View Full Version : OT - vacuum vs. pressure vessel
10-21-2009, 03:06 PM
I have a question about reservoirs used for vacuum systems. I don't mean sweep the floor vacuum, but vacuum used for things like control systems. What is the difference between a vessel used to store compressed air and one used for, well, a vacuum? Since both are used to maintain a pressure differential, and from systems I've seen, they look similar, I would think that mechanically, there may be no difference. From an OSHA standpoint, though, are there different ratings? Or can a container used to store compressed air at, say, 125 psig, be used to "store" a vacuum of, say, 25 in?
The reason I ask is that we have a piece of equipment at work that uses vacuum to engage a test fixture into a "bed of nails". The software only allows 2 seconds (or so) for the engagement to occur or it gives up and errors out. Experience has shown that a vacuum pump alone is not typically able to evacuate the space below the fixture and pull the fixture down quick enough to satisfy the software. So, typically, the system employs a vacuum pump along with a reservoir. Most of these are purchased as a system. However, we have a need to demo the equipment, and if we don't need to spend $3-4k, we would prefer not to. We can buy a pump capable of sustaining the vacuum (read - keep up with the systems inherent leaks), but it is not likely to be able to pull the fixture down quick enough. I am able to find pressure vessels used in compressed air systems quite readily, but even Google has not turned up many readily available, appropriately sized vessels specifically use in a vacuum system.
Any help is appreciated.
10-21-2009, 04:53 PM
Vacuum vessels are similar to, but not the same as pressure vessels. The primary difference is that the design is for external pressure.
ASME SecXIII and ASME SecI both address components subject to external pressure, and Sec XIII, and some other sections, specifically address vacuum components and systems, but I don't recall anything specifically addressing vacuum receiveres. Some jurisdictions may have requirements (federal/military sites, some states, etc)
The stresses are different for external pressure than internal pressure. For example: A 12"dia cylindrical shell, 1/8" thick in 70KSI (at service temperature) material is suitable for at least 200PSI with a safety factor of more than 5 (depending on the service, and other design factors such as reinforcement from heads, etc). It wants to stay a cylinder. If it is a little out of round when made, it will round out in service (hence the allowable limit on out-of-round: in service, the flexing may lead to yield of the material or cracking from stress cycling)
The same shell is likely to collapse for vacuum without reinforcement (often internal rings). The shell isn't stiff enough to maintain shape if there is a significant force imbalance, such as from mounts (the weight of the vessel will lead to an imbalance if not compensated for), manufacturing imperfections (such a s a slight out of round), service damage (a small ding from a dropped wrench during installation), or a variety of other factors. Buckling of the material is the principal (but not the only) concern.
Consequently, design for external pressure, such as a vacuum reservoir, tubes running through a boiler barrel, etc, needs to be designed to avoid failure from buckling. This often means the material is thicker than for a similarly sized pressure vessel (until you get to vessels with very high design pressures) or has internal or external reinforcement.
Vacuum piping with standard pipes is less of a concern, since the wall of most standard pipe and tube is fairly thick relative to the diameter (though very thin wall pipes,tubes are a concern... never dealt with real thin stuff, so I don't know where the limits are.)
Another example: examine a soda can. The internal pressure may be about 50PSI (http://hypertextbook.com/facts/2000/SeemaMeraj.shtml ) if the cans are stored in a warm place, and higher at elevated temperatures (like a sunny summer day in a car). The wall thickness is about 0.010" (I just measured a can. It varied, but this is the ballpark) This gives a material stress of about 7.5KSI, a safe stress for the drawn aluminum.
On the other hand, I can sack the can flat while drinking from it without needing to squeeze. The wall begins to buckle at any imperfection, and the wall material is then in bending rather than compression.
In the end, the pressure differential is small even at a high vacuum, and for a relatively small reservoir, failure isn't likely to cause injury (directly, at least) for a vacuum vessel made from a ductile material, so the risks are not great, compared to vessels with internal pressure. For reliable service, design IS important.
