Correct operation of attemperators
Attemperators have been a key component of combined cycle power plants for many years . Back when the units were base loaded , spray water control was a non-issue .
Text by VMCS
In today ’ s world , plants are ramping up and down daily , putting a strain on steam temperature control regulation . Attemperators are being cycled more than ever before , introducing unforeseen problems in the Heat Recovery Steam Generator ( HRSG ) steam system . Service is demanding for this equipment , as hardware can remain at elevated temperatures for extended periods of time , and then be cooled quickly when spray water is required . Located between the primary and secondary superheaters and reheaters , attemperators are the final control element before the HP , IP , and LP turbines . They are responsible for controlling steam temperature in accordance with steam turbine design guidelines . Most plants use an attemperator control logic program for tighter temperatures and to prevent overspray , and subsequent carryover of water into the secondary superheaters . It is critical that proper temperature is maintained during plant start-up and shutdown to prevent thermal damage to superheater and reheater tubes , as well as downstream steam piping .
Key to success
The key to success for precise temperature control at the turbine , is to provide the exact amount of water needed to reach design conditions for optimal efficiency . Even small fluctuations in spray water flow will cause an operator to lower steam temperature set point to the turbine , reducing steam cycle efficiency and electrical output . During low load operation , it is critical for precise control at valve travels 10 % or less . Reducing or eliminating spray valve trim erosion , especially in the HRH lines , plays an important role in predictable temperature control .
The problem
Maintaining accurate spray water valve position in a cycling plant can be a challenge . Excessive temperature variability caused by
92 Valve World November 2022 www . valve-world . net