Principles for selecting safety valves

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Principles for selecting safety valves 　　1. Type and material 　　The type of safety valve is chosen based on the manner of loading. For containers with low working pressure but high temperatures, lever-type safety valves are generally used, while spring-type safety valves are more commonly selected for high-pressure containers； Choose based on the type of gas emitted: if the medium inside the container is a toxic, flammable, or explosive gas, or a refrigerant or other gas that can pollute the atmosphere, a closed system should be used; for air or other harmless gases, a semi-closed or open system can be employed ； Based on the degree of opening, full-opening safety valves can be used for high-pressure vessels, as well as medium and low-pressure vessels that have a large safety discharge volume but limited strength margin, in order to reduce the opening area of the vessels. When the medium is air or water vapor and regular opening tests of the safety valve are required, a safety valve equipped with a wrench should be used; for toxic, flammable, or explosive media, a wrench-free safety valve should be used unless there are special requirements. If a wrench is needed, then a closed-type safety valve with a wrench should be selected. 　　If there are no special requirements regarding the material of the safety valve body, and when the nominal pressure of the valve is less than or equal to 1.6 MPa, gray cast iron should be used ； Carbon steel is selected when the nominal pressure is less than or equal to 2.5 MPa ； For corrosive media, stainless steel should be chosen; for higher temperatures, chromium-molybdenum steel is preferable. 　　 　　2. Rated discharge capacity The rated discharge capacity of the safety valve must be greater than or equal to the safe discharge capacity of the container. The rated discharge capacity of a safety valve can be found on its nameplate, while the safe discharge capacity of a vessel can be calculated in accordance with the relevant provisions of GB150-1998. 　　An appropriate rated discharge capacity ensures that when the safety valve opens, its disc opens quickly and remains in the discharged state stably; the maximum pressure inside the container always stays within the range it can withstand. Once the pressure drops, the valve closes without any oscillation of the disc. If the rated discharge volume is too small, it can cause the pressure inside the container to exceed the allowable level, posing a danger; this is especially true for liquid media with very low compressibility. However, if the rated discharge volume is too large, the large discharge area causes the medium to be discharged rapidly when the valve disc opens, resulting in a sharp drop in pressure near the safety valve inside the container to the seating pressure. The valve disc, which has just risen, quickly falls back onto the seat, causing a violent impact that closes the valve. Yet the high pressure far from the valve remains unchanged; when the pressure near the safety valve rises again to the opening pressure, the valve disc opens once more. This cycle repeats, causing damage to the valve disc and the sealing surfaces. When used with liquid media, it can also lead to water hammer effects in the system. 　　 　　3. Operating pressure range: The maximum operating pressure of a safety valve must not be higher than its nominal pressure; otherwise, the strength of the valve body will not be sufficient. Nor should valves with excessively high nominal pressures be selected, as this results in high costs and affects the accuracy of valve operation. The nominal pressure indicated on the safety valve’s nameplate is the maximum pressure that the valve body can withstand when the operating temperature does not exceed 200°C; above 200°C, the pressure it can withstand will be lower than its nominal pressure. The nominal pressure of the safety valve and the maximum pressure it can withstand at different temperatures are shown in Table 7-1. 　　 　　Table 1-1 Nominal pressure and allowable pressure of spring-loaded safety valves
Valve body material Nominal pressure Maximum pressure allowed at the following temperatures /MPa
PN/MPa 200℃ 250℃ 300℃ 350℃ 400℃ 450℃ 500℃ 600℃
Carbon steel 1.57 1.57 1.47 1.28 1.18 0.98 0.69 ---- ----
2.45 2.45 2.26 1.96 1.77 1.57 1.08 ---- ----
3.92 3.92 3.63 3.24 2.94 2.75 1.77 ---- ----
Molybdenum-containing steel 1.57 1.57 1.57 1.57 1.57 1.47 1.28 0.88 0.59
2.45 2.45 2.45 2.45 2.45 2.26 1.96 1.37 0.88
3.92 3.92 3.92 3.92 3.92 3.53 3.14 2.16 1.37
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(III) Precautions for the installation and adjustment of safety valves