If you were to make one in house, and the needed volume is fairly small, take a look at the chart in ASME SecI for cylindrical components subject to external pressure (steel boiler tubes) in pressure vessel construction, and this will give you an idea of appropriate sizing of material. For reference, a 0.110" wall 2" tube is pretty typical in a boiler at 200PSI, and has a reasonable allowance for corrosion. Standard Sch40 steel pipe and standard fittings will likely be suitable, if heavy, for smaller vessels (Check the codes--- I make no claim that this is sufficient information to use for design purposes)
Here is one company that supplies vacuum rated receivers (first hit on google: http://air-tanks.com/ )
Example of site requirements re: pressure and vacuum systems (both fall under the same heading, since vacuum components may be inadvertantly pressurized) http://www.jlab.org/ehs/ehsmanual/6151.htm Note that if there is no chance of pressurization, then ASME code doesn't apply.
Nice vid: http://www.youtube.com/watch?v=Zz95_VvTxZM
10-21-2009, 05:26 PM
Less scientific but good info
10-21-2009, 10:41 PM
Thanks for the information. This confirms what I thought about the differences. It also confirms that there is less of a danger in the event of a failure (collapse) than in a pressurized system. It's funny how a container that can hold several hundred PSIG from inside can be destroyed by 15 PSIG from the outside.
I'll have to check and see if there are any code issues, but since it is relatively small, I think I may try to use a portable air tank (air bubble). I suppose, if we really wanted to get creative, I could weld girders around the outside to give it a little added strength. If it fails, and it could (according to my weight/size calculations, the thickness must be only about 0.08"), I am only out about $30. And since it'll be ductile steel, there isn't much chance of shrapnel. We only need this demo once in awhile. And I don't think it really requires more than 10 or 15 inches of vacuum, though we do spec 20-25, I think. The biggest issue we have is getting the fixture pulled down quickly, which means the air needs a place to go immediately. I think this may be a workable solution.
That PVC vacuum receiver is a little scary. It probably isn't going to fail, but if it does, there could be shrapnel. That's the reason it is not allowed for pressurized air systems. An implosion can cause the ruptured, and probably shattered, PVC to come together at a high velocity. Then it heads back outward. We did a science experiment in high school with a 5 gal glass water bottle. We swirled a very small amount (less than a capfull) of alcohol inside, and then dropped a match in. It did not create great amounts of pressure, but as it ran out of oxygen, it sucked in air from outside. Eventually, after a few days of this, a couple hours after my class, it broke and sent shards of glass across the entire lab. Many kids were taken to the hospital and a couple were lucky (glass millimeters from the jugular, in one case) to have survived. Flying PVC can be just as deadly. Again, it probably isn't likely to happen, but it can.
10-21-2009, 10:45 PM
BTW, Enlpck, there was a link for an ad for a vacuum trailer. I can see that DOT stop.
Okay, well it seemed funny to me.
Actually, I think it is a trailer that is used to suction up spills. But it's funny how we use the word suction and vacuum interchangably.
10-21-2009, 10:51 PM
How does 14.7 PSI external pressure on a cylindrical chamber, where the inside is a vacuum, or very close to it, differ from a cylindrical chamber, where the inside has not been evacuated?
10-22-2009, 08:37 AM
While I'm not sure what your fixture looks like or what size/speed/type of pump your using I do have a few thoughts on this. Vacuum system speed is a combination of pump capacity and the mean free path of the tubing its connected to. Generally if I wanted to have a fast system with an undersized pump I would design the tubing from the fixture to the pump to be as large a diameter and as straight as possible. Place your actuation valve at the fixture end of the vacuum line and the entire length acts as an accumulator chamber(with a minimum number of connections and corners) and will also speed the evacuation times(remember to get the air out of the system it has to get to the pump first). Two to four inch copper plumbing pipe will stand up to vacuum down to 20 millitorr easily and is relatively common and easy to with. Soft solder your connections and its vacuum tight with very little effort.
10-22-2009, 12:40 PM
Well, I got some additional information from the engineers of this system and they say that the pump they have used in the past without a receiver is capable of 60M3/hr. That's a heck of a pump, and likely to cost a bit more money than my boss is going to want to spend. However, they also mentioned that using 10M of hose (I think they are talking about the hose that we provide with the equipment, which is, IIRC, 1 1/4" ID.) seemed to provide enough reserve to pull the fixture down quickly enough. I'm going to bring in my vacuum pump that I use for A/C work for proof of concept and then we can proceed from there.