Venting valve working principle and classification analysis

- May 23, 2018-

Venting applications is one of the most important applications in combined cycle power plants. These valves must exhaust the main steam of the waste heat boiler bypassing the turbine to the atmosphere at the beginning of operation to ensure that the unit temperature does not rise too fast. This is done because the process line has already warmed up before allowing steam to pass through the turbine bypass valve to the condenser.

working principle

In this application, the high-temperature main steam (high pressure/medium pressure/low pressure) is bypassed to the atmosphere when the waste heat boiler starts to fire and the pressure and temperature rise to a certain point. As the control valve is subjected to a full differential pressure, the valve and downstream piping generate high noise and vibration. Therefore, the downstream is usually

Limiting devices such as diffusers or silencers are installed. This design provides a back pressure for the valve to limit the pressure drop and subsequent noise and vibration. It can also be used to reduce noise before leaving the system to be introduced into the atmosphere. The control valve noise requirement is usually 85 dB. In some cases, insulation can be used to meet noise requirements. However, in many applications, the downstream piping valves do not have any insulation. Far-field noise requirements depend on where the factory is located, but generally require 60 decibels on the fence line. In addition to noise and vibration, another issue is strict shutdown. Because these valves are commonly used in the event of a trip or factory trip, they must remain closed during normal operation. If there is a significant leak in the valve, steam will be lost. In many cases, the lost steam is enough to affect the performance of the turbine.


High temperature (1850 psig) and intermediate pressure/heat reheat (600 psig) applications experience temperatures approaching 1055F. At these temperatures, C12A or F91 (9% Cr) body materials are required. In addition, at these temperatures, the bolt material of the valve body needs to be explored. As the temperature rises above 1050F, the bolt load on steel will be greatly affected. Steel bolts will be elongated faster than C12A, causing the valve body and upper bonnet connections to leak. Therefore, Fisher recommends using Inconel 718 alloy material at temperatures above 1050F. As temperatures rise, tight shutoff becomes a bigger challenge. Since in the past the stability of pilot-operated counterbalanced spools was used in the industry, Fisher designed a C-type seal that utilizes pressure-assisted metal sealing along with the internals of metal-to-metal seats. This "double" shutoff provides Class V protection up to 1100F and provides long-term shutoff for high/medium pressure/hot reheat vent valves. It should be noted that the application conditions of the medium/hot reheat vent valve may vary depending on the position of the valve in the boiler line. If the valve is placed behind an intermediate pressure superheater (medium pressure vent valve), the maximum valve temperature tolerance is 850F. If the valve is located behind the reheater (hot reheat vent valve), the maximum valve temperature tolerance is 1055F. The pressure of the low pressure vent valve is very low (150 psig), but the temperature can still reach 500F. At this temperature, Fisher uses a spring-loaded seal ring and a polyetheretherketone anti-extrusion ring structure to ensure that the seal ring does not deform out of shape. In order to solve the noise problem, noise attenuation technology is recommended instead of limiting the valve fluid capacity. The system's optimization calculation is essential to ensure the best solution. With this in mind, valves and mufflers need to be combined and calculated. This eliminates the limitation of the valve's ability to flow, or the possibility of higher than expected noise levels.

High pressure vent valve: 4"-8" ANSI 2500 EH body in C12A chrome-molybdenum alloy steel with Whisper III or WhisperFlo noise reduction trim, Type C seal tight shutoff structure, combined with downstream diffuser/muffler.

Medium Pressure/Thermal Reheat Vent Valve: 6" to 12" ANSI 900 EH or EU valve body in C12A chrome molybdenum alloy steel (heat reheat) or 6" to 8" ED valve, WC9 chrome molybdenum Steel valve body material (medium pressure) with noise reduction internals using Whisper III or WhisperFlo, C seals tightly shut off the structure, combined with downstream diffuser/muffler.

Low-pressure vent valve: 6" to 12" ANSI 600 ET body in WCC carbon steel with Whisper III or WhisperFlo noise reduction internals, high temperature tight shut-off structure, combined with downstream diffuser/muffler.