Drum Boilers

Drum BoilersIn a drum boiler, the circulation of water is produced through the density difference of water in the downcast pipe and the water/steam mixture in the furnace water-wall. In low-pressure boilers where this density difference is large, the circulating force is high and a high volume of circulation can be ensured, but since it becomes difficult to maintain sufficient circulation volume when the density difference between the two drops due to higher pressure, a pump is installed in the downcast pipe to supplement circulating force. The type that circulates water using only the density difference is known as a natural circulation boiler, while the type that includes an installed pump is known as a forced circulation boiler.hrsg boiler design
Typically, for redundancy, there are three different methods used to measure drum level. In the "Boiler drums/level measurement" example, the bull's eye technology is a direct reading level measurement. The differential pressure transmitter represents the level control measurement, and the probe type sensor is a common method for level alarms and low and high level shutdown. Note the connections in the second illustration are not realistic.
The chamber with the probes is for drum level alarms and boiler trips. The longest probe is the common one. The one above it is low water trip. The one above that is the low water alarm. The short probe can be a high level alarm or a boiler trip. The length of the probes is determined by the boiler manufacturer. My experience is the low water shutdown probe is 1½ to 2½ inches above the water tube boiler connections.
The basic indication of the drum water level is commonly shown in a sight gage glass (bull's eye) connected to the boiler drum. The American Society of Mechanical Engineers requires a direct reading of the drum level. Due to the configuration of the boiler, and the distance the boiler drum is from the operator, a line-of-sight indication may not be practical. The gage glass image can be projected with a periscope arrangement of mirrors. There are a number of methods for drum level measurement. Other methods are a closed circuit television and the use of fiber optics.