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PV with an "H Stamp" inside a shield.

PV with an "H Stamp" inside a shield.

PV with an "H Stamp" inside a shield.

We currently have a pressure vessel on our site with an H stamp inside a shield. There is also an N with a circle around it.
There is not a lot of additional information on the stamping.  (See attached picture)

What code is indicated by this stamp if any?

The vessel specs are as follows:
No documentation
Hydrogen Service
2300 psig operating
Ambient temperature operating.
No weld seams can be seen.
PMI verified to be plain carbon steel (grade unknown)
No hardness data taken.
No evidence to indicate it is a laminated construction vessel.
24'' OD
~38' T.T.
UT thicknesses are coming back way too thin for pressure containment (0.5'' for shell.  0.65'' for heads).

RE: PV with an "H Stamp" inside a shield.

H-stamp (heating boilers) is part of section IV of ASME code.

RE: PV with an "H Stamp" inside a shield.

First, welcome to the forums and thanks for filling out your profile info!

I would suspect that the shield-H is a fabricator's trademark. It certainly lacks the ASME cloverleaf and thus is not what I would expect as an ASME stamp.

If you don't know that material of construction... how would you know how thick the vessel should be for internal pressure? Allowable stresses for CS can vary quite a bit. Although I tend to agree with your asessment - looking at allowable stresses for Section VIII Div. 1 the highest allowable stress for carbon steel materials would be around 27 ksi.

I'd say its time for a new vessel. Depending on your jurisdiction, continued operation of this vessel may be illegal.


RE: PV with an "H Stamp" inside a shield.

I have been a long time reader but this is my first post.  I really appreciate what you all do here at eng-tips.  I have been a refinery engineer for almost 2.5 years now and I have used this site to get me going several times.    I agree that it is not an ASME stamp.  I suspect because of the dates stamped on the vessel that it was manufactured in the 40s.  I used the stress allowable for likely materials from the 1940 ASME code to come up with the required thicknesses.  We are not in a coded state but I believe OSHA 1910 requires that gaseous hydrogen cylinders be built to Section VIII 1968 or newer.

It looks like it is a high pressure gas cylinder that was hot formed on both ends.  Check out this video and zoom to 2:15 to see what I mean. I could post pictures of the neck if you want me to.


I attempting to confirm that it is indeed unsafe.

RE: PV with an "H Stamp" inside a shield.

It sounds like a high pressure storage vessel for H2 built like the cylinder used for "Rolling Pipelines" trailers. like the ones used.
These cylinders were first made by USS.
The spec's I have are for 22" OD Seamless Trailer tube.
.536" wall.
34' 4" OAL
11" radius hemispherical heads with center nozzles
From the end of the nozzle to the head tangent line has a 15" reference.
They reference the ICC

A design pressure of 2640 psig.

They are still made and more are coming for the H2 economy whether we want it or not.
At one time you didn't have to use ASME, you used the DOT or some part of it.  They came under the heading of cylinders not pressure vessels.


We had USS to make us some shorter ones with heavier wall for use in H2 service at 5500 psig.  

RE: PV with an "H Stamp" inside a shield.

Had one of them brain things that sparked a recall that brung back some thoughts about the mark you found on your cylinder.  If the shield is the same shape as the Federal Seal there is a good possibility that it is the DOT stamp of approval.  I was unable to find a direct reference to this idea but decided to look a little further and boy did I find a mess.
From what I found your vessel is not a cylinder, it is a tube, specifically a ground tube as it isn't used to transport gases over the highway.   These are built under the same stamp as the transport cylinders except installed to be used for on site storage.
It seems that when the use of seamless steel tubes to transport high pressure gases over the highway was proposed the ASME didn't want any part of it so the DOT took it on to have some control of the design.  Apparently the DOT just considered them just like a standard cylinder only longer.  There were also rules on the transport trailer design.
The current regulations are sort of, kinda, sort of, convoluted as there is the DOT regulations  concurrent with UN regulations that is controlled by the CFR xx-xxx.  The ASME has gotten involved to the extent that you can now purchase a tube with the ASME stamp or the required DOT.  One thing I couldn't ascertain for sure is will an ASME design meet the DOT criteria.  I think all states exemtt the DOT stamped tubes from the local Boiler and Pressure Vessel Regulations.
I didn't completely check The CGA site.  It Appears that your tube was made the DOT Class 3A or 3AA standards. The CGA site has information on the the testing and requalification  and the life of such vessels.  I believe the transport and ground tube have and indefinite life with testing.
The cylinders I mentioned in previous post we had made would come under what the trade now calls Hydril cylinders.  Which again can be ASME or DOT stamped.

