Overpressurizing / Overfilling steel tanks

[Charlie19]

[QUOTE
What is your justification for overpressurizing[/QUOTE]


Because most dive shops can't get a tank full. Invariably, my LPs end up having about 2300-2400 psi, when they should legally be filled to 2640. I have found that if the shop will fill the tanks to 2850, I can usually end up with 2600 or so in the tank.

I don't feel that 2850 is any higher than the pressure would be if the tank was filled to 2640 and left in the sun or in a hot vehicle for a few hours.

Finally, the tanks all have burst discs for an added saftey feature.

 

[Scubakevdm]

How about everyone fill up their tanks as much as they want to, and I'll take notes until we get a better feel for actual burst pressures, rates and conditions. Kinda Darwinian.

 

[FredT]

I don't feel that 2850 is any higher than the pressure would be if the tank was filled to 2640 and left in the sun or in a hot vehicle for a few hours.

Finally, the tanks all have burst discs for an added saftey feature.

The tank rated pressure is for contents at 70°F. While a hot overfill to cool down to rated pressure is not the best way to fill, it is an economic and legal one.

The burst disk is a red herring in this discussion as the rated pressure of that part is significantly higher than would be used in any form of "overfilll hot" condition.

As a side note I have _underfilled_ tanks to warm up to rated pressure at 70°F. Prechilling an AL 80 to near freezing before and during a slow fill will allow a full load in the tank with a lower pressure in the bank. This works ONLY with VERY DRY air in the bank. A 3000 psi fill at 32°F grows to almost 3500 in an open boat under a hot tropical sun. Better to prefill cold to 2700 and let it warm up to 3200 on deck. When quenched in the seawater at entry the pressure rapidly drops back down to the 3000 it's supposed to be.

 

[roakey]

I just merged the old, existing overfilling thread with this one; if you're following this thread you might want to go back and read it from the beginning, since replies are placed in chronological order, so all the replies from the old thread come first.

Roak

 

[Mverick]

Also we'll do the dive in a Missouri cave on trimix. You'll have to carry all the gas you need because there isn't an air fill for 100 miles and no trimix fill for 300 miles. Oh, on a week end the closest fill is my house in Indiana period. LOL

On a Weekend, Try Mermet. I think they're mixing TriMix. Although I mix my own. Pat would know. If you're diving Missouri caves. You would Know Pat.

 

[MikeFerrara]

On a Weekend, Try Mermet. I think they're mixing TriMix. Although I mix my own. Pat would know. If you're diving Missouri caves. You would Know Pat.

I mix my own too. Doesn't every one? LOL
Mermet is a very long way from where we dive. Only been to mermet once though.

We are diving Missouri caves but I don't think I know Pat so I guess I don't understand why you state that I would. We usually don't run into any one unless we bring them and I've never brought a Pat. LOL At Roubidoux we run into some of the OCDA guys once in a while but I don't know their names.

Can you explain?

 

[-hh]

Engineers are often wrong especially when they're doing what an MBA tells them to do. Also they usually know it's wrong when they're doing it..

Its not that they are "wrong": it is that there is always tension between the technical side and the business side for what constitutes an acceptable or unacceptable risk, which ultimately boils down to money and profits.

For an illustrative case study, current Mad Cow disease bit, in the context of the 5-year ban the USA placed on Alberta Canda for what will prove out to be a lesser QA infraction than what is being found in Washington State.

As an engineer...One of the things some engineers spend lots of time doing is argueing test specs with QA and sometimes even legal people.

Yes, this can happen when there is not clear understanding of the relevant risks, and their degree of criticality. As a fellow engineer I can say with firsthand experience that when you win the lawsuits afterwords, you are vindicated, and you have fewer arguements with the QA and Legal guys

While overfilling might cause a tank to have a shorter life (fail hydro sooner?) some might be able to accept that.

First, there is absolutely no question from an Engineering standpoint: this is classical fatigue cyclic loading, so a higher operating pressure will manifest itself as fewer load cycles (eg, "fills") at the same predetermined level of acceptable risk.

And while I agree that a shorter life may be acceptable, the problem generally found in scuba diving is that no one has any clue as to what the risk and/or revised lifespan is, becuase they've never done the formal Engineering Analysis.

At best, we have anecdotal data from Joe Nobody's who didn't do a scrap of analytical analysis, but just put maybe 1000 overfills on their tanks, and have turned around and declaired that it is enough to declare it "perfectly safe" for anyone and any application.

Finally, let's not forget that the Engineering Analysis isn't complete until it formally includes the Engineer's name, Discipline, PE licence#, State of Registration, and the document franked with his stamp and signed, because from a Professional Engineering standpoint, anything less falls short of Doing It Right. If anyone has a problems with this, they can go take it up with the ASME and NSPE.


-hh

 

[NAM001]

i havvnt read the whole thread but heres my input.

lp steel @ 2400 100% rated per stamp 40% of tank burst presure 5970
3000 125% of stamp rating 50 % "
3500 150% " 58% "


how much safety buffer is needed??? filling to 3K or slightly above is no where near any real danger. i may not run mine up to 3500 but a hot fill to 3200 and cooling to 3000 is no biggy.

ox tox was once at 2 atm then 1.6 now 1.4 how long befor we we teach folks they die at 1 atm. precaution on top of precaution on top of precaution on top of precaution is as dangerous as no precaution at all. because of loss of credibility. at least someone is here questioning validity of standards. good thread.

KWS

 

[hex92]

A LDS operator sells LP steel tanks with HP valves (typically the combo DIN yoke valve -rated to 3300 PSI). He will then overfill them to 3000+ psi. He fills his own tanks with regular DIN valves to ~3500 psi. As a result alot of the people who frequent this shop are diving with these tanks since you are getting 120 cuft of air in a 95 cuft container - don't quote me on the math here....

I asked around at another LDS and they said they wouldn't fill to more than the rated 2400 psi and looked at me like I was crazy to even suggest it. So they sell mostly HP tanks.

I am trying to decide if I should go with the LP 95s. What do yall think? And have any of you had luck finding people willing to overfill LP tanks?

 

[cornfed]

I am trying to decide if I should go with the LP 95s. What do yall think? And have any of you had luck finding people willing to overfill LP tanks?

With the new PST E series tanks this isn't really an issue. I would suggest looking at those before trying to over fill tanks. You might find places to do it but it's not worth the hassel.