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Associate

Join Date: Apr 2008
Posts: 53

Pipe minimum allowable thickness

04/12/2008 12:05 AM

We have just completed an ultrasonic testing of a firewater line. How do i compute for the minimum allowable thickness? the pipe remaining life? The tabulations in ASTM 106 and A53 are manufacturing criteria?

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Guru
Engineering Fields - Piping Design Engineering - New Member Egypt - Member - Member since 02/18/2007

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#1

Re: Pipe minimum allowable thickness

04/13/2008 3:25 AM

Refer to the following example calculation for Determination of the max. allowable working pressure for a given pipe wall thickness.

EXAMPLE

Given: Code : ASME B31.4

Design Temperature : 140 oF (60 oC)

Pipe Material : API 5L Grade A25, ERW (in accordance with ASME B36.10M)

Allowable Tensile Strength of Material at Design Temp., S: 18000 psi

Joint Efficiency for pipe ERW, E : 1.0

Nominal Pipe Size, NPS = Do : 48 inch (1219 mm)

Pipe Schedule (Wall Thickness): STD. (9.53 mm)

Assume a corrosion allowance, C = 3 mm (0.11811 inch)

(1 psi = 1 pound per square inch = 0.070306955 kg/cm2)

Find: Max. allowable working pressure, or what we called design pressure, P.

Data:

Nominal pipe wall thickness, tn = 9.53 mm (Schedule STD.)

Pipe Wall Thickness Tolerance = ± 12.5% tn

Remaining thickness after deducting of tolerance, t = (1-0.125) tn = 0.875 tn = 0.875 x 9.53 mm = 8.33875 mm (0.328297 inch)

Corroded Thickness = 8.33875 – C = 8.33875 – 3 = 5.33875 mm ( 0.210187 inch)

Solution:

t = PDo / 2 SE or P = 2 t S E / Do

P = 2 t S E / Do = 2 (0.210187 inch)(18000 psi)(1.0) / (48 inch) = 157.6403 psi (10.869 bar)

So, the pipe is designed to resist a design pressure P = 157 psi (11.0382 kg/cm2)

Therefore, the operating pressure shall not exceed Pop = 120 psi (8.4368 kg/cm2)

For Remaining Life (years), please refer to the API 570 Piping Inspection Code

Inspection, Repair, Alteration, and Rerating of Inservice Piping Systems.

7.1 CORROSION RATE DETERMINATION

7.1 .1 Remaining Life Calculations

The remaining life of the piping system shall be calculated from the following formula:

Remaining Life (years) = (tactual – trequired) / Corrosion Rate

Where,

tactual = the actual thickness, in inches (millimeters), measured at the time of inspection for a given location or component as specified in 5.6.,

trequired = the required thickness, in inches (millimeters), at the same location or component as the tactual measurement computed by the design formulas (e.g., pressure and structural) before corrosion allowance and manufacturer's tolerance are added.

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#2
In reply to #1

Re: Pipe minimum allowable thickness

04/13/2008 3:56 AM

Blimey, that is an answer and a half. One question related to your answer but not the original question. Is this what they always do on oil refineries before we shoot the pipe x-ray photographs?

We always measured from the pictures and went by tables for that section of the pipe but we were given those tables by the refinery engineers. I worked for a ndt company for a few years but never gave this much thought.

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Guru
Engineering Fields - Piping Design Engineering - New Member Egypt - Member - Member since 02/18/2007

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#3
In reply to #2

Re: Pipe minimum allowable thickness

04/13/2008 5:57 AM

Yes case491,

My answer was sent 2 hours ago in a hurry, because there is no data available from the threader inspectorjoe. But if a such full data were given related to original thickness, current thickness, year of built, design pressure, design temp., pipe material, ... etc., I'm ready to proceed a full solution and clear explanation, since it is my play. And, I'd like you to know that before proceeding any evaluation to any piping system, there will be an Inspection Report including full data for thicknesses, RT for weldment and/or UT, ... etc.

Also, instead of spending time to tell me what I have to do, the chance is still available to send your recommended solution.

Note. The min. required thickness can be easily calculated (as shown at my post #1)from equation, t = PDo / 2 SE, or you can use the exact equation of the code you follow.

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Guru
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#4
In reply to #3

Re: Pipe minimum allowable thickness

04/13/2008 6:30 AM

I think that there is no problem, since I have a solved example explaining how to calculate the min. required thickness and the remaining life of a piping system, but as PDF file, and ready to send, just let me know your e-mail.

