Whats your opinion on pressure vessels with different Flange Ratings?

[caper656]

I've always been taught that you want to keep all nozzle connections on a pressure vessel the same for safety and engineering 101 principles. I currently have a customer that wants all nozzles 1-2" to be 300#, and all 4-8" to be 150# when specified as minimum pressure,say 50psi. I always understand keeping the flanges the same ensures people on the site wont mix and match flanges but is there any actual code or document that states this as good practice?

thanks

 

[JStephen]

If that's how all their other flanges onsite are done, it'd be confusing to make them all the same on this one tank.

 

[moltenmetal]

There is nothing (nothing!) wrong with putting a higher class flange on a lower class vessel. The vessel of course needs proper relief design to handle whatever it is connected to, so if it's connected to lines designed to a higher class for a reason, that will mean a potential back-flow or source of over-pressure that has to be considered in the relief design. But changing flanges on nozzles to higher classes to mate with an instrument or valve which happens to come with that higher class flange? Nothing wrong with that whatsoever.

 

[caper656]

Hey molten. I think you may have misunderstood. The request is inlet and outlet process nozzles to be 150# but vents and drains to be 300#. Typically I have always kept flange ratings the same, so I would update all to 300#

 

[moltenmetal]

Wouldn't matter. As long as the 150# flanges are OK, so are the 300# flanges. Doesn't matter- they don't all have to be the same. If the client wants it that way for a reason, and are willing to pay the extra money for otherwise unnecessary metal for reasons opaque to you but likely obvious to them, then let 'em fill their boots and do it the way they like.

 

[mk3223]

IMO, this's the Owner's design requirement for the pressure vessel, which is complied with the ASME BPVC Code, of course. The 300# flange used for the vessel nozzles 2" and smaller can have higher bolting torque to prevent leaking.

 

[moltenmetal]

It is a puzzling specification of course. Who knows why they like 300# for small flanges- something associated with some sort of standardization that made sense to someone at some point. Why they wouldn't get rid of the acknowledged poor design of the 3" 150# flange in favour of the 8 bolt 300# version is a puzzle. But hey, whatever they like. Nothing wrong with it in terms of safety, just a waste of money on metal- a fairly minor one.

 

[r6155]

Design flanges not only for the design pressure, but also for other loadings that will be applied to the joints during the project life (e.g., externally applied bending moment and axial thrust loadings).

Some specifications say: Flange bolts shall not be less than 3/4 inch (19 mm) nominal diameter.

Regards

 

[LittleInch]

I've seen this before where they want the extra metal in the smaller sizes. It can make it a bit confusing but a PV will be plated with the MAWP so I wouldn't be too worried.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[chicopee]

I agree with r6155. External loading could be a factor.

 

[r6155]

1)LittleInch. More information in API 660 7.7.14 The maximum allowable working pressure shall not be limited by flange bolting.

2)caper656. Consult the customers

Regards

 

[DriveMeNuts]

Process nozzles with as yet unknown external loading are often bumped up a rating as an insurance policy, especially when the MAWP of the vessel is close to the rating of the flanges.
Small flanges are bumped up for additional corrosion allowance and because it is inexpensive.
This means openings like manways typically remain at 150#.

 

[SnTMan]

all nozzles 1-2" to be 300#,

Very common practice.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[moltenmetal]

Stiffening the flange just transfers the problem to the nozzle neck...not a good strategy in my opinion. I'd prefer a leak which leads to fixing the problem with excessive external pipe stress by better support design and flexibility analysis, to having a nozzle neck crack!

 

[SnTMan]

I suspect the practice is intended to more or less eliminate 5/8" bolting for these small flanges. Off topic, but also common that these small nozzles be LWN.

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

This practice is due to the following:
1)Vibrations of: tall towers, mixers, safety valve, etc.

2)Environmental corrosion can affect small diameter bolts.

3) Most joints are tightened manually by ordinary wrenching, and it is advantageous to have designs that require no more than this. Some pitfalls must be avoided, however. The probable bolt stress developed manually, when using standard wrenches, is

S = 45000 / d^0.5

where
d = the nominal diameter of the bolt
S = the bolt stress
It can be seen that smaller bolts will have excessive stress

See ASME VIII Div 1 NONMANDATORY APPENDIX S

Regards