HP Vs. LP and failing hydro tests..

[GeorgeC]

My local dive shop stated it is better to purchase a LP tank rather than a HP tank because the HP tank has a greater chance of failing a hydrostatic test. I don’t see any logic to this statement. I would assume the manufactures of the tanks would build the tank to handle the 5/3 hydrostatic pressure regardless of HP or LP. I think my LDS has an inventory of LP’s he is trying to push. I checked with the owner of my local oxygen supply company and he does not see any validity to the LDS’s claim.

Has anyone experienced a higher incidence of HP tank hydro failures as compared to LP hydro tank failures?

 

[SparticleBrane]

Sounds like he's pushing a product...

 

[hunter991]

Some of the PST HP tanks have some sort of finish on the inside that requires the hydro facility to follow a special protocal before testing. It they don't do this there is a higher chance a new tank will fail hydro. They could be talking about this, but I would bet they are just giving you a sales pitch.

I don't remember exactly what the procedure is, something about filling it to a certain pressure and draining just before hydro testing it.

Brent
www.hidawayscuba.com

 

[roakey]

Some of the PST HP tanks have some sort of finish on the inside that requires the hydro facility to follow a special protocal before testing. It they don't do this there is a higher chance a new tank will fail hydro. They could be talking about this, but I would bet they are just giving you a sales pitch.

I don't remember exactly what the procedure is, something about filling it to a certain pressure and draining just before hydro testing it.

Here's the stuff about special hydro testing -- it's for BOTH HP and LP hot dipped galvanized cylinders from Pressed Steel.

PST rounding out procedures

Roak

 

[GeorgeC]

Thank you for your responces and the PDF's.
George

 

[roakey]

Actually George, I failed the answer your original question. There should be no difference between failure rates for low pressure and high pressure cylinders. However, specifically for pressed steel cylinders the high-pressure cylinders are manufactured under a DOT exemption. You will find much discussion on this site about the possibility of pressed steel going out of business. If they were to go out of business they would no longer file their exemption and after a number of years the ability to fill high-pressure pressed steel cylinders would expire.

In such a case individual could file an exemption on their own but in my opinion the local dive shop would not accept such an exemption even though perfectly legal as per DOT regulations.

Roak

 

[CompuDude]

In other words, he's either pushing product, or he really has the wrong idea in his head.

 

[oxyhacker]

Not necessarily. A lot of shops will tell you that HP steel tanks fail hydro more often than LPs, though I have never seen any hard numbers. There are enough differences in metallurgy, thickness and heat treatment between LP and HP tanks that there is no reason to expect them to perform identically, and the HPs are not overbuilt the way the LPs are. Still, the difference, if it really exists, is probably not great enough to base a purchase decision on. However, it is worth being sure, when hydro time comes, that the hydro shop is aware of the special "exercise" pretest cycle PST stipulates.

This cycle, BTW, is not a specially permitted or required procedure, but reallly just PST taking advantage of a loophole in the regs to make it a little easier for their tanks to pass hydro - it could be done with equal benefit to other steel tanks.

 

[pescador775]

Yeah, it's called "rounding out". The tank is pumped to about 85% of TP, deflated and then hydroed in the normal way. Incidently, George, HP tanks are not tested to 5/3 rated press, they are tested to 3/2.