Seizing of cylinder threads 2

[Prometheus21]

Hello everyone,

A filling plant is experiencing some issues when mounting fill adapters on the cylinder valves, see illustration image below.

Simply stated: The valve is supplied by another company; we supply the O-ring,connection piece and union nut.

The customer tightens the union nut with 4 Nm, at a rate of 100-800 rpm (automatic installation tool - customer built).
The problem is that approx. 1 in 5 union nuts tends to seize, and require around 20-30 Nm to loosen, requiring the need of a spanner to loosen again.(This happens immediately, no pressurization and/or product usage has been performed yet.) The interesting part is that the customer reports that after the first initial assembly; if one just handtighten the union nut a bit forcefully by hand (no tool, no 150-750 rpm) the nut still seizes and require a spanner to loosen again.

The valve is made of CW617B brass, the connection piece is made out of CW724R brass and the union nut is made out of CW614N brass. The thread in question is W21,80 x 1/14 per DIN 477.
The use of lubricant or anti-seize on the threads are forbidden due to regulations.

My first thought is galling of the threads, and/or other damage to the threads. I am visiting the customer next week and will be checking both the valve and the union nut, and wonder if there is something you guys think I should keep in mind? Anything to look out for in particular? It must be noted that this problem has just recently occured, so I will have to go through the process history and see what could have changed to make this problem appear in the first place.

Thank you for any input you may have.

 

[desertfox]

Hi Prometheus21

Apart from the fact that running the same material over each other is bound to have high friction similar to running steel on steel to start with.
However how good is this installation tool? because you are assembling threads dry and the friction coefficient will be all over the place, so that machine could be over tightening the joint and getting friction welding of the threads.

Bottom line torque is the worst and most inaccurate method for tightening screw threads.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Jboggs]

I see no provision in the design for any kind of "pre-alignment" of the two engaging components. That could help a lot.
Obviously look at the quality of the thread surface finishes.
Are they dissimilar materials? (Steel on brass, Stainless on aluminum, etc.) That would help too. Similar materials are more likely to gall up.

 

[Tmoose]

I hope you get to see some parts pairs that have stuck. And of course some new, unused parts.
I'd bring magnification and a bright light. And camera that that does macro (?) zooming.

I'm guessing you may find differences in the finishes of newer threads that stick and older parts that don't.

Who cleans the parts last, prior to final assembly? And how are the cleaned, and with what solvent?

I'm anticipating some detailing, like buffing with a very fine wire brush, may improve things.
Looks like that would be relatively easy on the male threads on the valve.

 

[Tmoose]

"the union nut is made out of CW614N brass"

Looks like that material would have about 3% lead, which I'd think //might// help prevent seizing.

=========.
CW724R
"Alternative brass alloy for drinking water installations, as the alloy is considered resistant to stress corrosion cracking and dezincification, but without lead, arsenic and nickel."

 

[zeusfaber]

Assuming multiple cylinders and multiple filling points: Is the problem evenly distributed around the filling points; what happens if you move a cylinder that's experienced a problem round the other filling points?

Have you got any suitable thread gauges?

A.

 

[mfgenggear]

Promethetheus
When assembling soft threads that can bind I like to apply anti seize. Then I don't have issues.
The other is the procedure must be hand tighten first, then lightly torque, and never try to start with a power tool. This will cause issues every time.
Third brass on brass is an issue, have the thread dry film coated for a permanent solution, and start assembly by hand for proper alignment

 

[Prometheus21]

Thank you for the rapid responses!

Desertfox -Yes, I am acutely aware. Honestly I am surprised we dont see this problem more often in the industrial gas industry since similar metal pairs are quite common due to the limitations put forth by regulations and standards.
I will inspect the tool in person, it seems like a cheap solution IMO to speed up the connection process. From what the customer is saying they press a button. Depending on how hard they press it the faster it is, therefore the wide span in rpm. So your point of "so that machine could be over tightening the joint and getting friction welding of the threads." does seem very reasonable. I will also check if the torque actually is 4 Nm.

