How is a plate with a hole in it sealed

[Jarfi]

Say I have a simple 2 or 3mm thick sheet of plexiglass or aluminum, and I have a closed chamber box out of these sheets.

Now, I want to build a pressure inside the chamber, but the prototype only has a simple O shaped "hole" on the box. I can choose the diameter of the hole, and I have a rubber tube or plastic pipe entering the box, but there needs to be a O-ring type of sealing.

How would I go around doing this in the most simple way, in what way are pipes/tubes generally connected to holes in tanks so that a seal is created.

 

[russ_watters]

How much pressure and how big of a box? This is potentially dangerous.

 

[Jarfi]

How much pressure and how big of a box? This is potentially dangerous.

30x40cm. I think I may have to go for a full on flange with screw holes, it's mostly the guys I work with rushing too fast and being ignorant on how a seal works. So they just want to build a box with "holes"

 

[berkeman]

30x40cm. I think I may have to go for a full on flange with screw holes, it's mostly the guys I work with rushing too fast and being ignorant on how a seal works. So they just want to build a box with "holes"

You didn't say how much pressure. Is it positive or negative pressure with respect to the atmosphere? How are you planning on welding/joining the sides of the box? What experience do you and your team have with pressure vessels?

 

[Jarfi]

You didn't say how much pressure. Is it positive or negative pressure with respect to the atmosphere? How are you planning on welding/joining the sides of the box? What experience do you and your team have with pressure vessels?

Them? zero. The box will be made air sealed I am sure, but they are biologists.

It's little pressure, maximum 1.5 atm positive off the top of my head

 

[berkeman]

Them? zero. The box will be made air sealed I am sure, but they are biologists.

Well it's a lot harder to "seal" a plastic box than it is to seal a metal box. That's why pressure vessels are usually made out of metal.

Now, I want to build a pressure inside the chamber, but the prototype only has a simple O shaped "hole" on the box. I can choose the diameter of the hole, and I have a rubber tube or plastic pipe entering the box, but there needs to be a O-ring type of sealing.

Is the hole for the tubing that pressurizes the vessel? Or is the pressurization done separately, and the hole is for withdrawing samples or something?

 

[Jarfi]

Well it's a lot harder to "seal" a plastic box than it is to seal a metal box. That's why pressure vessels are usually made out of metal.

Is the hole for the tubing that pressurizes the vessel? Or is the pressurization done separately, and the hole is for withdrawing samples or something?

Both. Hole for pressure tube and hole for pressure sensor, it's a feedback system

 

[berkeman]

Both. Hole for pressure tube and hole for pressure sensor, it's a feedback system

Something like these?

https://www.ceramtec.com/_img/content/produkt.us.druckdurchfuehrungen.1.jpg

 

[russ_watters]

It's little pressure, maximum 1.5 atm positive off the top of my head

Absolute or gauge? How is it generated? Is there any risk of overpressurization?

 

[jrmichler]

Time for some calculations. 30 X 40 cm = 11.81" X 15.75", so the area is 186 square inches. One atm = 14.7 PSI, so 1.5 atm = 22.05 PSI. Area times pressure = 186 X 22.05 = 4,100 lbs force. Flat sheets of aluminum or polycarbonate that's 2 or 3 mm thick will stretch out of shape and spring a leak if you are lucky. Plexiglas, on the other hand, likes to shatter exactly like window glass - into many razor sharp pieces. This much pressure in a box of this size is a bomb.

Back when I was about 12 or 13, I soldered two tin cans together, soldered in a piece of copper tubing, then hooked it up to an air compressor. I was at least smart enough to put the can in a garbage can, cover the garbage can with 2 X 6's, then put an anvil on top of that. I did not have a pressure gauge, but estimated the peak pressure at about 40 PSI based on displacement and running time. The explosion bounced the anvil, and was loud enough that my mother, on the second floor, thought that she had a dead kid in the basement. My second try was with a gallon can. That did not work at all - the can stretched out of shape until it sprung leaks at the corners.

 

[mfb]

150 kPa * 30 cm * 40 cm * 30 cm (the last one is a guess) is 5400 J, a bit more than the energy contained in 1 gram of TNT.
30 cm * 40 cm * 2mm = 240 cm³, with plastic that gives a mass of about 200 grams, that means we can accelerate such a plate to about 200 m/s. The actual speed will be lower, but I wouldn't expect it to be safe.

 

[Jarfi]

150 kPa * 30 cm * 40 cm * 30 cm (the last one is a guess) is 5400 J, a bit more than the energy contained in 1 gram of TNT.
30 cm * 40 cm * 2mm = 240 cm³, with plastic that gives a mass of about 200 grams, that means we can accelerate such a plate to about 200 m/s. The actual speed will be lower, but I wouldn't expect it to be safe.

Well,

I think I misexplained.
We are talking about 1.5atm inside and 1atm outside, so the total pressure balance is 0.5 atm, but from my understanding they will need much lower over pressure, perhaps 0.1 atm.

I agree though, plexiglass is a weak choice of material, I think I have to talk to them further and make them think it through a bit before wasting money on a useless prototype.

Also, I've decided to just build a full on flange type of sealing with 5 m3 screws and an O ring, around a fitting module which is connected to a tube, perhaps with a metal hose clamp.

It's mainly an issue of campatibility, they want to use it with some older equipment with specialized medical hoses/sensors which need to be fitted, quite a sorry job for the engineer.

 

[berkeman]

Do you need to be able to see into the enclosure? If so, the metal enclosure with a small window is probably a better choice compared to transparent Lexan. But it sounds like you already know that.

 

[256bits]

I think I have to talk to them further and make them think it through a bit before wasting money on a useless prototype

It certainly sounds necessary.
Laying down the exact requirements will save everyone grief.
You wouldn't want to present to them a box, and then have then inquire "So how do we open it and our put stuff inside?"
Ship in a bottle.

 

[Rive]

Biology and such... Are you (they) sure this thing should be airtight? Or just it should be able to hold some kind of special atmosphere at a small overpressure?

 

[Baluncore]

What is in the box?
Can you have “through bolts” passing through the box to hold flat walls together?

Or can you use a cylindrical clear tube with heavy square flat metal ends pulled together by four external rods, like a pneumatic cylinder.