Hydrostatic testing...how much can the tank expand and still pass?

[fweber]

Very good...
smart as_ ...

Sorry,..... I wasn't intending to be an ***......it just comes naturely.

Luis, I unfortunately lost a long typed response to your post 12 from yesterday.

Reader Digest Version:

Essentially, we agree on most points. We should be careful to seperate the discussion of the characteristics and concerns with regard to AL vs ST cylinders.

AL pose a real catastrophic issue beyond hydro, due to the fact that hydro does not detect neck cracks.

Steel tanks don't all require "roundingout/leak checks". 3AA cylinders can be tested all day with no need for such. It's PST's E series that cause headaches. And how exactly does a manufacturer get authorized to "require" that a "leak check" gets used as a modification to a DOT procedure?

Sorry for the breavity, but here comes the boss.

 

[Luis H]

Sorry,..... I wasn't intending to be an ***......it just comes naturely.

Luis, I unfortunately lost a long typed response to your post 12 from yesterday.

Reader Digest Version:

Essentially, we agree on most points. We should be careful to seperate the discussion of the characteristics and concerns with regard to AL vs ST cylinders.

AL pose a real catastrophic issue beyond hydro, due to the fact that hydro does not detect neck cracks.

Steel tanks don't all require "roundingout/leak checks". 3AA cylinders can be tested all day with no need for such. It's PST's E series that cause headaches. And how exactly does a manufacturer get authorized to "require" that a "leak check" gets used as a modification to a DOT procedure?

Sorry for the breavity, but here comes the boss.

No need to apologize.


Below is the PST D100-E bulleting. If you read the second paragraph you will see that it applies to all PST hot dip galvanized cylinders (all cylinders since 1952).

See highlighted quoted sections below:
“This bulletin describes procedures that must be performed prior to retesting each cylinder. The cylinder must be prepared and test system checked in order to obtain accurate results in the hydrostatic expansion retest for galvanized steel cylinders. All PST scuba cylinders made starting in 1952 are protected from corrosion by a hot dip galvanized coating. This hot dip galvanized coating may affect the readings from the hydrostatic test, therefore failure to prepare the cylinder and perform the test system check in accordance with this procedure may result in rejection of a perfectly acceptable cylinder.”


Manufacturer requirements are not normally legally binding (unless there is a contract). Like you don’t have to change the oil in your car as required by the manufacturer, but don’t expect any warranty or sympathy from the manufacturer (or anyone else).

In a similar fashion, for a hydro facility to receive my business, they are required to follow the manufacturers requirements (per PST D100-E bulleting) on all my steel cylinders. I make sure they have a fresh copy. This requirement is compatible with CFR49. So it is very simple, if they don’t meet the requirements, they don’t get my business.


A better copy can be downloaded here:
http://www.vintagedoublehose.com/downloads/PST-D-100.pdf

 

[guruboy]

Is tank permanent expansion relative to tank spec or the actual value prior to that hydro test session?

In other words, can a tank keep getting bigger by 9% each year?

 

[Luis H]

Wow… this is a 9 year old thread. I can’t believe that I have been writing in ScubaBoard about cylinders for that long…


Keep in mind that the allowed permanent expansion is only a percentage of the total expansion. It is only 9% of the total expansion not a percentage of the total cylinder.

Yes, the cylinder can permanently expand a miniscule amount during every hydro-test, but don’t count on any measurable amount.


Let me give an example with one of my typical steel 72’s. I measured the actual internal volume using water.

This particular cylinder has an internal volume of 728 inches cube (= 0.421 cubic feet).

BTW, the volume of the compressed air in this cylinder is 71.4 cu ft (at 2475 psi).


During the most recent hydro test this cylinder had the following results:

Maximum expansion: 57.0 cc (3.48 cu inches or 0.002 cu ft)

Permanent expansion: 3.6 cc (0.22 cu inches or 0.000127 cu ft)

Elastic expansion: 53.4 cc (3.25 cu inches or 0.00188 cu ft)

The % permanent expansion: 6.3%

This is BTW my worst steel 72. The results on most of them is less than 4%.


If we assumed a 9% of 57 cc, we would get a permanent expansion of 5.1 cc (0.311cu inches or 0.00018 cu ft).


The actual permanent expansion is only 0.311 cu inches (assuming 9%). If we divide that by the actual total volume of 728 cu inches, you get 0.043% increase in volume. Therefore the cylinder that was 71.4 cu ft will grow to 71.43 if it had that much permanent expansion.

That 0.03 cu ft of air is impractical to measure, therefore the cylinder will still be considered a a 71.4 cu ft (after rounding the numbers).


BTW, my steel 72 range in actual volume from 70.0 to 72.7 cu ft.

 

[guruboy]

I see. That makes a lot more sense than the entire cylinder expanding 9% each time. Thanks.

 

[Luis H]

The maximum expansion (the elastic + the permanent) on that cylinder was 57 cc or 3.48 cu inches. That is 0.48% of the total volume (728 cu inches). Most of the expansion was elastic.

The actual permanent expansion (also know as plastic deformation) was 3.6 cc or 0.22 cu inches. That is 0.03% of the total volume.


To actually measure with this level of precision, the hydro machine has several lab style burette for different ranges. A small leak on the neck fitting can produce erroneous results. If the water temperature is changing, it can also affect the readings.