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Hydro-static Test or Pneumatic Test?

Hydro-static Test or Pneumatic Test?

Hydro-static Test or Pneumatic Test?

Hi All,

In standard practice, we will carry out the hydro-static test (HT) for equipment such as vessel and piping. HT pressure is equal to 1.3/1.5 x design pressure which specified in the codes and standards.
When are we allowed to do the pneumatic test?

Thanks in advance.


RE: Hydro-static Test or Pneumatic Test?

Your design code will determine if and under which conditions, pneumatic testing is allowed. Please note that jurisdictional requirements may set additonal (more stringent) requirements, which can waive Code requirements.

B31.3 (2018): 345.1(b)
VIII-1 (2019): UG-100(a) (thus referring UG-99(a)(1) and UG-99(a)(2))

RE: Hydro-static Test or Pneumatic Test?

Pneumatic testing is much more dangerous due to the stored energy. It should only be done when hydrotesting can't be done, and after full consideration of the possible consequences of a failure during the test. A rather extreme example can be found here : link

ASME PCC-2 Article 5.1 Appendix II and III provide calculations for the stored energy as well as the safe distance for workers. This distance accounts for hearing damage due to the blast wave. Workers should also be behind a solid wall for protection from flying debris. The calculations do not account for damage to surrounding equipment such as that shown in the link above.

RE: Hydro-static Test or Pneumatic Test?

Geoff13, Yowza!

The problem with sloppy work is that the supply FAR EXCEEDS the demand

RE: Hydro-static Test or Pneumatic Test?

It depends from the stored energy. See the attached explanation

PS : I have a video of a failed hydrostatic test, but I have to remember where I stored it. ASAP I'll revert

RE: Hydro-static Test or Pneumatic Test?

Hi XL83NL ,

I have studied ASME B31.3 before I posted but I don't get what "impracticable of hydrotest" means in the clause. Hydrotest shall be always practicable if the manufacturer fabricated at workshop..

ASME B31.3,Clause 345.1,
"(b) Where the owner considers a hydrostatic leak test
impracticable, either a pneumatic test in accordance with
para. 345.5 or a combined hydrostatic-pneumatic test in
accordance with para. 345.6 may be substituted, recognizing
the hazard of energy stored in compressed gas."

And as per my understanding to the statement, does it depends on the client's decision/justification if the hydrostatic leak test is impracticable?

RE: Hydro-static Test or Pneumatic Test?

Very informative thread. Thanks, folks.


RE: Hydro-static Test or Pneumatic Test?

Wow Geoff2.
That was 1.5 tons of TNT equivalent. Nasty!

Keith Cress
kcress - http://www.flaminsystems.com

RE: Hydro-static Test or Pneumatic Test?

Quote (itsmoked)

That was 1.5 tons of TNT equivalent. Nasty!
How do you know it's 1.5 tons of equivalent TNT? I couldnt find much details in the comments.

This one is less impressive due to the cover. No idea how much energy is stored here though.

Quote (Sam_Low)

I have studied ASME B31.3 before I posted but I don't get what "impracticable of hydrotest" means in the clause.

The definition of impracticable is of course open for discussion. An example we found, and whats accepted in the industry, is that for our (pilot) plants and units, tubing is used a lot for main process lines. 99% is twin-ferrule compression type tubing joint, the other 1% is orbital welded tubing joints. Hydrotesting this with water impracticable. Same holds for lines where even the slightest amount of water can damage equipment which is installed (inline) with the piping system alter on.
We manufacture all our plants on modular, skid-built basis, so all manufacturing and assembling is done in the workshop.

Quote (Sam_Low)

Hydrotest shall be always practicable if the manufacturer fabricated at workshop..

As per my remark above, that's incorrect

RE: Hydro-static Test or Pneumatic Test?

Hi Geoff13& robyengIT,

Thanks for the reply and info!
It seems like pneumatic test is more critical than hydrotest.

Have you guys experience any pneumatic test before?

RE: Hydro-static Test or Pneumatic Test?

