Pressure testing - Leakage testing (Hose assemblies for industrial gases) 1

[Prometheus21]

Hello everyone, I would appreciate some input on this topic if you have any:

My company (Based in Europe) produce, assemble and test flexible hoses lined with fluoroplastic tubing (PTFE and PFA) for the industrial gas industry, (amongst many other products used for this industry as well). This has been done with no issues for decades.

Every hose is cleaned for oxygen service, and is tested according to the following specs:

1. Hydrostatic testing at twice the maximum working pressure.
2. Leakage testing at 1x the maximum working pressure (oil-free air (pneumatic)).

To verify the resistance to ignition in oxygen, 3 sample hoses are tested at a third party testing facility. A customer can at random intervals request that one or more hoses gets tested at a third party facility (Adiabatic compression test in pure, heated oxygen)

When filling gas cylinders with 300 bar working pressure, there is a short period of time where you can get pressure ratings far above the working pressure (usually 380 bar, but sometimes over 400 bar). Now this has for a long time been recognized as a short spike, and nothing to worry about as long as the hose has a burst pressure 4x the rated working pressure.

For 300 bar working pressure the following is used:

6 mm flexible hose with kevlar reinforcement. Burst pressure 1660 bar.
This particular type of hose is pressure tested hydrostatically at around 685 bar. (Minimum 600)
Followed by a leakage test (pneumatic) at 300 bar minimum (often around 310-330 bar).

The hose is also certified by a third party (laboratory): Adiabatic compression test with 60°C oxygen (min.99,5%) at 360 bar (1.2 x WP).

Now this is where the fun begins:

One of our customers have recorded a spike of 416 bar for a short period of time (no accident, pure measurement). He is now convinced our hose is not tested properly, and wants us to change our testing procedure. He now wants us to test each and every hose at:

832 bar pressure (hydrostatically)
420 bar pressure (pneumatic).

Now remember, this has worked fine for decades with no issues. We have no interest in upgrading our facility to enable us to test pneumatically at 420 bar (as adiabatic compression heat becomes a serious issue at this pressure rating), and would rather drop the customer if push comes to show.

Now being professional we would like to at try to avoid this..

What would you recommend? Try to convince the customer is it actually safe? That what has been working for decades is in fact still working as intended?

Any input is greatly appreciated!

 

[EdStainless]

Let me get this straight, the hose is actually rated for 300bar, correct?
It sounds like your customer has an issue and he really needs a 450bar hose which is a different item altogether.

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P.E. Metallurgy, consulting work welcomed

 

[geesaman.d]

I agree, and if test pressure is not defined by a code or agreed in the product specifications, make it so.

 

[LittleInch]

Looks like the cart before the horse a bit. Surely the thing to do is to look at why this spike is happening and do something to stop it.

One of my phrases about something like this "Now this has for a long time been recognized as a short spike, and nothing to worry about .." is that in the words of the great Richard Feynman you appear to be normalizing failure.

Or maybe it was Diane Vaughan on the space shuttle who said this is the normalisation of deviance - "‘The gradual process through which unacceptable practice or standards become acceptable. As the deviant behaviour is repeated without catastrophic results, it becomes the social norm for the organisation.’

Simply because you've not had a failure in "decades", doesn't mean it is the correct thing to do.

When it comes to these sort of things you need to consider yourself in court trying to explain why your hose which is only really certified for 300 bar, was being used with your direct knowledge in devices where the pressure could exceed 400 bar, if only for a short time. "We've never had one burst before" tends not to go down too well.

I don't know what the codes and standards say for hoses, but why is the test pressure more than twice the MWP?
What does the code or standard say about short term exceedance of the MWP?
Does it provide any guidance on the amount or time span of the exceedance?

Can you raise the MWP and at the same time figure out why this spike is happening?

IMHO, you need to look at this from the other angle if you don't want to uprate your MWP of your hose.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Prometheus21]

EdStainless: Not correct. The hose has a burst pressure of 1660 bar. This means the maximum allowable working pressure is 1660/ 4 (Safety factor)= 415 bar for this particular hose. Now the actual working pressure in the application is rated as a nominal value of 300 bar, which is what we use as a baseline for all of our testing. The customers argument is that the "working pressure" that we should be testing at is the maximum pressure they record (Simplified example: let's say the filling takes 5 minutes, the pressure is steady 300 bar for 4 minutes and 45 seconds. The last 15 seconds of filling the pressure spikes to 420 bar to ensure that the cylinder actually has 300 bar at standard conditions. The customer states that the working pressure we need to test at is 420 bar).

