B16.5 Blind Flange - Best Practices

[marty007]

I dug through many old threads hunting for the best path forward to handling inspection for B16.5 blind flanges, and can't find any solid recommendations. I'll lay out the issue, and provide some discussion, but I would love to hear how you all approach this issue.

When blinding a standard B16.5 flange, it is normal to just grab a standard B16.5 blind flange. This is done without a second thought, and life moves on.

When considering long-term inspection however, things get more complicated. ASME B16.5 is nearly silent on the topic of corrosion. The thickness of a blind flange in B16.5 is depicted as being exclusive of the raised-face, and paragraph 6.3 states you are allowed to remove the raised-face to create a flat face flange. This leads me to believe the corrosion allowance would equal the height of the raised-face (1/16" for CL150/CL300, and 1/4" for CL600->CL2500). Many times however, I see manufacturer's supplying a standard CL150 or CL300 B16.5 blind flange for manway covers, even when the corrosion allowance on the vessel is specified as 1/8".

Fast forward to on-site inspection. When monitoring the thickness of a blind flange, what is the best practice for specifying a t-min?

I see two valid approaches:
[ol 1]
[li]Set t-min equal to tf from the B16.5 code. Essentially this means the corrosion allowance becomes the height of the raised face.[/li]
[li]Set t-min equal to the thickness obtained by performing an ASME VIII-1 calculation. This will be specific to every vessel as it would take into account the vessel MAWP, temperature, and gasket selection. As many of us know however, many B16.5 flanges fail ASME VIII-1 calculations, so there is a real chance the flange will fail.[/li]
[/ol]

Long story short, what approach do you use in specifying a t-min for B16.5 blind flanges?

 

[cbPVme]

I take a similar approach.

If the corrosion allowance does not exceed the raised face thickness, the B16.5 rating is acceptable.
If the corrosion allowance exceeds the raised face thickness, calculate the minimum thickness per UG-34. This may require a blind cover of custom thickness.
Alternatively, specify a lower corrosion allowance for the blind flange and replace once that value has been exceeded.

 

[jtseng123]

I believe your question can be simplified as: what is the maximum corrosion allowance allowed for the standard blind flange based on pressure-temp rating table.
If it is 1/4", then replace the blind when the thickness corroded more than that. Shall not have anything to do with the height of raised face.
But I have no idea what is the maximum allowance. Even we have 1/4" corrosion allowance on a vessel, we still specify flange based on rating table, not bumping up to higher rating for thicker blind.
Someone in flange fabrication business may know more of this issue. .

 

[KevinNZ]

Flange thickness is often limited by the bolting stress at the edge of the gasket rather than thickness in the middle of the flange. The bind flange may be fine with corrosion on the inside face as along as the there is no thinning at the flange edge. You can see that the required thickness for a spade is less than a blind flange.

The problem is calculations for a blind flange usually tell you that standard flanges not is thick enough for the bolting stress. Some how you would need to isolate the calculation for thickness required inside the gasket for pressure.

 

[BJI]

It is an interesting question. I guess it hasn't really been addressed to date since a blind flange can be relatively easily replaced. Like most B16.5 flanges, the blinds would be able to accommodate more corrosion than the raised face permits. It will be a leakage based failure mode, and would require significant material loss before it had an appreciable effect on the joint integrity.

For design though, it seems the only practical approach would be to use option 1 and have a separate CA for the blind flange. Then cross that bridge with FFS if you get to it, or replace the blind if more economical.

 

[XL83NL]

Not sure if this is of any interest, but B16.5 (as well VIII-1, B31.3, etc.) is for new construction. They obvious are silent on subjects that matter from an in-service point of view. Have you checked if any of the ASME PCC codes (which stands for Post Construction Codes, hence suit the scope of the question better) provide any useful insight?

Huub
- You never get what you expect, you only get what you inspect.

 

[marty007]

I take a similar approach.

If the corrosion allowance does not exceed the raised face thickness, the B16.5 rating is acceptable.
If the corrosion allowance exceeds the raised face thickness, calculate the minimum thickness per UG-34. This may require a blind cover of custom thickness.
Alternatively, specify a lower corrosion allowance for the blind flange and replace once that value has been exceeded.

Very clear and concise. I think this approach cleanly follows all code rules.

I believe your question can be simplified as: what is the maximum corrosion allowance allowed for the standard blind flange based on pressure-temp rating table.
If it is 1/4", then replace the blind when the thickness corroded more than that. Shall not have anything to do with the height of raised face.
But I have no idea what is the maximum allowance. Even we have 1/4" corrosion allowance on a vessel, we still specify flange based on rating table, not bumping up to higher rating for thicker blind.
Someone in flange fabrication business may know more of this issue.

