"fishmouth" in hot / chilled water piping

[hvacSLOG]

Distinguished Technical Experts--

Have you come across the practice of welding a "fishmouth" instead of using a "tee" for branch piping?

I don't like a "bastard fitting" in ductwork...but should I have any objections for hydronic piping system? in terms of pressure drop or other reason to not allow? I want to go faster/more economical but not if this is bad practice. Piping is schedule 40 steel with mechanical couplings - victaulic. chilled water main starts at 8" with 3" takeoffs. HW main starts at 4" with 2" branches. Pressure no more than 75 PSIG.

We are on a job and my fitter doesn't want to use the victaulic Tee he would rather cut and weld - says it will go faster. And those Victaulic Tees are expensive...especially the reducing T.

Thanks in advance...

 

[MJCronin]

I believe that you are referring to "unreinforced fabricated tees" (Thats what ASME B31.1 and B31.3 calls them)

Will your fitter stand behind the system if there is a little vibration and cracking/leaks occur ??

....or will he be long gone ???

Some things to note about these "unreinforced tees"

- They are the weakest type regarding pressure

- A lot depends on the skill of the welder

- They are almost impossible to inspect the welds

Most fabrication shops use weldolets, which are stronger and guarantee proper joint fit-up and easy welding.



MJCronin
Sr. Process Engineer

 

[LittleInch]

Picture?

Drawing?

Design code?

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

 

[LittleInch]

Tees have a large stress concentration factor and unreinforced tees even larger than fabricated ones.

75 psig is no laughing matter if the tee decides to break - if you'd said 5 I might not be so worried.

You don't allow it because your design code doesn't allow it or your piping specification doesn't allow it. Or try asking your fitter if he's done a stress analysis on the tee and watch his face.

what materials and thicknesses are we talking here?

As said above- a direct pipe to pipe connection relies on the fillet weld at the connection point. It is very difficult to establish the thickness, strength and QA of that joint.

If fishmouth refers to the profiling on the branch pipe then at least there is less pressure drop compared to just stabbing the branch pipe into the header pipe, but it is a sharp edge / turn compared to a tee.

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

 

[hvacSLOG]

Thank you for your responses. I appreciate the insight and the code references - will review.


Thanks