Boiling fluid level measurement

In case of boiling, the density rises and the level drops as the rate of boiling is reduced. Therefore, the measurement of hydrostatic head alone can determine neither the level nor the mass of liquid in the tank, and hence, the volumetric percentage of bubbles must be determined by a separate measurement. For a more detailed discussion of the measurement of the level of boiling fluids, refer to Figure 3.116. 

Many of the problems in operation and control of a given evaporator system will be specific to the application. However, all systems need to answer such questions as how to evaluate performance, how best to schedule periodic boil outs (cleaning), how to measure and control variables typified by temperature, pressure, fluid level, fluid flow rate, composition, etc., and how to detect faults in evaporator operation quickly and efficiently. ... 

Measurement of liquid level is usually noisy, because of splashing and turbulence of fluids entering the vessel. As we have seen, loops that resonate respond to random disturbances by oscillating at their natural period. As a result, level measurements are rarely quiet, often fluctuating 20 or 30 percent of scale. This is particularly true in vessels containing boiling liquids, where turbulence is high. 

Some error in this measurement of NPSH may result due to heat leak down the stainless tube to the vapor bulb. However, it is expected that at low liquid levels the fluid is essentially boiling throughout its full depth and that zero NPSH will be present. In fact, some slightly negative value may be possible at the pump inlet due to possible instantaneous superheat. 

The method selected to determine the effective density was to utilize the equation of fluid statics relating pressure, density, and vertical distance in a fluid. Since it would haye been difficult to determine the surface level accurately, the difference in pressure between stations in the tank was measured while the liquid level dropped, due to boil-off, from above to below the upper station. 

Two-Phase Systems. The combination of low boiling point and low heat of vaporization increases the possibility that cryogenic fluids become boiling, two phase systems. The influence of this fact upon pumping, liquid density, and level determination are obvious. Any sensor adding energy to the system is in fact creating a vapor-liquid interface at the very point of measurement. 

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