In boiler steam drum, actual water level at the drum midpoint is often substantially higher than the indicated gauge level at the drum end, because of the water flow in the drum and steam entrainment of the drum water, and it is necessary to evaluate the actual level height for the reasonable designing of drum and steam separator.
This report describes the results of experiments on the actual water level in relation to the drum internals. The experiments were carried out with two phase flow of air-water mixture in the drum model of 1/2 scale. Also, this paper reports the test results on actual water level measurement by sampling method in boiler drum in operation.
The test results indicate that the difference between the actual drum level at the drum midpoint and the indicated gauge level may be divided into the level rise caused by water flow along the drum axis and the level swelling due to steam (void) entrainment in the drum water, and the total level difference may be estimated by summing up these both kinds of level swelling. The level rise caused by water flow depends mainly on the flow resistance at the plates installed across the drum for the checking of drum water pitching, and the effect may be calculated with the equation for submerged weir or orifice. The level swelling by void entrainment depends chiefly on the air velocity and to some extent on the velocity of circulating water discharged into the drum.
Influence of pressure on void fraction in drum water has been investigated applying the equation by Mitsuyoshi et al. for two phase flow of steam and water mixture in tube. The results indicate that void fraction increase with pressure. This tendency is remarkable when the pressure is higher than 100 atg.
Actual drum level of marine boiler in operation was measured by sampling the drum fluid, and the difference between actual and indicated water level was in excess of 200 mm at 360 m³/m²h of volume rate of steam. These measured values were in satisfactory agreement with calculated values by the procedures shown in this paper.

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