Chapter 20
Continuous level measurement
Many industrial processes require the accurate measurement of fluid or solid (powder, granule, etc.) height within a vessel. Some process vessels hold a stratified combination of fluids, naturally separated into di erent layers by virtue of di ering densities, where the height of the interface point between liquid layers is of interest.
A wide variety of technologies exist to measure the level of substances in a vessel, each exploiting a di erent principle of physics. This chapter explores the major level-measurement technologies in current use.
20.1Level gauges (sightglasses)
Level gauges are perhaps the simplest indicating instrument for liquid level in a vessel. They are often found in industrial level-measurement applications, even when another level-measuring instrument is present, to serve as a direct indicator for an operator to monitor in case there is doubt about the accuracy of the other instrument.
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CHAPTER 20. CONTINUOUS LEVEL MEASUREMENT |
20.1.1Basic concepts of sightglasses
The level gauge, or sightglass is to liquid level measurement as manometers are to pressure measurement: a very simple and e ective technology for direct visual indication of process level. In its simplest form, a level gauge is nothing more than a clear tube through which process liquid may be seen. The following photograph shows a simple example of a sightglass:
20.1. LEVEL GAUGES (SIGHTGLASSES) |
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A functional diagram of a sightglass shows how it visually represents the level of liquid inside a vessel such as a storage tank:
gauge valve
Level gauge
(glass tube)
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Liquid column |
Liquid column |
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Process liquid
gauge valve
A level gauge is not unlike a U-tube manometer, with equal pressures applied to both liquid columns (one column being the liquid in the gauge sightglass, the other column being the liquid in the vessel).
Level gauge valves exist to allow replacement of the glass tube without emptying or depressurizing the process vessel. These valves are usually equipped with flow-limiting devices in the event of a tube rupture, so too much process fluid does not escape even when the valves are fully open.
Some level gauges called reflex gauges are equipped with special optics to facilitate the viewing of clear liquids, which is problematic for simple glass-tube sightglasses.
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A weakness of glass-tube level gauges is the glass tube itself. The tube must be kept in a clean condition in order for the liquid level to be clearly visible, which may be a problem in a dirty-liquid service. Also, glass tubes may rupture if subjected to thermal or mechanical shock. One solution to this problem is to eliminate the glass tube entirely, replacing it with a non-magnetic metal tube (e.g. stainless steel) containing a magnetized float, with magnet-sensing indicator flags outside of this tube to visually indicate level. Here is one example of such a level gauge, manufactured by MagTech:
In this instrument, you can see red-colored flags toward the bottom of the scale which have been “flipped” by the motion of the magnetic float inside the stainless-steel tube. The height of the red zone (i.e. how many flags have been flipped to show their red sides) indicates the height of the liquid inside the tube.
Some magnetic level gauges even have highand low-level magnetic switches located at strategic points along the tube’s height, providing discrete sensing capability for alarms and/or shutdown controls, if the liquid level ever goes outside of safe operating limits. These switches will open and close as the magnetic float passes by, remotely signaling liquid level at that height.