LOF v LOP

Date 07-09-2009 12:28

Shane,

I dont really see what you are asking ;( other than ASME tolerate more defects than EN standards.

I hope your exam went well.

3.2

By Shane Feder (****)

Date 07-09-2009 12:41

Thanks 3.2,
Sorry, I was not comparing US v British/European codes, it was just that it came up in the exam.
I am trying to understand why 37 mm (nearly 1 1/2") of unfused metal on the surface on the inside of the pipe is OK but 6 mm (1/4") or less of unfused metal in the middle of the weld is not OK.
There may be a metallurgical or mechanical reason that I have completely overlooked but it just doen't make sense.
Just hoping to see what everyone thinks,
Regards,
Shane

By 3.2 Inspector (***)

Date 07-09-2009 13:03

I agree that it sounds strange. I have always been tought that surface (inner or outer) defects are more serious than internal defects.

when looking at different welding criterias it is important to understand that the criteria is not evidence that a weld is sound for the intended purpose, but rather an indication of what the welders/operators can do over and over again with todays equipment.

3.2

By CWI555

Date 07-09-2009 18:13

You have to take the code as a whole. For that reason one code may have an allowance for an indication or a different definition etc.
What B31.1 says for design criteria (materials, flexibility, support, pressure etc is taken into account for design of the acceptance criteria.
If you want an apples to apples from one code to the other you have to perform an analysis and reconciliation between the two to level
the playing field.

This is why the difference in codes. It's also why you have to be careful not to cross one acceptance criteria to the other.
What worked for one, may be dangerous or over the top for the other.

Regards,
Gerald

By Shane Feder (****)

Date 07-09-2009 22:34

Gerald,
I am not talking about crossing over between two codes, I only mentioned the British / European because it came up in the exam.
I am talking about the acceptance criteria for B31.3 Normal Fluid Service.
Table 341.3.2 states Lack of Fusion = zero allowance and Incomplete Penetration = 1 1/2" maximum
A 1 1/2" notch or notches (one or two missed edges on the root) is deemed acceptable whereas a 1/4" (or less) of Lack of Sidewall or Lack of Interrun is deemed not acceptable.
I cannot understand how something that is very small and may be 1/2" from either surface can be more detrimental than something that is 6 times as big and is exposed to the surface.
Is there a metallurgical / mechanical reason for this ?
Regards,
Shane

By CWI555

Date 07-09-2009 23:51

Lets stay within B31.3 then. The previous example was meant to clarify, but we'll do it this way.

Even within B31.3 there are differences in acceptance criteria that depend on the service. Normal fluid service/severe cyclic/category D etc.

Ask yourself why there are different criteria within the same code?

It would be much simpler just to have 1 criteria for all. The reason they have different criteria is one fluid service has more risk than another.
Metallurgically; with a sound PQR/WPS, the difference between a normal fluid service and a severe cyclic weld is zero for = T, Diameter, and grade of metal.
The differences would be in the class of service, stresses, etc for the system as a whole.

Chapter 2 details some of those differences.

The acceptance criteria is based on the design criteria as modified by seismic, support, fluid class etc.
What the authors of B31.3 are telling you that if you have followed the code, then you have reasonable assurance of no failure.
Using your example, they are saying that if that 1 1/2" notch for the relevant fluid class were to receive the maximum design stress, that joint would hold.

It's been my experience that many inspectors overlook the importance of ancillary systems such as supports, hangers and sliders. For that matter, not to many owners these days understand that either. It is there where an owner can find themselves in hot water.
Without the ancillary systems being properly constructed and welded, all of the design calcs go out the window, and then that 1.5" notch becomes a problem because the rest of the system is compromised.
It's the overall system of piping, supports, etc that count.

From my view, I don't really care one way or another. An inspector needs to remember they are a reporter. They compare conditions in the field against a known standard and report them. It's up to the owner's engineer then. They can accept it as is, or could require a new weld replace a perfectly good weld if they so chose.

I hope that clarified the intent of my post.

Regards,
Gerald

By Shane Feder (****)

Date 07-10-2009 00:13

Gerald,
Thanks for the response but it is still not clarified.
I am not talking about the differences between classes in B31.3, only Normal Fluid Service.
If a weld in NFS has 1/4" of internal lack of fusion say 1/2" from the surface it fails but it can have up to 1 1/2" of unfused edges on the surface at the root and it passes.
I can't understand why and I am asking for any opinions.
There must be a reason, was just hoping someone could clarify.
Regards,
Shane

By CWI555

Date 07-10-2009 00:40 Edited 07-10-2009 01:00

Ok, your trying to compare volumetric to surface criteria within the same same system. The use of 1/4" internal lack of fusion implies your comparing radiographic criteria as a comparison.
Lets try the link instead.
http://cstools.asme.org/csconnect/pdf/CommitteeFiles/15243.pdf
B31 Case 181-1
Use of Alternative Ultrasonic Examination
Acceptance Criteria in ASME B31.3
You'll note that you can in fact have 1/4" length or less LOF, with the depth to height
Code case 2235-9 and now -10 also shed some light on this.

