[Technical Series #02] Choosing the Right Safety Valve: Types, Features, and Understanding 'Back Pressure'

nick so
Jan 13
3 min read

Hello, this is NEFSS, your specialist in industrial valves and piping materials.

In our last session, we covered the definition and basic structure of Pressure Safety Valves (PSV). Now that we understand how they are built, it’s time for the most critical step: deciding which type to install at your site.

Today, we will take an in-depth look at the three main types of safety valves and the key variable you must consider when selecting one: Back Pressure.

*Types of PSV: Three Main Formats

Safety valves are generally classified into three categories: CONVENTIONAL, BELLOWS, and PILOT.

1. POSRV (Pilot Operated Safety Relief Valve)

As shown in the image, the Pilot type consists of a main valve connected to a pilot unit via thin tubing.

**Structure of POSRV (Source: Wikipedia)**

Operating Principle: The pilot unit senses the pressure from the main valve inlet through the tubing and controls the opening and closing of the main valve.
When to use:
- Typically used when back pressure exceeds 50% of the set pressure.
- When space is limited and a compact valve is required.
- When you need to operate the valve by sensing pressure from a remote location rather than the inlet itself.

(※ Note: For this series, we are focusing on the theoretical characteristics of the Pilot type.)

2. CONVENTIONAL vs. BELLOWS TYPE

These are the two types most frequently compared in the field. The most visible difference is the presence of space for the Bellows component between the body and the bonnet.

**Visual Comparison: CONVENTIONAL (Left) & BELLOWS (Right) (by NEFSS)**

Theoretical Difference: A Conventional valve is directly affected by back pressure, whereas a Bellows valve is designed to be isolated from the effects of back pressure.
Selection Criteria (Back Pressure Ratio):
- 10% or less: Conventional Type is applicable.
- 10% to 50%: Bellows Type is recommended.

3. Types and Characteristics of Back Pressure

What exactly is Back Pressure, the standard for type selection?

Back pressure refers to the pressure existing at the outlet of the safety valve. In other words, it is the pressure present at the valve outlet due to discharge piping, headers, or atmospheric pressure, both during discharge and under normal conditions.

**Systematic Classification of Back Pressure (by NEFSS)**

① Superimposed Back Pressure

The pressure that exists at the outlet before the safety valve operates.

Constant: Pressure that remains steady at the outlet.
Variable: Pressure that fluctuates depending on the situation.

② Built-up Back Pressure

The pressure that develops after the safety valve opens. It is caused by resistance (friction) in the discharge piping as the fluid flows out.

💡 Nick’s Pro-Tip: "Nature of Pressure is More Critical Than Numbers"

While we use the 10% and 50% ratios as general guidelines, there are always exceptions in the field.

If only Constant Back Pressure exists, a Conventional type might still be usable even if it exceeds 10%.
Conversely, if Variable Back Pressure is present, a Bellows type is recommended even if the back pressure is below 10%, to ensure system stability.

If you have any questions regarding type selection due to back pressure, please feel free to contact us. We will provide detailed answers and, if necessary, consult directly with the MAKER to provide the most accurate technical solution.

🛠️Technical Consultation & Request for Quotation

If you are having difficulty selecting the right safety valve or need a fast and reliable quote, please feel free to contact NEFSS at any time!

Technical Consultation & Request for Quotation

[Coming Up Next]we will return with [Part 3] The Operating Mechanism and The Differences in Form: Full Bore vs. Lift Type.

At NEFSS, we go beyond simple material supply to become a partner that cares about your site's safety.

Thank you. Best regards, Nick, NEFSS