature leaving the reheater will not be too high.

For the best efficiencies and overall plant heat rate, tight shutoff is recommended. If the bypass valve leaks, steam that could be driving the high pressure turbine will be lost to the cold reheat piping. Another concern is thermal gradients as the high temperature steam leaving the bypass valve will mix with the cold reheat steam. Because of the high pressures involved, poor shutoff will lead to significant valve damage in a relatively short amount of time.

There are several possible solutions to address this application. The most common device used is called a steam conditioning valve. This device incorporates both pressure and temperature reduction into one control valve. The steam conditioning valve controls the flowing pressure drop the same as any control valve, but controls the temperature via a spray water section at the exit of the valve.

Another alternative is a separate pressure reducing valve and a separate desuperheater downstream from the valve. This arrangement works well when space is a consideration allowing the user installation flexibility. The downside is that deadtime or lag can be introduced into the system because of the distance between the pressure reducing valve and downstream cooling section.

Fisher recommends the use of the Fisher CON-TEK CVX valve design that combines both noise and temperature control into one valve body. The valve is designed to withstand the thermal transients typical in this application. Included in this design is extensive research into thermal mixing of water and steam ensuring the best spray patterns and nozzle designs on the market.

For applications where installation flexibility is required, the Fisher design HP or EH with a downstream cooler or desuperheater is recommended. Noise abatement trim can be installed into the control valve to eliminate any damaging noise and vibration. The same type of cooler used with the CVX design can be installed a given distance downstream of the pressure reducing valve to provide the proper temperature control.

Fast acting actuation is required to protect the steam turbine in the event of a load rejection with most requirements ranging between one and two seconds. The Fisher pneumatic actuation system with the 2625 booster arrangement will meet stroking requirements of one second. In applications where faster acting times are required, electro-hydraulic actuation packages are available.

To maintain tight shutoff at high temperatures, Fisher recommends the use of the C-seal tight shutoff trim. This trim provides Class V shutoff at temperatures up to 1100 degrees. The true balanced construction minimized actuator force requirements and eliminates problems with installation, setup and removal.