Design of the safety valve

The design of a safety valve always includes two main components - a closure element and a setpoint device that provides a force on the sensitive element. The closure element consists of a disc and a seat, while the setpoint device can be a spring or a lever mechanism.

Spring-loaded safety valves - in these valves, the force of the spring opposes the water pressure on the disc. The spring force determines the set pressure, and the adjustment range is determined by the spring's elasticity.

The reliable construction and low cost of spring-loaded safety valves have led to their widespread use in engineering systems and small industrial installations.

Some safety valve manufacturers supply their devices with factory settings for a certain pressure, and adjustments for others can be made on-site during commissioning by changing the compression force of the spring.

Lever safety valves - the force created by the lever mechanism opposes the water pressure. The weight and length of the lever determine the set pressure and adjustment range. Lever safety valves are no less reliable than spring-loaded ones, but they have a slightly higher cost and are available in connection sizes DN50 and larger. Lever valves have found widespread use in industry.

Safety valves are divided into low-lift and full-lift based on the disc lift height.

Low-lift safety valves - the disc lift height does not exceed 0.05 times the seat diameter. Proportional opening mechanism is usually used in low-lift valves. They have low flow capacity, simple construction, and lower cost compared to full-lift valves.

Full-lift safety valves - the disc is lifted to a height that is greater than or equal to the seat diameter. A two-position opening mechanism is typically used in full-lift valves. Such valves have high flow capacity and are installed in systems with compressible media, such as steam or compressed air. The construction of a full-lift safety valve is more complex than that of a low-lift valve, so its price is correspondingly higher.

Based on the action when reaching the opening pressure, safety valves are divided into proportional and two-position valves.

Proportional safety valves - open proportionally to the pressure increase, with the lift of the disc increasing the volume of the discharged medium. Proportional valves are used for water and other non-compressible media, although the design does not exclude the possibility of use with compressible media.

Compared to two-position valves, proportional safety valves have the following advantages:

Low cost
Simple construction
Suitable for use with water and other liquids
No oscillations occur in systems with variable emergency flow rate
Ability to open to the extent necessary to establish the working parameters

Two-position safety valves - open instantaneously to full stroke when the opening pressure is reached. They are recommended for compressible media, such as steam, air, or gas.

The main disadvantage of two-position valves is the valve chatter. The operation of the safety valve in the chatter mode is possible:

If the valve size is oversized
In systems with variable emergency flow rate
If the time of emergency flow rate rise to its maximum value exceeds the time of full valve opening

Using two-position safety valves for water or other non-compressible liquids has two peculiarities:

The sharp opening of the valve leads to a large discharge of water and a sharp drop in pressure.
After a sharp drop in pressure, the valve closes instantly and provokes a hydraulic shock.