I currently have a project I'm working on which is an existing power plant designed to asme section 1 that has some letdown valves that have desuperheating capabilities. The pressure on the high side is 1500 psig and the low side is 425 psig. Boiler feed water is introduced within the valve to reduce the temperature of the steam to 700 deg F. The power plant is existing and I'm assuming the design velocity upstream and downstream of the valve was 200 ft/sec. I'm working on trying to increase the capacity through the valves which would require increasing the line size to maintain the 200 ft/sec velocity. My question is what is the velocity range you should keep the flow downstream of the let down valve for the boiler feed water to properly reduce the temperature? Also, is this in a standard or a guideline somewhere? One more thing; I'm also concerned about the straight length of pipe required as well as whether a reducer impedes the de-superheating.
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