A typical requirement in case of an upset is that the depressurizing system should reduce the pressure of the vessel to a certain value (e.g.50%) of the design conditions within a certain time frame from several minutes to almost an hour. The depressurized gas is led from the vessel to a properly designed disposal system, typically to the flare.
A slow depressurizing is used as first means of controlling the process before the fast depressurizing is taken into consideration. A slow depressurizing is typically used in parallel with a fast emergency depressurizing, and it should be closed when high rate emergency depressurizing valves are operated due to the capacity limitations of the flare.
What about the valves then..?
High safety integrity and reliability are definitely the most important requirements for depressurizing valves. The valve trim capacity should match the process conditions, so that the depressurizing is done fast, but not too fast. The valves should of course be very tight both from seat and packing to avoid leakage and emissions of toxic gases. The valve should open in case of air failure, that calls for single acting spring to open actuators with high safety factors. An operational depressurizing valve is normally equipped with a solenoid valve and a valve controller for dual function of the valve. Instrumentation related to an emergency depressurizing valve therefore may be more complex with redundant solenoids and voting logic to meet high safety and reliability requirements.
The valve type selection depends on the process design conditions and end user specifications. Metal seated rotary ball valves, such as Metso’s, are proven and widely used from operational to complex, highly demanding emergency depressurizing applications. Why is that?
Here is some arguments:
- Avoid unnecessary flaring due to long lasting metal seated tightness
- Availability in fire emergency conditions due to fire proof construction
- Reliable valve operation ensured even in conditions with particles in the fluid due to non-clogging design
- Reduced emissions due to rotary stem and spring-loaded packing
- Maximum availability and extended plant uptime due to on-line condition monitoring provided by smart positioned valves for predictive maintenance
- Hidden failures revealed without flaring due to partial stroke testing applied for emergency depressurizing valves by Neles ValvGuardTM ESD-valve controller
- Simple valve instrumentation (less equipment, such as solenoids) together with higher safety integrity and better reliability provided
- Long lasting tight shut off valve designs.
Trunnion mounted, metal seated ball valves have been successfully used in several projects with global oil & gas companies in depressurizing and blow down applications such as emergency depressurizing in hydrocracking reactors and steam crackers.