From what you say, the tank seems to be a rectangular one. Am I right? If it is, I doubt that it'll withstand 100 psi internal pressure, unless the plate thickness is increased and it's heavily stayed.
Giovanni S. Crisi
Sao Paulo - Brazil 

Hello Sourdough, Giovanni is definitely correct on the pressure issue and the size and design of your tank. If this is truly a tank that will see pressure, you will be looking at roughly 20 tons of force being exerted on one side of this tank if it is 20" x 20" and has 100 psi. of pressure exerted against it. I am wondering if you are miscommunicating the use of this tank, unless it is being used as an accumulator, in which case it is likely going to see even greater pressures. Is this an oil storage tank? If so, it will likely see little or no pressure as it will also likely have a vented fill cap or a seperate venting hose or filter assembly. If this tank needs to stand up to pressure, even as "little" as 100 psi it should be designed and built as a pressure vessel. If it is simply for oil storage and supply for the hydraulic system it will likely function just fine as you have described. My $.02. Best regards, Allan

Using a rough stress-based design for an internal pressure of 10 psig, this small rectangular tank would need to be about 1/2" thick to conservatively limit bending stress in the plates to a safe 15 ksi, assuming you use a decent grade carbon steel such as A-36.

Going the other way, a 14" x 20" x 20" x 1/8" wall tank (unstiffened) will safely only hold about 1 psig.  You can add stiffeners to the box or stay rods to the inside pretty easily if you want to keep with a 1/8" wall thickness at 10 psig.

Pressurizing any closed container over even say 1 or 2 psig can be tricky & dangerous business - so be careful!  There are lot's of things to consider - welding/joining of the walls, fatigue due to cycled loads, permissible deflection of the walls...

Regards

Hey Sourdough, everyone here is pretty much on the ball. A few years back I worked at a company that made hydraulic tanks of various sizes. All were leak tested using air pressure. We would use around 1 to 2 psi of air. These were much smaller than you are talking about. Small tanks were around 4" x 8" x .125 wt. Larger tanks were 12" x 14" x .125 wt. and 2 psi would really make those things bulge. One day a newer guy was doing the testing and turned up the pressure on the regulator to about 8 psi (if I recall), he was knocked unconscious after the tank blew up. Anyway, area is a big factor when making these calculations. To relate it to pipeline stuff, think of the SMYS's you deal with. A 2" .154 wt pipe can carry 650 psi at 20% SMYS where a 12" .250 wt can only carry around 250 psi to achieve 20% SMYS, given they are both the same grade of material. And these are cylindrical. Square edges add to the calculations. That's why there are no flat end caps, all round. Just things to think about when you're building your tank. Hopefully someone can give you a design formula, I'll do a little digging but no promises.