Pressure/Other limits for using Slip on Flanges?

[lookintomyeyes]

I am designing a piping system for plate and frame system pressure.

The system includes Type 316 SS pipe in diameters up to 10", and a design pressure of 230 psi.

I once had a reference book that gave limitations for using slip on flanges, but unforunately it's at my former work location. Can anyone provide their knowledge (or experience) instead?

 

[NozzleTwister]

My experience has been that slip-on flanges are normally limited to 500 deg. F. and for normal refinery piping, usually limited to CL150 systems.

Over my career, I've seen a drastic decrease in the use of slip-on flanges. Nowadays, I rarely see them used except for low temperature utilities.

I would be cautious using them with spiral-wound gaskets.

NozzleTwister
Houston, Texas

 

[moltenmetal]

They're commonly used at classes 150 and 300, to the limits of these classes- especially when the piping material is "exotic". I've used titanium piping to 900#/1500# with lap joint flange construction and stub ends of the correct thickness.

Suspect you'll see lap joint flanges becoming more popular again if nickel and molybdenum prices stay as high as they are currently for extended periods.

 

[NozzleTwister]

moltenmetal,

Slip-on and Lap-joint are different.

Slip-on flanges are welded to the pipe on the outside at the hub end and also on the inside at the flange ID. These two welds on a hot pipe and a cooler flange is what causes the problems at the higher temps. Also the flange is normally the same material as the pipe.

Lap-joint or Van Stone as some call them are never welded to the pipe. As you say the lap-joint configuration is used in alloy piping by weldeding an alloy hub to the pipe and using a carbon steel flange for economics.

NozzleTwister
Houston, Texas

 

[moltenmetal]

D'oh! Guess I should have read the OP's post more carefully!

Yes, I'm aware of the difference between slip on and lap joint flanges! Sorry for confusing matters!

Here too, we've seen slip-on flanges used as body flanges on small pressure vessels fab'd from pipe, to 1500# class. But in piping, where moments resulting from thermal expansion etc. have to be contended with, most people prefer to use weld neck flanges above 300# class.

I don't know if there's anything official limiting their use in piping below certain classes or certain temperatures- it seems to be rule of thumb. The only code reference I've seen is toward their use with a single fillet weld, which we never do anyway.

They're handy in piping because they allow a fair bit of angular adjustment during fit-up without a lot of fitting labour. They also conserve expensive metal somewhat relative to weld necks, though not as much as the lap joints I was babbling on about before NozzleTwister corrected me.

NozzleTwister's caveat about using them with spiral-wound gaskets is well taken: if you use a standard spiral wound gasket between these flanges, the inner diameter of the gasket is smaller than that of the flange faces, leaving an exposed lip which can catch in the fluid flow, break the tack weld and cause the gasket to "spool" into the bore of the pipe. Very bad situation, and not solved merely by specifying an inner ring. Special dimension spiral-wound gaskets are required for use with these flanges. The question is: even if you know it, will your customer's pipefitters be smart enough to know this in future?

One thing I've seen which is interesting: some fitters drill a small (1/16" or so) hole on an angle from the back face of the flange to about the midpoint of the air gap between the two fillet welds and the pipe wall. It allows them to complete the second weld without contending with hot air attempting to leave this space. It also acts as a telltale or weep hole to detect any leakage past the first fillet weld during the hydrotest, or subsequently during operation. You can imagine that this gap would be a pretty nasty place for crevice corrosion if it were left to fill with a corrosive process solution.

 

[KernOily]

A lot of owner's specifications forbid the use of slip-ons in anything except low-pressure and/or atmospheric pressure and temperature systems.

One thing to remember is that the B31 codes limit the fatigue life of a slip-on to 30% of that of a weld-neck. That may be the wrong %, but it ain't far off that. Suggest you spend a few minutes with the code book to verify.







Thanks!
Pete

 

[jte]

molten-

Not to hijack the thread, but with regards to the hole you observed in the flange going between the welds... Some outfits drill and tap the hole and hydrotest the welds by pressurizing the void space. This avoids hydrotesting the whole piping run back to a blind point which is occasionally a reasonable field necessity. So, I can understand the desire to call this a hydrotest. However, as an inquisitive engineer, I don't necesserely agree that the stresses induced in the welds during such a "hydro" are the same or substantially similar to those induced by internal pressure in the pipe. Yet it convinces some that the piping has been hydro'd and I've yet to see a failure in such a system. But I wouldn't be surprised if I did someday.

jt

 

[moltenmetal]

jte: sounds fishy. At first blush, that sounds like substituting a vacuum leak test for a pressure test! If I was the owner, I wouldn't accept such a test.