Here is a good paper on storage of H2 that covers essentially the whole nine yards, but no first down.

Here is another manufacturer of said tubes


Here is a reference for the using of the tubes as a ground tube. This is for CO Ground Storage Tubes but is applicable for any inert gas.


Here a company that leases and test said tubes


Here is a little information available at the CGA site.

Compressed Gas Association (CGA) Pamphlet C1, "Methods for
Hydrostatic testing of Compressed Gas Cylinders".

Compressed Gas Association (CGA) Pamphlet C5, "Cylinder Service Life-Seamless Steel High Pressure Cylinders".

Compressed Gas Association (CGA) Pamphlet C6, "Standards for Visual Inspection of Steel Compressed Gas Cylinders."

Compressed Gas Association (CGA) Pamphlet C8, "Standard for
Requalification of DOT-3HT, CTC-3HT and TC-3HTM Seamless Steel Cylinders"

RE: PV with an "H Stamp" inside a shield.

Great work, unclesyd!

RE: PV with an "H Stamp" inside a shield.

You all might be interested in the US Patent and Trademark Office Trademark Electronic Search engine:  

I didn't have a lot of time to play around with it, but using a structured search, I got a few 1000 hits with "full mark" contains "H" and "mark description" contains shield.

Also I believe that the CFR xx-xxx that Uncle Syd was referring to is 49 CFR Part 178.   Markings are covered in 49 CFR 178.35, Type 3A containers in 49 CFR 178.36, and Type 3AA containers in 49 CFR 178.37.  Here's a link to the CFRs: http://www.gpoaccess.gov/cfr/index.html

Patricia Lougheed


Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

RE: PV with an "H Stamp" inside a shield.

Thank you all for the responses especially you unclesyd.  I contacted one of the companies you referenced (fibatech) and here is the result:
- The tube was manufactured by National Tube in 1948. ( Their mark is the circled N)
- It was assigned serial number 6046.
- There are no markings to clearly confirm that the tube is manufactured to ASME code.
- The tube is certainly not manufactured to a DOT or ICC (old DOT) code.(In other words, it is not 3A, 3AA, 3AAX or 3T specification.)
- At the time of manufacture, the tube was tested to 2,800 PSI.  (This does NOT mean that the tube is rated for an operating pressure of 2,800-psi.  In fact, it is likely that the tube was intended for an operating pressure significantly less than 2,800-psi.)
- In 1980 someone pressurized the tube to 3,250-psi.  We have no idea why 3,250 was used, since the tube is clearly marked for a TEST pressure of 2,800.
- We suspect that the 17460 mark is for the Heat Number (HT. NO.) at the time of manufacture by National Tube, but that tells us nothing about the tube's material properties.
- The H shield is the mark for RW Hunt ("H" for Hunt, not for hydrogen!), which is a third party inspector still operating today.

There is still too much unknown about this vessel to assume it is safe.  I plan on replacing with an ASME coded pressure vessel.

Utilities Engineer
3 Years Experience

RE: PV with an "H Stamp" inside a shield.

Certainly appreciate the feedback as it might be called on again.
National Tube was a part of USS who has shrunk considerably from their heyday. National Tube still has 3 plants, none making cylinders.

"Reading a little further in another booklet I have:
The basic advantage in this design is obtained from higher mechanical properties of alloy steel with the pressure vessels being of the seamless type with swagged ends.  These cylinders may be obtained under the general class ICC specifications. With a 10% overfill, their design pressure may be as high high as 2640 psi for inert gases."

Your vessel may not have carried a DOT stamp as the ICC we the precursor to the DOT.


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