Calculating Vacuum Rating for Existing Vessel

[stuarbc]

I have been tasked with calculating the vacuum rating for two existing pressure vessels. Both vessels have been stamped but only list the MAWP and have no information for Vacuum or external pressure. Is this as simple as performing the Maximum Allowable External Pressure calculations from UG-28, using the actual dimensions and thickness of the vessel?

 

[vesselfab]

you should use the corroded thickness. thickness off U-1A less the listed corrosion allowance.

If you have a data book, external pressure calcs may have been done but not stamped on nameplate. Often done that way if only concern is steam out or pump out and vessel in not being operated in vacuum service

 

[SnTMan]

You will need to look at all the other relevant parts of the vessel as well, opening reinforcement for example. For large openings in thin shells this can often govern.

Regards,

Mike

 

[jte]

Mike-

I'm a bit perplexed... By "shell" I presume you don't mean flat covers, and unless the vessel is designed for less than 7.5 psi internal pressure, reinforcement for full vacuum won't govern. Or am I missing something? I've never seen vacuum govern reinforcement design, so I guess I'm curious as to your experience...

jt

 

[SnTMan]

jte, yes by "shell" I mean cylinders. In my experience (shell & tube HX) it is not uncommon for the external pressure (usually full vacuum) condition to govern nozzle reinforcement calculations, of course this is dependent on construction geometry, etc. I most often see it when a shell is "long" or "thin" or maybe has a "large" opening.

For example if a shell's external pressure allowable is close to the external design pressure it can occur that a nozzle will need no added reinforcement for the internal pressure, but it will for the external pressure calculations. It kind of depends on the difference (t-tr) for each of the cases.

A useful (but not 100% accurate) rule-of-thumb is that if the external pressure rating of a shell is under about 30 psi, opening reinforcment for external pressure needs a look.

My point in reply to the OP was that you cannot just consider the cylinder(s) and heads(s) when rating a vessel for presure, either internal or external. All relevant features must be condsidered.

Be glad to discuss futher if you want, I can even provide a current example

Regards,

Mike

 

[jte]

Ok, I think in the dim, dark past I did run into this... If the shell is governed by external pressure such that the required thickness for external is twice that required for internal pressure, then reinforcement would be governed by that. I guess I just haven’t run into such a situation in the last decade or so…

jt

 

[SnTMan]

Yeah, that's about it, I suppose it is more common in some classes of work than others.

Regards,

Mike

 

[jamesl]

jte,

Your math would be right only when a repad is used for 100% area replacement. There is other math here: the greater the required tr, the less available area from shell itself. In the other extreme case, if all available reinforcement area come from shell, when the ratio for tr-external pressure to tr-internal pressure is 4:3, you will see external pressure govern. In most of case, the cut off ratio is between 4:3 to 2.

In most of new designs, we usually add vacuum rings to keep the required tr for internal pressure and external pressure close enough. However I did run into this issue recently when we tried to put a full vacuum rating on an old vessel.

 

[jte]

jamesl-

I'm not considering at all where the reinforcement comes from. Simply what the required area of reinforcement is. If I need half the required area for external pressure vs that for internal pressure, then I believe my 2:1 logic stands. How do you develop your 4:3 ratio?

And yes... I have heard of the concept of vacuum stiffening rings. Really. I've even designed some. Wow.

jt

 

[jamesl]

jte:


I think you totally mis-understood my last statement. I have been to this forum long enough to know you are an very experienced expert. What I meant was, because we usually keep the required thickness for external pressure very close to required thickness for internal pressure, so we usually don't see the case that vacuum design governs nozzle reinforcement.

As I said in my last post, I agree with your 2:1 ratio between required areas. However that does not translate to 2:1 required thickness as you said in your previous post. My statement about 4:3 ratio was based on that all reinforcement areas come from shell thickness. Here is a simplified deduction:
For reinforcement for internal pressure:
d(ts-tr1)>=d*tr1
For reinforcement for external pressure
d(ts-tr2)>=0.5*d*tr2
for tr2 govern: 1.5*tr2>=2*tr1
so tr2:tr1>=2:1.5=4:3.