PSV for Thermal Relief on Piping?

Generally PSV not required for piping… however…API STD 521 stated that hydraulic expansion due to solar heating on “long pipelines” shall be assessed and checked if a pressure relief valve (PSV) is required. What is “long pipelines” ? Difficult…

This section allows that a PSV may not required for thermal relief if proper administrative procedures and addition of signs stipulating the proper venting and draining procedures when shut-down and block-in are implemented. These action are acceptable and considered do not compromise the safety of personnel or equipment.

System under consideration for thermal relief consists of piping only (does not contain pressure vessels or heat exchangers), a pressure-relief device might not be required to protect piping from thermal expansion if :

  1. piping is always contains a pocket of non-condensing vapour, such that it can never become liquid-full (even it is heated and compressed); OR

  2. piping is in continuous use (i.e., not batch or semi-continuous use) and drained after being blocked-in using well supervised procedures or permits; OR

  3. fluid temperature is greater than the maximum temperature expected from solar heating (the pipe temperature direct under solar heating can reached approximately 60 °C to 70 °C, it can be as high as 85 °C for certain area i.e. Middle east) and there are no other heat sources such as heat tracing; OR

  4. the estimated pressure rise from thermal expansion is within the design limits of the equipment or piping.