Now I get where he is coming from, but I simply disagree. The spike is never consistent and varies from cylinder to cylinder, not to mention between each and every gas installation and their internal standards and operating conditions. Stating that the working pressure is the same as the nominal cylinder pressure at standard conditions makes the whole process uniform across the board for all customers. Note that we always specify this in the product specification and the customer has accepted it previously. It's just that he wants us to revise our procedures to fit his paperwork..

I could be convinced that we should pressure test at 2 * 420 bar = 840 bar (hydrostatically). But to suggest that we should also leak-test with oil-free air at 420 bar is simply a bad idea and bad practise due to adiabatic compression heat. One solution would be to use N2, but the cost alone would make this unfeasible.

The question here is really two-fold I suppose:

1. Is the working pressure the nominal cylinder pressure, or the highest possible pressure that could be achieved during filling? (Not specified in local codes and standards)
2. Would it be enough to just test it hydrostatically at a higher pressure (Not a problem), or should the the pneumatic pressure also be increased accordingly? (Poses a challenge from a risk management standpoint).

I am tempted to just adjust the specs for this particular hose to the following:

"The hose has been tested hydrostatically with oil-free water at 900 bar".
"The hose has been leak-tested with oil-free air at 300 bar".
With an additional test report from a third party laboratory if applicable.

Ironically you could argue that 416 bar spike is outside our recommended maximum allowable working pressure at 415 bar, and our hose is not acceptable for use. This has worked fine for decades but who even cares about long term empirical evidence anymore when you have simple numbers to look at...

geesaman.d: The test pressure (hydrostatically) is given as 1.5 to 2.0 times (minimum) the working pressure. and 1x times the working pressure (leakage testing - pneumatic).

 

[Prometheus21]

LittleInch:

Good points.

"Surely the thing to do is to look at why this spike is happening and do something to stop it" - To make sure that they get as much gas as possible on the gas cylinder they usually "overfill it" to ensure a minimum nominell pressure at 300 bar. This is even more common in the winter time as the pressure tends to decrease in the cold filling stations. To compensate for that they overboost the filling pressure. Note that the hose is used both to fill the cylinder, but also to extract the pressure at standard conditions. At standard conditions the pressure is always 300 bar +- a deviation percentage.

""‘The gradual process through which unacceptable practice or standards become acceptable. As the deviant behaviour is repeated without catastrophic results, it becomes the social norm for the organisation.’" I like that one, this decsribes the industrial gas industry to the tee I'm afraid...

"When it comes to these sort of things you need to consider yourself in court trying to explain why your hose which is only really certified for 300 bar, was being used with your direct knowledge in devices where the pressure could exceed 400 bar, if only for a short time. "We've never had one burst before" tends not to go down too well." Agreed. Although we do have codes that state that a long successfull service history is enough evidence to allow for further use of the product... I don't agree wholeheartedly, but it is frequently used as an argument.

"I don't know what the codes and standards say for hoses, but why is the test pressure more than twice the MWP?" -Purely industry standards adopted into codes and procedures over decades.
"What does the code or standard say about short term exceedance of the MWP?" - Nothing.
"Does it provide any guidance on the amount or time span of the exceedance?" - No it does not.

"Can you raise the MWP and at the same time figure out why this spike is happening?
"IMHO, you need to look at this from the other angle if you don't want to uprate your MWP of your hose." - See my previous answer to EdStainless. The hose is rated for 415 bar working pressure (maximum). To test it hydrostatically at a minimum of 2.0 times the WP is fully doable. Pneumatic leakage testing however is a challenge.

 

[EdStainless]

Cost between dry oil free air and nitrogen?
Most every plant that I have worked in it turned out that N3 from our bulk tanks cost us less than clean dry compressed air did.
We used N2 for a lot of things because it was clean and cheep.
We had applications up to 700bar.
We used Corken or Hasken compressors, which lasted forever.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Prometheus21]

EdStainless: clarification: my company is not a plant. Just a simple manufacturing company that uses oil-free air for our testing facility and blow torch brazing operations. Our end users are mayor industrial gas plants. Our existing compressor system with booster tanks allows us a consistent line pressure of 350 oil-free air that we reduce down to 310 to 330 bar. At these pressures ignition is still possible, but mostly if 2 things happen at the same time:

1. The system is insufficiently degreased.
2. The system is contaminated with foreign particles that will provoke particle impact and ignition, creating a kindling chain.

