Seek advice on blast discs

Refer to standard HG/T20570 95, which provides detailed information on this topic; the relevant provisions are as follows: 9.0.1 A burst disc is installed at the inlet of the safety valve. In order to prevent significant loss of process material due to the rupture of the burst disc, in situations where the safety valve cannot be used directly (such as when the material is corrosive or leakage must be absolutely avoided), a burst disc is generally installed at the inlet of the safety valve. The calibrated burst pressure of the rupture disc is the same as the set pressure of the safety valve. The nominal diameter of the burst disc shall be no smaller than the inlet diameter of the safety valve. The use of blast discs reduced the relief capacity of the safety valve by 20%. The pressure drop of the burst disc, measured in equivalent length, is 75 times the nominal diameter. 9.0.2 The rupture disc should be installed at the outlet of the safety valve. If there is a possibility of corrosive gases in the discharge pipeline, the rupture disc should be placed at the outlet of the safety valve to protect it from corrosion. The maximum designed burst pressure of the rupture disc shall not exceed 10% of the set pressure of the spring-loaded safety valve. The nominal diameter of the rupture disc is the same as that of the safety valve’s outlet pipe. The burst disc is installed near the outlet of the safety valve. The pressure drop of the burst disc, measured in equivalent length, is 75 times the nominal diameter. 9.0.3 Parallel use of burst discs and safety valves: To prevent a rapid increase in pressure inside the pressure vessel under abnormal operating conditions, or to increase the discharge area in case of a fire, one or several burst discs are installed in parallel with the safety valve. The calibrated burst pressure of the burst disc is slightly higher than the set pressure of the safety valve, and must not exceed the design pressure of the container. The rupture disc must have sufficient discharge area to meet the requirements for protecting the container.

I have a question: don’t safety valves generally have some degree of leakage? In the configuration where the rupture disc is connected in series behind the safety valve, could long-term use lead to leakage of the medium located before the safety valve into the space between the safety valve and the rupture disc, thereby causing the rupture disc to be damaged?

The arrangement of installing a burst disc in series behind a safety valve is based on the premise that the discharge pipeline is exposed to corrosive gases, in order to prevent corrosion of the safety valve; therefore, the corrosion resistance of the material used for the burst disc should be higher than or equal to that of the safety valve. If a slight leak of the medium in front of the safety valve can cause the rupture disc to break, there must be an issue with the material chosen for the rupture disc.

There are issues with this standard: 1. The pressure tolerance for high-quality international rupture discs is 5%; although BS&B claims to be able to achieve 3%, rupture discs are devices that cannot be tested without being damaged, so it’s not possible to determine the exact tolerance of individual rupture discs. A tolerance of 5% is more appropriate for most applications. 2. As for lower-quality rupture discs, who knows? 3. It is recommended that positively curved rupture discs operate at 75% of the valve’s discharge pressure, while negatively curved ones can operate at up to 85% of that pressure. 4. Using rupture discs can significantly reduce the cost associated with valves, but it also lowers the operating pressure of the system. Designers need to find an appropriate balance in such situations

3. The burst disc safety device is connected in series on the outlet side of the safety valve. 4.3.3.1 If the outlet side of the safety valve may be subject to corrosion or affected by external pressure sources, a burst disc safety device should be installed there to ensure the proper functioning of the safety valve. 4.3.3.2 The rupture disc safety device installed on mobile pressure vessels shall not be placed on the outlet side of the safety valve. 4.3.3.3 When the burst disc safety device is installed on the outlet side of the safety valve, the following requirements must be met: a) The discharge capacity of the combination of the burst disc safety device and the safety valve shall be no less than that of the protected pressure-bearing equipment; b) The chamber between the burst disc safety device and the safety valve shall be equipped with a pressure indicator, an exhaust port, and appropriate alarm indicators; c) At the burst temperature, the sum of the designed burst pressure of the burst disc and the pressure present in the discharge pipe shall not exceed any of the following values: —— the set pressure of the safety valve; —— the designed pressure of any pipeline or fitting between the burst disc safety device and the safety valve; —— the designed pressure of the protected pressure-bearing equipment. d) The discharge area after the rupture disc bursts should be large enough so that the flow rate is equal to the rated discharge capacity of the safety valve; e) No pipes other than those connected to the rupture disc should become blocked as a result of its bursting.