A safety valve is a critical safety component for boilers, pressure vessels and other pressure equipment as it effectively prevents explosion incidents resulting from overpressure of production devices.
As such, safety valves play a crucial role in chemical plants.
To gain a comprehensive understanding of safety valves, it is essential to examine their classification, features, selection criteria, and installation requirements.
Classification and characteristics of safety valves
According to the difference of overall structure and loading mechanism
Safety valves can be categorized into three types based on their general structure and loading mechanism: heavy hammer lever type, spring type and pulse type.
Of these three, the spring-type safety valve is the most used.
1. Heavy hammer lever safety valve
The heavy hammer lever safety valve operates using a heavy hammer and lever to balance the force on the valve disc.
Taking advantage of the lever principle, a smaller mass can generate a greater force by increasing the length of the lever, and the opening pressure of the safety valve can be adjusted by modifying the position or mass of the weight.
Benefits:
The heavy hammer lever safety valve has a simple structure and is easy to adjust accurately. The load does not increase significantly as the valve disc rises.
It is suitable for high temperature applications and has been widely used in the past, particularly in boilers and pressure vessels that required high temperatures.
Disadvantages:
However, the heavy hammer lever safety valve has a bulky structure and the loading mechanism is prone to vibration and leakage.
Furthermore, it has a low readjustment pressure, making it difficult to close well after opening.
2. Spring-type safety valve
The spring-loaded safety valve uses the force of a compression spring to balance the force on the valve disc.
Spring compression can be adjusted by turning the adjusting nut, allowing the opening (adjustment) pressure of the safety valve to be modified as needed.
Benefits:
The spring-loaded safety valve has a light and compact structure, is highly sensitive, can be installed in any position and is resistant to vibrations. It is ideal for use in mobile pressure vessels.
Disadvantages:
However, the load on the safety valve changes as the valve opens, meaning that as the valve disc rises, the spring compression increases and the force on the valve disc increases. This may affect the quick opening of the safety valve.
Furthermore, prolonged exposure to high temperatures can reduce the elasticity of the spring, making it necessary to consider thermal insulation or heat dissipation when used in containers with high temperatures. This makes the structure more complex.
3. Pulse safety valve
The pulse safety valve is composed of a main valve and an auxiliary valve and operates using the pulsed action of the auxiliary valve to actuate the main valve.
Its structure is complex and is normally only suitable for boilers and pressure vessels that require large safety discharge capacity.
Classification based on average discharge modes:
Based on the medium discharge mode, safety valves can be classified into three types: fully closed, semi-closed and open.
1. Fully closed safety valve
The fully closed safety valve discharges all gas through the exhaust pipe, preventing any media from leaking out during operation.
This type of safety valve is mainly used in containers that contain toxic and flammable gases.
2. Semi-closed safety valve
In a semi-closed safety valve, part of the discharged gas passes through the exhaust pipe while the rest leaks through the space between the valve cap and the valve stem.
This type of safety valve is mainly used in containers that contain gases that do not harm the environment.
3. Open the safety valve
The open safety valve has an open valve cap that connects the spring chamber to the atmosphere, making it easier to reduce the spring temperature.
This type of safety valve is mainly used in containers that contain vapor such as medium or high temperature gases that do not harm the atmosphere.
According to the relationship between the maximum opening height of the valve disc and the flow channel
The safety valve is mainly divided into micro-open safety valves and fully open safety valves based on the relationship between the maximum opening height of the valve disc and the diameter of the flow path of the safety valve.
1. Microlift safety valve
The opening height of a micro-open safety valve is less than 1/4 of the channel diameter, generally 1/40 to 1/20 of the channel diameter.
This type of safety valve operates through proportional action and is used mainly in liquid applications and sometimes in gaseous applications with low flow rates.
2. Full lift safety valve
The opening height of a fully open safety valve is equal to or greater than 1/4 of the flow channel diameter.
The discharge area of a fully open safety valve is equal to the minimum cross-sectional area of the valve seat throat.
This type of safety valve operates via two-stage action and requires a lifting mechanism to fully open. It is mainly used in mid-gas applications.
According to the principle of action
Safety valve can also be classified into direct acting safety valves and indirect acting safety valves based on the principle of action.
1. Direct acting safety valve
The direct acting safety valve operates by the direct action of the working medium, which means that the pressure of the working medium opens the valve overcoming the mechanical load imposed by the loading mechanism on the valve disc.
This type of safety valve has a simple structure and operates quickly with high reliability. However, due to limitations in its structure, it is not suitable for high pressure and large diameter applications.
2. Non-direct acting safety valve
The direct acting safety valve can be further divided into two types: pilot safety valves and safety valves with auxiliary power devices.
Safety valve selection instructions
Determination of various safety valve parameters:
- Determine the rated pressure of the safety valve:
Choose nominal pressure based on valve material, operating temperature and maximum working pressure.
- Determine the working pressure level of the safety valve:
The working pressure level should be selected based on the design pressure and design temperature of the pressure vessel.
It is important to note that the safety valve working pressure has a different meaning than the spring working pressure.
The working pressure of the safety valve refers to the static pressure in front of the valve during normal operation, which is equivalent to the working pressure of the protected system or equipment.
In contrast, spring working pressure level refers to the spring's allowable pressure range.
Within this range, the opening pressure (i.e., set pressure) of the safety valve can be adjusted by changing the spring preload compression.
