March 15, 2008

Desuperheating valves take the heat

Geoffrey Hynes, Koso America Inc.

The cascading bypass system is perhaps the most common design for managing high-pressure steam in a combined-cycle plant. It is the hot reheat (HRH) bypass valve actuator that defines the valve's ability to respond to system demands. That makes it perhaps the most important component in the steam bypass system, which in turn is one of the most important control loops in a typical combined-cycle plant.

 

HRH valves play a critical role in the main and reheat steam loops, especially during unit start-ups and shutdowns. If your control loops can't closely follow a setpoint, chances are your plant is equipped with pneumatic actuators—and its heat rate is higher than it could be. Anything short of perfect control can also cause major operational problems that either extend start-up and shutdown times or increase the potential for unit trips. Both effects inevitably show up on the plant's bottom line.

In cascading bypass systems, steam from the high-pressure (HP) and intermediate-pressure (IP) drums that bypasses the steam turbine during start-ups, transients, and shutdowns does not go straight to the condenser (Figure 1). Instead, HP bypassed steam goes to the cold reheat (CRH) line on the HP turbine's exhaust and mixes with the output of the IP drum. This HP steam is then sent through the reheater and through another bypass pressure-control valve—the HRH valve—before going to the condenser.

 

 

Selecting the right valve

HRH valve requirements are complex from a mechanical design standpoint. The ANSI 600-lb-rated valves range from 12 to 24 inches in diameter. They must tightly shut off and be able to be throttled (conflicting requirements for such difficult service), and their body and trim materials must deal with rapid thermal transients. Noise control and extended trim life also have become very important design requirements.

Unbalanced HRH valves are typically not used in this application because the actuation forces required for valves of this size would be too large for conventional pneumatic actuators. However, because tight shutoff is a design requirement, pilot-balanced trim is common. This design allows for the use of relatively low actuator thrust at full differential pressure (balanced when open), while enabling full unbalanced forces on the valve seat in the closed position (installed in the flow-to-close direction) to ensure tight shutoff.

Special materials, tolerances, body/trim/bonnet arrangements, and flow paths (warming lines, for example) are used to address the thermal cycling issues that HRH valves must deal with, such as weld fatigue and internal reliability. Many designs have forsaken pneumatic actuators fitted with standard positioners and volume boosters to meet stroking speed requirements in favor of smart positioners with boosters that improve diagnostic capabilities and reduce overshoot.

What would be a good set of technical requirements for a HRH valve actuator? The use of pneumatic actuators poses inherent design challenges because air is compressible and therefore limits the response, positioning capability, and stability of an actuator. Nevertheless, it's still instructive to compare how a typical pneumatic actuator and a modern hydraulic actuator work. As long as you remember to include the effects of your plant's design in the comparison, the following discussion will point you in the right direction.

Let's begin the comparison by considering the following as typical HRH bypass actuator performance requirements:

• A stroke length between 6 and 12 inches.
• A stroking force between 15,000 and 40,000 pounds, depending on the valve design and the process parameters.
• A stroke speed typically less than 5 seconds between the full-open and full-closed positions.
• The ability of an input trip signal to stroke the valve fully closed within 2 seconds or less.
• High frequency response, repeatability, accurate setpoint control, and stability.

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