Question about pressure gauge

[mhorowit]

I'd like to test the integrity of a compressor air tank.
I"ll apply the pressure I need with a new grease gun (plug up one of the holes with a zerk fitting) filled with water.
Now, to measure the pressure, can I use an air gauge, or will the water ruin the gauge? - Mike.

[Hank Jarrett]

Re: Question about pressure gauge

Depends on what pressure you want and the volume of the tank. If you are looking for, say under 100#, you could put a stand pipe from the tank to higher than the top of the tank. Then you could actually use air pressure to pressurize the top foot of the stand pipe that pushes down on the water in the tank and rest of the pipe. The pressure gauge would be above the "T" you put the air in through. The idea is to have a VERY small volume of air so that when the tank bursts there isn't much "blast". If the tank is big (like 10 gal or more) and the air volume is low (like a foot of 1" pipe) there isn't enough air to cause an explosion if the tank bursts, you will just get wet from the spray if it splits. Where people get hurt is pressurising a tank where there is lots of air IN the tank. The bigger the air volume the bigger the "BOOM". A 2 liter soda bottle makes a REALLY LOUD blast. Take that as a guide. You DON'T want the air part to be very big and you DON'T want the failure in the air part!
Last warning. DON'T use pressurized air from a compressor to test a FUEL tank!!! Even with water in it! It WILL BLOW UP fromthe air presure. If you want to test a fuel tank put a condom over the filler neck and tape it to hold in place. Then use a HAND tire pump to presurize teh tank from the fuel outlet. If the condom inflates and holds, there isn't a leak. If you go to high on the pressure the condom will pop befoethe tank is damaged.
Hank
Can't use a balloon, the neck won't streach over the fill neck. ;-)

[mhorowit]

Re: Question about pressure gauge

I believe the tank's working pressure is 125 lbs, so I should test it to 250.
Thanks for the info - Mike

[Hank Jarrett]

Re: Question about pressure gauge

250 is WELL above 100 and the BOOM goes up by the cube of the pressure so be REAL careful! Put a stand pipe on it and keep the air OUT of the tank! NO AIR AT ALL in the tank. Using any air at that pessure is getting dangerous if there is any volume at all. What the heck are you making that uses a 250# tank pressure anyway? You have more projects than I do!
Hank
Get me some details and we can do a proper test. I don't want you blowing yourself up.

[mhorowit]

Re: Question about pressure gauge

I was at the airport talking about compressors and spotblasting and someone said they had one that didn't work, but I could have it, so I took it home.
Speedair Model 3Z394. The oil level in the compressor had been allowed to go down, so the connecting rod broke and the shaft was severely scarred.
Looking around for replacement parts, Grainger had the shaft ($70), but not the connecting rod, so I threw the compressor, saving the gauge, tank, electric motor.

Asked about testing the tank and someone suggested hydrostatic testing to 2x the rated pressure. Further suggested using a new grease gun to get to that pressure. That's when I posted the question about the compatibality of water and the internals of a pressure gauge.

You know that even with your suggestion, as I add water, the air in the 'stand pipe' will compress and water could get in that gauge. - MIke

[Ron Babos]

Re: Question about pressure gauge

mhorowit: Pressures pressure for all practical purposes. Won't hurt the guage as the bordon tube is brass or copper. Using a hand held hyraulic pump with a quick disconnect in the tank is what I have used at the shop sometimes. If you have a small air pocket someplace the grease gun volume may drive you nuts trying to reach the 250 lb mark.
Ron

[Hank Jarrett]

Re: Question about pressure gauge

Picture it this way, the verticle stand pipe comes up AT LEAST a couple of feet above the top of the tank you are testing. At the top of the pipe you put a "T" sideways with another 1' pipe coming out the top. You put the gauge there at the top and add the air at the side of the "T" so the gauge is a foot above the top of the water. Your stand pipe has to be set up so there is NO AIR above where it attaches on the tank. That is the important part, NO AIR IN THE TANK. (For the other engineers reading this, I know, there is an amount that won't hurt anything but NONE is safer than a little too much, so NONE in the tank.)
When you fill the tank the water will fill to the point where you will add the air and then run out. No water a foot higher where your gauge is. The air in the foot above the hookup point will act as a cushion and regulate the pressure. You CAN pressurize with the air IF there isn't any in the tank, only in the hose and 1' pipe to the gauge. It really does work and what I'm telling you is safe IF there is no air pocket in the tank. If there is a pocket even the grease gun method will compress it and have the same result (BOOM!)
Hank

[mhorowit]

Re: Question about pressure gauge

Hank - trust me on this. The only air will be in the top 6" of standpipe leading to the gauge.

Let's talk pipe now. What at HomeDepotAerospace will withstand 250PSI? Will the galvanized water pipe work?

Then I need to worry about threads matching. - Mike

[mhorowit]

Re: Question about pressure gauge

mhorowit: ... Won't hurt the guage
Ron

Thanks - MIke

[Guest]

Re: Question about pressure gauge

I believe the tank's working pressure is 125 lbs, so I should test it to 250.
Thanks for the info - Mike

Mike,

Does it have a relief valve? It should. If it does and the tank was tested when new and the relief valve works now I would skip it.

Look at Northern Tool for a replacement compressor. I saw some nice ones in there.

Dave

[VictorBravo]

Re: Question about pressure gauge

Mike, have a very thorough look at all the websites and news items for the "Darwin Awards" before you start pressurizing anything to 250 psi....

I can still hear my Aircraft Hydraulics/Pneumatics instructor (Mr. Richard Virgin, West Los Angeles Trade Tech College) telling us students the story of the Air Force mechanic who was inflating a KC-135 tire without the safety cage over it, and who had forgotten to put all the nuts on the wheel bolts... "and if you are anywhere near March Air Force Base on a still night, you can still hear the top half of his body screaming as it flies overhead in low Earth orbit!"

[Hank Jarrett]

Re: Question about pressure gauge

Ever see pictures of the KC-135 they were doing a cabin pressure test on and "over-inflated" the whole fuselage? Ruptured like a "Plump-When-You-Cook-Em" hot dog. How do you explain THAT to the "Old Man"!
Hank

[Hank Jarrett]

Re: Question about pressure gauge

By the way, I don't have a CLUE what a water pipe will hold safely. I SURE AS HECK wouldn't hold the water pipe!
Hank

[mhorowit]

Re: Question about pressure gauge

I think I've solved my problem. Instead of putzing around, I'll just build a ramp to move a rented compressor into the back of the Explorer. $25/day. End of problem. - Mike

[NY86]

Re: Question about pressure gauge

$25/day. End of problem.

Why don't you pay someone to test the tank?