Generating steam at higher pressure facilitates meeting peak steam demands on fluctuating loads.
Most plants have fluctuating steam loads, where a peak load occurs only for a few hours in the day or maybe even in week. When these peak loads occur the boiler needs to deliver the maximum steam in a very short duration. If the boiler is not able to meet this demand in the required time, steam starvation or pressure drop will be seen in the plant. This will has a negative effect on the process.
As with most gases, the volume occupied by a gas decreases directly with the pressure. Thus at a higher pressure steam occupies lesser volume. Thus if steam is generated at pressures lower than the design, the quantity of steam stored in the boiler drum will be lesser than when steam is generated at a higher pressure, closer to design pressure. Thus when peak loads occur, a boiler operating at a higher pressure will be more capable in meeting the demand than when the same boiler is operating at a lower pressure.
While the fuel used in the boiler, the burner provided in the boiler and the characteristics of the boiler themselves have a role to play, when these factors are held constant the boiler pressure is the important factor.
As can be seen from the above illustration, at a higher pressure the boiler drum will hold greater quantity of steam.