The maximum operating pressure of any boiler is that stated on the boiler design documentation, and subject to review, downwards, depending upon the condition of the boiler as inspected at intervals by the Engineer/Surveyor responsible for burst indemnity insurance on the vessel. The pressure relief valves should start to lift at that pressure.

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For any fluid that is above its critical point, H_fg is zero and there is no phase change between liquid and gas. Thermodynamic efficiency calculations determine the economics of a boiler design that operates near the critical point of water/steam .

There was a supercritical boiler designed for about 5300 psig (365 bar) back years ago. Find a copy of the B&W Hdbk "Steam", even the old ones are very good. It will help you learn more about boilers than maybe you want to. Such high pressure boilers are again being discussed. Most SC boilers operate at about 3500 psig (241 bar).

So then directly sublimation takes place ? Are supercritical boilers really used in power industries? how are the calculations done? The steam table provides information ony upto 221 bar, right? I'm sorry but i don't know much about this, tried finding online. I'm a rookie in this.Can you please clarify my doubt, sir?

Yes, steam is formed without boiling; direct liquid to vapor formation. During SC boiler statup they go through a conventional boiling cycle as pressure builds up but as they pass the 221 bar pressure the process becomes a contineous phase change without boiling. Read this;

http://www.imteag.com/2-PGA-2004.pdf

page 6-23

These plants are "once through" so require condensate polishing. All BF water is vaporized.

Check this out:

http://www.babcock.com/library/steam.html

You seem interested so this book is a huge help.

That is USC (Ultra super critical boilers at 375bars and 700 deg C

For supercritical usually at about 250 bars and 566 deg C

for Sub-critical, it is at about 160 bars and 538 deg c

It could be anywhere from 15 PSI to 2000 PSI or more for super critical type boilers. but. Boilers in the USA at least are required to never be operated over their max. design pressure or MAWP(maximum allowed working pressure) and must have appropriate Pressure relief valves/system to keep an over pressure situation from occurring. We operate the boilers and try to keep the steam header system at 850 PSI in the mill where I work and the MAWP or design pressure of these 3 boilers is 1175 PSI and the PRV's are set to open at 865 PSI. We also have a pretty large package boiler for back up steam that operates at 150 PSI.

I think the word "boiler" may be an issue since USC and SC boilers are really "steam generators". You are correct in that the "boiling process" can not exist above 221 bar.

By the way, complex cross coupled turbines are required for USC and SC use since the increase in specific volume of steam can not be expanded in one turbine, generally.