Do you think it is good to use sch 40 half inch PVC pipe to distribute compressed air in a shop? I think the max air pressure is about 100psi.
I tend to recall that sch 40 half inch PVC pipe has rated pressure above 120 psi, may be even 200 psi.
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Replies
No!
PVC will carry the pressure with no problem until the pipe is fractured by being accidently bumped or struck with a heavy object. The fractured pipe can produce flying plastic shards that you will not want to be around. My first choice of material would be type L copper pipe: almost as fast as hanging PVC and you don't have to wait for the cement to dry.
You might want to check previous threads over in Breaktime. There was recently a discussion on this topic.
Best!
-Nazard
I'll second the NO. Even increase the font-- NO. Way too many first hand stories of disasters or near disasters from using PVC for compressed air. The worst part is it can fool you. It will hold the pressure, that's not the problem, the problem is the mode of failure with schrapnel flying around to put out eyes and do other serious mischief.
Agreed use metal pipe or even rubber air hose, the PVC will shatter if it blows!
A technical aside: Why is it okay to use PVC pipe for water at 100 psi, but not air at 100 psi? It's because gases are compressible, while liquids are not. Compressed gases at high pressure contain much, much more stored energy than liquids at the same pressure. When the containment vessel ruptures, that stored energy is released. If the contents are air, the result is KABOOM. If the contents are water, the result is boop.
-Steve
The part about mechanical wealness pf PVC is valid so I won't use it. I will live with the droopyness of a hose.
The part about stored energy is interesting to consider. Once a pipe breaks the water no longer has the lust to expand but gas does.
But a water heater can explode and cause serious injury if the pressure relief valve is not working. It is the flying debris of the water heater that hurts.
"But a water heater can explode and cause serious injury if the pressure relief valve is not working. It is the flying debris of the water heater that hurts."
But once again, it's because there are always air pockets inside the water lines. That's where the energy to propel pieces of the water heater comes from. If the entire water system were completely full of water, with no air whatsoever, it would just go boop.
These days, there is an additonal danger that's a side effect of a safety feature: water heaters are equipped with expansion tanks. While the expansion tank does protect the water heater and plumbing by preventing pressure from building up too high, it also adds quite a bit of stored energy to the system, so if a rupture does occur, it's more dramatic.
-Steve
"If the entire water system were completely full of water, with no air whatsoever, it would just go boop."
True enough, unless the pressure valve was plugged and the thermostat was bonkey. Then, the water can become superheated (i.e., over 212 degrees) and you get a massive steam explosion instead of "boop".
This was posted over on Breaktime last week or so -- check it out.
http://www.youtube.com/watch?v=akqvaZ5J1aY
Mike HennessyPittsburgh, PA
"True enough, unless the pressure valve was plugged and the thermostat was bonkey."
Ah, yes. With enough effort, you can make anything blow up. ;-)
-Steve
"Ah, yes. With enough effort, you can make anything blow up. ;-)"
Much of my ill-spent youth was dedicated to proving that hypothesis.
;-)
Mike HennessyPittsburgh, PA
Dear Steve,Boop?John
"Boop?"
Yeah, boop. You know, as in Betty Boop or Boop-boop-be-doop. Don't you know boop?
-Steve
Dear Steve,
Betty Boop was bit before my time. I figured that it was an industry or perhaps a technical term.:-)Best,John
It is a technical term. I witnessed a pressure test once where the tank went tink, rather than boop. It was very disconcerting, because we couldn't find "tink" in our Manual of Pressure-Related Sound Effects.
-Steve
Dear Steve,
Oh...... OK, I think that I see it now. Being out of my field, I am learning..........Best,John
I saw a slow motion video of a glass window breaking as the pressure of the wind caused it to fail. As the standard glass window shattered, it formed long and pointed shards and the air pressure sent them flying.
When PVC and hotwater CPVC shatter from over pressure or an impact (table saw kickback, for instance) it forms the same type of pointed shards.... Way Too Dangerous to use either of them for air lines...
Bill
woodenfish.
Let me add a resounding NO to the use of PVC. I don't care if your cousin has been using it without a problem for a decade, it's stil something that is dangerous.
As I said, I am convinced to not use PVC. The point about mechanical weakness is strong enough.
I am not convinced about the "strored energy" incompressibility agrument, however.
A water heater explode quicker when there is NO air. The pressure exerted by heating water can be tremendous. Incompressibility of water is what causes tremendous pressure!
The water heater thing comes from the water being super heated and as soon as the tank breaks the water turns instantly to steam. This gives you the "power" to propel the tank.
No necessarily true, IMO.
There does not have to be steam in the water heater for it to explode.
Say if you have a tank at 70F completely filled with water and no relief valve, if you heat it to 200F, would there necessarily be steam in it? The pressure will be emormous.
Mythbusters did a show on water heaters. I think the water temp hit 350ish F when the tank let go1. stunning to watch!
"Say if you have a tank at 70F completely filled with water and no relief valve, if you heat it to 200F, would there necessarily be steam in it? The pressure will be emormous."
That's right. The pressure will be high (not really enormous, but high) and it will all be liquid water, no steam. And if it ruptures, it will go boop.
Explosions are caused by the sudden release of lots of energy. By itself, pressure means nothing. It's only when that pressure is associated with stored energy is there the potential for an explosion.
When boilers and other large vessels are pressure-tested, they're filled with hydraulic oil. Why? Because hydraulic oil is even less compressible (and therefore stores even less energy under pressure) than water. If your million-gallon boiler is going to rupture during a pressure test, you want it to go boop, not KABOOM.
