Mass Flow Meter Installation

What material is the pipe? Steel, aluminum, plastic, titanium, lead, copper?

What does the piping specification say?

What does your piping engineer say?

Process engineer?

Not to mention the previous questions?

The spec sheet says

What fluid, gas or liquid?

Any way 20 kg/cm2 is not a so high pressure that threaded connection can not be used. Only thing to be ensured that all the fittings and sealants used shall be designed for the pressure, temp and fluid intended.

Your pressure converts to 284 psi and you have 300 psi fittings - that's okay. Now as far as threaded fittings go, when I was a well analyst many years ago we had high pressure "hammer" unions for most fittings, but some still had to be NPT threads. On our control standpipe on 4000 psi gas wells, once properly tightened, we had no leakage problems, and if something is going to leak, it is high pressure natural gas. The gas companies just can't seem to get a nice slow pressure up on a standpipe - a quick hit of high pressure finds leaks really fast. Yes, we had to tighten some joints occasionally, but the threads held when snugged in tight.