[Technical Series #01] What is a Safety Valve (PSV)? A Guide to Definition and Structure

nick so
Jan 8
3 min read

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

In any plant or facility with pressure vessels, there is a valve known as the "last line of defense" for equipment protection: the Pressure Safety Valve (PSV).

Starting today, I will be publishing a technical series on PSVs, based on my hands-on experience, to assist professionals in the field.

In this first edition, we will cover the definition and basic structure of safety valves.

1. Definition of a Safety Valve (PSV)

First, let’s look at the definition provided by KOSHA (Korea Occupational Safety and Health Agency), a reliable guideline in the industry:

"A SAFETY VALVE refers to a valve designed to prevent fluid leakage by automatically activating a spring to release fluid when the inlet pressure reaches a set level, and returning to a normal state once the pressure drops below that level."

Nick’s Simple Explanation: "A stress-relief outlet for pipes and pressure vessels" Just as we need to release stress through exercise or conversation to prevent a breakdown, piping systems are the same.

When excessive internal pressure builds up, posing an explosion risk, the safety valve performs the critical role of protecting the entire facility by releasing that pressure at the right time.

Simulation image of a safety valve installed on a pressure vessel. by NEFSS

2. Key Structure of a Safety Valve: Body, Bonnet, and Cap

Just as an insect is divided into head, thorax, and abdomen, a safety valve is composed of three main parts for easy understanding:

BODY: "The Fluid Path and Foundation"
- The Seat and Disk are seated inside.
- It is the bottom part that connects to pipes or pressure vessels via Flanges or Threads for a secure installation.
BONNET: "The Housing for the Spring"
- The protective space where the Spring, the heart of the safety valve, is housed.
- It protects and supports the spring from the external environment.

CAP: The topmost part of the valve.
- Covers and protects the internal Adjust Screw.

The fluid flows in through the Inlet (where the seat is located) and exits through the Outlet to relieve pressure, as shown by the arrows in the diagram below.

3. Common Connection Types

① THREAD TYPE

Often referred to as a "Screw Type" in the field.

Connection: Inlet/Outlet connections can be Male or Female as per request.
PT or NPT threads are standard.
If you require special thread standards or non-standard specifications,
contact NEFSS. We will coordinate closely with manufacturers to quickly verify production feasibility.
Structure and fluid flow of a Thread Type safety valve. by NEFSS
② FLANGE TYPE
The most commonly used type, typically featuring an outlet size larger than the inlet. (e.g., 1" x 2", 1-1/2" x 2", or 25A x 50A).
- Standard: Mainly follows JIS (10K, 20K, etc.) or ANSI (150LB, 300LB, etc.) standards.
- We can also provide non-standard sizes, DIN standards, or non-standard connection upon request.
  Structure and fluid flow of a Flange Type safety valve. by NEFSS

🛠️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] Now that we know the definition and structure, it’s time to choose the right type for your site. In the next session, we will discuss the differences between Conventional, Bellows, and Pilot types, as well as the concept of Back Pressure.

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