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The title says it all, I'm just curious why my air tank is rated for 200psi at 400°F. It's a 30 gallon steel tank.

Wouldn't it still be able to hold 200psi at any other normal temperature? Like 100F or 500F? What does the air temperature have to do with the rating?

My best guess is that compressed air gets quite hot so they test it in that environment...though the last time I filled up the tank the fill tube and fittings were only about 180°F at the hottest. Maybe in a garage in Texas in the summer it would be closer to 300°F.

Does this mean if you go above the rated temperature it isn't safe to hold 200psi anymore?

Of all the things a person can google...this doesn't seem to be one of them!

Thanks ahead of time, Joshua

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  • The spec would probably be more accurately (and usefully) written as 200PSI and 400 degrees (not @).
    – isherwood
    Commented Dec 7, 2021 at 14:13

5 Answers 5

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Air tanks are pressure vessels. As such they fall under your local code authority. In the US, that would be ASME (American Society of Mechanical Engineers) and their ASME Boiler and Pressure Vessel Code, running to some 800 odd pages. To be certified as a pressure vessel the require certain ratings on the nameplate (see UG-119 Nameplates in the code). These include:

Name of the manufacturer, their serial number, and year built (required).

One of more of:

Maximum allowable working pressure (MAWP) as (Pressure) at (Temperature)

Maximum allowable external working pressure (MAEWP) as (Pressure) at (Temperature) - this one is pretty rare for vessels in air.

Minimum design metal temperature (MDMT) as (Temperature) at (Pressure) - accounts for ductile-to-brittle transition temperatures of the vessel materials.

So, the manufacturer is giving you the MAWP in a normal format. One could argue that it is not strictly required if the vessel is not a boiler, but this is a standard marking and they are laying out the limits of the vessel in accordance with code.

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  • Frankly, this is why it's labeled that way. If the manufacturer didn't have to comply with standards that include standards for labeling, this wouldn't be on the label.
    – FreeMan
    Commented Dec 7, 2021 at 17:02
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There may be seals, valves or other components that are temperature sensitive where they may fail at high temperatures. Materials may melt, deform or simply expand at different rates, and any of those situations could result in catastrophic failure.

Cold is a problem too. Consider what can happen if a simple O-ring fails due to freezing.

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    I believe this is the correct answer. The temperature rating is the highest temperature at which it was tested to hold 200psi. It's a certification test, not a regression test so that doesn't mean it can't go higher. Nothing to do with the relationship between temperature and pressure. Sure, that relationship is important but it is not why the tank is rated 400 degrees. If you fill it to 200psi at 40 degrees then heat it, it will go over 200 at 41, and fail long before 400. Similarly if it's empty at 450 and you try to fill it, it may fail for some reason, nobody knows.
    – jay613
    Commented Dec 7, 2021 at 12:56
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    400F is well into "affecting the heat treatment" of steel.
    – Ecnerwal
    Commented Dec 7, 2021 at 13:06
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    That O-ring link was due to not reading the instructions... Using things outside the range clearly specified.
    – Solar Mike
    Commented Dec 7, 2021 at 16:40
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My best guess is that compressed air gets quite hot

Correct. This is an inherent characteristic of compressing air.

Think about how air conditioning/refrigeration/heat pumps work.

  1. We compress a gas, which makes it very hot.
  2. We cool off this hot, compressed gas, in the hot-side heat exchanger.
  3. We allow the gas to de-compress; it becomes very cold.
  4. We run it through a cold-side heat exchanger.

Now, yes - in most installations there's some other stuff about a liquid-vapor transition, and yes, exploiting latent heat of vaporization is quite potent, and adds efficiency that makes it worth using weird gases. But the refrigeration cycle would work fine with straight air.

That's why your air-powered tools are cold.

Last time I filled up the tank the fill tube and fittings were only about 180°F at the hottest. Maybe in a garage in Texas in the summer it would be closer to 300°F.

Because you ran it to pressure then shut it off. Think about running it continuously for an extended time, e.g. if you had a sandblast cabinet.

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Air is a gas and pressure, volume and temperature are related.

If the volume is fixed, such as your tank and you increase the temperature then the pressure rises.

For the temperature range normally seen in your home workshop that won’t be an issue, but regulations require the information is stated.

So:

PV = nRT

Where P is Pressure, V is volume, n is mass or moles, R is the gas constant and T is temperature.

Engineers and scientists will often work assuming the pressure is constant or the volume or temperature depending what is being investigated.

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    That is all true. But the rating of 200 psi @ 400 deg F is taking temperature into account. In that regard, no additional derating is required unless the temperature exceeds 400 deg F. Also, it's important to note that in that ideal gas equation, the T in that equation is deg Kelvin, not fahrenheit.
    – SteveSh
    Commented Dec 7, 2021 at 11:07
  • The rating specified without a temp would be meaningless
    – bsd
    Commented Dec 7, 2021 at 11:12
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Pressure increases with temperature.

An LPG cylinder at 0°C has a pressure of 1.5 bar, the same cylinder at 70°C has 24.8 bar of pressure.

So your cylinder is able to hold 200 psi at 400°F, if you got your cylinder to 200 psi at standard temperature and got it to 400°F, it would at that point have a pressure over 200 psi.

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    You're right, but I don't think that applies here. 200PSI is 200PSI, regardless of temperature.
    – isherwood
    Commented Dec 7, 2021 at 14:16

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