Getting Relief Valve Sizing Right: Why It Matters & How to Do It

Skip To:
Avoid Overpressure & Instability with Correct Sizing
Why Proper Sizing Matters
The Variables Your Need to Know
Sizing Isn’t Just Math – It’s Applied Standards
A Smarter Way to Size Relief Valves
Need Assistance?
Resources

Avoid Overpressure and Instability with Correct Sizing

Pressure relief valves are a vital safeguard in any pressurized system. But they’re only effective when sized correctly. Too small, and they can’t vent enough to prevent overpressure. Too large, and you risk system instability or damage to downstream components.

Getting the size right is critical—and it’s more complex than it might seem.

Why Proper Sizing Matters

Relief valve sizing isn’t just about picking a valve that looks close enough on paper. It’s about matching performance to system demands with precision. Undersized valves can compromise safety. Oversized ones may cycle improperly or create backpressure issues.

The only way to protect both equipment and people is by accurately determining the system’s total discharge flow—and using that to guide valve selection.

The Variables You Need to Know

To size a relief valve properly, you need to understand a few key factors:

• Valve orifice diameter
• Discharge coefficient (Kd) – a real-world measure of flow efficiency
• Upstream pressure – typically 110% of the valve’s set pressure
• Downstream pressure – often 0 PSIG if venting to atmosphere
• Flow temperature
• Working gas

These variables work together to determine whether your flow is in a choked (critical) or unchoked (subcritical) condition. That distinction affects which equations you’ll use to calculate mass flow, and ultimately, how large your relief valve needs to be.

Sizing Isn’t Just Math – It’s Applied Standards

While the formulas behind relief valve sizing are well established, including those defined by EN ISO 4126-1, applying them correctly can be a challenge.

You need to:

• Identify the correct flow regime
• Use the appropriate equation for choked or unchoked flow
• Factor in real-world inefficiencies via the discharge coefficient (Kd)
• Maintain consistent units throughout the calculation

It’s easy to overlook something when working manually or to misjudge a variable that seems minor but has a significant effect on the result. And even if you get everything technically correct, calculating multiple scenarios—say, across a range of pressures or temperatures—can become time-consuming.

For engineers, technicians, and designers, this can slow progress or introduce uncertainty. That’s where a dependable sizing tool makes all the difference.

A Smarter Way to Size Relief Valves

Whether you’re in design, maintenance, or engineering support, having a reliable way to size relief valves quickly and confidently is essential. That’s why we developed the Generant Flow Calculator.

Built around the EN ISO 4126-1 standard and backed by actual test data, it helps you:

• Calculate accurate mass flow based on system conditions
• Select the correct Kd value for your valve type and set pressure
• Streamline your relief valve sizing workflow

For a more technical breakdown of how the calculator works, including the equations used and assumptions made,
click here for a detailed explanation.

With the calculator, you’re not just plugging in numbers—you’re gaining confidence in your selections. By walking through the same methodology our engineers use, it helps you validate your approach and catch inconsistencies before they become issues in the field.

Whether you’re sizing valves for a new system, troubleshooting an existing one, or running what-if scenarios for varying conditions, the calculator provides the repeatability and accuracy manual methods often lack. It’s designed to support both quick checks and more in-depth evaluations.

Relief valve sizing may be grounded in standards and equations, but applying them correctly in real-world systems takes experience—and the right tools. The Generant Flow Calculator helps bridge that gap, giving you a practical, efficient way to get it right the first time.

Need Assistance?

Our application engineers are here to help you select the right parameters and interpret your results. Sizing doesn’t have to be a guessing game. Contact us and we can help walk you through it.

Subscribe to our blog for updates!