High pressure piping life expectancy

[MechDesign105]

Hi,

I need to determine the life expectancy of high pressure piping (stainless steel 316), between 600 to 3600 lbs in a compressed air system maintained at -80C dew point. The system is located inside, so it is not subjected to rain, snow, or UV. The air flowing inside the pipes is very dry.

We have had a very good long term experience with these systems (>30 years), but in my search for life expectancy determination, I would like to be able to use technical data, standards, tests (maybe Xrays) to determine the life expectancy of the system. I basically want to be backed by real values. If a thickness test is done, what criteria should i use in determining the piping is still in good condition? Standard?

Thank you very much for your help!

 

[Jmoore1]

Mechdesign, you will need to calculate your minimum required wall thickness. since your maximum P is 3600 psi, austenitic stainless steel piping B 31.3 provide calculation....

Section 304 Para 304.1.2 provides general t calculations

Para K304.1.2 a (straight pipe under Internal Pressure for High Pressure Components) (>2500 psi)

I beleive you can design to either, however using the high pressure ratings required more info and would likely give you less thickness due to a higher accuracy calculation.

This is your absolute min thickness to contain your pressure boundary (subtract your mill tolerance and that is your Treq) [treq=t/0.875)

This is your retirement thickness.

Now determine remaining life take the WORST case corrosion rate over the life of this system. Divide your remaining CA by your max corrosion rate. And you have your life expectancy. Im assuming since this is very clean piping as well as stainless steel you will have very little corrosion. Take at least 4 or 5 readings to get a good gauge of the corrosion rate as mill tolerance could change this number a large deal.

 

[metengr]

What about conducting hanger surveys?

What about NDT of select butt welds using Liquid Penentrant testing?


There is much more to this than thickness testing!

 

[Jmoore1]

Fair enough, im not well versed in the QA world! would it be economic seeing as this is simply compressed air ? not that a leak at the weld would be catastrpohic or have any health risks.

 

[unclesyd]

I agree with the advice given above. There should be no corrosion with air as the working fluid. We have SS air line working at 1500 psig that are over 60 years old. One other thing I would look at is the pressure and temperature cycling.

Can you comeback with some cycle periods?

Depending on the size of the pipe with air as the working fluid could be consider a hazard due to stored energy.

 

[EdStainless]

You need to fist decide what would constitute end of life and how that might happen.
Since this is SS the only corrosion failures will be localized pitting. This will happen at welds that are in the bottom portion of the piping or at mechanical joints (threaded or flanges) since they create crevices.
Even then you will get a small leak before you have any major failures.
If the system is truly dry this should last 150 years or more. However there is bound to be dryer failure once in a while so pinhole leaks are the most probable, and as you say they will no be catastrophic.
Write a program to inspect for leaks once a year.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[MechDesign105]

@ Jmoore1

If I calculate the thickness, I have to then measure the thickness experimentally on all the piping in order to make sure it qualifies. Is there a standard that defines a procedure?
Also, it might be an impossible task as we have about 200 of these systems installed in the province and that their out of service time is very precious and difficult to obtain, especially for a system that might not have problems. Maybe a statistical approach at testing should be considered.

@metengr

If a crack is observed near welds wouldn't that crack be audible very early? we are talking about very high pressures. At the moment, whenever we have valve or electrovalve leaks, they are very audible.

@unclesyd

there are no pressure or temperature cycles in the system other than in the compressors and dryer but none in the piping that follows them. The piping consists of a distribution system of high pressure air throughout our installations through electrovalves.

The size of the piping does not constitute energy hazards, not as per canadian law.

What is the maintenance you have on your systems?

@Edstainless

I will try to obtain information as per problems that might have occured near welding or in the bottom of the piping. My survey of our maintenance crews only resulted in a problem with a specific electrovalve we have since replaced everywhere.

Our dryers are replaced when they are about 25-30 years old since they go through harsh pressure and temperature cycles and are subject to fatigue.

 

[unclesyd]

It's very good that the pipe isn't cycling anything like your dryers. A little surprised that you the the 20-25 year lifespan on a dryer. I was alluding to the same type fatigue in the piping as you see on your dryers if the line cycling.

After an initial UT survey the line was put on a ten year inspection schedule. For some RT of the welds and a complete
UT using the previous inspection points plus a few others.

In accordance with our piping specifications there was 100% RT on installation.

 

[Jmoore1]

MechDesign,as I said I don't know the QA field well, however I don't think "legally" there is some number you must follow. You could provide yourself a confidence that you can trust of say 95% within 10 thousands of an inch, with relatively few tests (not to brushed up on my stats from university).

B31.3 does have a step by step procedure for re-rating a line however. You could take a peek at that..

Again Im not 100% sure on a procedure like this.

As a side note, at many companies, a certain random end of life thickness is specified for each pipe size and original thickness.