"Bottom line torque is the worst and most inaccurate method for tightening screw threads." - I agree. And yet it is used in oh so many places and industries. Now for this configuration with an O-ring seal hand tightening is enough to create a seal. But lets say you wanted to have a surefire way each and every time you tighten this type of connection piece, what would be a feasible way of doing it in your opinion? My experience in the alternatives is limited. The main argument I hear for the torque method is that its fast. cheap and great if you are installing 50-200 of these per hour.

Jboggs - No. I have yet to see any kind of pre-alignment design for these types of connections. The entire connection piece/valve/nut follows strict design standards and usually the customer allows no deviation from the design itself. Now good industry practice dictates proper alignment before actual assembly to reduce the risk of introducing contaminants in the flow passage.
"Are they dissimilar materials? (Steel on brass, Stainless on aluminum, etc.) That would help too. Similar materials are more likely to gall up." - Different brass alloys (mostly) like i mentioned in the OP. The valve is USUALLY a brass alloy, but can be made of nickel alloys like Monel and Hastelloy. (SS, carbon steel and aluminium is being phased out at a rapid rate due to safety regulations). Same with the connection piece and union nut.

Tmoose - Me too. I hope to bring with me a pair back for further testing and inspection if possible. I will bring a bright light, magnification glasses (or a camera if I can get my hands on one quickly - though some fill plants have strict rules about taking pictures in their plant) and thread gauges/ (GO/NOGO/profile gauges).

"Who cleans the parts last, prior to final assembly? And how are the cleaned, and with what solvent?" - The valve is produced and cleaned by another company. It seems like a simple degrease with no surface treatment. Our parts are degreased, surface treated with a bunch of acid mixtures that adds a thin layer of corrosion inhibited copper oxide, before being degreased for oxygen service with an aqueous cleaner in an ultrasonic tank. Cleaned with DI water and dried with oil-free air to a maximum contamination level of 10 mg/mm^2. Then again, all of that means nothing the second the costumer opens the protection plugs and exposes the product to open air.

"I'm anticipating some detailing, like buffing with a very fine wire brush, may improve things.
Looks like that would be relatively easy on the male threads on the valve." - Could be. We round our threads slightly to avoid sharp corners that may slice open the skin of the technicians during handling. Still we do see from time to time certain manufactures that do not care about any of that.

Tmoose - "Looks like that material would have about 3% lead, which I'd think //might// help prevent seizing." Yes, to a certain degree. Then again this brass alloy is quite soft and easy to deform. CW724R is a great option whenever you need additional hardness, strength and/or corrosion resistance and is often used in fill adapters with RPV, due to the pin being 2-3 mm and often 4-14 mm long. CW614N will easily bend under load, while the CW724R will hold up nicely. Maybe a thought would be to try out a union nut made out of CW724R instead? Maybe the difference in hardness would make a difference(?).

Zeusfaber - Great point and something I will test out next week during my visit. As far as I can tell from the customer the problem is limited to (1 in 5) in all fill adapters using the W21,80 x 1/14 thread. W24,32 x 1/14/ and BSPP "G" threads are not having any difficulties in other similar cylinder valve connections. Of course I will try to verify this being the actual case.
Yes, calibrated GO/NO gauges (and rings for the valve) and profile gauges ready to go.

 

[Prometheus21]

mfgenggear - I wish I could. Unfortunately this is strictly prohibited due to ignition hazards if this comes into connection with the flow passages (could contaminate the gas which will often be used for food and medicinal uses - not to mention ignition hazards when dealing with industrial and medical O2 gas cylinders).

