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SarahBella
01-16-2008, 08:08
I was talking to the owner of an LDS the other day about which tanks would be a good investment when he told me the most common type of tank that fails Hydro is a high pressure tank. Anyone ever heard this? I am not sure if this was his personal experience or if it was based on some statistic but I alway thought tanks would last many years if they were taken care of regardless of the pressure.

fireflock
01-16-2008, 08:24
What kind of tanks was he selling? :)

I'm far from an expert on the subject, but I think some of the 3442 series tanks have a slightly different hydro procedure than others. I've heard of facilities using the wrong procedure and failing those tanks when they likely would have passed using the correct procedure.

Hopefully someone with more details can chime in.

Rich

in_cavediver
01-16-2008, 11:10
The only tanks I have seen/heard fail more often is the older 3500psi high pressure tanks. Comudude I believe has had one or two fail. One of my buddies has 1 fail. (first hydro too)

Most of the common 3442 tanks are just now coming due for thier first hydro so I doubt there is a wealth of data for them. (PST introduced them in 2003)

Puffer Fish
01-16-2008, 16:56
The only tanks I have seen/heard fail more often is the older 3500psi high pressure tanks. Comudude I believe has had one or two fail. One of my buddies has 1 fail. (first hydro too)

Most of the common 3442 tanks are just now coming due for thier first hydro so I doubt there is a wealth of data for them. (PST introduced them in 2003)
We are coming up on the time where the first of these chrome Moly tanks will get Hydro'd. Will be very interesting if someone has done that with one of the 3442 tanks...(as I have a bunch)

marchand
01-16-2008, 17:23
Strange, I have three of the old 3500psi tanks made by PST in 1991 and they just passed their third hydro.

It may actually be true because there is really no difference between the HP and LP tanks, but during hydro they are both filled to 5/3 of their working pressure. This means that the HP tanks get filled to ~5800psi during hydro while the LP tanks are only filled to ~4000. Even thought the tanks are essentially the same the HP tanks are under more stress than the LP tanks during use, and under even more when they are hydroed.

cummings66
01-16-2008, 21:28
If there really is no difference then why does the LP version of the tank weigh MUCH more than the HP version?

texdiveguy
01-16-2008, 21:53
The true question to be asked is how many pass the VIP after passing the hydro, this is really were the true test is.

Puffer Fish
01-17-2008, 11:13
If there really is no difference then why does the LP version of the tank weigh MUCH more than the HP version?
You have to compare equal volume (internal volume) and not capacity. Have not actually done that, so I don't know.

The new generation of hp tanks (the worthington and Faber XF) are a very different alloy, one that should be much stronger and not have any issues (the XF I have are thru their second visual and they don't even have flash rust in them). But "should" and "Don't" is sometimes very different.

marchand
01-17-2008, 19:19
Because a LP120 is designed to hold 120cf at 2640psi while a HP120 is designed to hold 120cf at 3500/3442psi. A LP120 is really a HP160 if you can get cave fills.

SarahBella
01-18-2008, 07:09
I had no idea a little question would kick off such a discussion-thanks for the input.

marchand
01-18-2008, 18:02
I don't think any thread has actually stayed on topic.

cummings66
01-19-2008, 07:35
Because a LP120 is designed to hold 120cf at 2640psi while a HP120

Neglecting cave fills, the HP tanks have thinner walls and that's about the only reason I can think of for them to fail hydro before a LP tank which has thicker walls. That's why a tank with the same physical outside dimensions will vary that much in weight.

marchand
01-19-2008, 23:12
Because a LP120 is designed to hold 120cf at 2640psi while a HP120

Neglecting cave fills, the HP tanks have thinner walls and that's about the only reason I can think of for them to fail hydro before a LP tank which has thicker walls. That's why a tank with the same physical outside dimensions will vary that much in weight.

I think I see where you are confused. HP and LP tanks of the same capacity have very different physical outside dimensions. For instance, a HP120 is ~25" tall and has a diameter of 7.27" while a LP120 is ~29" tall and has a diameter of 8". The wall thickness is about the same, but the size of the tank is very different. The internal volumes are also different which is why a LP120 @ 2640psi and a HP120 @ 3500psi have the same amount of air in them.

skdvr
01-20-2008, 06:49
Because a LP120 is designed to hold 120cf at 2640psi while a HP120

Neglecting cave fills, the HP tanks have thinner walls and that's about the only reason I can think of for them to fail hydro before a LP tank which has thicker walls. That's why a tank with the same physical outside dimensions will vary that much in weight.

