EN13480-3 and corrosion

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rickand
Beiträge: 12
Registriert: Mi 14. Sep 2011, 09:49

EN13480-3 and corrosion

Beitrag von rickand » Fr 4. Okt 2013, 10:25

Hi to all EN13480-3 - users

I would like to start a discussion about how to interpret the corrosion allowances in the EN13480-3?

For dimensioning the wall thickness in the internal pressure design the integration of the corrosion allowances is clearly defined.
You need to increase the ordering wall thickness so that even after corrosion there still is enough wall to resist against pressure.
EN13480-3_fig431.png
but for the longitudinal stresses (chapter 12) the description is much more fuzzy:

Code: Alles auswählen

Stresses shall be determined for nominal thickness. 
NOTE  Wall  thickness  reductions,  allowed  by  the  technical  conditions  of  delivery  for  seamless  and  welded  pipes  are covered by the stress limits.
At the first look it seems clear: Manufacturing tolerance + Corrosion allowance does not need to be considered
but in the NOTE there suddenly is a clarification for manufacturing only: Normal manufacturing tolerance does not need to be considered.
So what about corrosion?

Lets have a look at some other codes: (all translations are done by me and are not official)

FDBR – Richtlinie Berechnung von Rohrleitungen
9.4.1 Rohrelemente
"... Dabei sind für die Ermittlung der Querschnittswerte (Trägheitsmoment, Querschnittsfläche) der Bestelldurchmesser und die Mittelwanddicke einschließlich Plattierung zugrundezulegen"
"... the sectional characteristis (moment of inertia, surface) are based on the ordered wall thickness including plating."

10.1:
"Die Spannungsermittlung wird daher bei bestelleter Mittelwanddicke mit der Bestellwanddicke abzüglich der Abnutzungsabschläge durchgeführt."
"The stress calculation is done with the ordered wall thickness minus corrosion and erosion allowances."

CODETI
C 3.2.1.4
Les calculs de flexibilité doivent être effectués en utilisant les épaisseurs nominales et la vérification des contraintes en utilisant les épaisseur définis en C.3.2.6.3
The flexibility calculation must be done using the nominal thickness and the stress analysis based on the thickness defined in C.3.2.6.3 angegebenene Wanddicken.

C.3.2.6.3
"e_f : Epaisseur de référence utilisée pour le calcul des contraintes: epaisseur de demi-plage des tolerances c1- et c1+ diminuée des surépaisseurs de corrosion ou d'érosion eventuelles."
"e_f : reference thickness for the calculation of stresses : average thickness between tolerances c1- and c1+ reduced by allowabces for corrosion and erosion"

ASME B31.3

302.4 Allowances
In determining the minimum required thickness of a piping component, allowances shall be included for corrosion, erosion and thread depth or grove depth.

So basically all other codes require to take into account the corrosion allowances for the longitudinal stresses as well.

How do you deal with the corrosion in your pipe stress calculation?
CODETI even take into account the reduced wall thickness for the calcualtion of the SIFs.

Any comments are welcome.

Best regards

AR
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poschfri
Beiträge: 1
Registriert: Mi 19. Dez 2018, 11:23

Re: EN13480-3 and corrosion

Beitrag von poschfri » Do 20. Dez 2018, 08:39

How do i deal with the corrosion in my pipe stress calculation?
Short answer: In new plants I make a standard Rohr2 calculation. For support design I consider the corrosion allowance (the corrosion allowance leads to higher support loads when the system is new) by adding a certain percentage to the load results (percentage depends on the amount of corrosion allowance).

BUT it is more complex when partly modifying existing plants and I try to give technical input (without considering the standards) for further discussion …

A) GREENFIELD-APPROACH
“simply consider both worst cases (for pipe and support) in your calculation
All the worst cases, that may occur in the system, to be considered in the calculation results, to avoid system failure.
So from technical aspect in my opinion the calculations A1) and A2) should be considered:

A1) For pipe STRESS results take the MINIMAL wall thicknesses!
If I want to calculate the (worst case) stress of any pressurized part, I have to calculate with the theoretical minimal wall thicknesses (sc_stresses = s – c0 – c1). Especially in those areas, where the system is weak (e.g. many bends).

A2) For support REACTION FORCES take the MAXIMAL wall thicknesses!
If I want to know the maximum possible reaction forces (worst case) of a piping system, I have to consider the theoretical maximal wall thickness (sc_forces = s + c0) in the calculation.

This greenfield approach A) is only “save”, if there is a “homogenous” green-field system. (e.g. new plant = all pipe components to be newly installed)

When working in “brown-field” this approach does not consider all the worst cases, therefore approach B has to be chosen.

B) BROWNFIELD-APPROACH
“ …think what you have to do, to ensure that every worst case is considered in your calculation”
When revamping old pipe systems, probably the remaining old pressurized parts are thin due to corrosion. The new piping parts are probably delivered with maximal wall thickness including manufacturing and corrosion allowance .
To evaluate the IMPACT of the replacement parts ON THE STRESS IN THE REMAINING OLD SYSTEM, take the minimal wall thicknesses for the existing remaining system and take the maxima wall thickness for the new replacement parts!
(sc_existing = s – c0 – c1; sc_replacement = s + c0)
The old system is “weak” due to corrosion – the new replaced subsystem is “stiff” due to the bigger wall thickness. Replacement of some parts of an existing system may lead to higher deformations and stresses in the “old” system, although the subsystem was just replaced, without(!) design changes. With this Approach the calculation will show the worst case for the existing system and the engineer can detect, if some additional measures have to be taken (e.g. wall thickness measurements) to ensure safe operation of the existing system.

practical experience:
As per my experience it never happened, that all the wall thicknesses of old remaining parts were completely down (s – c0 – c1) and it also never happened, that all delivered new parts had the maximum delivery wall thickness (s + c0).
In reality (e.g. in areas where we have corrosion allowance of 3mm), wall thickness measurements were performed in specified areas of the old system to get reliable (realistic) data for the existing system.

Both worst case approaches (A and B) are technical correct but expensive in engineering phase (calculation effort). In my opinion, if you do not get real figures (wall thickness) of the old system, you need to consider worst case acc. to approach B which may lead to expensive piping design.

my questions:
Is there any probabilistic evaluation on real wall thicknesses of delivered piping parts?
Is there any technical argument, which allows me (from technical aspect) to ignore approach B without performing wall thickness measurements?
Is there a standard describing the “brown-field problem”, mentioned above in approach B?

Abbreviations
s … wall thickness (as ordered) [mm] … Bestellwanddicke
c0 … manufacturing tolerance +/- [mm] … Fertigungstoleranz +/-
c1 … corrosion allowance [mm] … Korrossionszuschlag
sc … wall thickness for calculation [mm] … Berechnungswandstärke

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