Brief Understanding of the High Pressure Seal
When you come across the term high pressure, you should rest assured that it has been deemed a relative term that has been based on the perspective of the user. It would be pertinent to mention here that high pressure would be best defined as low as 200 and as high as 60k psi. Let us refer to the high pressure as any pressure outside the maximum sealing capacity of the 60k psi fittings or higher as 20k psi.
In case, the NPT has been unable to contain higher pressures, a better and stronger sealing fitting that has been specifically designed for high pressure would be needed.