Carbon steel 900# Orifice RTJ flange coating/paint 3

[dmertz]

A carbon steel 18"SCH40 900# orifice flange that we procured arrived coated with black paint. I am concerned that the coating will interfere with the RTJ groove on the flange. Is this a problem? Also, the inside of the flange will be exposed in the pipeline. Should I be concerned about flaking of the paint downstream in the pipeline?

 

[SNORGY]

What kind of coating is it?

It might be something similar to Impreglon (which would probably be red in colour), in which case I'd leave it alone. Other than that, your concerns are probably valid.

Regards,

SNORGY.

 

[dmertz]

It appears to be just black flange paint which was rather sloppily applied to the flange. Looks like they dipped it quickly and thats it. I can get the spec on the paint but I doubt it is anything special.

 

[dmertz]

Another thought: The black paint does protect the carbon from some corrosion. If we did need to strip this off, would it be better to instruct the site to do this on prep of the flanges for installation? We could do it at the factory but that would leave the groove vulnerable to corrosion.

 

[dcasto]

forget the paint, an 18" sch40 will not match an ansi 900# flange for pressure

 

[rneill]

Concur with dcasto.

Just to be on the safe side, and assuming that you might not need the full flange rating, I checked the 18" Sch 40 material against a Class 600 pressure rating and it does not meet that either.

Assuming typical A106-B pipe with A105 flanges, 18" Sch 40 material has a rating of about 7700 kPa (1115 psi) even with zero corrosion allowance (per B31.3).

 

[dmertz]

Rneil/Dcasto--great catch. I checked the spec and the flange is actually 18"SCH80. The calcs per B31.3 indicate that SCH80 satisfies CL900. If this isn't right, please let me know. Thanks.

 

[eadwine]

RTJ's require a metal to metal contact on the flg. groove and ring for proper sealing. Before assembly, clean the "paint" out of the groove and do not gouge or abrade that surface while cleaning (some people like using screw drivers or chisels). Also, check the ring to make sure there are no gouges in it and that it is round, not oval (some people also like to drop these things and try to beat them back into "round").

As for the Sch. number, oops! These numbers mean something, so be careful.

 

[rneill]

Not a lot of salvation in that either. Again, assuming regular A106-B / A105 material, I calculate that 18" Sch 80 material is only rated to about 13,050 kPa (1893 psi) again with zero corrosion allowance (per B31.3).

Class 900 flanges would typically have a rating of 15,320 kPa (2220 psi)

Depending on the actual required design pressure, and the required corrosion allowance, the Sch 80 material may or may not be acceptable.

If you could let us know the following, I could tell you what schedule would be required:
- Design Pressure
- Corrosion Allowance
- Material (e.g., A105)
- Design Code in use (e.g., B31.3)

However, if you want full flange rating with a typical zero corrosion allowance, with A105 material then you would need a min of Sch 100, if you wanted a typical 1/16" corrosion allowance then you would need a minimum of Sch 120 (all per B31.3).

 

[dmertz]

-Design pressure = 1885 psi
-Corrosion allowance = None
-Material = A105/A106
-Design code = B31.3

So it appears that SCH80 is technically acceptable. However, the SCH80 per B31.3 does not match the rating of the CL900 flanges per B16.5. Very enlightening--thanks for all the input!

I noticed on B16.5 that the flange pressure rating for CL900 drops with increasing temperature. Is there a similar temperature de-rating that applies for B31.3 piping? My process temperature range is for 60-75C. This particular flange is for use in an anti-surge system for a pipeline compressor.

 

[eadwine]

If this is a Raised Face Weld Neck Flg., which I assume it is, Sch. rating exists to match the Sch. rating of the pipe. So that the ID of the pipe bore and Flg. match. In the fabrication of pipe, within a nominal size, e.g., 18", the OD remains constant and the ID decreases with increasing Sch. no. and or wt. class. This is due to the increasing wall thk. which establishes the Sch. no. or wt. class.

Now for the P/T ratings. The calc's are based on a formula that results in a decrease in pressure with an increase in T or a decrease in wall thk. (smaller Sch. no.) or a change in material (due differences in the ultimate and tesile strengths of the various materials themselves). But this is all covered under B31.1 or B31.3 and is well established.

 

[rneill]

There is no requirement that the pipe be as strong as the flange. In this case, you require 1885 psig and a Class 600 flange does not have this rating so you have gone to a Class 900 flange which is correct.

The next step is calculate the required wall thickness and an 18" Sch 80 section of pipe (at less than 100 F) calculates as good to 1893 psig with zero corrosion allowance.

Consequently, you order an 18" Sch 80 bore Class 900 flange to match the pipe and everything meets code just fine.