Overpressure can occur with only a small rise in temperature. Thus all sections of lines that can be blocked off theoretically require provision for thermal relief. However, experience has shown that the probability of failure does not always justify installing a relief valve. As pressure is built up in a line, some types of valves may leak. Or conditions may be such that there is no possibility that the temperature of liquid in a blocked-off line can rise. Accordingly, there are services where provisions for relief never need be made, such as for cold water lines inside buildings.
Because of environmental and safety concerns, it is no longer acceptable to assume that flanges will leak to relieve thermal expansion pressures. Any relief design must assume that relief effluent is contained within the piping.