How high can steam pressure achieve?

Steam pressure can be raised as high as the rating of the steam generator. Some super-critical steam generators actually operate at over 15,000 psi. Critical pressure/temperature for steam is 3206 psia, and 705^F. This is the point at which steam and water are indistinguishable from one another. All critical and super-critical steam generators require forced flow, (the water/steam is pumped through the generator). The temperature or the pressure can be raised above critical values, with the use of specialized equipment. Generally speaking, non-forced flow boilers can only achieve about 1500 psia, as the density of the steam approaches the density of the water, preventing natural circulation.