I don't think this approach follows the codes. Take this to the extreme and think about a 1/2" CL150 blind flange with t=0.38"+RF=0.44". If you have a corrosion allowance of 1/4", your blind would be reduced to 0.19". This doesn't seem reasonable, I think you'd have to replace this blind earlier...

Flange thickness is often limited by the bolting stress at the edge of the gasket rather than thickness in the middle of the flange. The bind flange may be fine with corrosion on the inside face as along as the there is no thinning at the flange edge. You can see that the required thickness for a spade is less than a blind flange.

The problem is calculations for a blind flange usually tell you that standard flanges not is thick enough for the bolting stress. Some how you would need to isolate the calculation for thickness required inside the gasket for pressure.

One issue I see is that corrosion can occur right up to the gasket surface. The inside area of the blind flange can have a big impact on the rigidity of the blind and its ability to resist deformation.

It's true that many standard B16.5 flanges will fail an ASME VIII-1 blind calculation. For this reason, I'd like to find an approach that avoids the requirement for calculations as much as possible. This is where the idea of using the RF height as corrosion allowance comes from.

It is an interesting question. I guess it hasn't really been addressed to date since a blind flange can be relatively easily replaced. Like most B16.5 flanges, the blinds would be able to accommodate more corrosion than the raised face permits. It will be a leakage based failure mode, and would require significant material loss before it had an appreciable effect on the joint integrity.

For design though, it seems the only practical approach would be to use option 1 and have a separate CA for the blind flange. Then cross that bridge with FFS if you get to it, or replace the blind if more economical.

Yeah, that's what I was thinking. One situation that can make blind flange replacement more awkward is when there is a davit or hinge attached (say for a manway). For a hinged blind in particular, it could be difficult to ensure the replacement blind hinge lugs will line-up well with the existing hinge lugs on the matching WNRF.

Not sure if this is of any interest, but B16.5 (as well VIII-1, B31.3, etc.) is for new construction. They obvious are silent on subjects that matter from an in-service point of view. Have you checked if any of the ASME PCC codes (which stands for Post Construction Codes, hence suit the scope of the question better) provide any useful insight?

Thanks, I'll take a look when I have a chance. Any in particular off the top of your head that you think might address this question?

 

[r6155]

“When considering long-term inspection however, things get more complicated”.....

Change to short-term inspection,

Regards

 

[GD2]

Marty007,
When carrying out inspection and trying to determine the acceptance thickness for continued service, you can't any longer find the data either in B16.5 and Sec VIII-1.
These standard and code are for new construction.
Once in service, you have to do a fitness-for-service. The most popular practice is FFS-1/API 579, which is widely accepted by the industry.
Many times, it becomes a challenge for inspection engineers to do a FFS without guidance and training.
You can start with Part 4 -General Metal Loss.
Flanges are classified as Type B Class 2 component and a Level 2 Assessment has to be done. Acceptance Criteria is also given at the end of the part.
It's not easy but is the best practice way out.

Standard ASME B16.5 flanges are acceptable to use in Sec VIII Div-1 pressure vessels (UG-34)without having to do separate calculation to determine flange thickness.

If your question is to specify minimum thickness for the blind flange it will be tf as given in the B16.5 table. This thickness will be design thickness without Corrosion Allowance. Usually 3.0 - 5 mm + tolerance are applied for fabrication.
New construction codes will tell that the flange minimum thickness should be tf. So, if your to specify minimum thickness, it should be tf.

if you have an inline monitoring system in place and have previous inspection thickness readings, you can set-up an alert thickness (which most companies do)based on the corrosion rate estimated from the current process and service conditions and the nest inspection schedule.

Many times, you will be able to establish Remaining Life of the component based on the previous thickness monitoring results.

Now, whatever you do, the code and standards won't tell you if you can go below tf (minimum flange thickness).

To run below tf assessment, and determine if it's still suitable for continued service, you have to perform a FFS per API 579.





GDD
Canada

 

[marty007]

Thank you everyone, I think I understand where to go with this.

Change to short-term inspection,

I'm still new to the world of in-service inspection. I'll need to be careful with my wording in the long-term.

Many times, it becomes a challenge for inspection engineers to do a FFS without guidance and training.

I'm already signed-up for a course starting in May. I'm really looking forward to it, and getting some early exposure to it is generating a lot of questions that I plan to ask during the class.