Sam Low

During my career (35+ years) I'm only aware of my company doing one pneumatic test here in Canada. That was a co-worker's vessel, so I'm not familiar with why that was permitted.

I heard of a vessel from before my time that was pneumatic tested as the weight of water would have collapsed it. In that case we apparently had to get the local highway (the QEW) closed for the duration of the test as it is less than 300 feet away.

I'm familar with the PCC-2 rules as a couple of years ago I worked on an in-house study comparing our pneumatic testing rules with the rules of a company we bought in order to select which rules we'd follow in the future.

As XL83NL already noted, due to the explosive nature of a failure, it's no safer to do in your workshop than at the customer's site, unless you've have a really large empty field available.


The formula in ASME PPC-2 Article 5.1 Appendix II calculates the stored energy, and provides the conversion to TNT equivalent. I get about 4,500 lb of TNT equivalent at the failure pressure.

RE: Hydro-static Test or Pneumatic Test?

Geoff, thanks, I get the PCC-2 (used it myself to program our internal Excel-tool for this). I forgot to click the link to the ET thread where details on volume, pressure etc were given. Instead, I was looking in the youtube comments for that.

RE: Hydro-static Test or Pneumatic Test?

Hi Geoff13,

Thanks for your experience sharing! bigsmile

Appreciate it!

RE: Hydro-static Test or Pneumatic Test?

I have directed numerous pneumatic tests of piping systems where hydrostatic tests would be impractical or where the water, not able to be fully removed, would damage the process. Safety zones were provided based on calculations. Aslo calculations were made to define the applied stress to specified minimum yield strength ratio and the expected or known ductility (imact absorbed energy) of the material at the test temperature. When all factors were considered that the test could be done safely, a pnematic test procedure was written to be followed by all personnel and approved (signed) by corporate management and the Owner. In a number of instances, testing was performed during a time period when no other work was being done and only the test personnel wwould be involved at the site. None of the tests so performed caused any damage to pipe, or injury to personnel. We did find leaks during some of the tests, then depressurized the sysyem, made repairs and retested to confirm integrity.

RE: Hydro-static Test or Pneumatic Test?

Hydrostatic testing is always safer than pneumatic testing in terms of the sudden release of mechanical potential energy due to a catastrophic failure of the piping or vessel under test. It is not however always the safest way to test a given piece of equipment, vessel or piping/tubing system. THAT is why the word "practicable" is used in codes like B31.3 .

Hydrotests can sometimes be done with non hazardous liquids other than water, reducing the risks associated if water is the test medium. However, there are many examples where testing with any liquid would be hazardous to the process which will ultimately be carried out in the vessel or equipment. You have to weigh the risk of catastrophic failure detected during the test against the risk of water or other liquid to the process or materials of construction and design your test accordingly.

In practice, the risk associated with the pneumatic testing of small bore pipe or tubing lines in a shop setting is minimal IF the test is designed properly and the stored potential energy is taken into account. The risk obviously increases with stored energy which is more a practical function of volume than of pressure.

RE: Hydro-static Test or Pneumatic Test?

Hi weldstan/ moltenmetal ,

Thanks for the sharing!

RE: Hydro-static Test or Pneumatic Test?

One more thing to look at in the pneumatic test of a piping system; the NASA calculations are essentially correct for a pressure vessel but not for a lengthy piping system. Not all of the stored energy is released at the instant of burst. An initial release of energy does occur and then is followed by a slower decompression. I have personally used a length of about 200 ft of the largest diameter of the pipe system in the energy calculations at which safety zones are calculated.

I always use a leak before burst criteria in deciding whether a pneumatic test would be permitted. If I could not adequately define the conditions which could possibly result in bursting, I would not permit such testing to occur and would so state in writing to corporate management and the Owners representative. I must say that I was never overruled.

RE: Hydro-static Test or Pneumatic Test?

For long pipes, the issue or a break during test is not necessarily that all of the energy is dissipated (as mentioned by weldstan. Rather, the pipe may "whip", and that can be much more catastrophic.

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