Safety Relief Valve: Knowledge and Selecting Guide

What will happen when pressure vessels or steam boilers become over-pressured?
It might cause an explosion, malfunction, fire, or even death. Obviously, a device that can release excess pressure is the solution to this risk. This is the aim of the safety relief valve. They play a special role in the manufacturing system because they not only control the flow capacity but also protect the equipment and personnel.

Hence, the quality and reliability of the valve are crucial in the manufacturing procedure and safety. It is essential to know about it. 317 Valve will introduce the basic information and selection criteria for the safety relief valve. 

What is a Safety Relief Valve?  

A safety relief valve, also called a safety valve, is an essential assembly of pressure vessels. 
It prevents damage from excess pressure, gas, and fluid from the equipment in emergencies. 
It instantly opens and discharges the medium when the pressure is exceeded. If the medium 
pressure drops to a particular set pressure due to discharging, the valve closes to stop the stable release. 
After the pressure returns to normal position, the valve remains closed. 
They operate without an external power supply and are usually considered the last protection of the equipment.   

Working Principles

Safety Relief Valve Figure (Spring-Loaded Type)

1: Valve Body
2: Valve Seat
3: Valve Disc
3-1: Seal
4: Spring
5: Stem / Spindle

A: Body
B: Inlet
C: Outlet
Di: Diameter

As we mentioned, the safety relief valve opens at a fixed set pressure and releases the over-pressured medium. 
The working principle is the balance of forces from the medium and the spring. 

The spring-loaded valve is one of the most common valves, which 317 Valve also provides. 
For this reason, let us take it as an example. The opening and closing of the valve are activated by a spiral spring. 
When the valve lifts, the spring force increases. The spring spindle transfers the force to the disc. Once the spring 
force surpasses the medium pressure, the disc will enclose the nozzle.When the medium pressure exceeds 
the spring force, the disc will depart from the seat and open the valve to release the excess pressure. 
Every kind of valve may differ in its internal design; however, the basis of working principles is the force balance.

Types of Safety Relief Valve

Valves are classified according to various standards and codes around the world. 

Here are the 4 common types of them:

Full-Lift & Low-Lift:

The terms full lift and low lift mean that the quantity of disc moves from its closed position to the set position to discharge the determined capacity, which creates the discharge capacity of the valve.

*Low-Lift Safety Relief Valve: The valve lifts 1/24 of the bore diameter, which makes the flow capacity smaller than full lift ones. It is used in piping systems and pressure vessels.   
< Check out our low-lift type here >

*Full-Lift Safety Relief Valve: The valve lifts 1/4 of the bore diameter. They are mainly used for steam boilers.

Full Bore Safety Relief Valve:

The discharge flow depends on the nozzle, not the movement of the disc. The valve seat passage area equals the nozzle of the valve seat area when the valve opens. The distance of the valve from rising to falling affects the discharge flow amount.
< Check out our full bore type here >

Conventional Safety Relief Valve: 

Instant pop-up action is their feature. They are commonly used in high temperatures or applications without high back pressure. 
If the back pressure (the pressure in the valve outlet when discharging) changes, the conventional valve's working pressure and relieving capacity will be influenced.

Balanced Safety Relief Valve: 

They are loaded with spring and a bellow to maintain back pressure from affecting the set point. In contrast to the conventional type, they are suitable for high-back-pressure applications.

Application of Safety Relief Valve

-Gas & Oil 
-Steam or hot water boilers
-Liquefied petroleum gas carriers;
-High-pressure power generation equipment
-Pressure vessel

Selection Criteria: Things to Consider When Selecting a Safety Relief Valve

We have understood the basic information about safety relief valves. Now, how do we choose them?
As we mentioned at the beginning of this article, the safety relief valve plays a critical role in the pressure vessel system. 317 Valve suggests you pay attention to below 6 points when buying it. 

Required Capacity

The safety relief valve should have a specific capacity to accommodate excess pressure. Normally, each country has different standards and regulations for valve required capacity. We must consider the medium temperature and the discharge area when deciding the valve's capacity. Furthermore, we need to ensure the valve’s design can resist the maximum pressure of the vessel. 


Depending on the system and environment, a valve may need to be made from a particular material. Bronze and stainless steel are the most common materials for making valves. Here at 317 Valve, we also provide WCB material. 

Set Pressure

The set pressure is the pressure at which a safety relief valve opens.
The set pressure of the valve should be, at most, the system's maximum allowable working pressure (MAWP)

Back Pressure

Back pressure is the pressure on the outlet of the valve when it discharges. If it is too high, the back pressure will damage the valve, reduce the set pressure and make the valve pop open again and again. It is recommended to maintain it 10% below the set pressure. 

Design and Size

It must decide the valve size according to the inlet and outlet size. The inlet and outlet pipeline connected to the valve must be the same size as the valve’s inlet and outlet opening. The design of the connection between the valve and the vessel system also determines the selection.


The condition of the medium and pressure levels are controlled mainly by temperature. We decide on the valve material depending on the operating temperature, too.


This is the basic information you need to know about safety relief valves. Safety relief valves are selected based on equipment, fluid properties, operating temperature, set pressure, and discharge capacity. It is necessary to apply various conditions, such as the use environment, and must be carefully evaluated. 

Need advice on valve selection? Please do not hesitate to contact 317 Valve.