Thought question 3: How should safety valves for vacuum systems be designed?
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Thought question 3: How to design, calculate, and select safety valves for vacuum vessels. Are there any exact standards? There is a statement in GB150 that it is for preventing overpressure; it should not cover protection against vacuum, right?
Example of safety valve calculation: The safety valve is one of the safety accessories that enable pressure vessels to release excess pressure during operation, and it is also a mandatory item in the regular inspections of such vessels. It includes two main series: overpressure protection and vacuum protection. One series is designed to discharge the overpressured medium inside the container to prevent it from failing, while the other is intended to draw in external medium to prevent a loss of the container’s stiffness. Pressure vessels that fall within the scope of application of the Code for Pressure Vessels shall be equipped with safety valves in accordance with the requirements of the design drawings. I. Methods for selecting safety valves
a) Determine the safety valve based on calculations. The nominal diameter must ensure that the discharge capacity of the safety valve is ≥ the safe discharge capacity of the pressure vessel.
b) Determine the pressure rating of the safety valve according to the design pressure and design temperature of the pressure vessel.
c) For safety valves used with steam at a opening pressure greater than 3 MPa or for gases with a medium temperature exceeding 320°C, a type equipped with radiators (fins) should be selected.
d) For media that are flammable or extremely or highly hazardous, closed-type safety valves must be used; if a valve with a lifting mechanism is required, it should be a closed-type valve equipped with a wrench.
e) When the back pressure that the safety valve may experience varies by more than 10% of its opening pressure, a safety valve equipped with a bellows should be chosen.
f) For non-hazardous media such as air, hot water above 60°C, or steam, a safety valve equipped with a wrench is appropriate.
g) For liquefied gas tankers, built-in safety valves should be used.
h) Select an appropriate material for the safety valve based on the properties of the medium: for example, copper or copper-containing safety valves cannot be used for ammonia-containing media; acetylene cannot be handled using safety valves made of 70% copper or pure copper.
i) For applications with high discharge rates, fully open-type safety valves should be used; for applications with stable working pressures and low discharge rates, slightly open-type safety valves are suitable; for high-pressure applications with high discharge rates, non-direct-acting safety valves such as pulse-type valves are preferable. For containers longer than 6 meters, two or more safety valves should be installed.
j) For fixed containers with low working pressures Pw, gravity-type (pressure cooker type) or lever-weight type safety valves can be used. Mobile equipment should use spring-type safety valves.
k) For media that are thick and prone to clogging, it is advisable to use a discharge device that combines a safety valve with a rupture disc.
l) The spring operating pressure rating should be selected based on the nominal pressure of the safety valve.
a) Determine the safety valve based on calculations. The nominal diameter must ensure that the discharge capacity of the safety valve is ≥ the safe discharge capacity of the pressure vessel.
b) Determine the pressure rating of the safety valve according to the design pressure and design temperature of the pressure vessel.
c) For safety valves used with steam at a opening pressure greater than 3 MPa or for gases with a medium temperature exceeding 320°C, a type equipped with radiators (fins) should be selected.
d) For media that are flammable or extremely or highly hazardous, closed-type safety valves must be used; if a valve with a lifting mechanism is required, it should be a closed-type valve equipped with a wrench.
e) When the back pressure that the safety valve may experience varies by more than 10% of its opening pressure, a safety valve equipped with a bellows should be chosen.
f) For non-hazardous media such as air, hot water above 60°C, or steam, a safety valve equipped with a wrench is appropriate.
g) For liquefied gas tankers, built-in safety valves should be used.
h) Select an appropriate material for the safety valve based on the properties of the medium: for example, copper or copper-containing safety valves cannot be used for ammonia-containing media; acetylene cannot be handled using safety valves made of 70% copper or pure copper.
i) For applications with high discharge rates, fully open-type safety valves should be used; for applications with stable working pressures and low discharge rates, slightly open-type safety valves are suitable; for high-pressure applications with high discharge rates, non-direct-acting safety valves such as pulse-type valves are preferable. For containers longer than 6 meters, two or more safety valves should be installed.
j) For fixed containers with low working pressures Pw, gravity-type (pressure cooker type) or lever-weight type safety valves can be used. Mobile equipment should use spring-type safety valves.
k) For media that are thick and prone to clogging, it is advisable to use a discharge device that combines a safety valve with a rupture disc.
l) The spring operating pressure rating should be selected based on the nominal pressure of the safety valve.