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Impact velocity of blown off cover

I have a vertically oriented vacuum vessel with a bottom cover held to an o-ring seal with a quick release device. The device is just enough to hold the weight of the cover for lowering and raising during normal operation, vacuum pressure holds the cover in place during the normal process time.

We have the potential for an overpressure condition (steam) in the vessel which I want to relieve by letting the cover "blow down". The cover will be guided and directed to a set of shock absorbers (hydraulic type) to absorb the impact. I am trying to define the impact velocity at the shocks by adding the energy stored in the vessel at the quick release device setting, to the potential energy of the cover mass and equating that to the kinetic energy seen at the shocks. I think this is overkill because the stored energy when the cover leaves the o-ring seal will start dispersing outward and will not continue to contribute to the cover dynamics. I guess I am concerned about having shocks too stiff. Any thoughts on the proper direction to take? I left out actual operating values but could supply them if anyone wants. I am mostly looking for direction not an answer. I think I'm starting to see integration in my future (which I left by the side of the road some years ago).

Thanks,

Ralph

Reply to
Ralph
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Here are some thoughts - they may be too naive for your purpose:

For a vertical cylinder with diameter D meters and with a bottom plate H meters above the catcher assembly, the initial force F newtons is given by pi times D squared times pressure P divided by four. This force will vanish over some number of diameters dropped, I'll suggest that drop is one diameter D. So the work expended on the bottom plate is 0.5 times D times F in joules. (This supposes a linear force drop) The other energetic contribution is conversion of potential to kinetic energy in the bottom cover mass. This is m times g times H for m is the cover mass in kg, h is the height fallen to the catcher in meters, and g is 9.81 newtons/kg

The total energy to dispose of in a reasonable catcher is

0.5 D pi D ^2 P / 4 + 9.81 m H in joules

One reason this expression is naive is that it is well known that disks acted on by a uniform pressure topple. Try dropping a quarter from the Empire State, for example.

This means that the force around a catcher assembly won't be uniform.

For what its worth.

Brian Whatcott Altus OK

Reply to
Brian Whatcott

Hi Brian, thanks for the response. I'm not worried about disc topple because the cover is guided and moves only about 6 inches. It is actually about 6 feet in diameter. The problem I see is that, whether I look at the internal stored energy in the vessel or the force from pressure over a distance, neither are constant during the drop because the steam will start to disperse horizontally as soon as the cover is free of the seal. I guess it might be more like a compressed spring inside the vessel where, as the spring extends, the force from it decreases. The problem is: how do I define the spring constant? Ralph

Reply to
BobKat

Oh wait, you did mention the force vanishing over some number of diameters dropped. Not sure how to make that assumption. I guess I'll solve it for a few different distances and see how much it matters.

Reply to
BobKat

Did I mention I was offering a naive model? :-)

Here's another step in playing with this issue: if you go to some science museums, you will see a hole in the ceiling for a blower, and a disk to set under the hole (sound familiar?)

The interesting result for a disk reasonably bigger than the hole is that the disk pulls up towards the hole. The side flow at lower pressure provides the suction effect.

So....if you could put a flat skirt round the foot of the cylinder, and around the bottom cover - say a margin of eighteen inches all round, to make a disk of maybe 9 ft diameter, you might need only a buffer for the potential energy in the six inch fall - because the disk might well hold up during the initial outflow!

Brian Whatcott Altus OK

Reply to
Brian Whatcott

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