Relief Valve Sizing

I have come across a shell and tube heat exchanger in the basement of my office building that has no pressure relief valve on it and I would like to know how to select the right size relief valve for it. The heat exchanger provides hot water for the buildings HVAC system, 15psi steam on the shell side, 30psig water in the tubes, centrifugal pumps. Its always been my understanding that the relief valve in this context protects against thermal expansion either from a blocked in condition or failed expansion tank. A tube failure on these units just results in heating water being lost into the condensate line and making the boiler plant operators annoyed at the hard water coming back to the plant.