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.
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Most air compressors have a number of precautions built in to avoid the risk of a tank rupture. The compressor itself will probably have an automatic shut-off control that turns off the motor once the maximum tank pressure is reached.
In case the auto-shutoff switch fails, there will typically also be a safety valve built into the tank. Such safety valves, such as the Conrader hard seat valve shown above, are rated to specific pressures. A 100 PSI valve will open up at ~100 PSI in order to vent excess air to keep the pressure at or below 100 PSI.
Essentially, safety valves have spring-loaded pistons. Below their factory-set pressures, internal springs hold the pistons downward, creating a seal. But once the pressure inside a tank or device overcomes the built-in pressure limit of a safety valve, the piston is pushed upwards, opening the valve seal to lower and equalized the air pressure.
Safety valves typically also have loops attached to the pistons so that you can manually rapidly depressurize a device or air tank. You should check safety valves every now and then to ensure they can open freely.
There are two main types of safety valves – hard seat and soft seat valves. Hard seat valves are rugged and inexpensive, but typically leak a little bit. Soft seat valves are a little less rugged and more expensive, but they are built with better seals that aren’t as prone to leaking.
I went with Conrader hard seat valve for a recent project, but there are other good brands as well. Safety valves are available in a wide range of pressures and in different styles.
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Amazon carries a couple of valves directly and through 3rd party vendors, but your best bet is to check with Grainger, Zoro, McMaster Carr, or another industrial suppliers that have wider selections.
Hang Fire
I have an ancient air system that could really use this. It does have a pressure activated power cutoff, but that slowly goes out of adjustment. Something like this would tell me when it needs adjustment again.
On a related note, can someone point me to a small (5-10 gallon) portable air tank that does NOT have that weird constriction in it that makes it useless for air tools and take forever to charge? I’ve been through two imported models, two different brands but obviously from the same Chinese factory, they both had the same problem.
Dave
I’m not sure what you mean by “…that weird constriction…” but I’ve been having a consistently good experience with HF 10 gallon compressor #67708 for everything I do, including paint sprayers. Takes less than 5 minutes to charge from zero to maximum 125 psi. I use Milton V-style Hi Flo fittings (http://www.amazon.com/s/?url=search-alias%3Daps&field-keywords=milton+v+style) which are designed for spray guns, that allow greater CFM for whatever size fitting you’re using. I also use 3/8 or 1/2 ID hose for the same reason, which also (slightly) increases the total air capacity of your system. If you use the V style, it should be used in the entire system. First thing I did was replace the stock (M style) quick connect from the HF compressor and went from there. One warning: most folks use M style, and while the V-style female quick connect will accept a M-style male fitting and not leak, the V-style male nipple will not fit into a M style female quick connect. Hope this helps, kind of off topic, I know. BTW, that compressor does have a safety valve just like the one pictured, probably not THE same one.
Hang Fire
By constriction I mean watching my big compressor tank build pressure 4 times faster than the HF tank downstream being “filled”. I also mean waiting ten minutes for a 100PSI charged HF tank to bring up a low passenger car tire by 5 PSI.
This was about a decade ago. I junked both tanks. Nowadays I would probably try to find new hardware for them. The tanks themselves were OK and didn’t leak, but the hardware up top (name?) was JUNK.
Dave
Sounds like a regulator issue. Seems to be the weakest link in HF compressors. I bought a quality (PneumaticPlus) replacement I’ve yet to install.
Caleb
My 3 year old daughter likes to “help” in the garage. We make sure she is safe. Her grandfather and I were doing something on the car and she pulled the pressure relief valve open on the air compresser. It did not hurt her, but needless to say, she has been way more careful in the garage since then and does not mess things without asking first. . 🙂
Ian Random
My drunken dad relayed a story many years ago that someone put a high pressure air hose behind someone and killed them. Obviously, you can take some pressure or the Japanese playing with air pumps inappropriately would have a lot of fatalities.
fred
I recall that our Sullair compressor system had both a safety relief valve and a back-up rupture disk as a failsafe – sort of a high-pressure relief and a high-high pressure backup. Luckily the rupture disk was not something that we ever had operate.
Jimmie
For what it’s worth, you often find similar air safety valves on higher-quality pump sprayers used in landscaping. It’s a good thing,
Back to the original topic…What are folks’ opinions about leaving an air compressor tank pressurized? I’ve always let the air out of my tank (using the bottom drain so that any accumulated water is also drained) because I might not use it again for a week. But I’m wondering if repeated pressurize-depressurize cycles are harder in terms of metal fatigue than simply leaving the tank pressurized?
Opinions?
fred
This sounds like a topic that is right up Stuart’s alley.
For the industrial-rated compressor that I bought some years ago – the ASME coded tanks and piping were supposedly designed to “leak before break” – or so we were told. Nonetheless the compressor was housed so as to mitigate the consequences of a failure. I would think that for a compressor tank and piping system the propensity for cracking would also be influenced by corrosion – so regularly draining water out might help.
Stuart
Agreed. Gauges, hose connections, and other subsystems will often be greater weak points than air tanks. Seals wear, and plastic parts age.
Regular draining is absolutely necessary. An air tank that rusts or corrodes on the inside could develop a high risk of rupture, as the rust would eventually deepen and spread and weaken the tank’s holding properties.
Automatic drain valves could be implemented to help reduce the need for manual draining.
Stuart
It depends on the recommendation of the manufacturer, but most contractor, mechanic, and industrial compressors with built-in tanks require regular drainage.
There shouldn’t be any issue keeping an air tank pressurized, as long as it’s dry.
I don’t see why standing pressure would cause metal fatigue. Remember, the max pressure does not equal the burst pressure. There is often a safety factor built in.
If standing pressure or pressurization cycles are affecting the physical or mechanical properties of the metal used to form an air tank, then that’s an air tank you probably don’t want to use.