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I would like to discuss what is used in modern day and old plant designs for temperature control valve on the outlet of the secondary reformer waste heat boiler (WHB) / inlet to high temp shift reactor.
Our plant is from the 1960’s. The secondary reformer WHB has an upper cooled gas outlet and a lower hot gas bypass outlet. Flow through these two outlets are balanced to achieve the desired inlet temperature to the high temp shift reactor. Temperature control is done by a 3 way tee mounted valve design. The tee contains two butterfly valves that are mechanically linked together and operated by a single pneumatic piston actuator.
I’m wondering if current designs still use a tee linkage valve or have moved to two independent control valves with a split range controller to blend the flow of fully cooled and hot gas bypass mixing.
What are the safeguards on the temperature control valves such hot gas bypass control valve failing closed and cooled gas control valve failing open, etc.?
Your valve set-up depends on the boiler lay out, but with gas on the shell side I think it is pretty common with two valves, but they don’t have to linked physically. However, you should limit the possibilities from the control system to not be able to close them both for example. I also think you neither want them fail close, nor open, but “fail in last position.”
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