A few years ago I posted about Conrader air safety valves, and how they can safely vent air tank pressure should the pressure exceed the valve’s manufactured threshold.
That is, if a compressed air tank rises above say 100 PSI, a 100 PSI safety valve will open and release a sufficient amount of air so as to lower the tank pressure to ~100 PSI.
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At the time of that post, I wrote that this was done to prevent an air tank from exceeding its recommended holding capacity.
What happens when you inflate a balloon past its holding capacity? It pops.
What happens when you pressurize an air tank beyond its rated pressure? It fails catastrophically and ruptures in a spectacular way that you wouldn’t want to witness firsthand.
Air tanks have safety air valves to prevent overpressure conditions.
But what does it do when installed to a portable air compressor? What could happen in a portable air compressor that doesn’t have one of these valves?
Why do you want to prevent overpressure conditions?
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I hope you don’t mind, but I need to think aloud for a bit, meaning none of this is at 100% confidence and should be considered as my opinion and not safety advice or complete understanding of any kind.
Yes, a safety valve prevents tank pressure from building up past its recommended and tested pressures. If a portable air compressor is rated to say 125 PSI max, you don’t want that tank to build to 250 PSI holding pressure.
I don’t think it’s so much the risk of the air tank failing, as there should be a sufficient safety factor built into its specs, but perhaps other components could be more susceptible to damage?
That is, well before a holding tank bursts, what else could happen?
Let’s say that a portable compressor’s pressure sensor fails somehow, and it misses its cut-off pressure. So instead of turning off an air pump once the tank pressure reaches its max operating pressure, the pump continues to run.
Could this be a more likely danger to system failure?
Let’s say you have a inflatable device and a hand pump. The higher the pressure in the inflatable device, be it a ball, tire, or other object, the greater resistance you will feel when operating the manual air pump. The parallel would be an electric pump seeing an increased load during air compression.
If the air tank pressure isn’t accurately measured or reported by the pressure sensor responsible for turning the compressor pump on and off, then in a hypothetical situation the compressor might continue to pump and pump and pump until something else stops it. What could this lead to?
Every component of every tool has a purpose. The implementation of safety valves in air compressor tanks make sense – it reduces the tank pressure should the pressure exceed an established maximum. Navigating the WHY has been difficult. What if a portable air compressor does not have one?
How often do air compressor pressure sensors fail? It perhaps seems more likely that safety valves are mostly intended simply to help regulate the maximum holding pressure. Let’s say you operate an air compressor outdoors in winter, and it continues to fill up until it reaches temperature equilibrium. Then you take it inside. If the temperature increases several dozen degrees as the tank reaches a new equilibrium with indoor temperatures, the pressure will increase. Having a safety air valve ensures that the tank pressure, which will rise with the temperature, won’t exceed the system’s maximum rating.
There are fewer questions when it comes to standalone air tanks, where the safety valve is there to protect the tank from exceeding its maximum holding pressure rating. So am I overthinking things when it comes to portable air compressors that have an integrated air tank?
This seemed like an interesting and worthwhile topic to think about aloud. I also know some of you are experienced with industrial air compression systems, and others might have experienced product failures or even taken part in testing portable compressors to failure. I would even bet that maybe one or two readers will have surely designed or engineered one or more portable air compressors, or taken part in the development of such products.
Please share any insights or corrections you might have!
Beyond the obvious, to protect tank integrity, what other causes or reasons might there be to have a safety pressure valve installed into a compressed air tank?
Or, what could happen to the components of a portable air compressor if the tank pressure exceeds its rated holding pressure? What are all of the possible modes of failure? Might tank overpressure cause other components to fail or break much earlier and with greater likelihood than the tank bursting?
One thing that’s for certain is that I wouldn’t use a portable air compressor with built-in air tank or standalone air tank that didn’t have a safety valve.
Also, consult your user manuals and test your safety valves as recommended.
Wayne R.
I don’t recall how I learned this, but: The valve is also a bleed, so that when you’re done using it you can return the tank to ambient pressure.
The value of this is that you’re also releasing excess moisture that will rust the interior of the tank – which, of course, will ultimately lead to a burst tank at a lower pressure than expected.
None of these portable tanks have a cage for that!
Nick
Most tanks have a specific bleed-off valve located on the “bottom” of the tank. The issue with using the safety valve as a bleed-off by manually pulling it open is it lets the air out, but it doesn’t let any condensed moisture out.
