'Hot bolting' a valve bonnet 1

[Bazinga]

Hi All,

We need to change out some studs on a control valve bonnet due to some concerns around stress corrosion cracking. The current bonnet studs are B8M's and we would be changing out with duplex studs. The problem is that there isn't a planned shutdown any time soon to do this and replacing all the studs at once would require changing the gasket and re-hydroing the valve. My question is, has anyone ever undertaken 'hot-bolting' of valve bonnets, were one stud is changed out at a time to maintain pressure on the bonnet gasket whilst the valve is operating. If so is there any guidelines for doing this or limitations on pressure rating? The valve in question is ASME #1500.

Thanks in advance

 

[KevinNZ]

Have changed flanges bolts one at time, but don't ask what code allows this.

 

[TugboatEng]

There are companies that specialize in hot repairs. They are not likely willing to share their processes. However, you can develop your own process. Think about ways you can maintain an equal amount of compression on the joint to substitute for the removed stud. I have used heavy duty C-clamps but my experience applies to ANSI 150 or otherwise unclassified joints.

Code is wonderful because it simply needs to be followed. If you want to make a deviation you can but you'll need to do some footwork.

Keep in mind that you can possibly remove a stud and the joint will still have remaining mechanical strength but the gasket will have lost resilience and could blow out leaving you in a worse situation. This is why I suggest figuring out some way to maintain clamp load while fasteners are removed.

 

[LittleInch]

Look at ASME PCC-2,2018 or later version, article 311.

In particular clause 311-3-4 states

311-3.4 Maximum Pressure/Hot Bolting
Unless otherwise justified by an engineering and risk
analysis, hot bolting may be performed only when the
operating pressure is equal to or less than 50% of
maximum pressure allowed as determined in para.
311-3.3.

So that is 50% of the maximum you somehow determine in 311-3-3 based on having one bolt missing. So for a class 1500 valve this could be as low as 50 bar or lower....

311-3.3 includes a number of items, but this one is probably the hardest to calculate.
(d) the reduction of effective minimum gasket
compressive stress between bolts associated with bolt
removal; e.g., that sufficient loading remains on the
gasket to ensure that it maintains a seal

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