The design pressure of the liquid nitrogen tank is 3.08 MPa; the designed burst pressure of the rupture disc is 3.38±5%, and the pressure used for routine pressure testing is 3.4 MPa

During pressure testing, this explosion-proof valve should be closed. Open it when in use. Additionally, this liquid nitrogen tank should have a vacuum insulation layer, and all the necessary procedures for its use have been completed at the time of manufacture. Check the regulations for the safe use of pressure vessels.

I mainly want to ask whether there is a safety risk when the operating pressure of the equipment exceeds the designed pressure (but does not reach the burst pressure specified for the rupture disc)

The phenomenon you mentioned does occur: usually, the safety valve activates first, and only when the pressure becomes even higher does the explosion-proof membrane rupture to release pressure.

I understand these points; I mainly want to ask whether it is necessary to replace the burst disc (design burst pressure)

It’s usually fine and doesn’t need to be replaced, but it’s advisable to have two. Sometimes, they also break if the time without overpressure is too long. It’s best to get in touch with the equipment manufacturer for purchases; there is a company in Shanghai that sells this product, but I’ve forgotten the name after all this time. It seems that the manufacturer can also be found on Taobao.

The designed burst pressure of the rupture disc is 3.38±5%; at a pneumatic test pressure of 3.4 MPa, the rupture disc may not burst under this pressure, but its service life will be **reduced** due to prolonged exposure to such high stress levels. The blasting pressure should be set to a value that is greater than the normal operating pressure and less than or equal to the design pressure of the equipment being protected.

1. The design temperature was not specified; the pneumatic test at normal temperature already exceeded the lower limit of the burst pressure. If there is a temperature factor, things could be slightly better if it can be adjusted for the rupture disc. 2. According to the operating instructions for the burst disc, it should be replaced even if it has not ruptured when the pressure exceeds the specified range for that burst disc. Because the rupture disc remains qualified even at a burst pressure of 3.22, it is impossible to predict whether its limit has been reached at this point. 3. It is also necessary to consider the type of rupture disc; if it is of the positive arch type, the pre-arch pressure is generally around 85% of the rupture pressure. The air pressure has raised the height of the rupture disc to its limit; a rupture disc that has reached this limit is likely to burst at a pressure lower than its bursting pressure in the future. There is unpredictability. 4. If it is an inverted arch shape, it can be slightly better. But such an increase is also undesirable.

1. The design temperature was not specified; the pneumatic test at normal temperature already exceeded the lower limit of the burst pressure. If there is a temperature factor, things could be slightly better if it can be adjusted for the rupture disc. 2. According to the operating instructions for the burst disc, it should be replaced even if it has not ruptured when the pressure exceeds the specified range for that burst disc. Because the rupture disc remains qualified even at a burst pressure of 3.22, it is impossible to predict whether its limit has been reached at this point. 3. It is also necessary to consider the type of rupture disc; if it is of the positive arch type, the pre-arch pressure is generally around 85% of the rupture pressure. The air pressure has raised the height of the rupture disc to its limit; a rupture disc that has reached this limit is likely to burst at a pressure lower than its bursting pressure in the future. There is unpredictability. 4. If it is an inverted arch shape, it can be slightly better. But such an increase is also undesirable.

So does this mean that the design burst pressure of the rupture disc needs to be adjusted?

First question: According to GB150.1 Appendix B 3.2.2, when multiple overpressure relief devices are installed, the operating pressure of one of them shall not exceed the design pressure. The rest are allowed to exceed the design pressure by 1.05 times. According to this rule, the designed burst pressure of this rupture disc is incorrect. However, if fire conditions are taken into account and the rupture disc is used as an auxiliary relief device, then according to B3.2.3, the design burst pressure of the rupture disc can be 1.10 times the design pressure. If the design specifications for the container refer to GB 18442-2011, then according to Article 4.2.1.3 of GB18442.6-2011, under fire conditions, the design burst pressure of the rupture disc as an auxiliary release device shall not exceed 1.16 times the design pressure of the container. In summary, if the design takes fire conditions into account, and provided that the setting pressure of the safety valve does not exceed the design pressure, then there is no issue with the designed burst pressure of the rupture disc, and no adjustments are necessary. Second question: The pressure for the pressure test you specified is the pressure required during the manufacturing of the inner cylinder. As for \"in-use containers\", it’s not possible nor necessary to achieve such high pressures, as these containers are connected to many valves and instruments, and it’s easy to lose pressure when such high pressures are applied. If a pressure test is indeed to be carried out, it is recommended to apply a pressure equal to 1.1 times the actual maximum operating pressure of the inner cylinder.