Minimum Flange Web Thickness 1

[agauth3]

In the course of maintaining aging process equipment, I often have to remachine gasket faces and at times there isnt enough of the RF left so I have to cut into the flange web. I need to determine how much metal I can machine off the web (boltcircle) of a flange based on the MAWP/MAWT of the vessel it is connected to. Is there a way to calculate the minimum flange thickness?

Thanks

 

[jte]

agauth-

The time has come for you to do some hot work.

Assuming these vessels are ASME VIII Div. 1 with B16.5 flanges, you can refer to Appendix 2 for flange thickness calcs. However, the typical response from folks doing these calc's the first time is "Even a new B16.5 flange fails the calcs." Most B16.5 flanges will not meet the thickness requirement when you go through with the effort to custom design them.

So, what to do... It seems that your best option is to do some weld buildup to bring the flange thickness up to the minimum req'd tf per B16.5. The tf does not include the raised face dimension (recent editions - older ones did include them but with a note that the raised face was included). Add enough rebuild a reasonable raised face. Start your machining process again. As an improvement, I'd suggest following the weld buildup with a weld overlay using a corroison resistant material - sounds as though the process is causing a lot of corrosion or you wouldn't be doing so much work. In essence, the entire raised face can be made from the corrosion resistant material.

ASME PCC-2, Repair of Pressure Equipment and Piping Article 3.5 provides some guidance on repairing flanges. 2006 ed available now, 2008 ed available later this year (I'd guess 3rd quarter).

http://catalog.asme.org/Codes/PrintBook/PCC2_2006_Repair_Pressure.cfm

jt

 

[MJCronin]

I don't understand...

What kind of process equipment requires a repeated machining of flange faces ?

What type of process affects these faces such that a re-machining is necessary ?

-MJC

 

[agauth3]

Thanks for the reply jte.

As for the process that destroys the flange faces...well, my experience is limited to 20-30 year old chemical plants with lots carbon steel equipment handling products that tend to form corrosion as they weep across faces over long periods of time, i.e. bridged corrosion. When we go into a turnaround every couple of years, we break hundreds of pairs of flanges apart and can only afford to replace so many. The rest are approved as is or are resurfaced. Occasionally, I've run into the problem above and have had to take metal off a bolt-circle in lieu of flange replacement when in bind.

 

[unclesyd]

We have five portable flange facing machines for in-situ facing of flanges up to 72" dia.
We have one area of our process were flange facing is almost routine. The process is very corrosive in general but it is exceptionally aggressive in crevices, IE flange faces. This problem is addressed first by having a heavier flange than required by design. This allows us the latitude of removing some metal without compromising the design parameters. This philosophy is used whether it is a Class Flange or a shop fabricated on. If the design calls for a 150 Class flange we will go with a Class 300 knowing it will be faced. If by design we have a flange approaching minimum thickness have the capability to overlay, machine weld, the face and remachine. Having a thicker flange by design gives us a little more leeway in using this approach.

We use this approach also on storage tanks as we make the flange thickness at least twice the thickness of the nozzle.
Here again we have the option to machine the flange face. We also by design have the cover plate thicker where we can flip it over after we spot face the nut landing area in the corroded area.

At one time we routinely machine flanges on 10' dia tubular reactors where we had problems with radial flange cracking that crossed the gasket surface into the process side of the tube sheet. We had to weld up the radial cracks while the reactor was still hot. This repair area would become a high spot and have to be machined. This was possible because the design pressures were 15" water on both sides of the tubesheet.

We also weld repaired and machined 30 foot diameter reactor flanges.

Though not routine we have machined our 650 steam line RTJ flanges were operation have allowed a leak to do a little wire drawing.

Before any machining is stared the OK has to given by the design group after a review and the minimum thickness determined.

Anecdotal:
In the same process area as in the first paragraph I was involved in a sometimes nasty pi..ing contest with our insurance carrier.

We had a 36" diameter SS vessel with 2 36" diameter and 2 24" diameter flanges. All we ever used the flanges for was open up to replace a gasket or repair/machine the flange faces. At the time I propose to use an all welded vessel and cut it apart every "x" number of years to inspect, repair and repack the column. It took several years before they agreed to using one 36" head flange and one 24" flange in the bottom section to effect a packing change out and inspect the column. The only thing we ever did was open the flanges and repair same as the packing had been changed to Ti.
The new Ti column has no flanges as the life expectancy is over 30 years.