Is 1/2 Inch tubing unsafe in some applications?

S

Thread Starter

Sam

Something that have always been bothering me is working with instrumentation tubing. How safe is it? I have seen tubing snapped before due to constant vibration on a medium pressure application only. So looking at a application on a pressure vessel running at 100 to 150 Bar which was built with 2 inch wall thickness and 1500lb flanges to contain this 150Bar product (condensate and hydrocarbon gas), surely using this ½ inch S/S tubing to connect to a level DP transmitter is the weakest point of the whole vessel. It defeats the object of having this big thick wall pressure vessel. Some say no, since the diameter of the tubing makes it safe and that the force per square inch inside the vessel is much more than inside the tubing therefore the thick walls are needed on the vessel. I don’t see that, surely 150 Bar is the same all over so the forces and pressure inside the vessel and tubing must be the same. Up to what pressure is it safe to use ½ inch S/S tubing? Vibration is present in most applications due to product flow so this must also reduce the max application limit.

I have brought this safety issue up with our design engineers before and they said that this is done, and the tubing is used all over the world and that it is safe, so basically shut up and leave it alone, this is the world standard so who are you to question it?
I know you can get Monel tubing as well and that will most probably safer to use but this is about normal thin and thick wall 316 S/S instrumentation tubing so let’s leave Monel out of it for now.
Some of your thoughts, opinions and technical knowledge on this issue would be appreciated.
 
Manfacturers supply pressure ratings for their products.

See page 4 for stainless tubing ratings in Swagelock's Tubing Data document, MS-01-107 at
http://www.swagelok.com/downloads/webcatalogs/EN/MS-01-107.PDF

Or here for Hoke's
http://www.hoke.com/pdf/79308A_TubingDataChart.pdf

Fatigue from vibration is a different issue than pressure ratings.

Circor lists these methods of dealing with tubing vibration:

It is suggested that impulse lines for instruments shall be arranged to avoid:-
* Movement through thermal expansion.
* Mechanical damage from impact.

When tube vibration is a problem, due to the closeness of machinery, process pressure changes or other excitation, it is recommended:

* Tubing is clamped to the monoflange, process take-off or process line using a suitable clamp with the minimum span between the compression fitting and the clamp.

* A vibration loop (pigtail) is installed in the tube span between the process line and any independent structure used to support the tube. The orientation of the loop should be chosen with due regard to the probable relative motion between the connections.

* The vibration loop crossover has a minimum clearance of 2 x tube OD to prevent fretting of the tube.

http://www.iceweb.com.au/Tubings/instrument_tubing.htm
 
Hi there,

Ok thanks for the reply but that is all pretty much common knowledge and all instrument techs and engineers are aware of these vibration points, installation procedures and vendor ratings. I am more interested to find out how a installation like this can be safe if you use a vessel with 2 inch thick walls to contain say 150Bar and then you use a thin 1mm wall thick S/S tubing. My question is that surely this must be the weakest point in the installation and you are defeating the object of the 2 inch thick walls and 24mm thick bolts of the vessel. If you say that it is safe because the S/S tubing manufacturer says the tubing can be used up to 150Bar burst pressure then you might as well built the vessel from the same 1mm thick 316S/S.

Is it possible to mathematically proof that the pressure inside the vessel is higher per squire inch than inside the tubing, therefore the tick walls are needed only on the vessel and not on the tubing?
 
You might be overlooking the point that temperature has a bigger impact on pipe and pressure vessel strength than pressure. The instrument tubing is uninsulated and outside and therefore is at a much lower temperature than the insulated piping and steam drums, etc.
Also, the tubing being much smaller diameter has much less surface area per unit length and thus much less total force to contain. Look at the wall thickness of standard pipe and you will see that the wall thickness increases with pipe size. Half inch schedule 40 wall thickness is 0.109 inch, while 16 inch schedule 40 wall thickness is 0.500 inch.
 
The walls need to resist tensile forces. Think about it this way. Imagine stretching a rope between two trees and having several people sit on it. Now imagine making the rope twice as long and having twice as many people sitting on it. It's the same force per unit area. However, which rope is in greater danger of breaking?
 
Sam,

The tables in the documents cited are based on tests of tubing of various materials at various pressures, and have some margin of safety built in to the recommended pressures for the tubing sizes in the tables.

To my mind and my way of thinking, the total forces in a large pressure vessel of a large diameter (large internal surface area) are much higher than in a tube of much smaller diameter (smaller surface area). It's not just about the force-per-area, it's about the total force for the given area.

Think of two identical sailboats with different sized sails sailing side-by-side on the same body of water on the same day in the same wind conditions, both unobstructed. Presuming the sailing crews are equally competent and the larger sail won't cause the boat to exceed its terminal speed in the water, the boat with the larger sail will be faster than the boat with the smaller sail. Same wind; same pressure; different total force because of the difference in sail area.

[Those of you who believe that hooey about the vacuum created on the opposite side of a sail pulling the boat instead of the wind pushing the boat, well, go ahead and believe in myths.]

That's just how I've always chosen to justify the use of tubing for high-pressure applications, besides the tables generated from using repeatable testing methods sanctioned by recognized organizations. All of these were arrived at using maths and approved methods. And when a tubing manufacturer says their product meets a certain specification, it's a safe bet that it's been designed and tested to meet or exceed the specification cited.