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#16
In reply to #1

Re: Pipe minimum allowable thickness

12/20/2009 1:40 AM

Dear sir,

as you have calculated design pressure as 157 kg/cm2 for pipe in this post, then you had calculated operating pressure as 120 kg/cm2. In your other posts I have also seen that some of our frns given operating pressure and temperature, then you calculated design pr from these operating pressure temp data. I want to know how do you calculate design pr and temperature from available operting pressure and temperature datas. Is operating pressure is 72% of design presssure or any other reference, Please explain.

regards

Rathore

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Guru
Engineering Fields - Piping Design Engineering - New Member Egypt - Member - Member since 02/18/2007

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#17
In reply to #16

Re: Pipe minimum allowable thickness

12/20/2009 6:58 AM
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#5

Re: Pipe minimum allowable thickness

04/13/2008 9:05 AM

If the pipe is made from some sort of steel and the structure suffers from corrosion, you could also consider relining or coating of the structure to stop the degradation. In that case for some part similar suggestions as stated by Abdel Halim Galala come into play, for another part specific considerations with regard to plastic or composite materials are important. Although the following link is related to lining of a high strength steel pipeline for the sake of hydrogen or carbon dioxide transport (and not liquid water, although some water vapour may be involved), it may be of your interest:

http://www.composite-agency.com/messages/Polyamide_Relining_Steel_Pipeline_Hydrogen.pdf

Regards,

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Guru
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#6

Re: Pipe minimum allowable thickness

04/13/2008 9:57 AM

My friends, I did my best to rewright the file into DOC instead of PDF.

Refer to the following example for determination of the min. required pipe wall thickness and calculating the remaining life of such a pipeline.

Based on API 570 for Piping Inspection Code "Inspection, Repair, Alteration, and Rerating of Inservice Piping Systems".

The remaining life of the piping system shall be calculated from the following formula:

Remaining Life (years) = (tactual – trequired) / Corrosion Rate (API 570, Para. 7.1.1)

Where, tactual = the actual thickness, in inches (millimeters), measured at the time of inspection for a given location or component as specified in 5.6.,

trequired = the required thickness, in inches (millimeters), at the same location of component as the tactual measurement computed by the design formulas (e.g., pressure and structural) before corrosion allowance and manufacturer's tolerance are added.

EXAMPLE

Given: A pipeline was constructed 15 years ago, the actual thickness measured along the full life was as follows:

Pipe Schedule:

Nominal Pipe Wall Thickness tnominal (initial thickness when the pipe was new)= 0.438" (11.125 mm),

● After 1ST interval of 5 years, tactual5 = 10.625 mm

● After 2ND interval 5 years, tactual10 = 10.125 mm (tprevious)

● After 3RD interval 5 years, tactual15 = 9.625 mm (tactual)

Code : ASME B31.8

Pipe material: API 5L XLS, ERW

Nominal Pipe Size: NPS 30" (Outside Diameter, Do = 30")

Specified Min. Yield Strength (SMYS), S = 52000 psi at the design temp.

Max. Operating Pressure (Design pressure), P = 910 psi

Appropriate Design Factor, F = 0.72 (ASME B31.8,Para. 841.114A)

Longitudinal Weld Joint Factor, E = 1 (ASME B31.8,Para. 841.115A)

Temperature Derating Factor, T = 1 (ASME B31.8,Para. 841.116A)

Find: The Remaining Life of the Pipeline in years.

Solution:

Min. Required Thickness, trequired, (or tmin. or tr, also called designed thickness or critical thickness) = P Do / (2 S F E T)

= (910 psi)(30 inch)/2(52000 psi x 0.72 x1 x1) = 0.364" (9.260 mm) mm/year

Long Term Corrosion Rate, LT = (tinitial – tlast)/time between last and initial = (11.125–9.625)mm/15 years = 0.1 mm/year

Short Term Corrosion Rate for Last interval, ST = (tprevious – tlast)/time between last and previous = (10.125–9.625)mm/5 years = 0.1 mm/year

According to API 570, Section 7, the higher of these two rates LT and ST should be used to estimate the remaining life. The interval between piping inspections shall be established and maintained depending on corrosion rate and remaining life calculation.