"The other is the procedure must be hand tighten first, then lightly torque, and never try to start with a power tool. This will cause issues every time." - I agree. The tool might just have to go. This will of course be a problem due to the large amount of cylinders these are put on every day. I can easily see a few tired wrists if that were to happen, not to mention long term damages.
"Third brass on brass is an issue, have the thread dry film coated for a permanent solution, and start assembly by hand for proper alignment" - I agree and good point on the coating. I have seen similar products coated in a thin layer of silver to reduce friction, so that might be feasible depending on the pricing of such an operation. I will look into coatings that the standards may or may not allow. I have a feeling it is frowned upon due to the risk of coating particles falling off and increasing the risk of particle impact and subsequent ignition - but worth a look!

 

[mfgenggear]

Yes silver and copper is used on gears for break in. Should work well on threads. If that's not go maybe electroless chrome or nickel plate.

 

[3DDave]

That specific thread can never be used for oxygen service and is used for combustible gases, so there is no ignition hazard to lubricating the thread. That aside, sure, contamination.

 

[desertfox]

Hi Prometheus21

I concur with what you saying, I don’t know if you can incorporate this in the design

https://www.stsindustrial.com/load-...f torque,, the washers' protrusions collapse.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[3DDave]

It would be more expensive I suppose, but the automated tool could do the same as people should do - turn the nut backward while applying slight forward pressure until there is a forward location change as the thread drops into alignment. This should be an easily detectable 0.07 inches. It should also advance the thread at speed to 1-2 threads remaining before full engagement and switch to a much lower speed that monitors torque.

What more sophisticated systems do is wait for a 10% final "torque" amount and then count number of degrees of turn to set the elongation of the threads. This has far better uniformity.

 

[Prometheus21]

3DDave -"That specific thread can never be used for oxygen service and is used for combustible gases, so there is no ignition hazard to lubricating the thread. That aside, sure, contamination." - ? Incorrect sorry to say.

The DIN477 standard is used differently in many European countries. W21,80x1/14 is used for industrial oxygen and CO2 in many northern European countries, while the very similar thread W21,70 x1/14 is used in the south like Italy. W21,80x1/14 LH is used for hydrogen. In contrast the Germans use DIN 477 #9 for oxygen which uses the G 3/4 thread.

Not to mention the the Danish standard, ISO, French, Swedish, NS, and so forth which may, or may not be fully integrated. And then of course you have the influx of Chinese cylinder valves that have started popping up that uses whatever they want. Its a solid mess.

 

[Prometheus21]

Desertfox - It would be difficult since the reusability would be limited. The existing O-ring allows for quick connecting to many, many cylinder valves a day, for weeks without replacing the O-ring. I would imagine adding a washer like that would be a hazzle for the plant to incorperate effectively.

3DDave - I like that idea. The tool would be expensive, but it would be nothing in comparison to the cost of damages, downtime and reduced assembly speed.

 

[3DDave]

W21,80x1/14 LEFT is used for oxygen, not W21,80x1/14.

If some idiots use it for Oxygen, they should stop.

 

[Prometheus21]

3DDave - Left handed threads for flammable gasses, like Hydrogen. Right handed threads for Oxygen and other non-flammable gases. (Exception being Nitrous oxide for certain offshore valves in the north sea- they use left handed threads for that)

 

[Prometheus21]

Then again, if a random country uses W21,80x1/14LH for Oxygen I wouldnt be surprised. I have seen many, many weird connections. Like left handed threads for a non-flammable gas mixture with 0.019% hydrogen. Or W24,32x1/14 for SF6 gas mixture.

 

[TugboatEng]

https://www.krytox.com/en/applications/oxygen-services

There are non-flammable lubricants.

 

[Prometheus21]

TugboatEng indeed there are! I spoke with a Krytox supplier and they could only offer Krytox NRT8908 for 230Bar oxygen cylinder pressure, which was crazy expensive. Other alternatives include types of Fomblin, Kluberalfa and Gleitmo that are all BAM/CTE/WHA certified for oxygen service. A thing they all have in common is their pretty bad lubricating properties and high pricing. That being said, it is an ongoing, never ending discussion if it should be used on cylinder valves or not.