Here’s What to Know About Pressure Relief Valve Sizing Before You Buy

Improperly pressurized systems can result in damage. This ranges from a small water heater leak to
full-blown disaster. To safely move liquids and vapors through different systems requires careful

There are several different variables to consider. Some systems require a liquid, gas or two-
phase relief valves.  Consider all possible over-pressurization scenarios.

Relief valves do not control or regulate the pressure in the system that they protect. They are an
important safety feature to eliminate risks to people and equipment. Correct pressure relief valve
sizing is essential.

Pressure Relief Valve Sizing Basics

Professionals spend many years learning the correct standards for system design. Engineering relief
valve size and placement are time-consuming disciplines. However, there are several computer-
aided solutions available to speed the process.

Step 1

Create your model. Pressure relief valves must operate in a variety of conditions. This includes
during a power failure.

A pressure relief valve opens at a predetermined set pressure. It allows a certain flow and then
closes when the system returns to a safe pressure level.

Determine what type of pressure relief valve you need. Do you need a single phase or multi-phase

Some valves are suitable for a simple air and water mix. Other valves are specially engineered for
corrosive chemicals.

Step 2

Determine your parameters. Use the applicable API and ISO standards. Again, there are software
solutions to help you assign values.

Choose the material, fluid pressure, and temperature.  Determine the design flow rate, set pressure
and discharge coefficient. Again, select the relevant standard for your industry and use.

If there are long distances involved, remember to consider pressure losses. Also, remember to
consider the liquid/vapor mix and viscosity.

Step 3

Perform the relief valve calculations. Compare the result with the nearest sized manufacturer
standard valves.  If possible, adjust your model to your specific pipework.

Using straight API standards can result in rigid parameters. These may not adequately cover all
scenarios. This can lead to erroneous values.

A correctly sized pressure relief valve has a discharge capacity to prevent system damage.  In most
cases, this is 10% above the maximum allowable working pressure.

For Safety’s Sake

An adequate safety system depends on the correct pressure relief valve sizing. Pressure relief
valves must protect your system against “worst case” conditions.

Defining the credible worst-case scenarios for over-pressurization should yield
conservative results.  Evaluation of failure probability is an important part of making correct

Book methods for pressure relief valve sizing are adequate. The results are likely to be conservative
with a margin for error. However, these book calculations are no substitute for expert analysis.

We have many years of experience with custom pressure release systems and innovative solutions.
We can offer your project a full catalog of products. Let us help you find the right
solutions. Contact us today.