Emerson Process Management

Reheat Attemperator Spray

Application Discussion
AD107
June 15, 2003

In order to increase the thermal efficiency of a steam generator and improve the overall heat rate of a unit, what is called reheat is incorporated into the power cycle. Reheat involves steam that is extracted from a turbine prior to complete expansion. This extracted steam is sent through the boiler again to gain more heat before being admitted into another turbine. In most plants, this involves removal of exhaust steam from the high pressure (HP) turbine, which is sent to the reheater prior to injection into the intermediate pressure (IP) turbine. The exhaust of the IP turbine is then injected into the low pressure (LP) turbine to expand completely prior to entry into the condenser.

What is called double reheat is also possible, but is more common in supercritical once-through units. This process reheats the exhaust from the IP turbine prior to injection into the LP turbine. As with many processes, this improves thermal efficiency, but additional reheat stages provide lesser and lesser gains on overall efficiency while still impacting the capital expense.

As with any superheated steam cycle, the temperature of the superheat needs to be monitored to ensure that the temperature does not exceed the material limits of the steam turbine. Temperature control is usually achieved by admitting a fine spray of water into the steam line through what is called an attemperator or desuperheater. The attemperator utilizes water from a separate control valve and incorporates a nozzle that creates a mist that mixes with the process steam, thus lowering the steam temperature. This process, however, reduces thermal efficiency as it takes heat away from the process steam.

In most coal fired power plants, the reheat attemperator is installed in the cold reheat piping between the outlet of the high pressure turbine and the reheat inlet header. A feedback control scheme tied back to the spray water control valve is used to control the temperature of the hot reheat steam. In combined cycle plants, the reheat attemperator is installed between passes in the reheat section with the most common installation point being between the two sets of reheater tube banks.

Most boiler manufacturers design their boilers so that reheat spray is not necessary at full unit load. In practice, however, operation at other loads, transients and slagging or fouling of the reheater tubes will make the use of spray necessary during most operating loads.

The reheat attemperator uses water pulled from the main boiler feedwater line. In some cases the spray water may be pulled from an interstage bleed in the boiler feedpump. Because the spray water is taken from the main feedwater line and the cold reheat is at a much lower pressure, concerns arise over the specification of the spray water valve.

The cold reheat pressures vary between 450 and 600 psig while the spray water pressures range between 2400 and 3500 psig. The inlet feedwater pressure can vary depending if the water is pulled from an interstage (1600 – 2500 psig) connection in the feedpump. Because of the high pressure drop taken across the control valve, anti-cavitation trim is generally required.

Not only is anti-cavitation trim required, the flow rate requirements are fairly low because the spray water is generally required at reduced loads and transient conditions. Couple this with the need for tight shutoff to protect against water carryover into the turbine, the reheat spray water valve selection needs to be explored thoroughly.

Fisher recommends the use of an anti-cavitation trim set that can control to very low flow rates. This is accomplished through the Cavitrol III/MicroFlat solution. This solution incorporates the staged pressure drop anti-cavitation design of the Cavitrol trim along with the low flow MicroFlat valve plug. This solution also provides tight shutoff by incorporating a matched plug and cage design along with inherent deadband that is built into the shutoff/throttling areas of the trim.

 

 

Product Bulletin: Fisher Cavitrol III One-, Two-, and Three-Stage Trims

Product Bulletin: Fisher Micro Trims for Globe and Angle Valve Applications

 

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