Tuesday, January 23, 2018

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Level Gauges (Sightglasses)

The level gauge, or sightglass is to liquid level measurement as manometers are to pressure measurement: a very simple and effective 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:

 

a simple example of a sightglass

 

A functional diagram of a sightglass shows how it visually represents the level of liquid inside a vessel such as a storage tank:

 

a functional diagram of a sightglass showing how it visually represent the level of liquid inside a vessel

 

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.

As simple and apparently trouble-free as level gauges may seem, there are special circumstances where they will register incorrectly. One such circumstance is in the presence of a lighter liquid layer existing between the connection ports of the gauge. If a lighter (less dense) liquid exists above a heavier (denser) liquid in the process vessel, the level gauge may not show the proper interface, if at all:

 

if a less dense liquid exists above a denser liquid in the process vessel, the level gauge may not show the proper interface.

Here we see how a column of water in the sightglass shows less (total) level than the combination of water and oil inside the process vessel. Since the oil lies between the two level gauge ports into the vessel (sometimes called nozzles), it cannot enter the sightglass tube, and therefore the level gauge will continue to show just water.

If by chance some oil does find its way into the sightglass tube – either by the interface level dropping below the lower nozzle or the total level rising above the upper nozzle – the oil/water interface shown inside the level gauge may not continue to reflect the true interface inside the vessel once the interface and total levels return to their previous positions:


f by chance some oil does find its way into the sightglass tube – either by the interface level dropping below the lower nozzle or the total level rising above the upper nozzle – the oil/water interface shown inside the level gauge may not continue to reflect the true interface inside the vessel once the interface and total levels return to their previous positions

Recall that the level gauge and vessel together form a U-tube manometer. So long as the pressures from each liquid column are the same, the columns balance each other. The problem is, many different liquid-liquid interface columns can have the same hydrostatic pressure without being identical to one another:


many different liquid-liquid interface columns can have the same hydrostatic pressure without being identical to one another

The only way to ensure proper two-part liquid interface level indication in a sightglass is to keep both ports (nozzles) submerged:


The only way to ensure proper two-part liquid interface level indication in a sightglass is to keep both ports (nozzles) submerged

Another troublesome scenario for level gauges is when the liquid inside the vessel is substantially hotter than the liquid in the gauge, causing the densities to be different. This is commonly seen on boiler level gauges, where the water inside the sightglass cools off substantially from its former temperature inside the boiler drum:


troublesome scenario for level gauges is when the liquid inside the vessel is substantially hotter than the liquid in the gauge, causing the densities to be different
 
Looking at the sightglass as a U-tube manometer again, we see that unequal-height liquid columns may indeed balance each other’s hydrostatic pressures if the two columns are comprised of liquids with different densities. The weight density of water is 62.4 lb/ft3 at standard temperature, but may be as low as only 36 lb/ft3 at temperatures common for power generation boilers.
Comments (2)Add Comment
0
Instrumentation
written by Grant, November 29, 2016
I have been looking for this sort of illistration for some time so thank you for posting it.
In all of the examples above, there are two ports for the sight glass (top and bottom) I have a vessel that someone has hooked up a 3rd port in an attempt to see the interface level in the sight glass. So the top port is gas, the middle port is oil and the bottom port is water.
Will this work? How does having the top (gas) port open to the sight glass affect my reading? or does it?

Any help would be greatly appriciated.
Thanks
Grant
0
sales
written by Duncan Phiri, March 17, 2017
Hello sales



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