So, the corrosion rate to be considered = Max. (LT, ST) = 0.1 mm/year

The Remaining Life of the Pipeline = (tactual – trequired) / Corrosion Rate = (9.625 – 9.260) mm / (0.1 mm/year) = 3 Years, 7 Months and 24 Days.

Before that interval, you have to inspect the pipeline.

Very Important Note. If there is a local corrosion or a pitting, you are requested to apply the procedures of ASME B31.G for determining the remaining strength of corroded pipeline. Or you can use the assessment(s) provided by API 579 Fitness For Surface, FFS.

Dear inspectorjoe, please let me know your comments and any additionals you need.

(Any example derived here is formulated by myself, may be from an actual case, and not extracted from any code or manual).

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Guru
Engineering Fields - Piping Design Engineering - New Member Egypt - Member - Member since 02/18/2007

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#7
In reply to #6

Re: Pipe minimum allowable thickness

04/13/2008 10:29 AM

♣ Sorry for typing mistake, in the following calculated equation, please delete the high lighted distinguish: mm/year

"Min. Required Thickness, trequired, (or tmin. or tr, also called designed thickness or critical thickness) = P Do / (2 S F E T) = (910 psi)(30 inch)/2(52000 psi x 0.72 x1 x1) = 0.364" (9.260 mm) mm/year"

♣ And, I'd like to clear that the pipeline expected remaining life is 3 years, 7 months after its old life of 15 years, i.e. total life shall be 18 years, 7 months, if the corrosion rate still as it is 0.1 mm/year.

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Associate

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#8
In reply to #6

Re: Pipe minimum allowable thickness

04/14/2008 1:34 AM

Thank you very much for all the quick response. The firewater line ranges from 6 to 12 inchess in diameter and the maximum allowable opearting pressure is 150 psi. Unfortunately, there are no data on the type or specification of the pipeline. Can i assume it is A 106 or A53?

So, how do i get started with the computation of the minimum allowable thickness?

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#9
In reply to #8

Re: Pipe minimum allowable thickness

04/14/2008 2:32 AM

You can assume a pipe material as ASTM A53, Furnace , Butt Welded & Continuous welded.

The Allowable Tensile Strength of A53, S = 0.72 SMYS = 0.72 x 25000 = 18000 psi

where, Specified Minimum Yield Strength, SMYS = 25 000 psi

Pipe Material Joint Efficiency, E = 0.6

The Min. Required Thickness, tr = P Do / (2 S E)

Operating Pressure, Pop = 150 psi

Assume a Design Pressure (more than the operating), P = 185 psi

For Pipes with NPS 6" : Outside Diameter, Do = 6.625 inch

For Pipes with NPS 8" : Outside Diameter, Do = 8.625 inch

For Pipes with NPS 10" : Outside Diameter, Do = 10.75 inch

For Pipes with NPS 12" : Outside Diameter, Do = 12.75 inch

To find the pipe nominal thickness t = tr + CA (where CA = Corrosion Allowance)

To find the pipe nominal thickness, tn = t / 0.875 (an increase represents 12.5%t)

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#10
In reply to #9

Re: Pipe minimum allowable thickness

04/14/2008 3:23 AM

Again, thank you very much for your quick response. Now i can finalize my report. Which standard should i refer to? ASME/ANSI 31.4???

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Guru
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#11
In reply to #10

Re: Pipe minimum allowable thickness

04/14/2008 4:05 AM

Yes, ASME B31.4.

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#12
In reply to #11

Re: Pipe minimum allowable thickness

04/14/2008 5:51 AM

Thank you very much to all of you. You have been very helpful.....

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#15
In reply to #6

Re: Pipe minimum allowable thickness

12/20/2009 1:34 AM

Dear Sir,

can you explain me RETIRING thickness of pipeline( LPG) designed as per ASME 31.4. as this is asked us during compliance of ISO documententation.IS THIS THE THICKNESS BELOW THIS PIPELINE WILL NOT SURVIVE AT MAX WORKABLE PRESSURE?

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#13

Re: Pipe minimum allowable thickness

04/14/2008 10:30 AM

Abdel: thanks for your valuable contribution to this thread.

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Guru
Engineering Fields - Piping Design Engineering - New Member Egypt - Member - Member since 02/18/2007

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#14
In reply to #13

Re: Pipe minimum allowable thickness

04/14/2008 12:42 PM

Dear friends, ELEMAN and inspectorjoe,

Thank you very much, and you are welcome. May be you need to evaluate that "valuable contributions" as you said through using of pushbutton "Rate".

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