Now for submerged systems (like flexible hoses) this is not a major issue as ignitions will simply be contained within the water filled tank. The benefit of this is that no hoses has ever left our company and ignited upon use, as any possible defects in the polymer, insufficient degreasing ect. will be picked up during testing. Think of it as a built in safety mechanism that has worked flawlessly the last 60 years. My collagues would be hard pressed to change something that has worked for that long with no issues. That and 30 % of our customers demand that we use oil-free air and not N2, while no customers demand the use of N2... Helium testing on the other hand.. But that is a different item all together

Now could we implement a 450 bar N2 testing station in addition to the one we have now? Probably. And that is something we are looking into for sure but I have a feeling that the powers that be might not grant any wishes in the near future.

 

[LittleInch]

Prometheus,

A comprehensive reply addressing the key points.

However for me there are too many variations of numbers going on here and I still can't believe these hoses are not made to some recognised standard, be it ASME, ISO or some industrial gas association.

Your definition of maximum working pressure drifts all over the place from 300 bar to 415. what exactly is it?

Why is pneumatic (leak) testing set so high. Chemical piping to ASM B 31.3 regards leak testing as Design Pressure or 1.1 x DP at worst. Why 2? Or as your reply says 1.5 to 2 - which is it?

But if this spiking of pressure is a regular event, then IMHO, the Max Working Pressure needs to be set at or more than what the hose is seeing.

Now how you test it seems to be bigger issue, plus the adiabatic compression heat. Can't you cool the air? or the tube? Or the incoming gas to avoid this overfilling issue?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Prometheus21]

LittleInch:

I try at least, though I see myself alway having to edit typos these days. Essay incoming:

"However for me there are too many variations of numbers going on here and I still can't believe these hoses are not made to some recognised standard, be it ASME, ISO or some industrial gas association." - There are some standards like ISO 7751: Rubber and plastics hoses and hose assemblies & Ratios of proof and burst pressure to maximum working pressure and
ISO 14113:2013 Gas welding equipment- Rubber and plastics hose and hose assemblies for use with industrial gases up to 450 bar (45 MPa).

These standards do somewhat inspire the internal standards for a lot of the major gas plants in Europe, but is severely limited once the cylinder pressure exceeds 285 bar cylinder pressure. Furthermore these standards are quite limited in their descriptions and practical value and are often disregarded in favour of the gas plants internal standards that they require us to follow. (No american standards has even been taken up - CGA G-4.4 for cleanliness is often referenced but is deemed insufficient for a lot of europeans gas plants, and require the old Praxair GS-38 standard to be followed).

Take ISO 14113:2013 as an example:

Where the maximum working pressure is defined as:

Now this is where the point of some confusion begins. The first line would indicate that the leakage test should be set to the maximum surge value, which is fair. But NOTE 1 clearly indicates that working pressure is to be defined at a uniform condition at 15°C and during service.

This standard is for hoses used to connect industrial gas cylinders to manifolds and bundles prior to any reduction stage. Note, not filling stage. During the usage stage the surge is limited to 315-320 bar pressure. So per the standard the leakage test pressure is usually set to 320 bar, and alot of our customers only demand testing at the nominal cylinder pressure at standard conditions (300 bar).

"Your definition of maximum working pressure drifts all over the place from 300 bar to 415. what exactly is it?" - My apologies for any confusion in my previous answers.

Lets take the 6 mm double braided kevlar reinforced hose as the example for pressure specifications:

* Minimum burst pressure: 1660 bar
* Maximum allowable working pressure: 415 bar (1660/4 - safety factor of 4)
* Real life maximum working pressure the hose will experience for the majority of its lifetime: 300 bar (nominal cylinder pressure - note excluding surges)
* Hydrostatic pressure: minimum 1.5 X maximum working pressure (We often go 2.0 or above with the hydrostatic, but below proof pressure at 0.7 x Minimum burst pressure).
* Hydrostatic proof pressure hold test: 0.7 x Minimum burst pressure = 1162 bar (deemed as a destructive test - x number of sample hoses per batch)
* Leakage pressure: minimum 1 x maximum working pressure (as before: usually set to nominal cylinder value = 300 bar. In reality we use 320 bar as leakage test pressure)
* Oxygen pressure surge test at laboratory: 1.2 * maximum working pressure (also deemed to be nominal cylinder pressure of 300. 300 *1.2 = 360 bar by third part laboratory engineers - we have no say in this).

(Note that Im not including vacuum testing, adhesion testing, flexibility, low temperature flexibility, ozone resistance, UV resistance, permeability to gas, eletrical conductivity, end fitting integrity non pressurized ect. as this is something else)

"Why is pneumatic (leak) testing set so high. Chemical piping to ASM B 31.3 regards leak testing as Design Pressure or 1.1 x DP at worst. Why 2? Or as your reply says 1.5 to 2 - which is it?" - See above note. Hydrostatic is minimum 1.5. Leakage (pneumatic) is set to 1.0 maximum working pressure. Now the question is if you define maximum working pressure as the nominal cylinder pressure (300), or the maximum allowable working pressure (415). I have yet to meet anyone who demands the last one to be performed, only the first one.

