The elusive "+" rating. . .

[JohnN]

Can someone please clear up my confusion about the "+" rating, as it applies to LP steel tanks?

I think I understand the process for Hydrostatic testing (best link I've found is: Hydrostatic). So when the permanent expansion is > 10%, then the tank should be decommissioned. Got it.


And the REE values for tanks (if it is not stamped on the tank neck) can be found at Compressed Gas Association Pamphlet C-5, "Cylinder Service Life, Seamless High Pressure Cylinders."


And the process for determining whether a cylinder can get a "+" is supposed to be in 49 CFR 180.209 (c) (http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&SID=f4795508d00345f7a6e14d7ff6a2c346&rgn=div8&view=text&node=49:3.1.1.1.3.3.1.5&idno=49)

I've slugged though this document without gaining any enlightenment.


So what is the relationship between REE, the Permanent Expansion and the "+" rating ?


Thanks!









---------- Post added January 10th, 2013 at 02:43 PM ----------

I dug further into the archives.

A good link (in case anyone besides me cares): http://www.scubaboard.com/forums/tanks-valves-bands/277461-plus-ratings-hydros.html

 

[halocline]

Post #8 on the thread you referenced is a good concise explanation of it. The + rating depends on whether the tank's total expansion during the test exceeds the REE number.

 

[JohnN]

Thanks. The whole thing seems to come down to providing the hydro facility with a REE number for the tank (if there isn't one stamped on the neck)

Do you have a copy of the CGA Pamphlet that lists REE's for different tanks? I've seen the copy of the PST sheet floating around the internet, but the shop isn't very likely to take that at face value. . .

Do you have a reference for looking up the site number from the number on the hydro stamp?


Thanks!

 

[00wabbit]

I heard recently that even though its technically possible to get the plus rating a second time, it is very difficult to find someone to do it.

 

[zider]

It is absolutely possible and is done by shops certified to do so. The issue is indeed finding a shop that does these ratings. In Indiana the are a couple shops but they require that the previous hydro was + rated. So they won't add the + back if it was lost. This is exactly why I go with HP tanks. Same fill without the hassle of finding a plus rating.

 

[JohnN]

It is absolutely possible and is done by shops certified to do so. The issue is indeed finding a shop that does these ratings. In Indiana the are a couple shops but they require that the previous hydro was + rated. So they won't add the + back if it was lost. This is exactly why I go with HP tanks. Same fill without the hassle of finding a plus rating.

From my reading of the process, it should be almost automatic and free, IF you can prove what the REE value should be. Without it stamped on the tank, the CGA C-5 pamphlet could be what was needed (the only catch is I don't want to spend the $47 just to satisfy my curiosity)

 

[spectrum]

I have written to Faber with cylinder numbers had they provided the REE. I was then able to get the+ rating on a cylinder that the prior owner had not done the REE legwork for.

PST and other gonzo brands are tougher but if you network a bit people have the information.

Find a shop that can do the extra calculation and explicitly request that you are seeking the + rating. The REE number is the factor needed for the extra calculation.

Pete

 

[JohnN]

I have written to Faber with cylinder numbers had they provided the REE. I was then able to get the+ rating on a cylinder that the prior owner had not done the REE legwork for.

PST and other gonzo brands are tougher but if you network a bit people have the information.

Find a shop that can do the extra calculation and explicitly request that you are seeking the + rating. The REE number is the factor needed for the extra calculation.

Pete

Thanks Pete,

My understanding is the go/no-go for certification is:

(pressurized volume -starting volume)/(starting volume) -- a measure of how elastic the tank metal is

and the Plus rating compares (pressurized volume - starting volume) the tank is metal is not too elastic


So there isn't much of a calculation. You compare the difference of two number to the REE value you're given. . .


Getting an agreed upon REE really seems to be the trick.

 

[captain]

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.
• Here is the "extra work:" -- Is Elastic Expansion < Rejection Elastic Expansion (REE)? If yes, then +. If no, then no + rating.

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.


Why some 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.

 

[idive2]

Why some 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.

Captain, I really believe it is the stamp of approval of a slightly higher fill pressure and the perceived additional
liability that may come with the plus rating. I really don't think it is the math or the additional time. While I don't
agree with some shops not giving this rating I can kind of understand their decision.