Hydraulic vs. pneumatic actuators
The scenarios outlined above represent real problems that combined-cycle power plant owners and operators are experiencing today. They will become even more common as more plants are forced into daily cycling service for which they were not designed.
Selecting hydraulic actuators instead of pneumatic actuators for critical desuperheating valve applications is one way to address cycling-related problems. Since oil is incompressible, performing the same response calculations as before, but this time for a hydraulic actuator, yields much better results: a dead time of just 0.00164 seconds and piston jumps in increments of just 0.00423, or 0.0564% of span.
Switching from pneumatic to hydraulic actuators virtually eliminates the lag in response to a control signal change and reduces jump to an insignificant level. Hydraulic actuation systems can be tuned for very fine setpoint control (down to 0.1% of span). In general, they feature very fast stroking speeds, 100% duty modulating service, unparalleled frequency response (millisecond dead times), immunity to dynamic instability and friction, and almost immeasurable overshoot.
But there are downsides to going with hydraulic actuators. Conventional hydraulic actuators have a reputation for being maintenance and reliability nightmares, and they cost much more than their pneumatic cousins. What's more, hydraulic systems require motors to run 24 hours a day, as well as an extensive network of very high pressure hydraulic tubing and fittings that may leak. Plant owners and builders tend to avoid hydraulic systems for those reasons, preferring instead to specify advanced pneumatic positioner technology, regardless of its performance limitations.
To get an idea of the benefits of retrofitting, take a close look at Figure 3 (p. 55), which compares the performance of a typical hydraulic actuator to that of a pneumatic actuator with a smart positioner tuned for maximum response (shortest dead time). The test whose results are shown was performed with actuators tuned for identical stroking speed on valves with polytetrafluoroethylene (PTFE) packing. A pneumatic actuator can be adjusted for less overshoot, but doing so increases its dead time and makes its slowdown to setpoint begin earlier.
3. Out of sync. A hydraulic actuator like the Electraulic can be tuned for almost instantaneous response. A typical pneumatic actuator has an inherently longer dead time and responds more slowly to a setpoint change. Source: Koso America Inc.
Another possible way to avoid cycling-related problems is to select a digitally controlled hydraulic actuator optimized for low fluid usage. The Electraulic actuator (www.rexa.com) is a good example of this type of device (Figures 4 through 7). It uses 20 times less fluid (standard motor oil) than a typical central system; it has no separate pumping systems, reservoir tanks, or high-pressure hoses; no fluid maintenance or filtration is required; and its motor(s) only operate when a position change is required. Sounds almost too good to be true.
4. IP attemperator retrofit. Retrofit of a conventional hydraulic system actuator (top) to an Electraulic actuator (bottom) on the IP attemperator of a typical combined-cycle plant. Courtesy: Koso America Inc.
5. IP bypass retrofit. Retrofit of a conventional hydraulic system actuator (top) to an Electraulic actuator (bottom) in the IP bypass line. Courtesy: Koso America Inc.
6. HP attemperator retrofit. Retrofit of a conventional hydraulic system actuator (left) to an Electraulic actuator (right) on the HP attemperator of a typical combined-cycle plant. Courtesy: Koso America Inc.
7. HP bypass retrofit. Retrofit of a conventional hydraulic system actuator (top) to an Electraulic actuator (bottom) in the HP bypass line. Courtesy: Koso America Inc.
Taking it to the bank
By retrofitting its pneumatic actuators to hydraulics one combined-cycle plant reduced its start-up times dramatically. It also reduced the time needed to blend a second GT/HRSG train into an on-line train from 2 hours to 50 minutes.
The plant decided to retrofit its pneumatic actuators after routinely experiencing hunting oscillations in the 35% to 55% stroke range. The oscillations created enough instability in reheat pressure to make it overshoot by 25 psi. Many efforts to tune the actuator and system (including the installation of a smart digital positioner) yielded no improvement.
Finally, the owner bit the bullet and replaced the pneumatic valve actuation system with a hydraulic system. The retrofit did more than eliminate the oscillations and instability; it also lowered the cost of starting up a second GT/HRSG train. The following bullet points detail the monetary savings and gains the plant continues to realize:
- Running the gas turbine at no load or low load for one fewer hour per restart saves $4,500 in natural gas priced at $10/mmBtu. The plant cycles one GT/HRSG train each night and brings it back on-line the next day during certain months. With 60 restarts every year, the plant conservatively estimates the annual value of this benefit at $270,000 in fuel savings.
- The faster the plant can restart in response to grid demand, the faster it can produce revenue. For example, one more hour of generation by the plant's 170-MW GT (at full load), and one more hour of generation by its steam turbine at 80 MW (full load is 160 MW) at 5 cents/kWh adds up to $750,000 a year in increased revenue from power sales.
- Shorter start-ups allow for more of them each year, because the plant has an annual start-up emissions cap.
—Geoffrey Hynes (ghynes@rexa.com) is international sales manager for Koso America Inc.
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