What is the reason behind SS316 Tube breaking, as this tube is installed in Instrument air line. it used to control the ON-OFF valve.

What is the reason behind SS316 Tube breaking, as this tube is installed in Instrument air line. it used to control the ON-OFF valve.
Air having a pressure of 6kg/cm2 and temperature is near to 30 degree.
 
Nikhil vaghela,

You can attach or include photographs and other electronic files/info to control.com threads.

One picture can be worth a thousand words. We really don't have any idea about the problem you are experiencing, and so a clear photo (and maybe a few words, if necessary) would be very helpful in trying to answer your question.
 
Nikhil vaghela,

Thanks for the photo! It does, indeed, help a lot.

When I see damaged tubing like this (and I think some part of this homemade "fitting"--a nut--is missing), most often I find it is caused because a backing wrench was not used when loosening or tightening the "fitting." It's amazing how few people know to use--and do use--backing wrenches on tubing fittings and devices when loosening and tightening them. A backing wrench can save a lot of trouble!

Also, when I see damage like this it is quite often caused by poor support of the devices on either side of the "fitting" (the solenoid-operated valve, SOV, and the control valve), allowing one or both of them to vibrate and wobble. Sometimes on device on one end of a piece such as in the photo is properly secured but the other side is not and it is free to "shake, rattle, and roll" and vibrate.

Alignment of the two devices a "fitting" like this connects is also often found to be a problem. And, it's very difficult to make a short piece of rigid tubing with any kind of bends in it to make up for misalignment. Sometimes, a braided hose, in a circle, is a great idea in such cases.

Combine this with not using a backing wrench to loosen--or tighten--the fittings' nuts and that's a recipe for premature failure.

The "corrosion" on the right side of the fitting also suggest that nut on the right was not properly tightened... It looks like the "fitting" is actually a piece of tubing with compression fittings on either side of it, and that one of the nuts used to set and tighten the compression rings (on the left side of the photo) was not in place for the photograph, probably to better show the damage to the tubing.

One last thing as I review the photo one more time. The tubing in the picture is "thin-wall" tubing; you could probably use a thicker-walled tubing for more strength. One of the things that often happens with thin wall tubing is that when the nut is tightened to set the compression rings it is over-tightened and that can weaken and even crack the thin-wall tubing. Many sites, and maintenance personnel, don't use go-no-go gauges for setting compression fittings the first time, or don't know how many flats to turn the fitting from hand-tight to initially turn the nut if a go-no-go gauge is not used.

There's a lot of possibilties for why the tubing might be damaged. Incorrect material--for dry, instrument air--is probably not one of them. Thicker-walled tubing, properly set fittings--done using a backing wrench, alignment of the two devices the tubing is connecting, and secure mounting of the two devices are probably one or more of the reasons for the failure.

Hope this helps!



Another thing about "fittings" like the one in the picture, usually they are made very quickly, sometimes using bits or pieces of tubing found laying around in the instrument shop--new and used.... Usually, they are not always cut to proper length (or if the as-found piece is used to determine the length, the as-found piece was not cut to the proper length when it was installed).

So, there's a few things to consider. Only you can know if the material you are considering using is right for the application/service. If it's just dry instrument air, it is probably okay--as long as it's at least as strong as the piece it is replacing. But, again--it would be interesting to see the two pieces of equipment the "fitting" is connecting--the solenoid-operated valve and the control valve, and the gap between them, and to know if both pieces of equipment (the SOV and the control valve) are mounted such that they are properly aligned, and one, or the other, or both aren't free to "shake, rattle and roll."
 
Nikhil vaghela,

Thanks for the photo! It does, indeed, help a lot.

When I see damaged tubing like this (and I think some part of this homemade "fitting"--a nut--is missing), most often I find it is caused because a backing wrench was not used when loosening or tightening the "fitting." It's amazing how few people know to use--and do use--backing wrenches on tubing fittings and devices when loosening and tightening them. A backing wrench can save a lot of trouble!

Also, when I see damage like this it is quite often caused by poor support of the devices on either side of the "fitting" (the solenoid-operated valve, SOV, and the control valve), allowing one or both of them to vibrate and wobble. Sometimes on device on one end of a piece such as in the photo is properly secured but the other side is not and it is free to "shake, rattle, and roll" and vibrate.

Alignment of the two devices a "fitting" like this connects is also often found to be a problem. And, it's very difficult to make a short piece of rigid tubing with any kind of bends in it to make up for misalignment. Sometimes, a braided hose, in a circle, is a great idea in such cases.

Combine this with not using a backing wrench to loosen--or tighten--the fittings' nuts and that's a recipe for premature failure.

The "corrosion" on the right side of the fitting also suggest that nut on the right was not properly tightened... It looks like the "fitting" is actually a piece of tubing with compression fittings on either side of it, and that one of the nuts used to set and tighten the compression rings (on the left side of the photo) was not in place for the photograph, probably to better show the damage to the tubing.

