Safety Valve: characteristics and categorization

March 18, 2019
Industry News

The safety valve is a valve type pressure relief device, which is mainly consistent of three parts: valve seat, disc and loading mechanism. The safety valve is an automatic valve that uses the pressure of the medium to discharge a rated amount of overpressured fluid through the opening of the disc to prevent the pressure inside the device from exceeding a predetermined safety value. When the pressure returns to normal, the disc automatically closes under the action of the loading mechanism to prevent the medium from continuing to drain.

Structure features:
1. According to the the overall structure and loading mechanism, the safety valve is divided into: lever type, spring type.
Lever safety valve: it balances medium action on disc with weight hammer and lever. The load on the valve is not increased by the rise of the disc. The structure is simple and cumbersome, limited the use in medium and low pressure situations. The loading mechanism is sensitive to vibration and often leakage due to vibration. This valve is suitable for high temperature cases, but the return pressure is usually lower than 70% of the working pressure to keep seal, so it is not suitable for continuous operation system.
Spring safety valve: the force of the compression spring is used to balance the force of the medium acting on the disc, and the opening pressure of the valve is adjusted by adjusting the nut and the compression amount of the helical coil spring. Its structure is compact and its sensitivity is high. Installation location is not strictly restricted. It is widely used and is not sensitive to vibration. And can be used for mobile containers. Spring force varies with valve disc opening, which is not conducive to the rapid opening of the disc. The long-term high temperature will affect the spring force. Therefore, the heat insulation or heat dissipation of the spring must be considered on the container with higher temperature, but the structure is complicated.

2. According to the ratio of disc opening height to flow diameter, the safety valve is divided into: full-opening type, micro-opening type.
Fully-opening safety valve: The nozzle type seat is used. With the medium flow rate increasing, the disc is given momentum. The recoil mechanism is used to change the flow direction of the ejection medium, and the momentum is changed into a huge valve lift, so that the disc increases the opening height (The opening height is greater than or equal to 1/4 of the flow diameter). This valve is suitable for applications where the relieving capacity is large and the pressure fluctuation is small.
Micro-opening safety valve: opening height 1/40-1/20 flow path diameter, it is convenient to manufacture, repair and adjust, and it should be used in occasions where the relieving capacity is small and the requirements are not high. The same relieving capacity, its diameter is much larger than the full-open type.

3. According to the gas discharge, the safety valve is divided into: closed safety valve, open safety valve.
Closed safety valve: the bonnet is closed, and all the medium are discharged through the discharge pipe and not leaked. The closed safety valve mainly used on containers with toxic and flammable media.
Open safety valve: the bonnet is open, and the inner chamber of the spring is open to the atmosphere. Without the connection structure of the exhaust pipe, the medium is directly discharged to the atmosphere. It is mostly used in containers such as air and steam media.

4. According to the principle of action, the safety valve is divided into: direct-acting safety valve, indirect-acting safety valve.
Direct-acting safety valve: It directly relies on the force generated by the medium pressure to overcome the mechanical load acting on the disc to open the disc.
Indirect-acting safety valve: It is consistent of a main valve and a pilot valve. The closing load applied to the main disc is provided by the working medium pressure and its size is not limited. The main valve acts on the piston by means of a medium discharged from the pilot valve, driving the main valve to open. The main valve is sealed.

5. According to the back pressure balance mechanism, the safety valve is divided into back pressure balance type and conventional type.
Back pressure balanced safety valve: using bellows, pistons or diaphragm elements to balance the back pressure, before the valve opens, the back pressure forces on the upper and lower sides of the disc balance each other. The spring and guide mechanism are separated from the medium by means of bellows to prevent the failure of these important components due to the corrosion of the medium. It can also prevent leakage before opening and back seat.
Conventional safety valve: without back pressure balancing elements.