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.

With more brownfield capital projects coming back to life many operating companies are mobilizing to increase their facility's charge rates.

If you are one of these companies looking for improvements, correctly evaluating your flare system's maximum capacity could "make or break" your project's feasibility studies.

Did you know, under certain circumstances, you need to verify if the Cold Differential Test Pressure (CDTP) is properly compensated for the superimposed backpressure from the flare header?

Have you come across a conventional pressure relief valve relieving to a flare header? Did you know that, under certain circumstances, you need to verify if the Cold Differential Test Pressure (CDTP) is properly compensated for the superimposed backpressure from the flare header?

The Opportunity:

After reading about flare header failures due to high velocities, an owner of a production platform in the Middle East wanted to ensure that the initial inerting/purging of the platform and pipeline would not result in flare header piping failure. Smith & Burgess was hired to review the flare systems for safe operation under the high flow rates associated with the initial facility start-up.

Did you know, a relief device in liquid service does not always mean that the device is liquid-certified?

Liquid-certified devices are mechanically designed and sized by an engineer to relieve liquid. Per UG-125 of ASME Section VIII, "Vessels that are to operate completely filled with liquid shall be equipped with pressure relief devices designed for liquid service, unless otherwise protected against overpressure."