A blocked-in liquid line is exposed to solar heating. Which protection is often appropriate?

When a section of piping containing a liquid is blocked or isolated (e.g., between two closed valves) and exposed to a heat source like the sun, the liquid will expand. Because liquids are nearly incompressible, this thermal expansion can generate extremely high pressures, potentially rupturing the pipe. A small, dedicated thermal relief valve or device is the appropriate engineering control. It is designed to open and relieve the small volume of liquid created by expansion, thereby protecting the piping from overpressure.

B. A lower audit frequency is an administrative change that weakens safety oversight and does not address the physical hazard.

C. Removing insulation may be inappropriate for the process (e.g., for personnel protection or energy conservation) and does not prevent overpressure if the line is blocked-in.

D. A larger operating crew is an administrative control that does not prevent the physical phenomenon of thermal expansion overpressure.

1. American Petroleum Institute (API). (2014). API Standard 521: Pressure-relieving and Depressuring Systems (6th ed.). Section 5.13, "Thermal Expansion," pp. 65-67.

2. Center for Chemical Process Safety (CCPS). (2017). Guidelines for Pressure Relief and Effluent Handling Systems (2nd ed.). Wiley. Section 3.3.8, "Hydraulic Expansion," p. 121.

3. Mannan, S. (Ed.). (2012). Lees' Loss Prevention in the Process Industries (4th ed.). Butterworth-Heinemann. Volume 2, Chapter 19, "Pressure Relief," Section 19.5.10, "Thermal Relief," p. 1388.