I think I see where you are confused. HP and LP tanks of the same capacity have very different physical outside dimensions. For instance, a HP120 is ~25" tall and has a diameter of 7.27" while a LP120 is ~29" tall and has a diameter of 8". The wall thickness is about the same, but the size of the tank is very different. The internal volumes are also different which is why a LP120 @ 2640psi and a HP120 @ 3500psi have the same amount of air in them.

I do not think he is comparing a LP120 to a HP120. HP tanks do have thinner walls than LP tanks. In my opinion what Cummings66 is talking about is if you compare two tanks that look the same while they are sitting next to each other and one of them is a HP tank and the other is a LP tank. For example lets take a look at Worthington cylinders.

LP 85
7.25 inches diameter
24.7 inches tall
34.3 lbs empty weight

HP100
7.25 inches diameter
24 inches tall
33 lbs empty weight

Now as you can see the two tanks are not identical tank with one being filled to 2640 psi and the other being filled to 3442. They are infact two completely different tanks. The .7 inches is going to make up the 1.3 lbs of extra weight. It is the fact that the walls of the LP tank are thicker which is where the extra weight comes from.

If you compare all the LP and HP tanks from Worthington none of them are an exact match. They are competely different tanks.

Now all I am familiar with is Worthington so I am not sure about Faber or others.

Phil

cummings66
01-20-2008, 07:39
skdvr got what I was saying about tanks of equal physical size, not volume.

While I haven't crunched the actual amount of steel in the tanks I wonder what it would be. IE I don't have a LP tank handy to actually do the math and see how it works out. Plus I don't know the exact dimensions of them to boot.

The weight of sheet steel is measured by the following formula;

The thickness (in decimal format, ie .25) multiplied by the width in inches multiplied by the length in inches muliplied by .2836

marchand
01-20-2008, 15:42
If you only considered the difference in weight of those two tanks then the difference in wall thickness would be less than a thousandth of an inch. However, I am almost positive that the difference in weight is coming from the .7" height difference; remember, the .7" is coming out of the center of the tank. I would think a 7.25" ring of steel would weigh around 1.3lbs.

From an engineering standpoint making the HP tanks have thinner walls than LP tanks is basackwards. Do you have the actual specifications for wall thicknesses?

cummings66
01-20-2008, 20:03
I saw a website somewhere once that showed the tanks cut apart which is why I contend the LP tank is thicker for the same size tank. Tell you what, my local dive shop is also a hydro facility. It might take me a while but I'll track him down and ask him, if anybody knows it's him.

A person could do the reverse math, ie algebra and figure out the wall thickness. I know by spec they should start out from steel that's about 4 to 5 mm thick. Beyond that I do not know for sure about LP and HP tanks.

In Europe the tanks actually have the minimum wall thickness as part of the stamp where in North America we have the bare basics on ours. If you have access to a couple European tanks you could also answer the question.

in_cavediver
01-20-2008, 21:08
If you only considered the difference in weight of those two tanks then the difference in wall thickness would be less than a thousandth of an inch. However, I am almost positive that the difference in weight is coming from the .7" height difference; remember, the .7" is coming out of the center of the tank. I would think a 7.25" ring of steel would weigh around 1.3lbs.

From an engineering standpoint making the HP tanks have thinner walls than LP tanks is basackwards. Do you have the actual specifications for wall thicknesses?

First, for to clarify, if you are talking HP vs LP tanks of the same size, but not capactity then you are closer to apples to apples. Even then, the steel alloy is differerent hence the 3AA versus some exemption number. It is quite possible the 3AA designation, which goes back to WWII with + ratings, might require a thicker wall thickness. The exemption paperwork will detail the specs specific to the 'HP' tank. It does not have to 'jive' with the 3AA specs, just be sound engineering. Remember, we know a lot more now then we did then. Our alloys may allow a thinner tank with higher pressures than was previously possible.

There are some metallurgists on this site as well as several materials engineers. Hopefully one of them can chime in as well.