Air Tank Questions

[David]

I should know the answer to this stuff, but I don't. So help me out.

 

I have a 7-gallon pancake style air tank rated to 200 psi. It has three female NPT outlets: 1 x 1/4" and 2 x 1/8".

 

I'm all set with hose and chuck, but have some (probably silly) questions:

 

1) Should I have a regulator to automatically release any pressures above 200 psi to protect the integrity of the tank?

 

2) Should I have a regulator to control the outlet pressure?

 

3) Can I skip a regulator and simply put a fixed gauge on the thing, and monitor that I don't fill it at pressures above the tank's rating?

[Boffin]

I should know the answer to this stuff, but I don't. So help me out.

 

I have a 7-gallon pancake style air tank rated to 200 psi. It has three female NPT outlets: 1 x 1/4" and 2 x 1/8".

 

I'm all set with hose and chuck, but have some (probably silly) questions:

 

1) Should I have a regulator to automatically release any pressures above 200 psi to protect the integrity of the tank?

 

2) Should I have a regulator to control the outlet pressure?

 

3) Can I skip a regulator and simply put a fixed gauge on the thing, and monitor that I don't fill it at pressures above the tank's rating?

 

How big a bang do you like

 

I would play safe with a blow-off valve set to less than the max rating - say 180 psi

 

Andy

[David]

I would play safe with a blow-off valve set to less than the max rating - say 180 psi.

 

Okay, that answers #1. ANy idea on #2? I'll be using it to fill m/c tires, mainly. But other things, too.

[Boffin]

If you connect 180PSI straight to a tyre with a hose, the two will try to get to the same pressure quickly. I would go with a regulated fill hose, like you get on compressors

 

Andy

[ShovelStrokeEd]

David,

I run a single stage regulator on my Nitrogen tank set at about 80 PSI for things like filling tires and running my air tools. For me, it is essential as the N2 tank is at about 2500 PSI when full.

 

A similar regulator is easy to find from someplace like Parker Hannifin to take your 200 or so PSI down into the 70-80 PSI range. Makes filling a little more precise.

 

As to the 200 PSI thing, I would certainly consider a pop safety valve set at about 90% of the tank's rated capacity. Remember, at the track you are likely to have the thing sitting in the sun and, while there is a safety margin, 6 gallons of air at 200 PSI will make a pretty big bang.

 

A suitable regulator can be found here or at a local supply store. Sears has 'em hanging on the wall and you might even be able to find a suitable blow-off valve.

[Joe Frickin' Friday]

1) Should I have a regulator to automatically release any pressures above 200 psi to protect the integrity of the tank?

 

Others are right: get a pop-safety valve. Standard equipment on every compressor/tank I've ever seen for sale. McMaster is my favorite vendor: go to www.mcmaster.com and search on "pop-safety valve" to get a page full of options for pressure setting and NPT size.

 

3) Can I skip a regulator and simply put a fixed gauge on the thing, and monitor that I don't fill it at pressures above the tank's rating?

 

If there's a fire or it gets left out in the sun or something, you'll want it to be able to relieve the excess pressure. Granted if there's a fire you've got bigger problems, but you don't want a bomb thrown into the mix.

[2wheelterry]

Others are right: get a pop-safety valve. Standard equipment on every compressor/tank I've ever seen for sale. McMaster is my favorite vendor: go to www.mcmaster.com and search on "pop-safety valve" to get a page full of options for pressure setting and NPT size.

I believe its also good practice to compare the flow-rate of the pop-safety valve at the desired set pressure versus the maximum supply source flow to ensure the safety valve will flow more than the supply source. There should be a chart on the website for each valve.

[RFW]

Others have answered your questions, I think, but there is a more general question about air tanks....

 

What kind of idiotic system rates tanks in "gallons", and compressor output, and air tools in CFM??? It's no wonder that Europeans who are used to the more sensible metric system, think our North American systems of measurement is simply hillarious!

 

Bob.

[ShovelStrokeEd]

As I recall, Tennessee is part of North America. What other units do you suggest we use to express volume given that we use this system? Cubic inches results in large numbers, cubic miles small numbers. A gallon is an accepted volumetric measurement although I agree that the math is easier with metric.

 

Ditto for CFM as a unit of consumption/capacity. It accuratly describes the requirements or performance of the tool/compressor and makes just as much sense as l/hr. Actually, to really compare performance flow rates should be in SCFM which converts into standard temperature ratings. Of course, just to complicate matters, there are two different SCFM thingies as well, differing in temperature.

 

So long as both parties agree on which units to use, there is no problem and none of this has anything to do with David's question.

 

[steveknapp]

Usually if you need to ask, you know the answer.

 

Regulators are cheap, plus most hoses and air accessories aren't speced for 200psi.

 

Safety valves...safety good.

