In the world of boiler PSV design, the “length” of a vent line is less about a specific length in feet and more about BACKPRESSURE. According to ASME Section I (Power Boilers) and Section IV (Heating Boilers), the moment you add a vent line to a Pressure Safety Valve (PSV), you are introducing flow resistance. Here is the breakdown of when “standard” sizing is no longer sufficient.