Historically, RT has had a tighter criteria for lack of fusion and cracks because it's POD (probability of detection) is a) not as efficient as UT for planar type flaws and b) because fracture mechanics were not understood as well as they are today when the criteria was first written.

If you can see it all with RT, there is a good chance there is more to it than meets the eye.
Therefore your going to see a less restrictive criteria for UT that does in fact allow a small amount of planar flaws. (as reflected in the code cases listed above.

On the visual side, the acceptance criteria is well known and accounted for with regards to the specific system in question.
It's almost always a mistake comparing volumetric criteria to surface criteria.

Regards,
Gerald

By Shane Feder (****)

Date 07-10-2009 03:18

Gerald,
Thanks again for the response but I think we are going around in circles.
If you radiographed a joint and there was clearly defined incomplete penetration in the root you would be allowed 1 1/2" but if there was a clearly defined straight line outside the root area it would generally be classed as lack of sidewall fusion and none is allowed.
That is where I am confused, one is allowed and one is not but they both seem to be detrimental to the joints soundness,
Regards,
Shane

By CWI555

Date 07-10-2009 04:39

Shane,

I think your overthinking it a bit. It might be a good idea to have another look at my previous post.

I'll desist on the topic, and see if anyone else has something different to add.

Regards,
Gerald

By kipman

Date 07-10-2009 13:30

I believe Gerald has it. I've always understood that the seemingly strange difference in allowable discontinuity lengths between incomplete fusion and incomplete penetration was due to the probabilities of detection with radiography. High POD for IP and lower POD for IF. I can imagine the code writing committee sitting there debating this and arbitrarily coming up with 1/4" for IF because it gave them a bit of a safety factor for the lower probability of detection.
Mankenberg

By Metarinka (****)

Date 07-10-2009 05:34

It might be noted that often times acceptance criteria are based not on what can be discovered by NDT but by the implications of those discoveries. Or simply put; the finding of certain defects may make other more serious defects more likely thus the minor problems are rejectable. My guess is this is a case where lack of sidewall fusion may indicate a more serious metallurgical problem like seams or lamenations in the plate.

From my understanding of code work there are several types of indications that have otherwise artificially high acceptance criteria because they may indicate more serious (and often times non identifiable defects)

perhaps the best example I can think of is preheat and interpass temperatures which differ in tolerance based on the likelihood of defects due to metal or filler composition.

The other human guess is that the standards were made or revised at differing times based on more recent data and as such one is under more modern engineering understanding.

By Shane Feder (****)

Date 07-11-2009 00:00

Kip, Metarinka, Gerald,
Thank you for your responses.
I realise I may be overthinking but my curiosity has been aroused and the only way to learn is to keep asking questions (until everyone ignores you. LOL !.)
Kip,
The 1/4" I noted was only an example, zero LOF is allowed.

I will try once more to explain.
I have a 20 mm thick Single Vee weld with 60 degree included angle.
The width of the weld on the face is approx 30 mm.
Depth of the weld is 21 mm on one side of the root (fused) and 17 mm on the other side of the root (unfused).
The radiograph clearly shows a dead straight line slightly off centre measuring 37 mm long.
12 mm from the centre of the weld there is another dead straight line measuring approx 10 mm. This appears to be lack of sidewall fusion in the last or second to last fill run.
One is a reject and one is acceptable.
IMHO the discontinuity at the surface is more detrimental than the internal defect but the code committee feels the opposite. I am just trying to find out why.
I realise LOSWF is difficult to detect with RT but I have seen numerous cases, generally on GMAW on thick plates.
Thanks guys,
Regards,
Shane

By Eutectic (**)

Date 08-04-2009 14:11

Good afternoon Shane,

My opinion, LOP can only be as deep (high) as the root face ie 1-2.5mm, however unless you have AUT it it difficult to measure the height of LOF.
Meaning that LOP can be 1-2.5mm high for a length of 1.5" but if the LOF is more than 1 pass height or if 1 pass height is a substantial % of the wall thickness it might be more detrimental.
For example you have your 20mm Plate. LOP = 2mm therefore effective WT =18mm.
LOF= 8mm high, effective WT = 12mm.
I would therefore aggree that this is intended for general NDT detection methods.
Regards
Hanre