Threadolet Pressure Rating (MSS SP-97) 3

[AtticusMech]

Looking to get clarity on something that has bothered me for years. There seems to be a giant misalignment that nobody talks about with threaded weld-on branch fittings (threadolets) per MSS SP-97 vs ASME B16.11's class system.

First, for most threaded fittings covered by ASME B16.11, Table 7 provides the basis for how pressure ratings are determined. Class 2000, 3000, and 6000 fittings have pressure rating matching Sch 80, 160, and XXS pipe respectively. Then Clause 2.1.1 explains that you still need to reduce wall thickness by applicable allowance (e.g. thread depth, corrosion). So a Class 3000 threaded fitting per ASME B16.11 is equivalent to a Sch 160 pipe minus thread allowance and corrosion.

But then MSS SP-97, rather than referencing the ASME B16.11 system makes it's own. Table 1 of this code defines Class 3000, 6000, and 9000 threaded fittings (9000 added in 2019 code) to match Sch 80, 160, and XXS pipe respectively. Again Clause 2.2 and 2.3 mention you need to remove wall thickness allowance for things like threading and corrosion. So a Class 3000 threaded fitting per MSS SP-97 is equivalent to a Sch 80 pipe minus thread allowance and corrosion. A totally different pressure rating.

Each of these codes make sense on their own but the differing uses of Class 3000 and 6000 is excessively confusing and I always get some very confused looks when I try to mention this to others. I have reviewed numerous client's piping specifications and noticed they don't account for this and generically specify Class 3000 fittings.

I'm curious on other people's takes on this? Have you noticed this also? Am I interpreting things incorrectly?

 

[AtticusMech]

Also I should probably clarify for others if needed: Most threaded fittings (elbows, crosses, tees, etc.) are governed by ASME B16.11 and use their system. But weld-on branch fittings (threadolets, weldolets, sockolets) aren't covered by ASME B16.11 but rather a different code MSS SP-97.

 

[1503-44]

See Note #1

Allowable pressure of pipe varies with code, material strength and temperature anyway and corrosion allowances may be different depending on service fluid and design lifetime, so there is no need for fittings to have a pressure rating to match the pressure of whatever pipe they used to get the2000,-9000 ratings.

In gas piping design, pipe can have different design factors depending on location, so a pipe with the same wall, same temperature, same service, same corrosion allowance will have a different pressure allowable simply depending on its location.

There is no standard pressure for any given sch. Neither do Flange Pressire Classes match threaded, or welded fittings pressures, nor any pressure allowable you might calculate for any particular pipe sch, except by pure luck.

Note #1 Rules for piping design

 

[LittleInch]

Never noticed it before but kind of makes sense.

All the B16.11 fittings are essentially joining the same size pipe to the same size fitting.

The branch fittings are joining a smaller pipe to a much bigger pipe. Hence the smaller pipe is more likely to be mechanically stressed more.

Hence the pressure rating of the connecting pipe is going to be lower than the B16.11 fitting.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[david339933]

MSS-SP97 - The minimum wall thickness at the root of the thread at the hand tight plane shall be equal to or greater than the nominal wall of the pipe schedule for the appropriate fitting class.

 

[1503-44]

Yes, you can match thicknesses, but hardly ever the pressure ratings. I'm usually never over 60-75% of the fitting pressure rating.

 

[AtticusMech]

david339933 thanks for the input. I see now Clause 6.3.1 explicitly says that threaded fittings shall still have wall thickness greater than that of the nominal wall of the pipe schedule. That would give justification to using the pipe wall thickness without thread allowance when calculating the allowable pressure. The mention of "threaded allowances" in Clause 2.2 caused my confusion.

Thanks for the help all.