Be careful about operating at reduced pressures. The steam drum may not be adequately sized and may cause excessive water carryover into the steam piping.
Yes, it is possible to determine your energy savings, but you need a lot more info. I assume that you must not know what your fuel input is? If you assume that your Boiler Efficiency doesn't change (it shouldn't change more than a percent or two), then based on the boiler duty at 8.5 bars and 10 bars you can determine the reduction in fuel flow or energy input.
Hope this helps. Post more info if things are still unclear.
In addition to the particulars of operating the boiler at off-design conditions, it is useful to consider the overall "thermal efficiency" of the plant to which the boiler is supplying steam.
If this boiler is for power generation, the overall thermal efficiency is typically referred to as the "heat rate" of the cycle; the amount of "heat" into the cycle per amount of electric power out of the cycle. Operating a steam turbine at lower-than-design inlet pressure is typically not going to improve the turbine efficiency (unless, perhaps this is a plant derate and you would otherwise be throttling at the turbine inlet), but you should review the particulars for your application.
For some sort of process application, the calculation is similar in concept: (energy in)/(product out).
This ultimately translates into $$$ in each case. My point is that there is more to your question than the operation of the boiler.
Spirax Sarco have an FAQ section on their UK website (spiraxsarco.com) in which they talk about this subject for shell type boilers producing saturated steam - as follows: Saturated steam boilers are designed to evaporate dry saturated steam at a rated pressure. Design calculations consider the velocity that steam is released from the surface of the water. Reducing boiler pressure increases this velocity above the design figure, which tends to increase turbulence of the water surface and create wet steam. As a result, the boiler's capacity is reduced. The maximum dry steam output is proportional to the ratios of steam's specific volume between rated and running pressures. For instance, if your boiler is rated at 150 psig (v = 2.76 ft^3/lb) but runs at 100 psig (v = 3.9 ft^3/lb), output is effectively reduced to 2.76 / 3.9 = 0.7 of rated output. In this case, 70% of 5 000 lb/h at 150 psig = 3 500 lb/h at 100 psig. At the lower pressure, steam loads above 3 500 lb/h will cause the boiler to produce wetter steam, potentially reducing the performance of plant.
The question of whether savings can be made by running at the lower pressure will depend on the load on the boiler. For instance, if a steam load near to 5 000 lb/h has to be sustained, then another boiler may have to be brought on line. This would offset any savings made by running at lower pressures. If, however, the maximum steam load can be guaranteed to be near 3 500 lb/h, the boiler will be able to cope and it is possible that fuel savings will occur. These savings are due to a higher heat transfer efficiency and lower radiation losses, but it should be stressed that such savings are likely to be marginal (probably less than 0.5%).
