I am having difficulty in correctly modelling pressure safety (relief) valves that discharge to the atmosphere. Typically, I model the valve connection with a weldolet, weld neck flange and then the valve. The discharge of the valve typically has an LR elbow that discharges into an open vent stack. I model the valve as a rigid body with a vertical force to account for the valve weight. I also apply a Dynamic Load Factor of 2 to the valve discharge reaction forces quoted by the manufacturer. I have seen most valves in the field installed this way but I usually get high stresses resulting from high bending moments when the valve opens.

Various suggestions have been made to me such as installing a dummy leg below the discharge elbow to counteract the reaction forces or use a reinforcing pad but none of these suggestions seem really satisfactory. For example, the dummy leg will lift off its support due to thermal expansion as the pipe and valve heats up. Also, use of a re-pad does not significantly help with the stress intensification factors (SIF).

Also, I have found that sweepolets offer much lower SIF's than weldolets but I have been told that they are expensive, hard to get and the fitters and welders find them difficult and expensive to intall.

Any suggestions or comments?