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Vacuum resistance of two similar vessels

Vacuum resistance of two similar vessels

Vacuum resistance of two similar vessels

Hi another question on resistance to external pressure

If I have water two storage vessels, one marked 'atmospheric' and protected at 100 mbarg and one rated at 1 barg, of a very similar shape, size and diameter and the same material (stainless steel), and having been in the same service conditions (fluid, temperature etc) can I assume that the one with the lowest vacuum resistance rating is the one with the lowest pressure rating?

In other words, if I ask the vessel fabricator of the atmospheric vessel to determine by calculation the vacuum rating I can protect them both at this setpoint? Or is it imperative to remeasure the wall thickness of the other vessel.


RE: Vacuum resistance of two similar vessels

Assume nothing. Measure the wall thickness.

And get a competent mechanical engineer well-versed in pressure vessel design and fitness-for-service to perform the evaluations.

RE: Vacuum resistance of two similar vessels

I was expecting that answer. They (site) don't really want to pay for as it involves removing insulation from the tank, measuring all the way up (5m), putting it back on, paying for all this & will probably argue that we can size the vacuum protection on current operating data.
If it was my money I wouldn't hesitate but this site is mean with money. I suppose I need to make the case that the consequence might be that we have to change quite drastically their operating mode to avoid any slight underpressure.
I have found an engineer from the vessel fabricator who is willing to do the work so he should be competent.

RE: Vacuum resistance of two similar vessels

Lots of good pictures in here to (hopefully) scare them into parting with some of their money.

RE: Vacuum resistance of two similar vessels

Very similar is not identical.

However if the pressure rating or relief rating of the atmospheric tank is only 100 mbar, I can't see the vacuum rating being anything higher than 20-30 mbar. At the sort of level the risk is very low so although you really should find out what the wall thickness is it won't in reality collapse.

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

RE: Vacuum resistance of two similar vessels

Quote (erbu)

I have found an engineer from the vessel fabricator who is willing to do the work so he should be competent.

Excellent! Competence tends to be a good thing.

You could always suggest to the client that they proof test the external pressure capability. Maybe replace the vacuum breaker with one with 1/3 the capacity, then drain the tank. That'll give you an opportunity to help them procure a new tank!

RE: Vacuum resistance of two similar vessels

Not common to strip a vessel of it's insulation just to take a few thickness measurements - you cut 2-inch ports and measure there.
Try hiring a certified API Inspector with 510 & 653 cert's. What is being discussed [and agonized over] is a typical day in the tank farm for an API. Tank or Vessel? What Code, if any? What thickness, and where? Any features to give vacuum support? All pretty simple IF you have ever evaluated a vessel [tank?] of unknown origin.
You need an API evaluation. So you need an API Inspector who is ready to do some calculations. The engineer you found might be able to get this done, but if he didn't correct your idea of stripping off all the insulation, probably not.

RE: Vacuum resistance of two similar vessels

the setting of the system relief vessel it could be for a rated of flow that is filling or unfilling, check what receive or deliver (pumping rated)

RE: Vacuum resistance of two similar vessels

Depending on the circumstances- on API-type tanks (not pressure vessels)- internal pressure mainly affects the roof and anchorage, but not the shell. However, vacuum rating may be governed by the roof, bottom or by the shell. So it is possible to have two tanks with different pressure ratings but the same vacuum rating.

In both API-650 and API-620, they have a statement that tanks built to those codes can be used up to some nominal vacuum rating (I think 1" water column) without any additional vacuum design. If the tanks were built to one of those codes, that could save a lot of work if you can furnish vacuum protection within that limit.

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