Compressed air going to ambient air cools, and this causes condensation on the sides (inside and out) of the tank. If you just pull the bleeder valve (usually NOT at the bottom of the tank), you release the pressure, but the water/condensation pools at the bottom of the tank with no way to escape.
The purpose-built bleed-off valves are on the bottom of the tank, which allows not just the air, but also the condensation/water to flow out of the tank.
Stuart
The bottom valve is a condensate drain valve. Some products seem to have guidelines as to when (with respect to fill or pressure level) the drain valve could or should be opened for draining.
The safety valve could potentially be used for quickly venting a tank if needed, but it’s not ideal.
I tend to use the drain valve, although I can’t say whether it’s recommended or not. I keep meaning to add a blower to a quick connect coupler for pressure bleeding purposes, so that the drainage valve only needs to be open for the final 10 PSI or so.
I generally don’t like uncontrolled venting except if absolutely necessary.
PB
I have seen a manual specifically state to leave the drain valve open when the device is in storage.
I use it to bleed off all of the pressure and try to open it fully and as quickly as possible to allow the air to push the condensation out. Obviously a potential hazard to the eyes and lungs if your area isn’t clean enough, but I take preventative measures for that.
Since the tanks are likely to be steel, it is best to store the compressor indoors with the drain vent open. This will result in little temperature differences between the ambient tank temperature and the internal tank temperature.
Scott Burke
If a tank is rated 150 psi, should the pop value also be 150 psi, or something less (e.g. 125)?
Stuart
You’ll have to look at safety requirements.
If a 4-to-1 safety factor is required, for instance, a 150 PSI max pressure tank would have to have pressure relief at 37.5 PSI. The safety factor would depend on any engineering requirements or safety regulations.
I don’t know what safety factor is standard, nor would I feel comfortable guessing.
All I can say for certain is that if a tank is rated at 150 PSI, a relief valve would have to be rated at much lower than that. I can’t tell you what pressure to select, but 150 PSI would absolutely not be appropriate.
If a compressor is rated at 150 PSI, that might be a bit different, but it depends on the circumstances and safety factors.
Peter Fox
The primary reason for the over pressure relief device is as you stated. to protect the pressure vessel from possible rupture due to an over pressurization event. Most pressure vessels are required to have on even if they do not have a a compressor or pump. if you ever look close at the valves on propane cylinder or high pressure gas cylinders you will find that they have a pressure relief valve or bust disk as appropriate. The main exception to the requirement would be if the cylinder contains toxic or poisonous substance, depending on the substance it may be prohibited from having an over pressure relief device.
The results of non functioning or bypassed pressure relief devices can be quite spectacular. Although a bit less common than the basic air compressor the story of the dammage done by a liquid nitrogen tank rupturing is a good reminder of the engery that can be stored in pressure vessels https://blogs.sciencemag.org/pipeline/archives/2006/03/08/how_not_to_do_it_liquid_nitrogen_tanks
Any other benefit such as preventing motor or compressor burn out in the event a pressure switch would fail closed is just a secondary benefit. I suspect that the weak point in term of burst pressure on most compressors is the tank, most common brass fittings and associated tubing have fairly high burst pressures. Just by nature of its size the tank has much greater forces applied to it at a given pressure.
lastly as a related topic, the common failure of compressor tanks due to rusting out has spurred my interest in hydrostatic proof pressure testing mine to verify its integrity. I am currently building a has high pressure pump to accomplish this.
Stuart
When TAing for physics labs as an undergrad, one demo I loved doing involved a beverage can (larger Arizona iced tea cans worked best), bunsen burner, and pot of cool water.
Fill the can with a little water.
Heat the can over flame (holding it with tongs).
Continue heating until water boils and water vapor fills the can.
Once all of the water has been converted into steam, quickly flip the can upside down into the tub of water.
What happens is that the can is now filled with water vapor. Upon cooling of the lid, the water vapor instantly starts to condense back to liquid water. Very rapidly, the can goes from being filled with water vapor to liquid water. The same amount of water, but a much smaller volume. The pressure in the can drops and the can implodes spectacularly to fit the volume of water. It pulls up some water from the bath, but not fast enough to fill the near vacuum inside.
There’s another version of this that involves a metal drum, but I’ve never seen it successfully demonstrated in person.
The volume ratio between gas to liquid is on the order of 1000 to 1.]
It’s not hard to image boiling of liquid nitrogen leading to catastrophic results without proper safety venting.
There was a demo we did where a test tube was lowered into the top of a liquid nitrogen dewar. The top of the test tube is exposed to air.
