why the "LP" versus "HP" service pressure rating in the US?

[Lorenzoid]

Okay, I know this sort of question has been beaten to death, but even using my best Google-fu I was still unable to find a concise answer to the question I have in mind. Let me get specific:

Why does a manufacturer rate Steel Tank A that is 7.25 in. in diameter, 24.7 in. in height, and 34.3 lbs.in weight in air at a service pressure of 2640 psi, while a manufacturer rates Steel Tank B that is 7.25 in. in diameter, 25.4 in. in height, and 34.3 lbs. in weight in air at a service pressure of 3442 psi? Are Tank A and Tank B not, structurally speaking, nearly identical?

If (over-)filled to 3442 psi, Tank A contains 108 cf. Approximately the same as Tank B at that pressure. That fact has been mentioned in many threads in which someone was trying to decide which tank would best suit their needs.

The eagle-eyed among you recognize that Tank A is a Worthington LP85, and Tank B is a Faber FX100. They have essentially the same dimensions and the same in-air weight. Are they made of the same alloy? Is the wall thickness ever so slightly different? (It can't be THAT different, given the dimensions and the in-air weight, right?)

In another thread, @tbone1004 mentioned that the so-called high-pressure tank has "thicker walls." However, I would think that if the FX100 had thicker walls that it would weigh MORE than the Worthington LP85, since it is an inch taller than the Worthington LP85.

I realize that the buoyancy characteristics are not EXACTLY identical, which would suggest that the wall thickness accounts for that difference. The Worthington LP85 has a buoyancy full (without valve attached, I believe) of -7.1 lbs and a buoyancy empty of -0.7 lbs, while a Faber FX100 has a buoyancy full (WITH valve) of -8.41 lbs and a buoyancy empty of -0.59 lbs. Even accounting for the valve (what, 2 lbs. or so in air?), this seems like an inconsequential difference.

The bottom line: Are the two tanks in fact nearly structurally identical? And if so, why would a manufacturer not rate a tank at the same pressure as a structurally identical tank?

 

[Blasto]

A spec sheet for Worthington tanks shows the same pattern - their HP tanks consistently weigh less per unit water volume than their closest in volume LP tank. (E.g., X7-100 at 12.2L is lighter than LP77 at 12.0L) Going by the specs, the LPs seem to have no purpose. Similarly uncertain about the reason, though, sounds like better steel, but no confirmation.

 

[Bob DBF]

The bottom line: Are the two tanks in fact nearly structurally identical? And if so, why would a manufacturer not rate a tank at the same pressure as a structurally identical tank

Without knowing the alloy(s) used there is no way of knowing if the tanks are structurally identical. A high pressure tank wall need not be thicker if it is made from a stronger alloy. The new Faber tanks seem to be lighter, for the same volume, than the older Faber's so they are probably using a new alloy, or found they could use less of the old alloy and still certify the tanks.

Since there are two manufacturers involved, I think you just ran into a coincidence.

The point, to me, of trying to show equivalency, is to justify overfills. Either you overfill or you don't, any justification is just a personal decision. Regardless if one finds they are identical tanks, the LP tank is only certified for LP.


Bob
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Not necessarily against overfills.

 

[Charles2]

Let me try to answer from my limited understanding.

• Tank A appears to be manufactured according to DOT 3AA specifications. Very old specification. https://www.gpo.gov/fdsys/pkg/CFR-2012-title49-vol3/pdf/CFR-2012-title49-vol3-sec178-37.pdf
• Tank B appears to be manufactured according to DOT SP13488 specifications. Relatively new specification approved by DOT for Faber Industrie. https://www.phmsa.dot.gov/staticfiles/PHMSA/SPA_App/OfferDocuments/SP13488_2010040981.pdf

Both specifications appear to be very similar with a few notable exceptions.

• Steel compositions specified in both are very similar and exhibit similar strength properties.
• Wall thickness design calculations are based on differing maximum tensile strength properties.
  • DOT 3AA maximum of 70,000 psi
  • DOT SP13488 maximum of 90,500 psi
• Test pressures vs. service pressure
  • DOT 3AA Test Pressure = 5/3 Service Pressure
  • DOT SP13488 Test Pressure = 3/2 Service Pressure

In other words:

• The HP tank specification is allowed to take advantage of a steel that possesses higher strength properties.
• The HP tank specification is allowed to use a greater ratio of design strength/maximum strength.

The bottom line: Are the two tanks in fact nearly structurally identical? And if so, why would a manufacturer not rate a tank at the same pressure as a structurally identical tank?

The two tanks appear to be structurally very similar, almost identical. I suspect the manufacturer produces either specification depending upon the specification chosen by the purchaser. I would not recommend overfilling either.

 

[Schwob]

Quite interesting. Thanks for thread and input....

 

[boulderjohn]

It is a question that has intrigued me as well. Just what is the difference?

Here is a chart for Worthington tanks: http://www.drexler.ca/diving/images/products/worthington_steel_specs.pdf

I have Worthington LP 108 doubles, and someone I dive with has Worthington X8-130s. Sitting side by side, you would have to look closely to see a difference. According to this chart, my doubles are about an inch longer and weigh about a pound more. They have about a 5% greater internal volume. If I were to fill them to the rated pressure of his tanks (and I have), I would have about 10 cubic feet more gas--about 8% more. The slightly larger internal volume would almost account for it.

 

[Lorenzoid]

So far, @Charles2 's observations come closest to what I'm looking for.

• The HP tank specification is allowed to take advantage of a steel that possesses higher strength properties.

IF the two tanks are in fact made of alloys possessing different strength properties, then I think that would answer the question. But are they?

As for DOT or other government specifications, the tank itself isn't aware of specifications. A tank has its physical/structural properties and that's all, as far as any of us end-users are concerned. If, assuming for the sake of argument, the interior and exterior dimensions and the alloy material are nearly identical, why would the tanks be subject to different government specifications? I could be wrong, but assuming there is no government regulation preventing it, I would think that labeling a tank "LP" would not be as effective a marketing tool as labeling a tank "HP." If "HP sells better than "LP" why would all manufacturers not label all their tanks "HP"?

 

[MichaelMc]

For tanks from the same manufacturer, Blue Steel specs for Faber tanks: Blue Steel Scuba - Cylinder Specs

......fill L... cu in ..l. .d.. wt.. full/ empty
LP85. 2640 13.. 793 25.98 7.01 31.2 -3.80/+2.32 3AA
FX100 3442 12.9 787 25.39 7.24 34.3 -8.41/-0.59 SP 13488
All stats without valves, salt water.

Near identical internal volume, FX100 is heavier. Might not help as diameters are different and their shape at the ends might differ in its efficiency.

 

[Scared Silly]

Are Tank A and Tank B not, structurally speaking, nearly identical? Are they made of the same alloy? Is the wall thickness ever so slightly different?

I am too lazy today to type much, but different alloy, different heat treatments, different wall thickness, different mfg process, etc. all determined the WP of a cylinder.

Years back people were saying Faber LP cylinders were LP cause they were being sold in the USA whereas their cylinders that had the same form factor were HP cylinders in Europe. The bottomline is that it was bovine excrement because of different alloy, different heat treatments, different wall thickness, etc.

 

[Lorenzoid]

I am too lazy today to type much, but different alloy, different heat treatments, different wall thickness, different mfg process, etc. all determined the WP of a cylinder.

I agree. That's the first thing that occurred to me as likely accounting for the difference. Can anyone confirm these are different between a Worthington LP85 and a Faber FX100, or between other "LP" and "HP" tanks that are nearly identical in dimensions and weight?