Total heat supply rate of the boiler to the steam?
Boilers are rated in horsepower. The pressure increase is about 1 lb for each 2 degrees rise in temperature over 212 degrees. the water input pressure must be above the boiler operating pressure,so a high pressure pump is used to push the water in. The more steam you take out the more water you must put back. If your recovery rate isn't high enough you run out of steam and pressure. I hope this answers your question.