One last thing as I review the photo one more time. The tubing in the picture is "thin-wall" tubing; you could probably use a thicker-walled tubing for more strength. One of the things that often happens with thin wall tubing is that when the nut is tightened to set the compression rings it is over-tightened and that can weaken and even crack the thin-wall tubing. Many sites, and maintenance personnel, don't use go-no-go gauges for setting compression fittings the first time, or don't know how many flats to turn the fitting from hand-tight to initially turn the nut if a go-no-go gauge is not used.

There's a lot of possibilties for why the tubing might be damaged. Incorrect material--for dry, instrument air--is probably not one of them. Thicker-walled tubing, properly set fittings--done using a backing wrench, alignment of the two devices the tubing is connecting, and secure mounting of the two devices are probably one or more of the reasons for the failure.

Hope this helps!



Another thing about "fittings" like the one in the picture, usually they are made very quickly, sometimes using bits or pieces of tubing found laying around in the instrument shop--new and used.... Usually, they are not always cut to proper length (or if the as-found piece is used to determine the length, the as-found piece was not cut to the proper length when it was installed).

So, there's a few things to consider. Only you can know if the material you are considering using is right for the application/service. If it's just dry instrument air, it is probably okay--as long as it's at least as strong as the piece it is replacing. But, again--it would be interesting to see the two pieces of equipment the "fitting" is connecting--the solenoid-operated valve and the control valve, and the gap between them, and to know if both pieces of equipment (the SOV and the control valve) are mounted such that they are properly aligned, and one, or the other, or both aren't free to "shake, rattle and roll."
 

Attachments

Yes, It is Exactly what you have told about the thickness of the SS tube,
as the Tubing Breaks at the Back Ferrule point, images attached, and there is no any missing of Nut, Both the site nut is available.
Refer Attached images at the Breaking point,

one doubt came to my mind, may I use SS 316L inplace of SS316? and I can not face any problem in future.

is there any doubt about the SCC(Stress Corrosion Cracking)?
 

Attachments

Nikhil vaghela,

I believe you said this was for dry, instrument air service. I'm not a metallurgist, so I can't comment on what material might be best or even better.

If you don't have a need for high flow-rates (such as for opening or shut-off or control valve position changes), I would suggest using thicker-walled tubing of the same material. It's readily available (or should be in most plants/instrument shops/pipefitter's shops) and if the compression fittings are set properly initially (not overtightened) using a backing wrench everything should be fine. Again, this presumes proper alignment of the two devices being connected, and secure mounting of the two devices being connected, and the proper length of tubing. Using such a short piece of tubing means that installation and disassembly (proper installation and disassembly) of the length of tubing will require that one device or the other (probably the SOV) will have to be loosened from its secure mounting to allow for insertion of the tubing during assembly and removal of the tubing during disassembly. And, of course the loosened device will have to be re-secured after assembly or re-assembly.... And the fittings will have to be properly tightened using a backing wrench.

I can't stress enough the use of backing wrenches--it's so often overlooked, and it's so simple and logical and intuitive--once the need is understood. If you don't have a go-no-go- gauge for proper initial tightening of the compression fittings on initial assembly, then use the "flat count" method; after the fitting nut is screwed hand tight and using a backing wrench tighten the fitting nut 8 total flats (1-1/3 turn on a six flat nut). That should be sufficient to set the compression fittings ferrules without causing damage--presuming there is not angular misalignment between the two pieces of equipment (the SOV and control valve). Do this on both ends--using a backing wrench when tightening--and you will be fine. Subsequent re-tightening of the fitting--using a backing wrench after reassembly--should turn the nut no more than half a flat, if that.

Many people mistakenly believe that over-tightening compression fittings to account for misalignment and/or loose devices will "solve the problem" of misalignment or loose devices. NOT TRUE. Over-tightening only serves to increase the probability of premature tubing failure. And, again, I cannot over-stress how important--and simple--using a backing wrench is on tightening (and even loosening!) tubing fittings, especially when initially tightening a compression tubing fitting.

Best of luck!
 
T
Nikhil vaghela,

I believe you said this was for dry, instrument air service. I'm not a metallurgist, so I can't comment on what material might be best or even better.

If you don't have a need for high flow-rates (such as for opening or shut-off or control valve position changes), I would suggest using thicker-walled tubing of the same material. It's readily available (or should be in most plants/instrument shops/pipefitter's shops) and if the compression fittings are set properly initially (not overtightened) using a backing wrench everything should be fine. Again, this presumes proper alignment of the two devices being connected, and secure mounting of the two devices being connected, and the proper length of tubing. Using such a short piece of tubing means that installation and disassembly (proper installation and disassembly) of the length of tubing will require that one device or the other (probably the SOV) will have to be loosened from its secure mounting to allow for insertion of the tubing during assembly and removal of the tubing during disassembly. And, of course the loosened device will have to be re-secured after assembly or re-assembly.... And the fittings will have to be properly tightened using a backing wrench.

