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Aug 9, 2012
Reduce that process gas overpressure – and do it safely
Does your oil & gas processing facility include risk scenarios involving overheating due to fire, exothermic or runaway process reactions? Are you concerned that your process vessel metal temperatures can reach a level where stress ruptur can occur? Do you need more risk reduction than what a pressure relief valve can provide to depressurize your process vessel? If the answer is yes, please read further.
What is depressurizing?
Depressurizing reduces the internal stress, extending the life of the vessel at given temperature when a relief valve cannot provide the adequate risk reduction. Therefore, depressurizing valves are used to reduce the risk of losing equipment integrity. What kind of requirements and practices there is for the depressurizing system and valves in particular? In many cases, a process plant is equipped with two different kind of systems: a slow depressurizing system also known as an operational depressurizing system and a fast system, that can also be called an emergency depressurizing system.
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 Neles’, are proven and widely used from operational to complex, highly demanding emergency depressurizing applications. Why is that?
Here are 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 ValvGuard™ 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.
Neles trunnion mounted ball valve.
Alok Kishore heading sales & services team in India, explains the importance of depressurizing valves for Indian market:
“It is very important for Indian Refining & Petrochemical customers to achieve process safety & reliability. That is why recent refining & petrochemical projects in India have chosen Neles valves for their Depressurization and HIPPS (High Integrity Pressure Protection System) applications. I would like to highlight the importance of correct valve selection and sizing to these high demand applications, where Neles is capable of taking clear responsibility as a single manufacturer of all components tested and configured as a complete assembly”