"But if this spiking of pressure is a regular event, then IMHO, the Max Working Pressure needs to be set at or more than what the hose is seeing." I tend to agree with you. And increasing the hydrostaic test pressure is no problem. The leakage test pressure is.

"Now how you test it seems to be bigger issue, plus the adiabatic compression heat. Can't you cool the air? or the tube? Or the incoming gas to avoid this overfilling issue?" - As of today we have do not have a way to pre-cool the air we use. So the heat you experience is pure adiabatic compression heat when the air is compressed from standard conditions to 320 bar in a split second. More than enough to provoke an ignition in contaminated systems (And as long as the hose is submerged during the leakage testing this is a nice final failsafe if I may say so. Luckily we have only had 2 hoses being insufficiently cleaned in my time here). Is it a weakness? Of course. Will the powers that be make a change? Not likely, although I keep trying to convince them

I think the best solution here is to just sit down with the customer and hopefully come to an agreement. The industrial gas plants in Europe have so many internal standards that they deem "the one" that we usually just pick the strictest one and perform testing from there. I have no doubt about the hoses structural integrity mind you, but if the time comes to upgrade our systems to meet new requirements I want to be ready and not dilly-dally.

 

[LittleInch]

No problem with essays...

In respect of item 3.6 that you list, my interpretation is different.

The first line is as you say quite clear about the definition of Maximum Working Pressure.
Note 1 though just states that this is consistent with the definitions used in a different code for gas cylinders and does not IMO alter the definition in the first line.

The third line in your bullets I think is better just called Working Pressure or maybe Normal Working Pressure

In piping codes there is sometimes Maximum Allowable Operating Pressure and then you can set Maximum Operating pressure to MAOP or lower, with sutable protection devices, relief valves etc set to protect at MOP.

One way is maybe to differentiate between hoses used "normally" and ones specifically used for filling. This one may then need extra testing to 415 leakage, but you then need to do something about the heat of compression.

"As of today we have do not have a way to pre-cool the air we use." This is the real issue for me - that's an excuse and a bit of a poor one for me (sorry). There are many ways to cool air which really shouldn't cost very much, but at the moment no one wants to pay for it. Only you know if this one off extra cost is required for the the number of hoses which need "extra" testing to be able to be sold with a working pressure of 415 compared to a working pressure of 300.

Is that a fair assessment?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Prometheus21]

LittleInch : Thank you for the response, and a fair assessment.

"Note 1 though just states that this is consistent with the definitions used in a different code for gas cylinders and does not IMO alter the definition in the first line." - I think what trips a lot of people up (including me) is when they state "the settled pressure", reading it as the nominal pressure rating, not the surge. Which again relates to my next point, which is that this standard only specfies emptying of cylinders, not filling (different surge-peak values).

"The third line in your bullets I think is better just called Working Pressure or maybe Normal Working Pressure" - agreed. We typically use WP or working pressure for this value, as this corresponds to the nominal pressure rating on the cylinder, which again correlates to what is marked on pressure regulators, relief valves ect. I was just being overly careful not to cause further confusion.

"In piping codes there is sometimes Maximum Allowable Operating Pressure and then you can set Maximum Operating pressure to MAOP or lower, with sutable protection devices, relief valves etc set to protect at MOP." - I have seen this. From what I have seen the piping codes are clear and defined (at least in comparison to flexible hoses with fluoroplastic tubing).

"One way is maybe to differentiate between hoses used "normally" and ones specifically used for filling. This one may then need extra testing to 415 leakage, but you then need to do something about the heat of compression." - Possibly. I am looking into using N2 and a small percentage of air at 300 bar, boost it up to 450 bar and use it for leakage testing. It really only depends what our system can handle, and what would need to be coorperated to make it happen. We already do it with helium testing on lower pressure ratings. Will we need to up the leakage test pressure at one point? Definitely.

"As of today we have do not have a way to pre-cool the air we use." This is the real issue for me - that's an excuse and a bit of a poor one for me (sorry)." - Don't be. I agree with you completely. Which is why I said I keep trying to convince the people in charge, being met with meager excuses in return. After the Covid pandemic the fear of using money has completely engulfed a lot of industries, including us, causing even the most mundane purchases to be a hill to die on.

