• The core technologies of safety valves

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The core technologies of safety valves

  • Mar 02, 2020
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A safety valve is an automatic valve that does not use any external force and uses the force of the medium to discharge a rated amount of fluid to prevent the pressure from exceeding the rated safe value. When the pressure returns to normal, the valve closes again and prevents the medium from flowing out. (TSG ZF001-2006) Safety valves are mainly used on boilers, pressure vessels, and pipeline equipment to ensure that no accidents occur in the system due to excessive pressure, which plays an important role in protecting the equipment operation and the safety of operating personnel.

As the core component of the safety valve, the spring needs to meet a series of detailed requirements and strict inspection procedures before being installed on the safety valve. When selecting the spring material for safety valves, the influence of working medium and temperature must be fully considered. Corrosion-resistant materials should be used or the surface of the spring should be coated (plated) with a layer of corrosion-resistant and rust-proof materials. For springs used in high or low temperature situations, it is necessary to fully consider the effect of temperature on the amount of spring deformation and the creep or cold brittleness of the spring material.

The spring of the safety valve should meet the following requirements:
1. A cylindrical spiral compressive spring should be used;
2. The structure of the safety valve can prevent the discharged medium from directly attacking the spring, especially for hot, corrosive fluids;
3. The spring design can ensure that when the safety valve is fully opened, the spring deformation is equal to 20%-80% of the maximum spring deformation, and the maximum shear stress of the spring is not greater than 80% of the allowable shear stress.

In order to ensure the stability of the spring during long-term work, the spring should be subjected to strong pressure processing and a strong pressure processing report should be provided. The results should meet the following requirements:
1. The spring is parked after being pressed from the free unsupported height to the height of doubling-up at room temperature, and the residual deformation of the free height is measured 10 minutes later, which should not be greater than 0.5% of the free height;
2. For springs of the same specification and the same heat treatment furnace, the deformation (or stiffness) under the specified load should be measured, and the deviation must not be greater than 10%.

The spring manufacturing unit shall provide the performance test report of the spring, the heat treatment report of the same specification and the same heat treatment furnace, and the qualification certificate one by one. After receiving the spring, the safety valve manufacturer should check the material report and download the relevant material specifications, test the size and material, and check the furnace batch number. The material report should include the material certification of the spring, heat treatment records, pressure treatment records and pressure test records.

After receiving the spring, the safety valve manufacturer needs to re-check the size, stiffness coefficient and deformation according to the national standard requirements. The test report should include the spring load, deflection, R value, free length, compressed length and other data. The spring can be installed on the safety valve after passing the factory acceptance test.

There are three main types of safety valve structures: spring-loaded, lever-operated and pilot-operated. The spring type refers to a safety valve where the sealing of the valve disc and the valve seat depends on the force of the spring; the lever type depends on the force of the lever and the hammer; the pilot-operated safety valve is designed to meet the needs of large capacity. It is a pulse safety valve composed of a main safety valve and an auxiliary valve. When the medium pressure in the pipeline exceeds the specified pressure value, the auxiliary valve opens first, then, the medium enters the main safety valve along the conduit and opens the main safety valve, so that the increased medium pressure is reduced.

There are many different classification methods of safety valves according to their structure, opening method, discharge method, and working principle. Among them, the common types of safety valves are:
Common spring-loaded safety valve: a safety valve that relies on direct mechanical loading devices (such as springs) to overcome the force generated by the pressure of the medium under the disc;
Balanced spring-loaded safety valve: a safety valve that takes measures to minimize the effect of back pressure on the continuity of action;
Spring-loaded safety valve with pneumatic device: a safety valve that opens when the pressure is lower than the normal setting pressure by means of a power assist device;
Pilot-operated safety valve: A safety valve that is driven or controlled by the discharge of medium from a pilot valve.

As the safety valve is a type of special equipment, it is necessary to strictly abide by international norms and standards during selecting. When selecting a safety valve, we need to consider the operating conditions at the job site, the valve parts material and the connection methods of the valve parts, and related media parameters.