Maximum test pressure

[duckbill]

For hydrostatic retesting, I read that:

"Minimum test pressure must be
maintained for at least 30 seconds, and
as long as necessary for complete expansion
of the cylinder. A system
check may be performed at or below
90% of test pressure prior to the retest.
In the case of a malfunction of the test
equipment, the test may be repeated at
a pressure increased by 10% or 100 psig,
whichever is less."

...and that the measuring device:

"The pressure-indicating device, as
part of the retest apparatus, is accurate
within ±1.0% of the prescribed test
pressure of any cylinder tested that
day. The pressure indicating device,
itself, must be certified as having an
accuracy of ±0.5%, or better, of its full
range, and must permit readings of
pressure from 90%-110% of the minimum
prescribed test pressure of the
cylinder to be tested."



Say the minimum test pressure for a steel 72 is 3750psi (2250 x 5/3). What is the maximum test pressure for this intial test?
3750psi + 1%?

 

[oxyhacker]

There is no maximum pressure. The operator can perform the test at as high a pressure as he or she likes, as long as it is at or above the minimum required.

Of course the higher the tester goes the greater the chance that the tank will fail, but that is no concern of the DOT.

In real life shops don't go any higher than they have to (other than the slight amount busy shops will usually overshoot by since creeping up on the exact pressure takes longer) since it takes longer and puts more load on the test apparatus. To say nothing of irritating the customers.

For hydrostatic retesting, I read that:

"Minimum test pressure must be
maintained for at least 30 seconds, and
as long as necessary for complete expansion
of the cylinder. A system
check may be performed at or below
90% of test pressure prior to the retest.
In the case of a malfunction of the test
equipment, the test may be repeated at
a pressure increased by 10% or 100 psig,
whichever is less."

...and that the measuring device:

"The pressure-indicating device, as
part of the retest apparatus, is accurate
within ±1.0% of the prescribed test
pressure of any cylinder tested that
day. The pressure indicating device,
itself, must be certified as having an
accuracy of ±0.5%, or better, of its full
range, and must permit readings of
pressure from 90%-110% of the minimum
prescribed test pressure of the
cylinder to be tested."



Say the minimum test pressure for a steel 72 is 3750psi (2250 x 5/3). What is the maximum test pressure for this intial test?
3750psi + 1%?

 

[duckbill]

Thanks, oxy.

O.K. I have 2250 psi and 1800 psi cylinders.

The 3AA 2250 psi cylinders should be tested at 3750 psi (minimum), yet a local hydrotester pressurizes them to 4166 psi (according to the paperwork he provided), or 11% over test pressure. He based this on a 2500 psi service pressure for some reason.

The 3AA 1800 psi cylinders should be tested at 3000 psi, yet this same guy pumps them up to 3633 psi. That is 21% over the minimum test pressure!

So, there is nothing in place to prevent these guys from damaging cylinders other than simple customer pressure?

 

[oxyhacker]

That's pretty much the case. The DOT wants the tanks tested at at least the specified pressure anything more is between the tankowner and the hydro shop.

Does the shop give you the "+"? If so it might be that it is confused about how it is properly done, and think they are supposed to do it by testing it at 10% over the test pressure. That's a fairly common misconception with hydro shops that will bother with the plus on older tanks. Still doesn't explain the TP for the 1800. Maybe he is just bad at math! I personally wouldn't get too upset over a short 21% overfill - as long as it didn't fail my tank. Have you asked the shop to explain? I'd be interested to hear what they say.

BTW, here's a fun question to ponder - which is safer, a tank which has passed hydro at the proper test pressure for that tank, or one which has passed hydro at 1000 psi more?

Thanks, oxy.

O.K. I have 2250 psi and 1800 psi cylinders.

The 3AA 2250 psi cylinders should be tested at 3750 psi (minimum), yet a local hydrotester pressurizes them to 4166 psi (according to the paperwork he provided), or 11% over test pressure. He based this on a 2500 psi service pressure for some reason.

The 3AA 1800 psi cylinders should be tested at 3000 psi, yet this same guy pumps them up to 3633 psi. That is 21% over the minimum test pressure!

So, there is nothing in place to prevent these guys from damaging cylinders other than simple customer pressure?