"Only you know if this one off extra cost is required for the the number of hoses which need "extra" testing to be able to be sold with a working pressure of 415 compared to a working pressure of 300." As of today, no. The cost does not justify the gain as far as I know, based on feedback from the industry. No specific changes has been required os of today. Simply put a measurement of an unusually large surge peak in filling pressure caused a customer to rethink our resting procedure, which is fair.

 

[LittleInch]

Not sure there is much more to say really.

Whilst I think if your hose really does have a MAWP of 415 bar and you hydrotest ALL of them to > 1.5 times MAWP then practically you should have no issues.

The issue therefore is what pressure do you clearly state as being MWP and test accordingly - 300 bar or 415 bar?

If I was you I would market the enhanced MWP hose as being tested pneumatically to 415 for leakage AND CHARGE MORE FOR IT.

Then it puts the onus on the buyer whether he needs this enhanced MWP hose for filling purposes or only needs the "normal" MWP hose for "normal" discharge only use.

Would that work? Give the high ups an incentive to add this option (more money) for what is actually the same piece of kit which costs the same amount of money, just that you need a small extra bit of equipment to do the extra test?

Anyway a great discussion - hope it's helped.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TugboatEng]

Has fatigue been taken into consideration? You are testing to a pressure for one cycle and your customers are not only excessing your test pressure but doing it many times.

 

[Prometheus21]

Update: I reached out to some individuals that I know is part of an ISO technical committee that oversees parts of the industrial gas industry to see if any applicable standards are in the works, and would you know they actually answered!

"A new standard is set to be proposed in 2025/2026 describing gas cylinders and flexible hoses assemblies - Specification and testing"
"Strength test should be an hydrostatic pressure test set at minimum 1.5 x PR, PR=PW x 1.25, where PW= nominal cylinder pressure"
"Leak test should be an performed at PR with an allowed leak rate of maximum 15 cm^3/h as a result of peak pressure spikes and temporary polymer creep"

In reality this would mean for a 300 bar hose should be tested at: PR=300x1.25=375 bar (Pneumatic) 1.5xPR= 1.5x375 bar = 562,5 bar (Hydrostatic). Which is doable with some moderate adjustments. I suppose we will have a sit down with our customers and try to unify the requirements for the different hose types and applications (emptying, filling ect.) (I would propose trying to implement the requirements in the proposed standard, with the caveat being that these values might be changed before the standard is actually released).

LittleInch: I agree wholeheartedly. I am currently looking into prices regarding third party pneumatic testing, inhouse upgrades to allow it ect. to have something to present to the higher ups, and will be including your arguments as well. I agree, the increased pricing will most likely be a nice incentive. Having talked to a lot of the major customers today they are giving signals that they are moving to higher pressure ratings requirements within the next 3 years, which would force us to upgrade our testing facility regardless. A bigger yearly purchase volume will also easily justify such an upgrade.

TugboatEng: Yes. With fittings made of brass, with (stainless steel union nuts) usually the threads give way long before fatigue happens. As a general requirement each type of hose needs to survive the following testing at a third party testing facility:

1. 5000 pressure cycles for flexible assemblies dedicated to non-filling applications such as customer installations. This applies to hoses with metallic and non-metallic liners.
2. 50000 pressure cycles for flexible hose assemblies with non-metallic liners dedicated to filling centers.

Now additional testing including torsion tests, fatigue cycling under test pressure (cyclic bending test), Tensile pull test, side impact test and kink test. So I think we are covered.

Now from a practical standpoint I have only ever had 1 hose break due to fatigue after 15 years of service (we recommend switching after 3 years depending on the application just to be safe), and it was at the stainless steel crimp sleeve. I have yet to see fatigue problems in fittings made of brass (alloy CZ121 and C69300).

 

[LittleInch]

Progress eh, but 2025/26 might stretch into 27....

Much better to have single hose for sure, but the extra WP hose and testing might be a stop gap until this new standard becomes the one everyone uses.

Better to be ahead of the game as well.

Let us know how it goes.

LI

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Prometheus21]

LittleInch

"...but the extra WP hose and testing might be a stop gap until this new standard becomes the one everyone uses" Hopefully. But just because I will propose we do it this way there is just no way everyone will be using the same specs. The industrial gas industry can't even agree on which standard to use on their cylinder valves (So far I have seen 36 different standards, not kidding, almost every European country has their own version, and they mix other standards from different countries and adopt different parts of it, it's a complete mess.

But I can at least try to unify all the specs out to our customers when it comes to testing; and as you say, stay ahead of the game. No downside in trying.

Will do. Who knows, maybe I'll even be able to finally convince customers to not use teflon tape on parallel threads hehe.