ASME B16.5 Raised face specs 3

[Thirlmere]

Hi Guys, i have been scratching my head for some time now with regards to the raised face height specs given in ASME B16.5, notable section 7.3 for facings.

It mentions a number of metric sizes and also gives an imperial conversion. EXCEPT, the conversation they give isn't correct.
For instance the give a size of 2mm and convert that to 0.060", clearly 20 thou out.
They also give a size of 7mm and convert that to 0.250"

When re-machining gramophones on flanges onsite, we need to know whether we should be looking at the 2mm height or the 0.060" (1.5mm) Height.

Any ideas, seems as clear as mud to me.

Regards, Jeff

 

[bljnv]

I dont have current B16.5 standard. In my view if the flange to flange overall dimension is kept within the tolerances specified by the standard, there should not be a problem whether individual flange raised height is 1.5 or 2.0 mm. While re-machining for raised face, ensure the flange thickness dimension is maintained as per standard.
Regards

 

[Thirlmere]

Hi, i'm really sorry if i wasn't clear, but i was asking about the raised face dimension only. Not a flange to flange dimension.
The problem i am up against is the client needs us to measure the raised face on a great number of pipe flanges and i'm unsure as to whether we should set the spec height at 2mm or 1.5mm

Thanks

 

[gerhardl]

A valid standard specification is a valid standard specification. Two different measures, both given as a valid, is equally good (foreseen that all other aspects / standard criteria is not unduly altered by the one or the other measure).

The buyer ( eg. end user (no pun intended!)) of your machining is the one that should be asked for an opinion here.

 

[bcd]

Hi. Please refer to paragraph 1.6 in the standard. It states that each type of unit (metric or imperial) are to be treated as separate. Because of numerical rounding during conversion, there are discrepancies so you cannot convert from one to the other. Note that there are errors in the standard. For example, the raised face dimensions in the mandatory appendix II are still shown as metric. The variation in raised face heights has been questioned before and was discussed bgy the committee responsible for maintaining this standard in its last meeting. These discrepancies and errors will be most likely cleared in the next revision. But for now, you need to pick one type of unit, metric or imperial, and only use those. Since the standards was originally imperial, and the bolts continue to be imperial dimensions, even for metric, it is safest to use Appedix II. Problem is the error in the figure in Appendix II which wrongly show metric dimensions. Use previous edition for clarification if necessary.

 

[racookpe1978]

The "real" dimension is the 1/16 inch, or 0.0625. Not even the .062, nor a converted metric value to any number of decimals. (1/4 inch 0.250 inch on high-pressure gasket faces).

And even that value is only needed to compress the gasket between two flat - key word being flat!) flanges. The actual dimension you need to disqualify a flange is uneveness (waviness) in the flange face or grooves through the face that allow leaks after compression. Those are the spec's you need to look at on each flange face.

Go back to the book of spec's. Use one system (ANSI is the original, best to use it.) Translate to metric using actual values (25.4 mm/inch) not a book's nominal 6 inch = 150 mm shortcuts. Get the value for the flange face tolerances and check those against a flat surface.

 

[jte]

racookpe1978 got it right. I'd modify "real" to "original" as technically, B16.5 is now a metric standard with US Customary as a secondary. But the point is... This 0.020" discrepancy as posted by the OP Thirlmere is of interest only to a machinist. Nobody actully putting a gasket in between the flanges and bolting it up will care about the height - just that they are flat.

Keep in mind that B16.5 is a new construction spec, not a post construction spec. One analogy would be for car tires: Their tread depth is set to a certain spec at the factory - but a mechanic checking for adequate tread depth a few thousand miles after being placed in service would be a fool to try to argue that the acceptance criteria is the new fabrication spec.

So my answer is one of two:
1) It does not matter. The discrepancy is irrelevant in a practical world. As long as the RF is high enough to avoid the OD of the flange touching when bolted up the RF is high enough. The better question to ask is "Will the flange perform as intended if either dimension is used?"
2) The correct answer depends on the client: If they are a US Customary plant, then the correct height is 0.060". If they are a SI or metric plant, then the correct dimension is 2mm.