And, I would have to believe that in a production-oriented process environment where hazardous and combustible liquids and gases are present at very high pressures that if the use of tubing was not safe that there would be some regulation or law to outlaw its use. Or, the lawyers and the insurance companies would prohibit its use or charge more for their services.

Generally we're talking about static lines for tubing runs to instrumentation and devices.

Can you just take any 1/2-inch tubing and use it for any application, regardless of the pressure or temperature? No. Tubing used for a particular application should be rated (and tested by the manufacturer, at least on a regular quality control basis one would think) for the application. Choosing a piece of tubing for a specific application should involve reviewing the rating of the tubing versus the requirements of the application. And, if one doesn't do that, then one is placing him/her-self and others at great risk.

Just because you go to Stores and get a piece of 1/2-inch tubing doesn't mean that the Sourcing people didn't do some research to ensure that the 1/2-inch tubing they buy and place in the warehouse meets the highest pressure rating required in the plant. You might even find that Stores has 1/2-inch tubing of different ratings for different applications in your plant, and one should be sure they are using the right tubing for the right application. (One-size-fits-all can be very expensive if one needs a very expensive type of tubing for a very limited number of applications, so it's probably very likely that there are multiple kinds of 1/2-inch tubing in Stores at your plant.)

And the material being used to manufacture the tubing also plays a large part in the strength and rating of the tubing. I doubt you'll find 1/2-inch copper or brass tubing on the application you cited; more like Stainless, or even Monel (oh, you said that wasn't being used; sorry).

There is tubing, and then there is tubing, my friend. Not all tubing is rated the same. And the right tubing should be used for a particular application. And tubing manufacturers usually publish tables showing the ratings of the tubing they produce, and those ratings are usually based on tests and calculations (using maths) to ensure there is some safety margin.

So, don't just blow off the tables in the references cited (you did look at them?); look at the references cited for the criteria in the tables in the documents cited. They are internationally recognized organizations who develop tests and ratings (using maths) to ensure that tubing can be tested using standards which are trusted and understood by everyone. And if their tests and maths weren't correct, their tests and methods wouldn't be used.

There are many methods to prevent fatigue failures of tubing, and that's what you should be most concerned about AFTER ensuring the properly rated tubing is being used for every application. If your site is experiencing a high number of fatigue failures of tubing on a high-pressure, critical application then someone should be doing an analysis of the mounting and support means being currently used and start looking for something different to reduce or eliminate the problem. (It's called Root-Cause Analysis, or Root Cause Analysis, and it has been proved extremely helpful in eliminating nuisance and costly and dangerous problems.)

You might even talk with the Sourcing people at your site and get the tubing manufacturer's representative to come to site to have a look at the application and make some recommendations. Some manufacturer's representatives can be very helpful; others aren't worth the money spent sending them an email.

Hope this helps!
 
B

Bruce Durdle

Sam,

You asked for a mathematical proof so here goes...

In a circular pipe or vessel, the force tending to burst the pipe is developed from the internal pressure acting over the full internal diameter.

This force is opposed by an equal force in the pipe walls which results in a tangential tensile stress (hoop stress) in the metal. Since pressure = force x area, the force developed in the pipe is equal to the pressure x diameter x length. The effective area of metal containing this force is 2 x thickness x length. So the stress in the metal is simply related to the internal pressure by

Stress = pressure x diameter / (2 x thickness)

It's not the pressure in the vessel of tubing that changes - it's the internal stress within the metal that is affected. So given a pressure vessel of say 2 m diameter and 50 mm thick, the thickness of 10 mm diameter tubing with the same internal pressure and the same hoop stress is only 0.25 mm.
 
Hi there,

Thanks a lot to all replies it was very interesting reading and the explanations about the rope and sailboats was great and they made a lot of sense.

Bruce, the reply and answer I was hoping for was the one you supplied, thanks a lot. I want to play with those calcs a bit more and see if I can actually proof every suspicious installation. I think it will be a interesting exercise to do and it will also make me feel better about these installations as well. Thanks again to all.
 
Sam... if you are going to delve into static pressure formulas, you may also want to investigate dynamic behavior, that is, Hydraulic-Pulse-Pressure phenomena.

I have a Nomagraph that determines the hoop-stress pulse pressure when oil or air thru tubing is suddenly shut-off!

Developed by Dr. D. J. Lapera of Marotta Valve Corp, way back when, it's available on request, off or on-list.

Regards, Phil Corso (Cepsicon [at] aol [dot] com)

 
W
For those of you who would like to have a bit more information about hoop and longitudinal stresses, please see the following links with some pictures:

http://en.wikipedia.org/wiki/Cylinder_stresses

http://www.codecogs.com/reference/e...spheres/thin_walled_cylinders_and_spheres.php

You can also find out about instrument tubing and ratings in these manuals:

http://www.swagelok.com/downloads/webcatalogs/EN/MS-01-107.PDF

http://www.restek.com/restek/images/external/59612A.pdf

ALWAYS REMEMBER THAT THERE IS A TEMPERATURE RATING

ASSOCIATED WITH ANY PRESSURE RATING!

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.
 
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