(1) Safety valves should be connected directly to the container itself and installed at the highest point of the container. For safety valves on liquefied gas storage tanks, they should be installed in the gas phase space; those used for liquids should be installed below the normal liquid level. Additionally, the nominal diameter of the inlet pipe of the safety valve must not be less than 15 mm. When it is difficult to install it on the container body due to special reasons, it can be considered for installation on the outlet pipeline; however, the pipeline between the safety valve and the container should be free of sudden bends or local reductions in cross-section, in order to avoid increased pipeline resistance and the accumulation of debris that could cause blockages. 　　(2) Regardless of where the safety valve is installed, the cross-sectional area of the connecting pipes and the openings in the fittings between the container and the safety valve must not be smaller than the inlet area of the safety valve. If a connection port is equipped with several safety valves, the area of this connection port should be at least equal to the sum of the areas of the inlets of those valves; ideally, it should be 1.25 times that sum. 　　(3) It is generally prohibited to install an intermediate stop valve between the container and the safety valve. However, for containers holding flammable, toxic, or highly viscous media, a stop valve may be installed between the container and the safety valve to facilitate the replacement and cleaning of the safety valve. The design and diameter of such a stop valve must not prevent the safe discharge of pressure from the safety valve; when the container is in normal operation, this stop valve must be kept fully open and sealed with a lead seal. 　　(4) If there is a possibility of condensate forming in the discharge pipe behind the safety valve, automatic drainage facilities should be considered ； The inner diameter of the discharge pipe of a closed-type safety valve shall not be smaller than the nominal diameter of the safety valve outlet pipe. In principle, each exhaust pipe should be equipped with its own valve, and it is prohibited to install any valves on the exhaust pipes. When two or more safety valves share the same exhaust pipe, the cross-sectional area of that exhaust pipe must be at least equal to the sum of the cross-sectional areas of the outlet pipes of all the safety valves. Additionally, the pressure drop in the exhaust pipe must be taken into account to prevent significant backpressure from being generated on the safety valves. Flammable gases such as oxygen, or other gases that can undergo chemical reactions when mixed, cannot share the same exhaust pipe. 　　(5) Before installation, the new safety valve should have its opening pressure and seating pressure adjusted on a test bench, and the sealing performance of the closing element should be checked. The opening pressure can be adjusted by modifying the load applied to the valve disc, such as the mass or position of the weight in a lever-type safety valve, or the position of the nut that compresses the spring in a spring-type safety valve (on the basis of properly selecting the spring pressure rating). The returning force can be adjusted by changing the position of the upper and lower adjustment rings of the safety valve. 　　 　　(IV) Use and Management of Safety Valves A properly selected and correctly installed safety valve must also be used and managed in a proper manner to fulfill its function. In daily use, to ensure the proper functioning of the safety valve, regular maintenance and inspection are necessary. The valve body should be kept clean to prevent rusting, and it must be protected from being blocked by oil residues or foreign objects. It is important to regularly check whether the seal on the valve is intact, to prevent the weight used in lever-type safety valves from becoming loose or displaced, and to avoid any unauthorized adjustment of the adjustment nuts on spring-type safety valves. In the event of a leak, the valve should be replaced or repaired promptly. It is forbidden to try to eliminate leaks by increasing the load, such as over-tightening the adjustment nuts of spring-type safety valves or adding weights to the levers of lever-type safety valves. To prevent the valve disc and seat from sticking to dirt such as oil residues in the gas, safety valves used for air, steam, or other gases that contain more viscous contaminants but do not cause environmental pollution when released, should be manually operated with a wrench on a regular basis to discharge air and wash away the dirt. 　　Safety valves should be inspected regularly, including their opening pressure, seating pressure, and sealing performance, with requirements that are the same as those for the calibration of safety valves. When the inspection fails, it should be disassembled; a detailed check should be carried out to determine whether there are any cracks, scratches, wear, corrosion, deformation, etc. in the various components, and after repair or replacement, it should be reassembled and inspected again. The periodic inspection interval for safety valves can be the same as that for the containers they are used in.