Locating the Level

Nwaoha Chikezie provides some advice on improving level measurement accuracy through proper technology selection.

Level measurement of a substance, whether liquid, gas, slurry, granular materials, etc, is a very critical aspect of process instrumentation. A level measurement system forms a very important part of a vessel overfill prevention system, being the tool that plant personnel make use of 24/7 to ensure that vessels are operating within designed safe working levels, to identify any abnormal events, and to ensure that product is loaded and unloaded safely. And of course, in these applications, very accurate measurement is required in other to determine the actual level of substance in a given container.

Level measurement can be either continuous or point type. Continuous level sensors measure level within a specified range and also determine the exact amount of substance in the container, while point level sensors indicate whether the measured substance is above or below the required sensing point or value.

In any good process operation, level measurement throughout the process must be regulated near the required values with minimal variability, and as detailed below, different types of level measurement devices using different technologies are available for the wide variety of applications that exist.

Bubbling up

With no moving parts, the pneumatically based air bubbler system is particularly suitable for measuring the level of sewage sludge and water with large quantities of suspended solids. It is also highly recommended for liquid level measurements where float and ultrasonic measurement techniques prove to be troublesome.

An air bubbler meter consists of a pipe, an air supply, a differential pressure regulator and transmitter. The regulator produces the air flow needed to prevent changes in calibration. A flow of air passes through the tube and produces a resultant air pressure in the tube which corresponds to the hydraulic head of the liquid in the vessel.

In cold climates where there is a risk of frost, the instrument air lines should be heat traced. Practically, the only part of the air bubbler that contacts the liquid is a bubble tube that is chemically compactable with the material that is to be measured. The calibration of the air bubbler system should be done at maximum temperature to avoid overfills, and its accuracy is dependent on a stable air supply controlled by a regulator.

This level measurement technique has the advantage of low purchase cost and simplicity in design. It is also somewhat self cleaning since it can be manufactured so that high pressure air is supplied via a bypass valve which will displace solids that might clog the bubble tube. And it is also a good choice for hazardous areas because the point of measurement has no electrical parts.

Its main disadvantages: is non suitability for use in non-vented vessels; calibration is highly affected by changes in the density of the product; and requires periodical cleaning because the hole at the tip of the pipe can be plugged by materials from the process liquid measured.

Displacement detector

A displacer level sensor is most applicable to measurement of relatively non-viscous and clean fluids. The displacer technique works on the principle of Archimedes, which states that when a body is partially or totally immersed in a fluid it experiences an upthrust which is equal to the weight of the fluid displaced. In this case, the level can be deduced by detecting the weight of a immersed displacer since, when the density of the liquid as well as the cross sectional area of the displacer is constant, a unit change in the level will give rise to a reproducible unit change in the weight of the displacer.

The technique has advantages of high accuracy and adaptability to wide variations in fluid densities. If repeatability is required, whenever a displacer level sensor is lined up for service, the process material to be measured must maintain its density. A disadvantage is that if the torque tube fails, it provides a path for leakage. And if the process fluid measured contains deposited contaminants, the effective displacement and mass of the displacer will change thereby causing a calibration shift.

Under pressure

The differential pressure (or DP as it is usually referred to) level meter is the most frequently used technique for the measurement of level. Based on the height of material in the vessel, a head pressure value is sensed by the diaphragm and then multiplied by the density.

To maintain accuracy with this type of measurement technique, the fluid density must be stable. If a change in fluid density is likely, a second DP level meter should be installed to measure the parameter and compensate for any changes.

Although, the DP level meter is less expensive than other level measuring techniques, there is usually additional hardware and labor required for its installation. The advantages are that it can be installed externally or retrofitted to an existing vessel. It can also be safely isolated from the process using block valves for testing and maintenance. For measurements where a total level in a separator vessel is needed, and which involve a wide range of variations in material composition of the upper phase, the DP level meter is the most viable option.

Its disadvantage is that the DP level meter is prone to errors due to changes in the density of the fluid to be measured. This density variation can be caused by product change or by changes in the operating temperatures. For accurate measurement, these variations must be compensated.

DP level meters are usually used for clean fluids. They should not be use for liquids that solidify due to increase in concentration. If so, the meter diaphragm will be coated and may require recalibration, which can be impractical and at same time add to the cost of ownership.

Float on

A float level sensor can operate well in a wide variety of liquids, including corrosives. But before it is applied, operators must verify the chemical compatibility of the process fluid to be, measured with the materials used to construct the sensor.

While a float level sensor is similar to a displacer level sensor in appearance, the float level measurement technique operates on the principle of buoyancy, which stipulates that as the fluid level in the vessel changes position, the float level sensor will also, providing it is less dense than the fluid.

The application of a float on the surface of the process fluid is a reliable and easy-to- install method of accurately measuring level. This measurement technique is optimized as long as there is nothing in the process fluid that could interfere with its free movement.

The float has a shape of a doughnut and it rides on the outside of a waveguide (tubular). It has a permanent magnet attached to it, which causes a disruption in an electrical pulse sent down the waveguide. The equipment then measures the point of the disruption with a remarkable repeatability.

Float level measurement has an advantage of having float switches with glandless design, and are capable of fail safe operation in the case of extreme process conditions. When a tank is equipped with two floats, a single transmitter can be used to measure the products level and interface level. It has a disadvantage of being an inherently contact measurement technology. For ease of installation and operation, end users must be given the necessary awareness.

Capacitance for coating

The RF capacitance level measurement technique – so called due to the radio frequency signals applied to a capacitance circuit – is most applicable for level measurements in coating applications, with the sensor designed to sense materials with a dielectric constant as low as one and as high as 80 or more.

