Power generation valve instability can ultimately lead to intentional unit trips , or reduced output . Often during start up , shut down or low loads , when trim in a pneumatically operated valve takes a beating , violently slamming closed when cracked open due the compressible nature of air , hysteresis , and excessive dead time . Maintenance frequency can be drastically increased for trim replacement every 6-8 months . If reheat spray valves become damaged to where they cannot control temperature within a specified range , the unit will usually be brought down to 50-60 % power where attemperation is not needed , and remain at that level until a rebuild can be performed . Catastrophic tube failure is also a result of leaking valves , adding too much water that carries over into the superheater and reheater during low loads and ramp scenarios . To get around spray valve inadequacies , operators program the control system to prevent low flow operation , which limits plant flexibility . Sometimes electric actuators with induction A / C Motors can also be found in attemperator applications . These types of actuators are not well-suited for continuous duty , as they are limited in starts / stops per hour . In this scenario , the plant must decide to either ‘ detune the loop ’ for much lower controllability to allow the actuator to survive , or live with the impact of high maintenance requirements due to frequent actuator rebuilds and replacements .
Before
The solution
REXA Electraulic™ Actuation offers a responsive and repeatable control solution for superheat attemperator spray valve applications . Designed for continuous modulating service , the patented self-contained , closed loop , hydraulic circuit provides stiff , stable control in the harshest conditions ( -40 ° C to 121 ° C ). The closed hydraulic system requires no filters , and has no requirement for any oil-based maintenance . A dedicated microprocessor control enclosure operates the drive unit , and is usually located in a convenient area . Set-up and calibration is made simple through a membrane key pad on the enclosure cover . Performance is unmatched in the industry with adjustable dead-band to 0.05 % of stroke , resolution of < 2 seconds ) with no hysteresis or overshoot .
The results
The most noticeable improvement that a combined cycle plant operator will immediately see , is precise spray water valve control on the low end of valve travel . This allows the plant to operate at lower loads efficiently , and without the risk of large temperature swings . With high resolution and repeatable positioning , REXA Actuators allow the steam temperature at the HP and LP turbines to be set closer to design than ever before . An increase of 5.5 ° C equates to approximately a 0.1 % increase in turbine efficiency for a standard condensing turbine .
After
About the company VMCS specialises in valves and actuators for heavy and / or precision applications . They focus their products on the energy , chemical and petrochemical industry and offshore applications . They also offer various services , such as performing calculations for clients to control valves and actuators , and reparing REXA drives . www . valve-world . net Valve World November 2022
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