On the 28th of March, 1979, thousands of people fled from Three Mile Island (near Harrisburg, PA) when the cooling system of a nuclear reactor failed. This dangerous situation

Figure 6-1: Click on figure to enlarge.

developed because the level controls turned off the coolant flow to the reactor when they detected the presence of cooling water near the top of the tank. Unfortunately, the water reached the top of the reactor vessel not because there was too much water in the tank, but because there was so little that it boiled and swelled to the top. From this example, we can see that level measurement is more complex than simply the determination of the presence or absence of a fluid at a particular elevation.

Level Sensor Selection

When determining what type of level sensor should be used for a given application, there are a series of questions that must be answered:

Can the level sensor be inserted into the tank or should it be completely external?

Should the sensor detect the level continuously or will a point sensor be adequate?

Can the sensor come in contact with the process fluid or must it be located in the vapor space?

Is direct measurement of the level needed or is indirect detection of hydrostatic head (which responds to changes in both level and density) acceptable?

Is tank depressurization or process shut-down acceptable when sensor removal or maintenance is required?

By evaluating the above choices, one will substantially shorten the list of sensors to consider. The selection is further narrowed by considering only those designs that can be provided in the required materials of construction and can function at the required accuracy, operating temperature, etc. (Table 4). When the level to be measured is a solid, slurry, foam, or the interface between two liquid layers, it is advisable to consult not only Table 4, but other recommendations, such as Table 5.

If it is found that a number of level detector designs can satisfy the requirements of the application, one should also consider the traditions or preferences of the particular plant or the particular process industry, because of user familiarity and the availability of spare parts. For example, the oil industry generally prefers displacement-type level sensors, while the chemical industry favors differential pressure (d/p) cells. (The petroleum industry will use d/p cells when the span exceeds 60-80 in.)

If the tank is agitated, there is often no space in which to insert probe-type sensors. Plus, because the liquid surface is not flat, sonic, ultrasonic, or radar devices typically cannot be used, either. Even with displacer or d/p sensors, agitation can cause the level signal to cycle. These pulses can be filtered out by first determining the maximum rate at which the level can change (due to filling or discharging) and disregarding any change that occurs faster than that.

Figure 6-2: Click on figure to enlarge.

The relationship between level and tank volume is a function of the cross-sectional shape of the tank. With vertical tanks, this relationship is linear, while with horizontal or spherical vessels, it is a non-linear relationship (Figure 6-1).

Table 4: Click on figure to enlarge.

If the level in a tank is to be inferred using hydrostatic pressure measurement, it is necessary to use multi-transmitter systems when it is desirable to:

Detect the true level, while either the process temperature or density varies;

Measure both level and density; and

Measure the volume and the mass (weight) in the tank.

By measuring one temperature and three pressures, the system shown in Figure 6-2 is capable of simultaneously measuring volume (level), mass (weight), and density, all with an accuracy of 0.3% of full span.

Boiling & Cryogenic Fluids

Figure 6-3: Click on figure to enlarge.

When a d/p cell is used to measure the level in a steam drum, a reverse-acting transmitter is usually installed (Figure 6-3). An uninsulated condensing chamber is used to connect the high pressure (HP) side of the d/p cell to the vapor space on the top of the drum. The steam condenses in this chamber and fills the wet leg with ambient temperature water, while the low pressure (LP) side of the d/p cell detects the hydrostatic head of the boiling water inside the drum. The output of the d/p cell reflects the amount of water in the drum. Output rises as the mass of water in the drum drops (because the steaming rate and the associated swelling increase). It is for this reason that a reverse acting d/p cell is recommended for this application.

Table 5: Click on figure to enlarge.

When the process fluid is liquid nitrogen (or some other cryogenic material), the tank is usually surrounded by a thermally insulated and evacuated cold box. Here, the low pressure (LP) side of a direct acting d/p cell is connected to the vapor space above the cryogenic liquid (Figure 6-4). As the liquid nitrogen approaches the HP side of the d/p cell (which is at ambient temperature outside the cold box), its temperature rises. When the temperature reaches the boiling point of nitrogen, it will boil and,

Figure 6-4: Click on figure to enlarge.

from that point on, the connecting line will be filled with nitrogen vapor. This can cause noise in the level measurement. To protect against this, the liquid filled portion of the connecting line should be sloped back towards the tank. The cross-section of the line should be large (about 1 inch in diameter) to minimize the turbulence caused by the simultaneous boiling and re-condensing occurring at the liquid-vapor interface.

Sludge, Foam, & Molten Metals

Many process fluids are aggressive or difficult to handle and it's best to

Figure 6-5: Click on figure to enlarge.

avoid physical contact with them. This can be accomplished by placing the level sensor outside the tank (weighing, radiation) or locating the sensor in the vapor space (ultrasonic, radar, microwave) above the process fluid. When these options are not available or acceptable, one must aim to minimize maintenance and physical contact with the process fluid.

When the process fluid is a sludge, slurry, or a highly viscous polymer, and the goal is to detect the level at one point, the design shown in Figure 6-5A is commonly considered. The ultrasonic or optical signal source and receiver typically are separated by more than six inches so that the process fluid drains freely from the intervening space. After a high-level episode, an automatic washing spray is activated.

When the sludge or slurry level is detected continuously, one of the goals is to eliminate dead-ended cavities where the sludge might settle. In addition, all surfaces which are exposed to the process fluid should be covered with Teflon®. Figure 6-5B shows such an installation, employing Teflon®-coated extended diaphragms to minimize material buildup.

In strippers, where the goal is to drive off the solvent in the shortest period of time, one aims to keep the foam level below a maximum. In other processes, it is desirable to separately control both the liquid level beneath the foam and the thickness of the foam. In the paper industry, beta radiation detectors are used for such applications (Kraft processing), while other industries detect the degree of foaming indirectly (by measuring related variables, such as heat input or vapor flow), or they use capacitance, conductivity, tuning fork, optical, or thermal switches, all provided with automatic washers.

Measuring the level of molten glass or metals is another special application. The most expensive (but also most accurate) technique available is proximity capacitance-based level measurement, which can provide a resolution of 0.1 mm over a range of 6 in. Laser-based systems can provide even better resolution from distances up to 2 ft. If such high resolution is not required and cost is a concern, one can make a float out of refractory material and attach a linear variable differential transformer (LVDT), or make a bubbler tube out of refractory material and bubble argon or nitrogen through it.
References & Further Reading
OMEGA Complete Flow and Level Measurement Handbook and Encyclopedia®, OMEGA Press, 1995.
OMEGA Volume 29 Handbook & Encyclopedia, Purchasing Agents Edition, OMEGA Press, 1995.
Instrument Engineer's Handbook, Bela G. Liptak, editor, CRC Press, 1995.
Instrumentation for Process Measurement and Control, Third Edition, N. A. Anderson, Chilton, 1980.
Measurement and Control of Liquid Level, C. H. Cho, Instrument Society of America, 1982.
Principles of Industrial Measurement for Control Applications, E. Smith, Instrument Society of America, 1984.