Merchant couplings are smooth on the outside so the user knows they are for thread protection. I worked for an idiot several years ago that pissed an inspector off so bad, he red tagged it and refused to sign off on the job until every stick of pipe was torn out, disposed of and replaced.
The larger diameter merchant couplings leaked, so the crackhead on the job fixed them with JB Weld. Apparently the same inspector was on the job inspecting the fire system and the crackhead did not know it. The inspector watched him fix the leaks with JB Weld and waited until final plumbing inspection to nail him.
The Builder made him hire a gas company and pay them to work around the clock until it was fixed and the store could open on time. I am just glad I wasn't on that job. All to save a couple of dollars on fittings.
Goto BH to know more.
Just a note about "Thread Protector Couplings" that some may not know. Of course their made of steel and are not malleable iron or cast iron made of the same material as the pipe
1/8" -2" these couplings are tapped with straight threads and are not to be used in a piping system although we have all seen them used (with a lot of Teflon and dope) Their leakers especially if you have any pressure. They make in too far.
2 1/2" and up the couplings come tapped with tapered threads the same as the pipe and are perfectly fine to use.
Why the couplings are straight thread 2" and under and tapered 2 1/2" and up I have no clue.
But you can look it up on pipe specification sheets.
Yes, I have had several bouts with gas inspectors who have questioned their use on larger gas line 2 1/2 and up.
haven't lost one yet
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