How does a boiler blowdown valve work?
Most blowdown lines are piped with two valves, each with a different design and purpose. The first valve (closest to the boiler) is a quick-acting valve followed by a slow-acting valve. The purpose of this configuration is that the quick-acting valve is opened first while the slow-acting valve is then opened slowly to allow the blowdown line time to acclimate to the high temperature and high pressure of the water being removed from the boiler. The slow-acting valve prevents the downstream piping and fittings from being thermally shocked by a large flow of hot blowdown water. Then, to ensure the blowdown is stopped with quick precision, the quick-acting valve is also closed first. The slow-acting valve can then be shut slowly since the flow of water out of the boiler has already been stopped.