Did you know, the allowable total backpressure for a conventional valve can be larger than 10% of the set pressure? According to API 520-1 §5.3.3.1.3:

Many people think a relief valve on a pipe should never see more than 10% backpressure for spring-loaded relief devices. In reality, that’s a guideline for normal cases, not a hard law of physics. That 10% is not a one-size-fits-all limit—it depends on how much extra pressure (overpressure) the valve can handle during an emergency or extreme case.

According to API 520-1 §5.3.3.1.3, “in a conventional PRV application, when the allowable overpressure is 10%, the built-up backpressure should not exceed 10% of the set pressure. A higher maximum allowable built-up backpressure may be used for allowable overpressures greater than 10%, provided the built-up backpressure does not exceed the allowable overpressure.”

So, what does this mean, and when is it applicable? In this blog, we’ll go over a few examples and explain the answers to these questions in simple terms. 

Did you know to exclude the exit pipe fitting in your PSV outlet pressure drop calculations if your PSV discharges to atmosphere?

This is because, according to research and fluid dynamic principles such as Bernoulli’s equation, there is no appreciable resistance once the flow exits to free air. Removing the exit fitting from backpressure calculations may lead to more accurate and slightly lower predicted backpressure on the valve, which aligns with current best practices within the industry.