I would think it is not proper to provide butterfly valve downstream of Pressure relief valve.
i) Isolation : This valve will shut close during Pressure relief valve inspection & testing. Wear-n-tear on butterfly valve seat will generally caused the valve can not be properly tie shut-off. Leakage and back-flow from downstream facilities i.e. flare poses potential hazard to operator and maintenance crew.
ii) Full passage area : Butterfly valve with valve disk in the middle can not provides 100% full area for relief flow passage. Full bore ball valve is generally can provide full area for relief flow passage.
iii) Internal blockage : Butterfly valve disk may be disconnected from the valve body without awareness of operator. It potential create partial or total blockage of the relief flow path which shall be avoided.
iv) Additional pressure drop : Even though the butterfly valve is full opened, as it can not provides 100% full passage area, additional pressure drop is observed on the line. It will crucial for inlet line with low set pressure system.
v) Solid accumulation : If the process fluid contains particle or solid, there is potential particle & solid settle at the valve disc, slow accumulation may increase partial blockage of the relief path.
vi) High potential of valve damage : As the valve disc is located in the center, large relief flow is impacting the disc. This increase the potential of valve disc disconnect from the valve body. Compare to full bore ball valve, this possibility is close to nil.
Ball valve is commonly used for Pressure Relief Valve isolation.
What do you think ?
Further Reading :
ii) Full passage area : Butterfly valve with valve disk in the middle can not provides 100% full area for relief flow passage. Full bore ball valve is generally can provide full area for relief flow passage.
iii) Internal blockage : Butterfly valve disk may be disconnected from the valve body without awareness of operator. It potential create partial or total blockage of the relief flow path which shall be avoided.
iv) Additional pressure drop : Even though the butterfly valve is full opened, as it can not provides 100% full passage area, additional pressure drop is observed on the line. It will crucial for inlet line with low set pressure system.
v) Solid accumulation : If the process fluid contains particle or solid, there is potential particle & solid settle at the valve disc, slow accumulation may increase partial blockage of the relief path.
vi) High potential of valve damage : As the valve disc is located in the center, large relief flow is impacting the disc. This increase the potential of valve disc disconnect from the valve body. Compare to full bore ball valve, this possibility is close to nil.
Ball valve is commonly used for Pressure Relief Valve isolation.
What do you think ?
Further Reading :
- Another descrepancy found in API Std 521Jan 2007
- Controlled and Non-controlled Type Depressuring.
- How to apply valve equation in HYSYS Depressuring ?
- 12 Features required for Shutdown Valve (SDV)
- ERRATA - API Std 521, Pressure Relieving and Depressuring Systems
- Requirement of overpressure protection devices on system design to PIPING code
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