In the period of 30-60 minutes, liquid will appear inside the test tube, as air (80% nitrogen) is cooled to where it changes state to a liquid. The amount of liquified air is not insubstantial, and so the experiment is always performed with windows and doors open.
I love cryogenics. I missed the “shattered rose” demo in high school physics, but college labs and teaching demos helped to make up for it.
Dipping balloons into liquid nitrogen was also fun as they shrink and bounce back.
The Leidenfrost effect is also fun, where liquid nitrogen beads around the floor as droplets ride around a cushion of boiled nitrogen gas.
Freezing one’s fingertip with liquid nitrogen, such as when holding a test tube whose insulated o-rings have failed, is not fun.
Rob
Mr. Wizard did this on PBS with a rectangular gas can way back in the early 80’s. Clearly, it has stuck with me for decades to still remember it. It is a great demo.
mla
https://www.youtube.com/watch?v=E299BkjxTY0
Rob
That’s it! Thanks.
Bart
Mythbusters did it with a rail tank car. It took over an hour for the pressure to drop, but once it did, it collapsed in on itself within a matter of a few minutes.
atomic
I know I’m really late, but here’s a more, uh, exciting (crazy?) demonstration of the Leidenfrost effect. https://youtu.be/3F-Tww516Cc?t=104
Gordon
I think it just saves money on a regulator. If the compressor is rated to 150psi, you can put a 150psi safety valve in and any regulator that is greater than 150psi will be safe.
It could also prevent catastrophic failure in rare instances. Imagine a maxed out tank in a garage that is on fire. Or a bozo that ran a compressor in sub zero temps, then brought the tank inside and put it next to the fireplace.
Alex
You talk about the safety factor but don’t forget that there are 2 factors that can greatly reduce the tank strength over time : metal fatigue and rust. Some people keep the compressor for 20-30 years (you need to have it hydrostaticilly tested after 10 years) and never drain it resulting on a very weak tank. Now just imagines with a compressor this old and rusted what would happen if the sensor happened to fail?
The valve is just a cheap insurance. Instead of exploding, the tank will just crack over time even if you do stupid things. Any pressured gas tank should have a safety valve anyway. They are pretty cheap and can save you from a painful explosion.
Paul
I’m thinking that the seals on the compressor piston are FAR weaker than the tank and that these are going to limit how far you can get the pressure in the first place. Tubing is usually rated to thousands of PSI. Threaded fittings are almost as high. So I highly doubt the fittings are going to get there. And as a pressure vessel the tank is going to have to be type tested quite high with a huge safety factor, probably 300% or 500%, rated to absolute maximum rating of the pump which is probably 125-150 PSI. So all integrity concerns aside, my money is on the seals.
Where are the Mythbusters when you need them? We wanna know…
In fact I even doubt the idea of the tank itself failing. That’s like watching piping fail. The seals blow out and/or the nipple threads blow out well before the pipe itself ruptures.
Peter Fox
That’s a good point.
Alternately the power required to run the compressor might exceed the motors out put power and it might stall.
Otherwise if the seals and motor hold up I imagine that there must be a upper limit to the pressure that can be developed in a single stage compressor. It probably depends on how much dead space there is above the piston and in the porting to the reed valves. It would be interesting to know just what the actual limit for a practical single stage compressor is.
alex
There are some small compressor explosions videos on youtube. It’s always the same thing : a very old compressor which was almost never drained.
Peter Fox
I was referring to the failure mode and or limits of the compressor not the tank or pressure vessel.
I do not doubt that pressure vessel failure is more catastrophic and therefore a much more interesting subject for a YouTube video
Mick
I have one of the Porter-Cable 6 gallon compressors. Using it one day to reapply the trim in the kitchen after having it retiled, the safety relief valve failed scaring the devil out of me. If that wasn’t bad enough, the replacement failed too. Maybe “failed” isn’t the correct term but they did the job of relieving the pressure. What was odd was that I was using my nail gun and the tank was in the process of filling, so I assumed it wasn’t up to the 150 psi.
Bruce
Somebody else said it best. A relief valve is cheap insurance. It’s not that the pressure switch is likely to fail, it’s that a cost of that failure is so high. Even a 1 gallon air compressor contains more metal than a fragmentation grenade. If it’s goes, the shrapnel is bad, possibly dead bad. For a 4 dollar part? Yeah, It’s worth it.
Alex
What I’m saying is that I have no doubt the seals will fail well before the tank on a new compressor.
But on an old one which was not properly maintained (drained and hydrostatically tested after ten years), the tank could totally fail first. It happens. There are some accident reports on YouTube like I was saying.