[David]

Thanks, folks! I'm all set.

[RFW]

As I recall, Tennessee is part of North America. What other units do you suggest we use to express volume given that we use this system?

It should be obvious that I am suggesting that the units for tank volume and the units for compressor output and air tool ratings, ALL ought to agree.

 

Since CFM is a pretty standard North American rating for the compressor output, etc, why the hell isn't the tank volume rated in cubic feet? Who on earth measures air in gallons?

 

Bob.

[ShovelStrokeEd]

Who on earth measures air in gallons?

 

Accepted unit of VOLUME is gallons and is used for tank measurement, not air capacity.

 

Accepted units for FLOW RATE is CFM.

 

They are NOT the same thing.

 

Depending on pressure, a tank may hold much more air than its volume might indicate. It is only at atmospheric pressure that the volume and air capacity of a tank are the same. Don't even get me started on moles, cause it makes my head hurt. Suffice that the amount of air in a tank of a given volume varies with the pressure in the tank. Therefore it makes sense to rate air flow in CFM which is, btw, more properly, cfm at 70 PSI which, I believe is the industry standard for such things as air tools.

[George Brown]

Don't even get me started on moles, cause it makes my head hurt.

 

Moles: A term that I haven't heard for years except for the ones that show up in turf grass occasionly. Moles, as in compressed gases, always made my head hurt too.

[RFW]

Accepted unit of VOLUME is gallons and is used for tank measurement, not air capacity.

 

Accepted units for FLOW RATE is CFM.

 

They are NOT the same thing.

Well, OBVIOUSLY!

 

Depending on pressure, a tank may hold much more air than its volume might indicate. It is only at atmospheric pressure that the volume and air capacity of a tank are the same. Don't even get me started on moles, cause it makes my head hurt. Suffice that the amount of air in a tank of a given volume varies with the pressure in the tank. Therefore it makes sense to rate air flow in CFM which is, btw, more properly, cfm at 70 PSI which, I believe is the industry standard for such things as air tools.

Actually, a standard yardsick of comparison is SCFM (Standard Cubic Feet per Minute) which is the number of CFM normalized to atmospheric pressure.

 

My point is that while it may be ACCEPTED PRACTICE to rate air tank volume in gallons, is pointless since you need to translate gallons to cubic feet to make any meaningful calculations. Hell, why not rate the tank volume in Bushels?? That would make just about as much sense as gallons of air!

 

Bob.

[ShovelStrokeEd]

Your still not getting it.

 

The volume of the tank is a descriptor to help in determining its size and could be expressed in any units you wish. If you prefer, call the tank 0.8 cubic foot capacity which is its conversion into US liquid measure.

 

The amount of air in the tank is still a function of its pressure and temperature.

 

Air tools are rated by their consumption of air at a given pressure, usually 70 PSI for things like impact guns or air driven drills and the like. They are not rated in SCFM, which is more properly used to measure flow in ducts, because they don't work at all at atmospheric pressure.

 

Units are chosen to suit the intended task of the tools. A tank may be used to store/transport other things than air, water for instance. In that case gallons make sense. Air tools, which are commonly used in conjunction with air compressors are both rated at CFM they can deliver or consume at a given pressure. Since both use the same units, it is easy to calculate the size of compressor and the lines needed to match the needs of the air tool in use.

 

Where is the problem? The same thing would exist in the metric system or even SI units if you prefer.

[Joe Frickin' Friday]

Thanks, folks! I'm all set.

 

And you thought that would end the discussion?

[wuli959]

buy a small compressor and call it even

[Firefight911]

buy a small compressor and call it even

 

Hand operated bicycle pump!

 

And that should end the thread, eh??

[RFW]

Your still not getting it......Where is the problem? The same thing would exist in the metric system or even SI units if you prefer.

The problem is obvious. When you are trying to determine how big of a tank is needed to drive a spray gun, or air tool, that consumes X CFM, at Y psi, when your compressor output will not keep up with the tool's air requirement.

 

This is a relatively common problem for those of us who have a medium duty compressor, but have some air tools that are used intermittantly that need a lot of air. The answer is to get a tank that is big enough to store enough air to allow the tool to work for as long as it it needed.

 

Thus it is absurd to try to work though the required details to select a suitable tank volume, when the compressor output, and tool/spray gun requirements are all nicely given in CFM at specified pressures, and the tank volume is given in some irrelivant units like gallons that need to be converted to cubic feet to be of any use at all.

 

The same problem would NOT exist in the metric system. In Europe, everthing is in liters/minute and liters or CCs (which directly translate to liters anyway).

 

But this is not a metric question. It is a common sense question. I don't care WHAT unit of volume is used; lets at least be consistent. The metric system may not be "better", but at least it is generally more consistent than the hodgepodge of arcane 18th century units we use here!

 

Bob.