Ideally you should aim for at least 1 D engagement, 1.5 D or 2 D would be even better.

These are usual parameters for threaded components subjected to axial loads.

In this case, going into 3 mm thick tube,,from the side, you don not seem to have much option and so must live with just less than 5/8 D. (0.118 / 0.1875) If the threads are axial, then aim for at 1 D engagement minimum.

Since it is a lubricator, in situ, it is unlikely to suffer much stress so it will probably suffice.

Don't overstress things by overtightening the locknut!.

If possible, I would try to avoid making the threads a loose fit.. In any case, a slack fit will allow leakage.

A sealant should be used, since you don't want steam or oil escaping

Howard, .

I am assuming you are screwing a male fitting on the lubricator into the side of a 3mm thick pipe? That beingnthencase you would be better off silver soldering a boss with a female thread onto the pipe.

Paul.

A couple of practical points, as I see it.

One cannot just hang a lubricator on a pipe – better for the lubricator to be suppported and a vibration proof/flexible pipe arrangement to connect the two.

Screwing in anything to a curved surface requires either a tapered thread or a boss (as per PK,) and sealing the connection with a washer (if a parallel thread). A curved surface would only support a locknut at two points.

But do you ever see these lengths of thread on plumbing?

Typical model nut and nipple probably less than 0.5D. Domestic compression fitting maybe only 0.25D and they are good for 7bar at 120degC at 15 & 22mm