NPT Temperature Rating

Check the specs, with the manufacturer. If the fittings are accordingly and the piping compatible. 250 Bar is a lot. What is the application?

Thanks for the comment. I have looked at the manufacturers' catalogues. They mostly give the pressure rating at room temperature and provide a derating factor at higher temperatures. But I believe those derating factors are for the pressure retaining strength of the fitting material, but does not necessarily correspond to the temperature applicability of the NPT thread. I mean, for NPT threads to be leak proof, a sealant needs to be used between the threads. I am not aware of any sealant that can be used at up to 600 degC.

I using the NPT fittings to measure the pressure in an autoclave.

If both male and female are NPTF, you should be able to seal without sealant. I did it also with NPT. Just be aware to choose the materials well. It the female expands more, there might a chance of leaks when hot. I would tighten the fittings also again when hot and under pressure. Take extreme caution.

EXPANDO will work. I have used it in the refrigeration and high pressure steam applications with dissimilar metals that react differently to temperature expansion and contraction. Expando produces a type of Ceramic seal to the pipe that will handle extreme temperatures.

What is the size you want to use? You probably will find the perfect fittings with SWAGELOCK or GYROLOCK. There ia quite a range of different materials and models. They also have seals, but I don't think you'll need any. I had applications in a pilot plant that exceeded your pressure and temperature. I connected with 1/4 O.D. stainless 316L. (SS ferrule type fitting) The pipe comes in 2 schedules.

Consider NPTF - where you actually have a metal to metal seal and no sealant is required.

You also need to be concerned about the thermal coefficients of the mating materials. If you get significant differential thermal expansion, it is unlikely that any available sealant will provide a reliable seal. I agree with the suggestion of using NPTF for this application...

There is also another option: use G thread instead where you have a metal gasket...

I believe G thread is in fact a metric straight thread. G thread and NPT are not to be used together as the thread pitch of each are different. They will screw together but only once and the "seal" is not to be relyed on. Just a thought.

G is not metric- it is BSP of course straight.

And obviously they can not be use together with NPT since the pitch in larger sizes and included angle in all sizes differ.

G is almost similar to BSPP, not completely. Of course you'll have to order an adaptor between the NPT male (of the instrument) and the G female socket on the pipe.

A better solution is to order the instrument directly with a G male thread.

You could also use a parallel thread with a metal washer as a seal