There's a little handheld device called a diamond anvil cell. It's used by physicists to create enormous pressures for experimentation. Just by tightening a thumbscrew, you can generate pressures as high as 50 million psi inside the cell. But there's no real danger, because the volume of the cell, and therefore the amount of stored energy, is miniscule.
-Steve
Steven,
I saw the Mythbuster's take on the Hotwater heater. They used an electric version with all the safety items bypassed or plugged.
The pressure guage read 315 Plus at the time of the EXPLOSION.... They had the hot water heater inside a house built to Calif. specs. The small shed.house was blown apart and the heater shed it's cover as the tank itself shot hundred's of feet into the air.
The waterheater failed as the concaved lower end of the tank became pushed outward before it blew.... no bloop.
The show ended with the boys saying...."and there's one just like that one in YOUR basement."...
Bill
Right. The Mythbusters "experiment" is straying pretty far afield from what we were originally discussing, but it still boils down (pun intended) to stored energy. Their scenario involved a two-point failure--both the overtemp and overpressure safety devices had been defeated. Under those conditions, it's possible to store additional energy in the liquid water because you're making it a lot hotter than you would under normal conditions. In other words, the additional stored energy is in the form of heat. The only thing the pressurization buys you is that it allows you to heat the water more without it boiling. (The amount of stored energy related to the pressurization of the water remains negligibly small. A quick calculation shows that the energy required to compress 50 gallons of water to 315 psi is about 200 J, which is enough to light a single Christmas tree light bulb for half a minute or so.)
When the tank ruptured in the Mythbusters experiment, the stored energy went somewhere. Some of it remained as heat--the water was still hot. But some of it was consumed in converting part of the water to steam, and it was the rapidly expanding steam (i.e., the conversion of heat energy into mechanical energy) that caused all of the excitement. In other words, the steam created its own pressure, after the rupture; the initial pressurization of the tank to 315 psi didn't play any direct role in the explosion.
The "...there's one just like that one in your basement..." part is a little silly. As spectacular as it was, that gallon of gasoline you have in your garage for your lawnmower has about twice as much stored energy as even that souped-up water heater.
-Steve
"The amount of stored energy related to the pressurization of the water remains negligibly small."
Actually, I realize that I had neglected one other component of the pressure-related stored energy: If water heaters were designed to hold 315 psi, then this wouldn't be an issue, but since they aren't, there was probably also a fair amount of energy stored in the strain within the steel tank itself. Still negligible compared to the energy stored in the heat, but maybe enough to light a few more Christmas tree bulbs.
-Steve
When boilers and other large vessels are pressure-tested, they're filled with hydraulic oil. Why? Because hydraulic oil is even less compressible (and therefore stores even less energy under pressure) than water. If your million-gallon boiler is going to rupture during a pressure test, you want it to go boop, not KABOOM.
Actually, boilers are tested with water, nothing else. Too hard to clean up oil from inside the boiler, and too expensive.
How do I know? Been to lots of boiler hydro tests. Never seen or even heard about using hydraulic oil.
Where would you get a million gallons of hydraulic oil for your million gallon boiler? And what would you do with the used oil?
Keep in mind with the boiler/hot water heater one little detail. You do NOT (under normal circumstances) have steam in a hot water tank. But you can have water above the boiling point (if your safety are messed up) this is because water boilers at a higher temp at higher pressure. This is why steam engines of old used high pressure on their boilers. Something in the range of 200 to 250 normally and sometimes as high as 300 psi (but rarely).
Now what happens when said pressure is relieved? Instant steam. That is what happened (in part) on MythBusters. Once the rupture happened the pressure dropped and the water turned to steam leaving the fastest way it could (out the bottom) thus sending the tank about 500 feet straight up.
Remember the power we are talking about. In the days of steam engines during WWII we had the most powerful trains we have had. Today we need several Diesels to equal one Allegheny (a very large steam engine used By the C&O) to pull coal in the Mountains of West Virginia. The thing weighed the better part of a MILLION pounds But when one of then ruptured the boiler it blew the thing up off the wheels and down the track hundreds of feet. Lots of potential energy in hot water that is above the boiling point.
Oh and yes their is steam in a steam engine boiler but not much. Mostly it is VERY VERY hot water, with a little steam at the top. Once the boiler is opened to the air (like in a rupture) the pressure dropped, the water turned to steam and come out the bottom (where the rupture was) and it took off like a rocket.
Doug
"Actually, boilers are tested with water, nothing else. Too hard to clean up oil from inside the boiler, and too expensive."
Yes, you are probably right about million-gallon boilers. It's been a long time since I worked with the ASME Boiler and Pressure Vessel Code, and it may be that the hydraulic oil was required only in some circumstances (e.g., if the design pressure was above a certain value). It was an issue on the project I was working on at the time, because we were designing and building ultra-high purity stainless steel containment vessels for nuclear materials, and there was no way that we were going to put hydraulic oil in them after we had taken so much effort to ensure that they were free of all contaminants.
-Steve
I got away with that for many years in my 4000sf shop in southeast Florida (rare freezing temps) with four primary work stations, a couple secondary outlets, and the compressor limited to 100 psi max.
Would never ever do that again though, knowing what I now know. Would go with copper.
The main thing here is simply the pipe is rated at 100 but domestic water pressure is only 40 (2.5 times the req'd load) which is an engineering standard. Therefor to carry 100 lbs of air you'd need a pipe rated at 250 lbs.
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