 

[Cylinder Tester]

Well, this is pretty scary...Although DOT does not specify a maximum test pressure, pamphlet C-1from the Compressed Gas Association specifies a limit of 10%. Using your 3AA1800 cylinder as an example, test pressure is a minumum of 3000psi. If your equipment has a leak during the test, you are supposed to retest at 100psi over the first test pressure (once the equipment has been repaired). Assuming the first test was 3000psi, this means your minumum retest is 3100psi. Should there still be a leak, you can make 2 additional test using this example.
Although DOT does not reference C-1 in the CFR, and therefore C-1 is not legally binding, all CGA pamphlets are considered "Industry Standards" and should be followed by all in this business.
If we pressurized a cylinder over 10% of test pressure, I would condemn the cylinder immediately. I suggest you find another retester and retire your cylinders.

Roger

 

[halocline]

If we pressurized a cylinder over 10% of test pressure, I would condemn the cylinder immediately. I suggest you find another retester and retire your cylinders.

Roger

I assume if you did that you would replace the cylinders for the customer, correct?

 

[fppf]

Well, this is pretty scary...Although DOT does not specify a maximum test pressure, pamphlet C-1from the Compressed Gas Association specifies a limit of 10%. Using your 3AA1800 cylinder as an example, test pressure is a minumum of 3000psi. If your equipment has a leak during the test, you are supposed to retest at 100psi over the first test pressure (once the equipment has been repaired). Assuming the first test was 3000psi, this means your minumum retest is 3100psi. Should there still be a leak, you can make 2 additional test using this example.
Although DOT does not reference C-1 in the CFR, and therefore C-1 is not legally binding, all CGA pamphlets are considered "Industry Standards" and should be followed by all in this business.
If we pressurized a cylinder over 10% of test pressure, I would condemn the cylinder immediately. I suggest you find another retester and retire your cylinders.

Roger

Why would you condemn a cylinder alone based on going over the minimum test pressure? If the cylinder does not yield over the limits dictated by the specs you have not done any damage to the cylinder. Now if the cylinder yields and has deformation then yes, it needs to be condemned.

And I'm guessing that if the tester over tested the piss out of a cylinder; causing it to then fail, you would just be told "yep it failed get a new one".

 

[rjack321]

Why would you condemn a cylinder alone based on going over the minimum test pressure? If the cylinder does not yield over the limits dictated by the specs you have not done any damage to the cylinder. Now if the cylinder yields and has deformation then yes, it needs to be condemned.

I would definately avoid hydro testers who condemn tanks which have not necessarily even exceeded their deformation specs just because they screwed up setting the TP.

 

[duckbill]

Does the shop give you the "+"? If so it might be that it is confused about how it is properly done, and think they are supposed to do it by testing it at 10% over the test pressure. That's a fairly common misconception with hydro shops that will bother with the plus on older tanks.

Yes. That could be an explanation. I will be looking for someone else to hydro my cylinders.

I had contacted DOT regarding this guys license status because I found out that when he tested my last steel 72 his license had expired. I just did a followup with the DOT to find out the outcome, and they issued him some letter to temporarily extend his license, or some such. The cylinder he did out of license should technically be retested, and I would think it should be on his dime. But, not only do I not want him touching my tanks anymore, he is also too far enough away to deal with easily. The DOT didn't seem all that concerned about his malpractice.

Thanks also to Cylinder Tester for providing the C-1 recommendation reference. I will remind the hydrotester about this if I get to talk with him again. From what I understand, he only does scuba cylinders. I'm sure the dive shops wouldn't care if he is shortening the life span of the cylinders they take to him. More cylinder sales, right?

BTW, here's a fun question to ponder - which is safer, a tank which has passed hydro at the proper test pressure for that tank, or one which has passed hydro at 1000 psi more?

I guess they'd be equally as safe, per se, though the one tested at the excessively high pressure would have undergone more fatigue and it's cycle life would have been shortened. Now watch this one have a punchline!

 

[fppf]

I would definately avoid hydro testers who condemn tanks which have not necessarily even exceeded their deformation specs just because they screwed up setting the TP.

Agreed, I would avoid anyone that can't do simple math and turn a knob.
But I also know that $hit happens, we are all human. The sad part is I don't think very many people stand up anymore and say "Yep, I screwed your tank, here is a new one".