Would you consider this an ASME Code vessel? 1

[PipingEquipment]

There is a customer that has a "design pressure" of 5 PSI, but then they are also asking to design for an 'extreme overpressure event' of 150 PSI in the event of a damaging event that is possible with this vessels service, they say they are adding this to the design so that the vessel would not become shrapnel as it fails, they say that if this event were to occur that would be the end of the vessel and it would be removed from service.
If you were given this job would you consider the 150PSI a design pressure (ie a stamped Code vessel)?

 

[SnTMan]

PipingEquipment, the overpressure is a condition the vessel must be designed for. Period

If the client and/or the jurisdiction requires, it would also be a Code vessel.

Regards

Regards,

Mike

 

[MJCronin]

Pipe...

A few questions and opinions.....

- What is the diameter, volume and desired material for this device ?

If the diameter is modest, say less than 36 inches, it may cost very little to upgrade the pressure rating for the item and go with an ASME vessel design ( with or without a code stamp. I would never design to 5 psig.... the calculated wall thickness becomes too small. For a while power plant deaerators were designed to too small of a pressure for the transient events that were commonly encountered. Failure resulted. This sounds like a similar situation.

- Have relief devices been considered..... tell us more about the system ?

Tinman is correct insofar as it is the responsibility of the client to specify design pressure of the system. However, it seems that you may want to consider the savings in capital costs from the addition of a relief device versus an expensive but thick relief vessel.

An ASME vessel designed for a design pressure 150 psig does not become schrapnel when pressurized above this number. Failure commonly will occur when pressurized to 2.5 to 3.0 times this limit. A lot depends on the specific design.

My opinions only....


MJCronin
Sr. Process Engineer

 

[GenB]

Non code vessel it is ar 5 psi or less tahn 15 psi operation with a safety device of 15 psi or less. The design can be over 150' 1500 does not matter. The user will penalized if nit fillowi.g rules. And depends on jurisductional Codes and enforcement.

 

[JStephen]

It sounds to me like they just want a 150 psi code vessel.

 

[Duwe6]

How big will it be? ASME VIII has some exceptions; 250 psi and <5-ft^3. Crossection of 6" [made from 6NPS pipe]. Either of these might apply; if not, it is a Code vessel.

 

[LittleInch]

Regardless of the answer, how exactly would anyone go about designing for such a huge difference in pressures? There is no such thing as going to yield but not ultimate stress, i.e. you can't design for "limit state".

Either design for 150psi (you might be able to justify 135 + 10% accumulation) or design a relief system which prevents overpressure (bursting discs, bent pin valves etc)

Start form the basis that this extreme event makes it a code vessel and then work the options from there.

Loads and loads of questions about normal operation and this extreme event need to be asked and understood to make an informed judgement and provide for a safe and efficient design.

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

 

[moltenmetal]

If the relief device is set at 5 psig and capable of keeping the accumulated overpressure during the 150 psig event to under 5.5 psig, then it's not a pressure vessel. But I'd bet that making this happen with a real relief device is about 1% likely if there is a 150 psig failure case being considered.

If the device is designed for 15 psig, relief set at 5 psig and the accumulated overpressure during relief is below 16.5 psig, then it isn't a pressure vessel either- and you have a hope in hell of designing a relief device- but if the flow chokes anywhere in the relief line, you're done- accumulation will increase beyond the safe limit.

If the device is designed for 150 psi and the relief set at 5 psi, what's your safe maximum accumulation? If it's above 16.5 psig, it walks and talks like a pressure vessel in my book and probably should be one. You need the code rules for design and fabrication and inspections (and AI involvement) required of a code vessel to justify that the design will safely withstand the higher accumulated overpressure. Design pressure doesn't matter per se- relief pressure is what matters, and relief pressure determines the hazard of a resulting failure.

Now if the relief consists of nothing but a properly sized pipe open to the atmosphere, that's a different story...

