Shell Radiography and joint efficiency 3

[Paulettaa]

Dear All,

(For vessels fabricated as per ASME BPVC VIII-1)I have seen many datasheets indicating joint efficiency for a cylindrical shell to be 0.85 corresponding to spot radiography. What is the reason for that? I mean is radiography so expensive that increasing shell thickness is a preferred option? Even if it is the cost due to radiography why not only do full radiography for LW's and do spot radiography for CW's? since in most cases the LW's are the more important ones in determining the thickness of the shells.

Warm Regards

 

[marty007]

In short, yes it's a cost consideration, but maybe for different reasons than you might expect.

In our shop, we have many different vessels being worked on in the same fabrication bay. When RT is required we have two options to get it done:
[ol 1]
[li]Cordon off the area, halting progress on all jobs in that area while the RT is being performed. This is required for large vessels that are awkward to move.[/li]
[li]Move the vessel to a dedicated outdoor location. This is our normal practice for smaller vessels or parts of large vessels. This allows progress to continue on other vessels, but can mean a lot of transportation outside and inside for larger vessels.[/li]
[/ol]

This transportation or halting of other work can really add up in terms of man hours, which translates to an additional cost for the project.

Then of course there is the additional cost for the RT contractors to perform their work.

When you're talking about equipment around 1/2" in thickness, the cost of all this handling and labor can far outweigh the savings in 1/8" of material.

 

[moltenmetal]

+1 on marty007's comments. I've also seen a lot of false positives from RT, i.e. flaws that weren't actually detectable when we went after them with a saw or grinder.

 

[MGZmechanical]

I think in ASME VIII division 1 you can choose to have E =1, 0.85, 0.7 or less depending on the inspection levels (UW-12).
In the European code the 0.7 option (no inspection) is only allowable for very low danger vessels (I think in ASME VIII div 2 this is not allowed). For medium or high danger vessels in Europe it is mandatory to perform some inspection so the E is always 0.85 or 1(and also in ASME VIII div 2).
Using EN or ASME VIII div 2 implies lower safety factors (it should led to less thickness and less weld --> less cost) but the code wants to be sure it is all fine --> More inspections and higher costs.
So, for the ASME is a cost analysis; in Europe it can be a cost matter or mandatory depending on the pressure x volume + fluyd combination of the vessel.

Best regards,

 

[Paulettaa]

Thank you all for your replies.

marty007, what you say about the costs of RT is understandable however, whether you pick a spot or full RT this comment applies. I mean radiography can result in pause in workshop activities be it a full RT or a spot RT.
Second, I just want to do full RT for the LW's not CW's . Then I can run a spot RT on the CW's. This way I can achieve the maximum joint efficiency as per table UW-12 without doing full RT on CW's where in most cases ( especially for large diameter vessels) the total length of LW's is much less than that for CW's. Remember I can achieve this maximum value of joint efficiency, E=1, using spot radiography for CW's as per UW-11 (a) 5.

 

[david339933]

Sometimes all you need is a spot to reduce the min. thickness below the next nominal plate thickness. E=1 is overkill.

 

[marty007]

Spot radiography can be performed during lunch breaks when nobody is in the shop. Full RT takes longer, and eats into production time.

A fabrication shop will always select the most cost effective level of RT for their shop. A customer is always free to specify whatever minimum level of RT they want, but need to understand that specifying higher levels of RT will have cost impacts. There can be very good reasons for specifying higher levels of RT (eg. sour service), but if the vessel is just holding high pressure water, they need to consider if it's really justified.

In general, customers need to understand that every note they put on a data sheet has a potential cost implication. Sometimes seemingly innocent notes can lead to significant cost impacts. In my opinion, the job of the customer putting together the equipment datasheet should be to identify the minimum requirements for the vessel, and give the manufacturer as much freedom as possible to come up with the lowest cost solution.

Coming back to your question, I would put it back to you. Why would you need a higher level of RT? What is the process reason? Have there been failures of similar equipment that can be linked back to RT examination levels? Do you have a weight limitation for existing foundations that require the equipment to be as light as possible?

Cheers,
Marty

 

[JStephen]

Since plate is usually stocked in standard thicknesses, increasing radiography will not always reduce thickness.
Also, reducing thickness with increased radiography may then require additional reinforcing at nozzles or manways or saddles.

 

[moltenmetal]

Amen marty007 - a lot of people add specs including types of inspections without understanding their implications to the fabricator in any detail.

 

[EdStainless]

Though as you work in more expensive materials (Ni alloys) 100% RT starts to make more sense.
It is a balance.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[marty007]

It is certainly a balance depending on material. Some materials even require 100% RT in ASME VIII-1 regardless of thickness (Titanium).

This also drives to my point to allow the fabricator to make the decision, as they will have much better tools for estimating the total cost of fabrication.

 

[Trestala]

Dear All, how about moving from spot radiography to full UT as per UW-51(a)(4). Could it make better sense in costs than full RT in some cases?

 

[r6155]

You can use UT instead of RT

Regards
r6155

 

[victorpbr]

Are there any special requirements to use UT in lieu of RT? I've heard that TOFD is not acceptable and only phased array is acceptable but did not find any reference in the code

 

[r6155]

See UW-11

Regards
r6155

 

[r6155]

RaymondN
Also see ASME V.

Regards
r6155

 

[Trestala]

r6155,

I understand we can use UT in lieu of RT. I am asking if going from Spot RT to Full UT would make more sense than Spot RT to Full RT in terms of costs and production.

Regards,
RaymondN

 

[SnTMan]

Answer: IT DEPENDS.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Don56]

Agree with SNTMan. It depends if you want to go broke or not. We attempted using PAUT on Section VIII Div. 1 vessels and got nothing but frustration. It was near impossible to get acceptable SAW welds. We RT'd PAUT rejected welds and they were acceptable with NO visible indications in the radiographs. PAUT is a very different technology than RT. The acceptance standards are in Sec. VIII Div. 2. Good luck.

 

[r6155]

I know several fabricators that begun the design with E=0,85
Then, due to a lot of defects, they arrive to RT 100% (E=1).
I suggested them begun the design with E=1

Regards
r6155