Safety valves with the same pressure rating may have different working pressure levels based on spring design requirements.
When selecting a safety valve, it is crucial to determine the working pressure level based on the required opening pressure value.
- Determine the relief valve discharge pressure (PD):
The discharge pressure of a safety valve is typically 1.1 times the set pressure (opening pressure), while the discharge pressure of a steam boiler safety valve is 1.03 times the set pressure.
- Determine the size of the safety valve:
The discharge capacity of the safety valve must be determined based on the required discharge, and the discharge capacity of the safety valve must be equal to or greater than the required discharge.
The required discharge of the protected system refers to the quantity that must be expelled to avoid overpressure in the event of abnormal system overpressure.
This value is determined by the working conditions, capacity and potential overpressure of the system or equipment.
- Determine material selection:
The material of the safety valve must consider the working temperature and pressure of the medium, the properties of the medium and the feasibility and economy of the material.
Determination of the Special Structure of Safety Valves
① For steam with an opening pressure of more than 3 Mpa or gas with an average temperature of more than 320 ℃, a safety valve with a radiator (fin) should be selected.
② In the case of a safety valve that supports additional backpressure and if the change in backpressure exceeds 10% of the setting pressure, a bellows safety valve should be chosen. In addition, for a safety valve that deals with a corrosive medium, a bellows safety valve must also be selected to prevent corrosion of the spring and guide mechanism by the medium.
③ For flammable, highly toxic or extremely dangerous media, a closed safety valve must be used. If a safety valve with lifting mechanism is required, a safety valve closed with a wrench must be used.
④ For non-hazardous media such as air, hot water or steam above 60℃, a wrench safety valve should be adopted.
⑤ For liquefaction tank (tank) trucks, a built-in safety valve must be adopted.
⑥ For working conditions with large discharge capacity, the fully open type should be selected. For working conditions with stable working pressure and small discharge capacity, the micro start type should be chosen. For high pressure conditions with large discharge capacity, an indirect starting type such as a pulse safety valve should be selected. For vessels longer than 6m, two or more safety valves must be installed.
⑦ For fixed vessels with low working pressure, a static weight type (pressure cooker) or lever weight type safety valve can be used. For mobile equipment, a spring-type safety valve must be adopted.
⑧ If the medium is thick and prone to blockage, a relief device combined in series of a safety valve and rupture disc should be selected.
Safety valve installation position and requirements
1. Installation position
① The safety valve must be installed vertically upwards.
② The installation position should be as close as possible to the protected equipment or pipeline.
③ The safety valve must be installed in a location that is easy to maintain and adjust, with sufficient space around it.
④ The safety valve for a pressure vessel must be installed in the gas phase space above the liquid level of the vessel or in the pipeline, with the connection point located in the gas phase space of the pressure vessel.
⑤ For containers and equipment containing flammable, toxic or viscous media, a shut-off valve can be installed in front of the safety valve. However, the flow area of the block valve must not be less than the minimum flow area of the safety valve, and a lead seal must be installed to ensure that the block valve is fully open and normally open.
⑥ To reduce the influence of self-gravity of the valve and avoid stress fatigue and discharge vibration fatigue, the safety valve should be installed opposite the block valve.
⑦ If the safety valve may be blocked or corroded by materials, a rupture disc should be placed in front of its inlet, an inspection valve should be installed between the safety valve and the rupture disc, and anti-blocking measures such as purging rear, heat tracing or thermal insulation must be applied to your inlet piping.
⑧ The safety valve installed on the pipeline should be placed in a place where the fluid pressure is relatively stable and at a certain distance from the source of fluctuation. It should not be installed in the dead corner of the horizontal piping.
⑨ For pipelines, heat exchangers or pressure vessels with liquid medium, when the valve is closed, thermal expansion may result in high pressure. To solve this, the safety valve can be installed horizontally to discharge the liquid directly downwards.
⑩ For emergency air discharge, the tube hole must be flat, not sharp and free from burrs to prevent electrostatic discharge.
The safety valve installation point must not subject it to excessive backpressure and must be within the specified allowable range. The safety valve body must be supported stably.
For containers with extremely dangerous or flammable and explosive media, the safety valve outlet must be led to a safe location and handled properly.
If more than two safety valves share a discharge pipe, the cross-sectional area of the discharge pipe shall not be less than the sum of the cross-sectional areas of the outlets of all the safety valves. However, oxygen or combustible gas and two other gases that can produce a chemical reaction with each other cannot share a discharge tube.
2. Installation Requirements
① For corrosive media, the safety valve must be installed in combination with a rupture disc.
② For highly toxic media, a safety valve with good sealing should be selected.
③ For high-temperature media, high temperature can have a significant impact on the spring, so spring-loaded safety valves should be avoided.
④ For important or fire-vulnerable safety valves, a spray protection system must be installed.
⑤ For safety valves for spherical tanks, double safety valves must be installed, and the pressure relief capacity of any safety valve must be sufficient to meet the pressure relief safety requirements of the spherical tank.
⑥ The safety valve must be equipped with an electrostatic jumper.
⑦ To prevent the safety valve from opening and closing repeatedly, causing vibration and damaging the valve, the pressure drop in the safety valve inlet pipeline must be reduced. This can be achieved by increasing the diameter of the inlet tube and shortening the inlet tube section.
