There have been accurate posts, speculative posts, and posts with other information. I'll add some of my daily experiences to the mix.
"Testing for the plus" is literally comparing one number to another to see if one is smaller than the other. One of these numbers is a result of a standard hydro test, the other is the REE number stamped into the cylinder.
If it is stamped on the cylinder. A very large percentage of older plus stamped cylinders do not include the REE. The next quote demonstrates that well.
NorthWoodsDiver, the reason you didn't get the + on LP72s is because no one seems to know the REE number that is required to test for the +.
Testing for the + is NOT a huge amount of extra work. Here's how a standard hydro test is performed:
- The cylinder is pressurized to hydro pressure, and the displacement in mL is recorded. This is the Total Expansion.
- The pressure is released, and the amount of expansion left over is Permanent Expansion.
- Total Expansion - Permanent Expansion = Elastic Expansion (how much the tank expanded and contracted during the test).
- Was the permanent expansion 10% or less than total expansion? If so, then the cylinder passes hydro.
Correct
[*]Here is the "extra work:" -- Is Elastic Expansion < Rejection Elastic Expansion (REE)? If yes, then +. If no, then no + rating.
That is incorrect. I wish it was that simple.
For a numeric example -- this is information from a hydro test on a PST LP80 I had awhile back:
Since this is an LP cylinder, the service pressure is 2400psi. 5/3 of 2400 = 4000. Thus, the hydro pressure is 4000psi.
- REE is 66.3mL as stamped into the side of the cylinder.
- Total expansion was 56.3mL.
- Elastic expansion was 56.0mL.
- Permanent expansion was 0.3mL.
First, did the cylinder pass hydro?
10% of 56.3mL = 5.63mL. Is 0.3mL < 5.63mL? Yes. Thus the cylinder passed hydro.
Does the cylinder get a + rating?
REE = 66.3mL
EE = 56.0mL
Is 56.0 < 66.3? Yes, thus the cylinder qualifies for the + rating.
Here I have to disagree, Section 173.302a clearly indicates that for a plus rating wall stresses are the limitations being solved for.
To qualify the 3AA cylinder the formula for the average wall stress is S=1.7EE/KV-0.4P and the formula for the maximum wall stress is S=(P(1.3Dsquared+0.4dsquared))/(Dsquared-dsquared). If either exceeds the limitations a plus sign may not be applied.
For the 3AA cylinder the Average wall stress Limitation is 67,000 and the Maximum is 73,000.
If the REE is known the process is rather simple, not quite 1st grade math, but simple enough. If the REE is not known CGA 5 offers several methods for determining that. None of it is simple or basic.
The last time I heard, DOT issued a fine of $1400.00 per occurrence for failing to show the math on the log. DOT's fines are extreme and when compared to the $25.00 bucks we charge for a Hydro, the time and labor cost of the operator can exceed that quickly. I can understand why a lot of Hydro facilities don't bother with the plus rating.
Hydro shops
don't "test for the plus" (effectively performing some 1st grade math) is completely beyond me.
It literally takes 2 seconds to see if one number is less than another number,
especially when most testers don't have to look far for these two numbers: one is usually stamped into the cylinder, and the other is a result of the hydro test.
As I posted above, they know it is not the simple math you described.
The short answer is basically yes, but the codes are a bit more complicated.
The wall hoop stress is proportional to the cylinder diameter and inversely proportional to the wall thickness.
A cylinder wall hoop stress can be calculated from the following equation:
Wall stress = (pressure x inside diameter) / (2 x wall thickness)
CFR49 paragraph 178.37 (Specification 3AA and 3AAX seamless steel cylinders) specifies the use of the following equation:
Keep in mind that wall thickness is = (D-d) / 2
I don't particularly like the equation used in the codes, but that is not my call.
Explaining that equation is more than I want to get into and I am totally sure why they are using it in that form. The equation is not complicated, but it is not really looking at plain hoop stresses.
I agree, I don't care for the use as well, unfortunately, the law is written and clearly states that both the average and the maximum wall stress must be calculated and be shown below the published tables before applying the plus rating after a retest.
In all business, the cost of producing a product is weighed against the liability and reward. In the Hydro market, a cylinder that does not have the REE stamped on it usually requires more time and effort to qualify it for a plus rating than acceptable market rates for the service allow. There you have it in the nut shell.
)) and go dive.
