1. What is a safety valve?

Safety valve, English name is Pressure Safety Valve (referred to as PSV), is a type of valve with the function to protect the safety of the system, namely to ensure that the pressure when passing the valve does not exceed the value for permission (pre-installed). During the working process, the safety valve is always closed. When the inlet pressure of the valve exceeds the rated value, the safety valve opens to allow a part of the liquid to flow through the valve and recirculate to the container, ensuring that the pressure through the valve does not exceed the set value. ensure the system is safe, no damage due to overvoltage condition.

2. Classification, structure & operating principle of each type of safety valve

Based on the structure of the safety valve, it can be divided into the following two types:

a. Direct-acting safety valve:  This is the most commonly used type of valve today, with a fairly simple structure, few details, suitable for installation in medium and small-sized systems. The reaction is quite fast and powerful.

* Operating principle:  Direct acting safety valve works based on spring elasticity. The spring compression will be changed by an adjustment screw on the top of the valve. The compression force of the spring is the set pressure to discharge the system. Valve designs all use three distinct states: equilibrium, open & closed.

- Equilibrium: When the system is operating normally, the pressure in the system is stable. Thanks to the compression of the spring, the valve disc is pressed down, the valve is completely closed. At this time, the system pressure acting on the valve disc is less than the spring compression force. The two forces balance each other causing the valve disc to remain in the closed position.

- Open state: When the pressure in the pipeline gradually reaches the pressure of the spring or preset pressure, the spring force and the pressure equalize, the safety valve begins to leak and make a sound. When the system overpressure occurs, exceeds the safety threshold or the set level of the valve, this pressure overcomes the compression of the elastic spring, the valve disc is pushed up. The valve will open quickly with a “pop”. Overpressure is usually expressed as a percentage of the set pressure. The codes and standards provide limits for maximum pressure. Typical values ​​are 10%, ranging from 3% to 21% depending on code and application.

- Closed state:  The liquid being discharged will reduce the pressure of the system acting on the valve. As the system pressure gradually decreases, the spring compression force will cause the valve disc to slowly compress until it is completely closed. In addition, the direct-acting safety valve is often equipped with a recoil arm, which is used so that the user can pre-discharge excess fluid even though the pressure has not reached the limit threshold. Pulling the lever will help open the valve disc to drain the fluid out.

* Structure of direct acting safety valve:  Direct acting safety valve has the basic structure including the following main parts.

-  Body:  The valve body is the part that occupies the most area of ​​the valve, protecting the valve & connecting the internal components. The body has a connection to the main pipe (Inlet) - the pipe needs to reduce pressure & the part connects to the pipe, draining the discharge fluid back to the source (Outlet). On the valve body there is a screw to adjust the nozzle. Depending on specific requirements, the valve body is usually made from cast iron, copper, steel or stainless steel, which directly determines the cost of the valve.

- Spring:  Spring is likened to the valve's controller, directly linked to the valve disc, creating push pressure and keeping the valve disc in the closed state.

- Valve cover: The valve body cap is assembled with the valve body, which is guaranteed to be sealed with a gasket to create a complete valve body without leakage.

- Disc shaft:  The valve shaft is located in the spring, with the purpose of directing the movement of the valve disc only in a certain stroke.

- Disc valve: Is the part in direct contact with the medium, usually made of materials with anti-corrosion, good pressure & temperature resistance. The valve disc is responsible for preventing the flow of fluid by sealing the nozzle, relying on the thrust of the spring.

- Adjustment screw:  Has the effect of regulating valve inlet pressure.

- Retractor arm: Depending on the design of each manufacturer, the valve is equipped with a recoil arm or not, it is used so that the user can pre-discharge excess fluid even though the pressure has not reached the limit threshold. Pulling the lever will help open the valve disc to drain the fluid out.

* Advantages & disadvantages of direct acting safety valves:

Advantages:

- Direct safety valve has simple structure, fast response speed.

- Suitable for small sized valves.

- Can be used in most systems.

- Lower cost compared to other safety valves.

- Diversified with many types of connections: threaded connection, flange connection, etc.

Defect:

- Valve size is limited by spring size, so this type of valve is not usually used for large sizes.

- Springs may corrode in the case of corrosive media.

Backflow pressure can affect valve function.

b. Indirect-acting safety valve: It is the first choice when it is necessary to protect hydraulic systems and large-capacity water in different locations.