Drum level settings are set by the boiler manufacturer. If you run at 20%, assuming this is above the low level alarm and trip levels, you will be more likely to get the low level alarms and trips. The alarm and trip settings are set to provide sufficient time to trip the unit without running the drum dry, which would likely result in damage to the evaporator and maybe any superheater and reheater.
If the reason for doing this is that you are getting high level problems, maybe you should consider correcting that problem.
In addition to what Otised indicated, it is to be borne in mind that the boiler drum has all sorts of tube connections going to it. The main area of concern are the tubes going to the evaporator section, where actually the water is boiled and steam generated. To ensure proper operation, the evaporator tubes' connection to the drum needs to be submerged in water. This is especially true in natural circulation boilers. Given the quite large number of such tubes, these normally occupy nearly all the bottom half of the drum. Thus the water level has to be above these tubes, so the normal 50% operating level. Forcing operation with a low drum level risks burning up some of the evaporator tubes due to insufficient or even no water supply to them from the drum.
If you ever lose the feed water pumps you'll see how fast people can move....the set point level is set by the manufacturer
too high and you get condensate carry-over...too low and on loss of FW supply or high steam demand you can expose the tubes. Exposed tubes fail instantly...with severe possibilities
You have redundant level indications and interlocks to alarm and shut down the boiler outside the safe zone.
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