Selection of the set pressure for safety valves in atmospheric-pressure reactions

The safety pressure relief measure for reactor loss-of-control scenarios is usually the installation of rupture discs. Following a reaction safety risk assessment conducted by a professionally qualified company, the conclusions and recommendations in the assessment report indicate that: through research and analysis involving reaction risk tests under various risk conditions, the worst-case scenario for accidental release and the corresponding release rate can be determined with reasonable certainty; thus, it remains feasible to select a safety valve. However, the poster should first determine the design pressure of the equipment in the reaction system, such as the reactor and condenser, before deciding on the setting pressure of the safety valve; the design process is carried out in reverse order.

It’s like this: the design pressure for the reactor under normal pressure is 4 kilograms, and the mechanical seal used is a 212 single-face mechanical seal; theoretically, it can withstand a pressure of 2.5 kilograms, but in actual use it can handle pressures ranging from 0.1 to 0.15 MPa. The condenser is designed to withstand a pressure of 0.25 MPa. Therefore, the setting pressure for the safety valve should be less than or equal to 0.1 MPa, so as to ensure that the pressure released during venting does not exceed the system’s minimum pressure tolerance. Is that correct? So, the value to use is 0.1x1.05=0.105MPa, right?

If the set pressure of the safety valve is set at a value close to or slightly above 0.1 MPa, equipment such as reaction vessels must be registered and managed as pressure vessels. If, from a ‘safety perspective’, a pressure higher than 0.1 MPa is indeed required, then there is no need to deliberately avoid regulating it as a pressure vessel. Additionally, the issues that need to be considered are: 1) How are the release conditions and amount under accident conditions in the reactor determined? 2) Can the opening speed of the safety valve adapt to the pressure rise rate under accident conditions in the reaction vessel? What is the maximum overpressure value that can be reached during the opening of a safety valve?

The local regulatory authorities here require that all reaction equipment be registered and managed as pressure vessels. In actual use, the weak point of the entire system is the sealing surface of the mechanical seal, which can withstand a pressure of only 0.1 MPa. What’s confusing now is that if the opening pressure of the safety valve is higher than the pressure that the mechanical seal can handle, then most of the pressure inside the reactor will escape through the mechanical seal. The relief conditions under accident conditions in the reaction vessel can be met by the safety valve; if not, we will replace it with a burst disc. The overpressure value has not been taken into consideration yet. The setting for the opening pressure of the safety valves in our plant (for reactions at normal pressure) is 0.1 MPa. I just want to understand why it is 0.1 MPa And not some other values?

Personally, I believe it’s difficult to find a proper justification for setting the set pressure of the safety valve in an atmospheric-pressure reactor system at 0.1 MPa; it also seems rather unreasonable. In that case, the condensers and gas-phase pipelines in the reaction system must be upgraded to regulated equipment or pressure pipelines.

As a supplementary note, we are a fine chemicals company, not a petrochemical company.

I was wondering, in a situation like ours, how should it be handled? For chemical processes subject to key supervision, is it necessary to install safety valves? Or what would be a reasonable solution?

Refer to the conclusions and recommendations in the reaction safety risk assessment report prepared by the professional company commissioned by you, as well as the project HAZOP analysis report. It is impossible to make a preliminary assessment without materials such as the process manual for the specific reaction system in your project.