What is the rule of thumb regarding PRV valve sizing?

The correct choice is that the valve size is smaller than the upstream piping. This is based on the principle that the pressure relief valve (PRV) must relieve pressure effectively in a situation where it is called into action, usually due to overpressure conditions in the system.

When the PRV is sized smaller than the upstream piping, it ensures that the valve will be able to open quickly and relieve pressure, thereby protecting the system from potential damage caused by excess pressure. A smaller valve allows for a quicker response to pressure increases, which is critical for safety.

The sizing also considers that the valve has to achieve its set pressure and flow capacity; a larger valve could open too slowly to react effectively in the event of overpressure, leading to system failures. This makes it essential for the sizing of the PRV to follow this guideline to maintain the safety and integrity of the piping system.