Why have dual tanks on an air compressor?

[Peter Kuhlman]

I work in the oilfield with equipment rated up to 30,000 psi - not an engineer. The choice of a single versus double tank can be an economic choice or a structural choice. For 150 psi - that is 150 pounds of pressure on every square inch of exposed surface - a 10 inch square plate having 100 square inches will have a total of 15,000 pounds against it!!!! That is scary. Many workers have been killed on low pressure vessels and piping with large surface areas. For example, a pipe flange on a large pipe might have 500 inches of surface and with 30 psi, that is 15,000 pounds total pressure on the flange. Easily enough to kill a guy.

For the dual tank, smaller tanks with less exposed surface will have less total internal pressure so the metal can be thinner. The engineers have to figure out if that is more economical to produce than a single tank with thicker heavier metal. As others stated, there are worksite issues where having a low center of mass has great advantages for stability.

You should see the pipe we use for 15,000 psi. It is called 4 inch triple X and while not exact measurements, has about a 1" inside hole in pipe with an outer dimension of a little over 4 " - and this is specially hardened metal where any welds have to be heat stressed and all connections and welds have to be XRayed. At 20,000 psi and above, no pipe is used, only huge blocks of metal so pressure drops need to be as quick as possible off of a wellhead.

[Paul Ryan]

I am not sure I will just throw out my belief.

I think everyone else is over thinking the situation. I don't believe it has anything to do with pressure per square inch or stability. I believe it is a way for the maufacturer to produce a machine that has the same volume and out put as a larger machine. 2 10 gallon tanks would hold the same volume of air as a 20 gallon thank, but could be postioned (stacked) to make the machine more compact and more portable. The machine would produce the same as the large machine but take up less room in the back of a pick-up or in the corner of a room. Just my .02

[Bruce Wrenn]

Many places require tanks over a certain size to be ASME rated. That means HEAVY, too heavy to be portable. I can't remember whether it is fuel, or air tanks that are designed to hold 119 gallons, as the regulations and standards change at 120 gallons.

[Ron Bontz]

Ok. It has been many years since I had any statics or dynamics but I think you guys are saying it is the ratio of wall thickness to volume that determines PSI max. Given the same shape of container. Yes?

[Ed Kilburn]

Please correct me if I'm wrong, be I've been told a tank with 6" or less inside diameter doesn't need to be ASME stamped, saving a lot of cost.

[Bob Genovesi]

The last time I turned on my compressor it worked..... nuff said...

[Bill Keehn]

In case anyone is interested, the stresses in a cylindrical pressure vessel are:
hoop stress = Pressure * radius / wall thickness
longitudinal stress = Pressure * radius / (2 * wall thickness)

In words, the stress resisting the end caps blowing off is half the stress resisting splittting the cylinder down the long axis. The radial stress is negligible compared to the hoop and longitudinal stresses. For any given pressure, a smaller vessel can utilize a thinner wall section. For any given wall thickness, a smaller vessel is stressed less, and will fail at a higher pressure.

The pool scenario is a bit different, being open on one end, and having a linear pressure gradient down the wall.

Cheers,
Andrew

Thanks for the tip Andrew. After reading about hoop-stress, I think I understand now.

I think Ed's argument about the smaller tank not requiring the same level of certification is probably a big factor.