Design Life factor for the New Pipe ? 2

[ET1983]

Dear All,

I came across a question from one of my colleague, asking for the Design life factor for the Newly Designed Pipe.
ASME B31.3, section 304.1, doesn't say anything about the design life consideration for a Newly calculated pipe wall thickness ( Allowance consideration is there for Corrosion, threading allowance & mill tolerance)....however, i found a calculation in API 570, for the Remaining Life years of the pipe installed in plant with a specified wall thickness, which is as follows:

Remaining life years = Tactual - Trequired / Corrosion rate

My question is, considering a severe sour service condition, with 3mm corrosion allowance, can we obtain or justify that, it will withstand for so much years with the minimum required thickness?>

TE

 

[LittleInch]

I think what you're really talking about is the use and calculation of corrosion allowance / corrosion rate.

This is a pretty blunt tool IMHO, but currently the only one we have.

To calculate your CA, you need the design life and the corrosion rate, with or without Corrosion inhibitors and some assumptions about fluid properties, pressure and temperature over time. Then some empirical calcualtions deliver a corrosion rate per year and you have your CA.

Reality is vastly different. The fluid composition changes, pressure and temp changes from assumed values. The actual corrosion tend sot occur in discrete locations, sometimes with much more corrosion than was "calculated", sometimes less.

The use of integrity assessments allows continued use at isolated locations with pits much deeper than the CA.

All you can do at the start of a project / design phase is the calcualtions noted above.

I still don't know what a design life FACTOR is though....

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

 

[bimr]

Most project cost benefit analyses for equipment use a 25 year design life for analysis. The actual design life could far exceed 25 years; it may be as high as 50 - 100 years.

In your application, since the design life of the pipe is controlled by corrosion exposure as compared to other failure mechanisms such as external corrosion, water hammer, abrasion, etc., and inspection has been performed after so many years of service, one could reasonable conclude that the design life of the pipe can be extrapolated into future years of service. In addition to the corrosion rate, when establishing maximum allowable pressure and temperature ratings, the material allowable stress and appropriate non-destructive examination factors from the original code of construction shall also be reviewed.

Your rationale for determining the years of service may be as simple as annual inspection to document the annual corrosion rate and the loss of pipe wall thickness. Assuming a corrosion rate based on the in-service life, the remaining service life will be at least x years into the future (use a spreadsheet to show the calculated corrosion rate based on in-service life). Once the wall thickness reaches a value below design (the minimum wall thickness calculation per ASME B31.3); that will be the future time when repair or replacement is required.

API RP 581 starting at Part 1, Clause 4 contains a methodology.

Note that failure processes in process equipment are complex. In the process industries, risk invariably applies to failure or loss of functionality and considers the probability of such failure or malfunction, and the consequences of the failure.

 

[Bohlool]

there are 2 lifetime related issues in the design,
first: corrosion allowance which is (yearly corrosion rate)*(years of design life), this issue is explained above by LittleInch and bimr

second item is expansion design and Flexibility Analysis of piping, this item in fact is not a continuous time depended item, it is related to the number of cycles the piping see during its design life, in the same unit a piping may go one cycle between every overhaul, while other has 1 cycle/day which will add-up 7000 cycles for 20 years others may see more cycles. here "Stress Range Factor" is introduced in the code which is some how a design life related factor!

 

[ET1983]

Thanks to Little inch, Bimr for the brainstorming explanations...

going thru the API RP 581 for more informations...

Thanks.