Should both sets of safety valves on a pressure vessel be in use, or one in use and one as a backup?

This post was last edited by ylb913 on 2016-9-17 at 11:54. Our two safety valves are put into use simultaneously, except for those with interlock switches. There are three situations in which two or more safety valves are required: 1. Due to limitations in the size of the safety valves, a single safety valve is not sufficient to meet the operational requirements ； 2. The two safety valves are provided to take into account different operating conditions; this helps to prevent large fluctuations in the device when only low-level discharge is required, and it also allows for different methods of handling emissions depending on the operating conditions. Two safety valves with different set pressures serve different purposes. A safety valve with a low setting pressure is called a control safety valve, while one with a slightly higher pressure setting is known as a service safety valve. The control safety valve activates first, serving to give an alarm signal and also helping to reduce the pressure in the boiler. If the pressure in the boiler continues to rise despite this, then the service safety valve activates. This approach helps to reduce steam waste and prevent excessive fluctuations in boiler pressure. 3. Sometimes, in order to meet the requirements for safety valve calibration without affecting production, two safety valves can be used; one of these valves must be capable of handling all operating conditions, and there should be a manual valve in front of the safety valve. In this case, the safety valve can also be verified online. API 520 and ASME both provide specifications for the set pressure of multiple safety valves. ASME VIII.1 specifies that for boilers (waste heat boilers or steam drums), if the gas generation rate is above 1800 kg/h (it seems to be this value), dual valves must be installed, and the maximum set pressure of these dual valves shall not exceed 3% of the vessel’s design pressure. For different operating conditions, API2000 provides two explanations for the use of dual valves in low and medium pressure storage tanks: 1) as a safety backup; 2) to provide an additional valve that meets the drainage requirements in extreme situations such as fires, thereby preventing a single valve from opening frequently due to non-extreme conditions ； Parallel double valves are installed on critical equipment to facilitate online maintenance; isolation valves are placed before and after each valve, with minimum mechanical interlocking required.

This depends on the calculation sheet provided by the manufacturer. The safe discharge capacity of the container requires several safety valves for release. When 2 safety valves need to be installed, the operating pressure of one safety valve should not exceed the design pressure, while the operating pressure of the other safety valve can be increased to 1.05 times the design pressure. Generally, spare safety valves are rarely installed on containers.

We keep all of ours fully open to prevent a safety valve from failing to open in case of overpressure

This is related to the design and selection of your safety valves; some systems require two sets of safety valves in order to meet the required discharge capacity, while others use one valve in operation along with another as a backup. You can find out by checking the safety valve design documents

The drawing indicates one in use and one as backup; having all of them switched on doesn’t serve much purpose

The design specifies that one can be in use while another serves as a backup

This is mainly for safety reasons, out of concern that the safety valves might fail to open; however, the possibility of both safety valves failing at the same time is extremely low. So they should be used simultaneously; it’s similar to the principle of having three sets of instruments installed on an airplane. Of course, it may also be the case that even the largest-sized safety valves are not sufficient to meet the requirements, or when the installation location does not allow for the use of large safety valves; in such situations, two safety valves are installed. But in any case, they should be used simultaneously. As for using one and keeping another as a spare, that doesn’t make sense. Safety valves are generally calibrated every six months; once they are installed, it’s assumed that they are in use, and they need to be recalibrated after six months.