 

[PipingEquipment]

Thank you for all the replies, to clarify it is 9' diameter, carbon steel and large enough to be considered a Code vessel. According to the Owners Agent, the failure condition is for a dust explosion (which would produce about 150 PSI). I recommended a burst plate or other means of safely designing this pressure into the system, they however said they didn't want to do this.
I guess my question stems from the definition of design pressure in the BPVC which is the "most severe condition of coincident pressure and temperature expected in normal operation", and according to the Agent this is not in normal operation but an unlikely event (that would result in the vessel being taken out of service if it were to occur).
Since we will not be building this vessel and the Agent is responsible for telling the designer/fabricator the design conditions I wasn't sure if I was interpreting the rules of the Code correctly. If in fact the design pressure is meant to cover the pressures in 'normal operation', and it is the owners responsibility to inform us of the design conditions it seems to me that this vessel could be considered non-Code.
Also, the vessel is designed at the 5PSI with the appropriate SF, but the explosion condition they want designed to SF=1.5 at UTS.
I told them that I thought it should be Code Stamped (which if it is the SF for the 150PSI would be 3.5), but ultimately they are the ones signing off on the design so I'm not sure if I've covered my bases or if I should be doing something else, which I guess would be declining to do these calcs if they insist on it being non-Code and not introducing a burst plate or other means of dealing with the overpressure condition. What do you think?

 

[SnTMan]

Ah, a little more information comes available

PipingEquipment, it seems that if designed for the overpresssure per the clients requirements, and if normal design condition is 5 psi, it could be justified as a non-Code vessel w/o overpressure protection. Big however, however.

If not a Code vessel, to what standards / methods is it designed in the first place? What IS the appropriate SF? What does the jurisdiction say? If designed as Code vessel at 5 psi, how shall UG-22(i) be considered? What's YOUR exposure in the whole thing?

Can o' worms. My preference would be to design to Code at 150.

Regards,

Mike

 

[LittleInch]

I'm not as familiar with BPV code as for piping, but isn't there also somewhere a definition of maximum accumulation or over pressure which you need to meet? (10%??) Hence although it meets one criteria in normal operation, it doesn't meet the occasional or emergency condition. In piping terms you are allowed to exceed DP by up to 33% for short periods. Hence in order to meet the max accidental pressure, it effectively drags up your DP to then still be code compliant.

I think my point remains which is how on earth do you actually design this given these wildly different criteria. The SF of 1.5 over UTS ( I assume this means the max stress levels are 66% of UTS?) will imply a much higher design pressure than 5 psi and result in a significantly thicker vessel.

I think you need to design it twice - one at 5 psi with suitable bursting discs / protection devices and cost that and one able to withstand the 150 psi without breaking apart (code vessel in all likelihood) and cost that, plus weight on the skid etc. Then you have done your job and they can decide based on rational designs.

So in essence you use the 150psi figure as your ultimate occasional short duration pressure and after allowing for the maximum percent over DP or MAWP you can, back calculate the DP to use in the code design (probably somewhere about 120-130 psig).

My other difficulty as a designer is that you cannot rely on the owners word that in the event of an overpressure event he would actually remove and replace this vessel. What happens if the overpressure only reached 100 or 125 psi - would he still replace it? Too many ifs and not enough answers...

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

 

[loilfan]

LittleInch, the maximum accumulation for overpressure is just for the PSVs. It must have full lift after the initial lift at set pressure which must be at or below MAWP.

The BPVC does not have the allowance for exceeding the DP the way the B31 codes.

I would design this as a non-code vessel with overpressure protection set at less than 15 psi. The 150 psi pressure buildup sounds akin to an external fire causing an overpressurization of the vessel. Since it would be a sudden increase in pressure, a very reactive pressure relief device is necessary. A rupture disc works.

 

[Guest102023]

PipingEquipment, you've answered your own question.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[georgeverghese]

You could interrogate the vessel owner on the measures(with adequate redundancy)they would have implemented to keep this dust explosion outside of " the normal envelope of operations " - if we were to take a corollary to safety systems for overpressure, these measures would need to be be of an integrity equal to a SIL 3 HIPS at least, depending on the risk to plant personnel resulting from this explosion. A safety audit / HAZOP / QRA may be mandated in many cases to demonstrate the integrity of this safety system.