High pressure "vessel" inside a vacuum chamber.

[A8yssUK]

Hi all I am looking at building a testing device. See simple image.
To measure leakage of helium from a test device with internal volume (V1) upto 200 bar with helium. (no explodey gases)
I want to use use our Helium Mass Spectrometer (HeMS) in full vacuum mode so I'd like a vacuum chamber around the test volume (V2).
Now V1 is a test device which may fail and blow out into V2. So I will have a blowout valve (Bout) on V2 to prevent it pressuring above 0.2 bar and a control on the pressure in (Pin).
The HeMS also has over-pressure protection and the blow out will be directed into a well ventilated lab or directly outside.

So does V2 only need to be rated for just vacuum to 0.2 bar or does it need to be rated higher to contain all of the V1 pressure if for some reason Bout fails ?

V1 will be around 30-40L. I am in Europe so it should be PED certified.
I have a similar setup already... but using much lower volumes (V1 < 1L) so not much risk of blowing anything up...

Thanks for any thoughts!

 

[LittleInch]

Yes. You need to calculate maximum pressure possible and either design your vessel to withstand that pressure or fit more reliable pressure reliefs.

My guess is that this will only pass muster if you use a Bursting disc as your Bout device. as a spring relief valve will probably just not react in time.
This is very similar to what shell and tube HZs do when the tubes have high pressure gas and the shell low pressure liquid. A tube rupture event generally overwhelms the relief valves and you need bursting discs to relieve the inner pressure fast enough.

Even then you might need to do some CFD analysis to show the peak internal pressure depending on the volume and assuming sudden rupture / explosion of V1.

My guess is that its really the relative size of V1 and V2 which is key as well as speed of response of your BD. I wouldn't be too sure your current set up contains unacceptable risk as 1 l of 200 bar is still a lot of energy and if your secondary vessel fails then you've got bits of metal flying about your lab destroying equipment and people. This "so not much risk of blowing anything up..." is being far too cocky in my opinion. Don't mess around with pressurised gases. You've probably just got lucky so far, but eventually the holes in the Swiss cheese align.

 

[Compositepro]

Your vacuum chamber can simply have a hinged lid and use the vacuum pressure to keep the lid closed. Any positive pressure in the chamber will just lift the lid.

 

[EdStainless]

And what pressure will it take to lift the lid?
Are you sure that it will seat and pull vacuum without clamping the lid?
Multiple relief devices will be in order.
And you need to design for some overpressure as there is always delay.
The other issue is energy containment in the case that the test article bursts.
That may be the biggest issue.

 

[A8yssUK]

So this is the plan, I'll add an sized blow out disc that blows out at say 2x more than the valve (Bout). (if i can find one that is ok for vacuum and pressure ??)

V1 should 90% of the time not fail, and if it does it'll likely be a whimper and not a boom 90+% of the time. But as littleinch rightly says the holes in the cheese can align, so I am planning accordingly. The lid will be hinged but also clamped in multiple places to ensure a tight vacuum. I don't think I want it to violently swing open if V1 fails.
Without any relief I estimate that V2 will go upto 10-15 bar if V1 fails at 200 bar (difficult to know V1's volume accurately at this stage, but I will minimise it when possible)

FYI the current smaller systems V2 chamber (its more of a bell really) is actually rated for 40bar, which if the smaller V1 blew out into it, it could only ever reach 10 bar due to the differences in volume..and the protection on the HeMS is active...

I've asked this question because off the shelf vacuum chambers are seemingly made from tissue paper....so clearly do not expect a high pressure to be inside... but I can add safety devices to them easily enough to avoid over pressure. If I keep V2 to be designed to stay below 0.5 bar it doesn't need all the upkeep of a PED covered piece of equipment. (Although I would of course now need to test the overpressure devices during annual service).

If I add the multiple blow out devices to reduce the risk, I can get away with not having to commission an expensive custom piece of rated pressure equipment as V2 (which would also be super bulky and difficult). With the additional blow out disc (which is a good idea, thank you!), the spring pressure release valve and the HeMS cut out. I would have 3 independent safety devices. I think this is a reasonable useability/cost/safety compromise. [Again this is for a lab/pilot not any sort of production environment]

Finally, just to cap it all off...I'll have a polycarbonate safety screen between the equipment and the user (i.e. me) because ship happens.

 

[LittleInch]

"ship [sic] happens"...

Indeed it does so good on you for realising that safety matters.

Bursting discs are often slightly domed so should be ok for vacuum, but define for sure. Twice the relief valve setting is good as BDs are not wildly accurate in set point and have a decent negative tolerance.

Thanks for coming back to us - many don't.

 

[EdStainless]

Sounds like a reasonable approach.
Make sure that you leave some documentation with it.
You want to people later to know why it has multiple relief devices.
There are vacuum rated rupture discs.