1. Relief should always be provided if there is more than 200 feet of pipe per valve even though the valves are of a type that may be expected to leak, such as typical solid-wedge gate valves. If the amount of pipe per valve is between 20 and 200 feet, use the “Doubtful Case” approach.
2. Relief should always be provided for lines confined by plug cocks, ball valves, double-block valves, or other valves which are not expected to leak, or by some kinds of line-blinds.
Note In blinding a line with a spectacle blind or with some styles of line-blinds, the line may be at least partially drained during the blinding operation. In such cases, thermal expansion of the liquid within the pipe will not cause any substantial pressure rise and no other provision for relief need be made.
3. Relief should always be provided for lines which are steam or electrically traced, except those that fit in the category described in item 4 above.
4. Relief should always be provided for cold side piping of heat exchange equipment (and for warm side if it normally operates below ambient temperatures) unless pressure relief has otherwise been provided to satisfy requirements of the Pressure Vessel Code. These relief valves must be located between the heat exchanger and the block valve, preferably at the outlet.
5. Relief should be provided for double seated valves of a type in which pressures exceeding the safe pressure of the valve can build-up in the cavity and blow the packing or damage the valve. (Note: Some styles of double block valves have spring loaded seats or other built-in provision for relieving such excess pressures.)