I can't stress enough the use of backing wrenches--it's so often overlooked, and it's so simple and logical and intuitive--once the need is understood. If you don't have a go-no-go- gauge for proper initial tightening of the compression fittings on initial assembly, then use the "flat count" method; after the fitting nut is screwed hand tight and using a backing wrench tighten the fitting nut 8 total flats (1-1/3 turn on a six flat nut). That should be sufficient to set the compression fittings ferrules without causing damage--presuming there is not angular misalignment between the two pieces of equipment (the SOV and control valve). Do this on both ends--using a backing wrench when tightening--and you will be fine. Subsequent re-tightening of the fitting--using a backing wrench after reassembly--should turn the nut no more than half a flat, if that.

Many people mistakenly believe that over-tightening compression fittings to account for misalignment and/or loose devices will "solve the problem" of misalignment or loose devices. NOT TRUE. Over-tightening only serves to increase the probability of premature tubing failure. And, again, I cannot over-stress how important--and simple--using a backing wrench is on tightening (and even loosening!) tubing fittings, especially when initially tightening a compression tubing fitting.

Best of luck!
Thank You CSA.
 
Dear CSA, I would like to add something regarding this Tube failure, we have installed 5 nos of valve, out of 5, 4 nos of valve have 500 mm distance between SOV and control valve, but particular in this case, the distance between SOV anc control valve is about 50 mm SS spool.
if I am not wrong, this might be the reason that over a period of time SS tube becomes Fragile and it breaks without any reason..

Regards,
Nikhil Vaghela.
 
Nikhil vaghela,

Again--a picture is worth many, Many, MANY words. Attach a picture of the SOV and control valve.

Use a thicker walled tubing. The only reason NOT to use a thicker walled tubing would be if the decreased ID of the tubing (because of the increase wall thickness) would act as some kind of restriction or orifice. And, that would likely only be a problem IF the control valve had some requirement for fast action to open or close--really fast. And, in that case there are other options which can be used to help the air pressure be relieved very quickly (namely: a quick exhaust valve). But, unless there is something unusual about this application (dry, instrument air being used for actuating a control valve (not a STOP valve, a control valve)) I would think using thicker walled tubing for this connection would be fine and would help alleviate the problem.

BUT, that presumes the SOV and control valve are properly mounted/attached and are not connected to each other SOLELY by the piece of tubing.

And, again, if the application is dry, instrument air I don't think the type of stainless steel really makes all that much difference (since you seem intent on wanting to replace one type of stainless with another).

Show us a picture of the specific application, maybe even three--showing how the SOV and CV are supported/mounted/attached to structure or bracket or support of some kind. We might have some better recommendations. But, thicker walled tubing and proper supports and proper installation (using backing wrenches ) is just as important as the choice of tubing material--if not more so.
 
To me this looks like that the fitting must have been through some undue stress as the point of failure is around the nut and ferrule which bites onto the tube for the sealing. The different causes for the failure might be
1. Over tightening of the fitting- Generally a rule of thumb for 12mm or 1/2" tube tightening is to do a 1-1/4 turn after finger tightening ....
2. Support- Generally fast acting shut-off valves when operated produces a lot of stress and vibration on the associated lines and fittings which are connected to it. So providing proper support on to the final control element is essential to nullify the thrust effect.
3. Installation- Personally I suggest to do a bend or a turn before connecting to the final valve fitting if the installation pertains as this lessens sudden radial stress effect when operated. Providing proper support and clamping (distance not more than 500mm) and also at every bend is essential.
4.Tube and fitting hardness is important to know when purchasing. A general purchase spec would be buying seamless stainless steel tubes conforming to ASTM A269 TP 316L, fully annealed with a hardness range of HRB 70 to 79. Keep in mind that the SS 316 fittings should posses with a 5-10 more hardness range than the tube so as to ensure good sealing. You dont want to use fittings with very high hardness as this would have a tearing effect on the tube when tightened.
5. 1/2" SS tube with wall thickness of 0.049" works for 250 Barg pressure at a room temp of 38 degc. So increasing the wall thickness more than whatever you have currently wont be much of a help as wall thickness is increased keeping the OD same for withstanding higher pressure ratings.
Also I would suggest you to give a thought about heavy walled PVC coated copper tubes conforming to ASTM B68M/B75 standards and with brass fittings. They have good corrosion resistance due to PVC jacket, good choice for normal temperature applications and air media. Also easier to install and much more hassle free from maintenance point of view also lower in cost that SS 316.
 
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