Consideration of thermal relief is necessary in all sections of liquid piping, regardless of length, when it is reasonable to expect that the liquid will be blocked-in while the line is subject to temperature rises from solar radiation, warm ambient air, steam tracing, or other external sources of heat.
A temperature increase will cause both the liquid inside a pipe and the pipe itself to expand in volume. Liquids have high thermal coefficients of expansion compared to metals. For example, oil will expand approximately 25 times as much as the pipe. Therefore, high pressures will build up when liquids are heated in a line sealed by block valves or blinds. Neither thermal expansion of the pipe, expansion of the pipe from internal pressure, nor compressibility of the liquid may be sufficient to relieve the liquid thermal expansion before pressures exceeding the maximum safe pressure of piping components are reached. Tests by the Manufacturing Department at El Segundo Refinery and by Crane Company, and verified by calculations, show that the pressure from thermal expansion of liquid hydrocarbons will increase about 70 to 100 psi for each °F temperature increase.
The length of line has no effect on the pressure that will result from thermal expansion of liquid in a blocked line. However, the volume of fluid that must be released to prevent excess pressure build-up will be directly proportional to the line length.
Calculations show that temperatures of 150°F can be reached in small lines (10 inches and less) containing liquid hydrocarbons before heat lost by convection equals heat gained from solar radiation. Therefore, if oil is initially at 50°F, a temperature rise of 100°F is possible during extreme exposure to the sun. Tests performed at El Segundo verified that even on a normal day the temperature of oil in a line can increase by 50°F.
The possible temperature increase from solar radiation is sufficient to raise the pressure in lines containing liquids by as much as 3,500 to 10,000 psi. See Figures 1200-19 through 1200-22. Such pressures may be considerably above the maximum allowable working pressure of valves and pipe, particularly if the pressure of the liquid, at time of blocking, could be at or near the maximum working pressure of the system. The principal reason more ruptures have not occurred in lines without relief valves has been that sufficient relief is usually afforded by inherent leakage of common valves. With the increasing use of positive shutoff valves, such as plug cocks and ball valves, double-block valves, and some kinds of line blinds, there is a greater likelihood of rupture.