Level-trols

This discrepancy between the apparent level, in the gauge glass, and the actual level (see Fig. 6.1), in the tower, also occurs in any other type of level-measuring device. This includes external float chambers, kidneys, displacement chambers, and level-trols. The one exception to this is level-measuring devices using radiation techniques. 

Ambient-heat loss from the external gauge glass or level-trol... 

The gauge glass will normally be somewhat colder than the process vessel as a result of ambient-heat losses (an exception to this would be a refrigerated process). For every 100°F decrease in the gauge-glass temperature or level-trol temperature, the specific gravity of the liquid in the glass increases by 5%. This rule of thumb is typical for hydrocarbons only. Aqueous (water-based) fluids are totally different. 

The same sort of problem arises in a level-trol, which measures and transmits a process vessel liquid level to the control center. As shown in Fig. 6.3, the level-trol operates by means of two pressure transduc-... 

Level-trol This is the most common device used to measure levels in vessels. It works by measuring pressure differences between two level taps at different elevations. 

Bridle Same as level-trol or stand-pipe. Used to measure liquid levels in a process vessel. 

The same sort of problem arises in a level-trol, which measures and transmits a process vessel liquid level to the control center. As shown in Fig. 10.3, the level-trol operates by means of two pressure transducers, devices for converting a pressure signal into a small electric current. The difference between the two pressure transducers shown in Fig. 10.3 is called the milliamp (milliampere) output. This milliamp output is proportional to the pressure difference between the bottom and top taps in the level-trol. To convert the milliamp output signal from the level-trol into a level indication, the instrument technician must assume a specific gravity ... 

But which specific gravity should the instrument technician select The specific gravity of the liquid in the level-trol, or the lower specific gravity of the liquid in the hotter process vessel The technician should use the specific gravity in the process vessel and ignore... 

After all, the external level indication chamber (level-trol) holds just water. When we look at the gauge glass, it has water, not steam. Hence, the indicated water level outside the drum is less than the boiling water level inside the drum. This discrepancy causes water carry-over with the steam due to high boiling water levels. 

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