Installation settings of the safety valve
Thread Content
As shown in the figure, this is a hot water flash evaporation system for generating steam as a by-product. Pressure vessel 1 operates at a pressure of 0.5 MPa, while pressure vessel 2 operates at 0.8 MPa (this operating pressure has been determined and the necessary inspections have been completed). According to the standards, the setting pressure for the safety valve on the flash tank should be 0.85 MPa; however, with such a setting, the safety valve cannot fulfill its role of protecting the equipment (since this system is influenced by pressure vessel 1 and will never reach a pressure of 0.85 MPa), rendering the installation of the safety valve pointless； In such a case, how should the safety valve be set? Should the setting be based solely on pressure vessel 2, with a setting pressure of 0.85 MPa, or should it take the entire system into account and use the device with the lowest pressure resistance (pressure vessel 1) to determine a setting pressure of 0.55 MPa? Is there any regulatory support for the second one?
It appears that the DN300 pipeline is lacking power equipment; otherwise, it would be impossible to transport superheated water in the direction indicated by the arrows.
It is recommended to first consider the following two questions: 1) Can fluid pressure increase be achieved solely through pipes, without any power equipment? 2) Can steam still be flashed out after the hot water is pressurized?
To determine the design pressure of pressure vessel 1, if it is below 0.8 MPa, an additional safety valve must be installed on pressure vessel 1. Furthermore, superheated water at 0.5 MPa cannot flash into low-pressure steam at 0.8 MPa.
Under normal circumstances, it should be as you said, but in practice, for certain reasons, container 1 cannot be equipped with a safety valve. In that case, the safety valve that protects container 1 is installed on container 2, with a setting pressure of 0.55 MPa; this way, the entire system can be protected. However, this does not comply with the regulations!
The entire system can also be protected by a safety valve, provided that there are no valves used for separation within the system, and that the effective discharge capacity of the safety valve is sufficient to meet the safety discharge requirements of all pressure vessels. If isolation valves must be installed, measures must be in place to ensure that they cannot be accidentally closed. What is the design pressure of pressure vessel 1? Is it below 0.8 MPa? If it is below 0.8 MPa, the setting pressure of the safety valve should be calculated based on the design pressure of pressure vessel 1.
The design pressure of pressure vessel 1 is 0.6 MPa, while the pressure used during normal operation is 0.5 MPa. In my opinion, the setting of safety valve parameters should not be based solely on the installation location; rather, those parameters should be determined after a thorough analysis of the objects that the safety device is meant to protect. If a safety valve is installed on pressure vessel 1, then it is not necessary to install one on pressure vessel 2, as their function is the same. However, the design institute believes that the safety accessories installed on pressure vessel 2 can only be designed in accordance with vessel 2!
The design pressure of pressure vessel 1 is 0.6 MPa, and the set pressure of the safety valve of pressure vessel 2 is 0.85 MPa. How can this safety valve protect pressure vessel 1? By the time the system pressure reaches the setting pressure of the safety valve, pressure vessel 1 would have already exploded due to overpressure. So what the design institute said is completely correct. The energy source for this system comes from pressure vessel 1, so a safety valve must be installed on pressure vessel 1.
One option is to replace pressure vessel 1 with a device that can withstand higher pressures. If you don’t make this replacement, then the setting pressure of your safety valve must be lower than the design pressure of the device with the lowest pressure tolerance (unless the two devices are not part of the same system); otherwise, you’ll have to stop production for repairs during safety inspections