JoeM
I can’t believe this question was asked at all… It’s a PRESSURE. SAFETY. VALVE. On an AIR. COMPRESSOR.
The REASONING and the NAME are Synonyms for eachother… It’s the thing that makes the pressure safe on the compressor… thus why it’s called a Pressure Safety Valve…
Wow… that ToolGuyd had to explain this to someone, ANYONE… Makes me very afraid that there’s someone, or MANY someones, out there with their minds set on making their tool shops into Bombs… Stupid… STUPID Bombs… But Bombs, nonetheless…
Stuart
This was a question I was asking/thinking about.
Asking “why” when the general answer is known makes the difference between information and insight.
What I concluded was that it seems unlikely for a tank rupture to be the earliest failure that could arise from overpressure. What could fail sooner? How? Why?
You can’t assume it’s for down-the-road protection when a neglected tank might be damaged by rust or other weakening.
PB
Many mechanical disasters occur due to lack of redundant safety measures.
As Gordon said, if the tank is pressurized or even if it is stored without the drain valve left open, in a fire the safest way to vent the tank would be the safety valve, something designed to release enough air.
You cannot say the seals will blow out and leak first because that is an unknown. Not only are they not tested for such a purpose, but they are specifically deigned to hold the pressure. Otherwise, people would not be very impressed with their compressors. Designing the seals to do such a thing could result in poor reliability or general function.
Also, those seals blowing may not release enough pressure to keep up with an over-pressure scenario.
The tank pressure factor of safety can account for fatigue (which with steel isn’t a never ending depreciation like it is with aluminum. Steel will only weaken to a point and then level off.)
Pressure holding capability after corrosion cannot be numerically accounted for. As Alex said, it needs to be hydrostatically tested. Where any deformations can be measured and/or observed.
But who is going to do that on a home unit? And the cost for doing so might not be worth it relative to the purchase of a new unit.
Any amount of rust will mean that the tank has weakened at some level. It is important to check the color and clarity of the condensation t drains out the bottom of the tank after every use.
With the very inexpesive USB boroscope cameras available, it should be possible to inspect the surface roughness ad color of the inside of the tank after removing the bottom drain valve. I haven’t tried this.
In general, if there is no visible corrosion, it is likely safe. It may also be beneficial to inspect a brand new tank as well to get a baseline or to make sure you didn’t just by a worn device.
Mac
I drain my compressor every time I am done using it. Thinking about replacing the valve…and this is a good reminder I should probably use a T…one side the screw valve and the other side a 12v solenoid valve so I don’t have to get on my knees every time I use the compressor.
bobad
I think the real reason is regulations.
I have 4 tanks, and I think all 4 of them will safely hold 4X the pressure my compressor heads are capable of making. If I were to try and run the pressure up to an even remotely dangerous pressure, it would blow every circuit breaker in my box or smoke the motors.
A far bigger worry with compressors is that 1 out in my shop could develop a leak. Since I don’t go in my shop but 2-3 times a week, it would run on and on for days and destroy itself. That’s why all my compressors stay OFF until I use them.
Pop-off valves are cheap, reliable, and not bulky, so I like them, but really, how many injuries have they saved? I’m betting they have caused more injuries than they have prevented. More than once I have been scared half to death when I brush against one and it vents.
Anderson johane
Can the malfunctioning of the MPV contribute to the blowing of the pressure relief valve ?
MM
The presence of safety relief valves on pressure vessels is required by law in the USA, it is part of the ASME Boiler and Pressure Vessel Code. There are similar standards for elsewhere in the world. These requirements date back to the days of steam engines when boiler explosions would cause serious accidents including fatalities. Of course any sort of pressure vessel or boiler is built with some factor of safety in (which is another legal requirement in its own right), but there is always the risk of corrosion weakening the vessel–especially with a steam boiler–and there is also the concern of whatever other fittings or pipes may be downstream of the tank.
Let’s say, for example, someone hooked up a compressed air system in their workshop which goes to various hose reels, tools, etc. Suppose the main pressure switch in the compressor fails and the motor just keeps running and increases the pressure above the setpoint. The tank could burst at some point but has a relatively high factor of safety….but that cannot necessarily be said for the tubing used to plumb the workshop, the QD fittings, the hose reel swivels, and the other various parts of the plumbing. Which is then preferable: having a self-resetting spring-loaded valve vent the excess pressure? Or bursting a pipe or fitting somewhere, potentially causing injury or further damage?