Hi! my name is Chris and I can't find any guidelines in API Standard Codes for PSV when to specify a vented bonnet. I'm current working with a PSV that has a relieving temperature of 105 degree C, the process fluid is boiler feed water. Although we know that vented bonnet is often used in steam service, why is that we cant used in a boiler feed water with a temperature of 105 degree C?
Could you help me find the exact point in the API Code in the used of vented bonnet. Your response is highly appreciated. Thanks so much!!!

Just to set the record straight, when dealing with pressure vessels (design or MAWP greater than 15 psig), API is NOT code but only Recommended Practice. Code falls under ASME.

Now that we got that out of the way....

A PSV with a vented bonnet is more commonly referred to as a balanced bellows relief valve. I suggest you re-read API RP520 and you will see many references to this type of valve and when to use it.

One other item to take note of if your plant is not familiar with balanced bellows PSV's. We had operators insist that the valves were going to leak and installed plugs in the bonnets so that they would "be like all the others."

The installation of plugs will cause damage and likely fail the bellows in the event the valve does try to lift. I say "try" because the plugged cavity can cause the valves to not relief at the intended pressure.

Have a short "tool box" discussion with the pipe fitters, maintenance and operations folks to let them know why there is no plug in those bonnets.