Which scenario necessitates a PSV downstream of a PRV?

A pressure safety valve (PSV) is installed downstream of a pressure reducing valve (PRV) primarily to protect the system from the risk of over-pressurization. In scenarios where the PRV may fail or malfunction, having a PSV in place provides an additional layer of safety by ensuring that any excessive pressure generated—due to unexpected fluctuations or failures—is safely relieved. This is crucial in preventing potential damage to equipment, piping, or personnel.

The requirement for a PSV is especially important in systems that handle varying pressure conditions, and inherent risks must be accounted for. This proactive safety measure adheres to industry standards and regulations designed to promote safe operational practices.

Other scenarios, such as changes in flow rate, high-pressure equipment ratings, or periodic testing needs, can have their own safety measures or operational protocols but do not inherently justify the specific necessity of a PSV downstream of a PRV as directly as the concern for over-pressurization.