An RF capacitance level transmitter has two important circuit additions: chopper drive and oscillator buffer circuits to accommodate separate measurement of capacitance and resistance. Since the capacitance and resistance of any coating are of equal magnitude, any error as a result of the coating can be measured and subtracted from the total output. Its resultant measurement is of high accuracy regardless of the amount of coating on the probe. In the case of insulating materials with dielectric constant changes, the measurement can only be made reliable if the material being measured is homogeneous.

This technique has the advantage of being the most versatile technology for continuous level measurement, able to handle a vast range of process conditions from vacuum to pressure of approximately 10,000 psi, and from cryogenics to 800 deg C. It is not subject to jam, wear or plug, because it has no moving parts. And for safety, it is usually installed at the top of the tank above the actual process level and with a single tank penetration.

However, RF capacitance has a disadvantage of being intrusive. And in the case of measuring insulating granular materials, special considerations such as location of the sensing element, and the moisture range are required in order to minimize errors caused by movement of the probe. Finally, for optimum accuracy, system selection and performance, a good knowledge of the electrical characteristic of the process material to be measured is needed.

Sound approach

The ultrasonic level measurement technique works on the principle of emitting high frequency (20 KHz to 200 KHz) sound waves from a transducer to the contents of the vessel to be measured. The ultrasonic level meter (sometimes called sonic level meter) measures the length of time taken for the reflected sound waves to be detected by the emitting transducer. A successful measurement depends on the reflection from the process material in a straight line back to the transducer.

Since the speed of sound in air changes with moisture level and temperature, ultrasonic level meters are naturally going to be affected by changes in moisture levels and variations in temperature and pressure inside the vessel. However, if used together with temperature and humidity sensors, correction factors can be applied. Accurate measurement also depends on the level meter being mounted in the correct position so that the vessel’s internal structures will not interfere with the sound waves reflecting back to the sensor.

In order to minimize measurement obstructions in the vessel, tank mapping should be used. This allows an operator to take a “sonic snapshot” of an empty vessel such that when the transducer emits a sound burst, the echo is recorded as a signature of the vessel. Any obstructions in the vessel will send an echo and create a profile, which is stored inside the memory of the meter so that it will not respond to echoes created by these obstructions.

The main advantages of an ultrasonic meter is that the transducer does not come into contact with the process material measured; it does not have moving parts; and a single top-of-the-vessel entry makes leaks less probable than a fully wetted technique.

Ultrasonic measurement is ideal for highly viscous liquids such as heavy oil, greases, and slurries, as well as bulk solids like sands, grain, cement, etc. The main disadvantage is that measurement accuracy is affected by powders, heavy vapors, foam, surface turbulence, and even ambient noise, which can affect the returning signal. And in general, ultrasonic level meters will not operate in high pressure and vacuum applications.

Making waves

The radar level measurement technique is ideal for use in dusty, moist and vaporous environments as well as in applications where there are temperature changes. Radar level sensors can be configured for non-contact applications, or with a waveguide that dips into the vessel content. The non-contact configuration is mostly used, but the waveguided configuration is suitable for applications where the liquid to be measured has a very low dielectric constant, and does not reflect the microwave signal well.

Radar level measurement can also be classified into two categories: frequency modulated continuous wave (FMCW), and pulsed wave time of flight (PWTF). The frequency modulated continuous wave continuously releases a signal, such that the distance is deduced from the difference in frequency between the transmit and receive signals at any point in time. In the case of tank gauging, the FMCW system is the method that delivers high accuracy measurement.

The pulsed wave time of flight, when applied, releases a microwave burst towards the material to be measured. This burst is reflected by the surface of the material and detected by the same sensor which also acts as the receiver. In this case, the level is deduced from the time of flight of the signal (microwave).

The radar technique has an advantage of non-contact application with high measurement accuracy, which makes it suitable for use in some process vessels like reactors and storage tanks. It is also a viable option for continuous level measurement. One major disadvantage is the high cost of the sensor, but this can be justified for inventory control and tank gauging.

The pulse radar technique has difficulty in making accurate measurement if the process material to be measured is very near to the antenna – because the difference in time between the send and return signal will be too fast to detect accurately. Finally, radar level measurement does not measure interfaces.

Going nuclear

The nucleonic level measurement technique is applicable for both continuous and point level measurements. It’s mostly used when other techniques have not proved successful. Requiring no product contact or tank penetration makes the technique suitable for high temperature and pressure applications.

The radio isotopes used emit gamma rays of high energy and short wavelength that penetrate the wall of the vessel to contact the process fluid, which absorbs the gamma rays in a predictable manner allowing a detector on the other side of the vessel to measure the radiation field strength and so deduce the level in the vessel. Depending on the penetration power required to detect the process fluid within the vessel, different types of radio isotopes can be used.

The technique has an advantage of not requiring physical penetration of vessel, and so is best considered for use where corrosive and toxic materials are within the process fluid. A disadvantage is that when no longer needed, nucleonic level sensors must be disposed of correctly via an external organization and appropriately licensed means.

Operating success In the application of any type of level measurement technique, the important thing to remember is that all technologies work when applied properly. It is imperative to take into consideration the principle of operation, and where the limitations of each technology lie.

The correct selection of a technology and the correct application of the technology will make the level measurement a success. Key factors to consider include process operating temperature, pressure, vacuum, dielectric constant of the medium, chemistry, specific gravity of the medium, electrical noise, contact/non-contact installation, response rate, instrument mounting, and accuracy.

Preventive maintenance is an essential part of high-accuracy level measurement. Understanding the causes that lead to a level sensor failure will help ensure proper level measurement. If the cause of a level meter failure is unknown, the end user should consult with the level sensor manufacturer for further analysis.

Nwaoha Chikezie is a qualified petroleum engineer and a member of the Nigerian Gas Association.