The need for accurate and reliable level measurement systems is increased by the demands of advanced automated processing systems, more stringent process control and strict regulatory requirements.
By improving the accuracy of level measurement, the variability in chemical-processes can be reduced, which, in turn, helps to improve product quality and reduce costs and wastes. The regulations laid down for electronic records are stricter in terms of electronic reporting, accuracy and reliability. These requirements are met using new level measurement technologies.
Level Measurement Technology in Transition
Sight glass is the oldest and the simplest level measurement technology used in industrial environments.
Sight glass is subject to many limitations as it is measured manually. The material used for constructing the sight glass can fail catastrophically, resulting in danger to personnel, environmental hazards and/or even fires/explosions. In case seals are present they may lead to build up or leakage, obstructing the line of view.
There is no doubt that in any installation, sight glass is easily the weakest link and needs to be replaced by advanced methods.
Some of the other level measurement methods are based on specific gravity, which is a physical property commonly used to sense the level surface. These devices consist of a simple float, whose specific gravity is between that of the process fluid and the headspace vapor that will float at the surface according to the rise and fall. The level measurement is based on hydrostatic head measurements.
In scenarios that involve more sophisticated physical principles, more advanced technologies that use computers for performing calculations are used. In such technologies, the data is transmitted in a format that can be read by the machine from the sensor to the control system. The various formats of the output signals from the transducer are analog voltages, current loops and digital signals. Although the setting up and recording of analog voltages is simple, they are affected by serious interference and noise issues.
4-20 mA current loops are the oldest and simplest form of industrial communication, and are also the most common output mechanism currently followed. Signals via current loops can be carried over longer distances with minimum degradation.
The most powerful format of digital signals are based on protocols such as Hart, Honeywell DE, Foundation Fieldbus, Profibus and RS-232. However, older protocols like RS-232 are effective only for short distances. The latest transmitter signals are characterized by wireless capabilities that allow signal transmission over very long distances without any degradation.
The more advanced digital encoding formats required by advanced technologies like ultrasonic, laser and radar require digital computer intelligence for formatting the codes. Such advanced requirements along with the need for sophisticated digital calibration schemes and communication capabilities have paved way for a new trend that embeds microprocessor-based computers in almost all level measurement products (Figure 1).
Figure 1. Level Measurement determines the position of the level relative to the top of bottom of the process fluid storage vessel. A variety of technologies can be used, determined by the characteristics of the fluid and its process conditions.
Established Level- Sensing Technologies
In all the scenarios that this article deals with, the density of the vapor present in the headspace is assumed to be a negligible value compared to the process fluid. Another assumption is that there is only a single, uniform process fluid in the tank. Some of the technologies described below may be used in multilevel applications in which a single vessel is shared by two or more immiscible fluids.
Glass Level Gauges
Glass gauges, which have been being used for more than 2 centuries now are available in a number of designs, in both armored and unarmored forms. They are the simplest of methods available for liquid level measurement. The clear visibility provided by their design is their biggest advantage, while the fragility of the glass that may result in spills or compromise on the personnel safety is the disadvantage.
The principle behind the working of floats is placing a buoyant object, with specific gravity between that of the process fluid and the headspace vapor, into the tank and attaching a mechanical device to record its position. The float stays on top of the process fluid, but sinks to the bottom of the headspace. Even though a liquid’s surface can be located by the float, reading the float still poses a problem.
Mechanical components like pulleys, tapes, cables and gears were used in early float systems to record the level. Currently, magnet-equipped floats are more popular. The level measurement provided by early float systems was in the analog or digital format through a network of multiple reed switches and resistor. This implies that the changes in the transmitter output were in the form of discrete steps. Hence, these devices cannot distinguish levels between steps, which continuous level measuring devices are capable of.
Displacers, Bubblers and Differential Pressure Transmitters
Bubblers, differential pressure transmitters and displacers are all different hydrostatic measurement devices. Changes in temperature cause a change in a liquid's the specific gravity of the liquid; similarly changes in pressure also affect the specific gravity of the vapor that is present over the liquid. As a result of these changes, the accuracy of the measurement is reduced.
Displacers are based on the Archimedes principle. Figure 2 shows a column of solid material placed in a vessel.
Figure 2. Displacement level gauges operate on Archimedes’ principle. The force needed to support a column of material (displacer) decreases by the weight of the process fluid displaced. A force transducer measures the support force and reports it as analog signal.
The density of the displacer is always more than that of the process fluid, so that it extends from the lowest level of the tank to the highest level that needs to be measured. The column displaces a volume of the fluid due to a rise in the process fluid; the displaced volume is equal to the product of cross-sectional area of the column and the level of process fluid in the displacer.
A buoyant force, whose magnitude is equal to the product of the displaced volume and the process fluid density, exerts an upward thrust on the displacer, which brings down the force required to support it against the gravitational force. This change is monitored by the transducer linked to the transmitter, and translates the change in force to level.
Figure 3 shows a bubbler-type sensor.
Figure 3. Bubblers sense process fluid depth by measuring the hydrostatic pressure near the bottom of the storage vessel.
The bubbler method is used in vessels that work under atmospheric pressure. A purge gas is carried in a dip tube with its open end near the opening of the vessel into the tank. The purge gas is usually air, but in some cases where there is danger of an oxidative reaction with the process fluid or contamination, dry nitrogen is used.
Due to the flow of gas through the dip tube outlet, the tube pressure rises till it is more than the hydrostatic pressure generated by the liquid level at the outlet. The product of the process fluid density and its depth from the end of the dip tube to the surface is equal to the pressure, which is monitored by a pressure transducer connected to the tube.
Figure 4 depicts a differential pressure level sensor.
Figure 4. Differential pressure sensors monitor for the process fluid level by measuring the total pressure difference between the fluid at the bottom of the tank and the vessel pressure.
In this sensor, the difference between the total pressure at the bottom of the tank and the head or static pressure in the vessel is indicative of the level. Similar to the bubbler method, the product of the fluid density and the height of the fluid in the vessel gives the hydrostatic pressure difference. Atmospheric pressure is considered as reference. In order to maintain headspace at atmospheric pressure a vent is provided at the top.
Unlike bubblers, DP sensors can be used in vessels by simply connecting the reference port to a port in the vessel, which is above the maximum fill level. The need for liquid purges or bubblers depends on the physical conditions of the process and/or the location of the transmitter with respect to process connections.
A mechanical support bracket or member that is provided with one or more sensors for detecting slight distortions in the support member is called load cell or strain gauges. The slight flexing of the member due to a force on the load cell produces a change in the output signal. Load cells are calibrated to make measurements ranging from fractional ounces to tons. The load cell must be placed in the support structure of the vessel for measuring the level. The force on the load cell increases as the process fluid level in the vessel increases. The load cell’s known output can be converted into fluid level by knowing the geometry of the vessel and the specific gravity of the fluid.
Although load cells are extensively used in many applications due to their non-contact operation, the expense and the need for designing the vessel support structure and the connecting piping as per the load cell’s requirements are significant disadvantages. Besides measuring the desired net or product weight, the weight of the piping, vessel and the connecting structure that is supported by the vessel is collectively measured by the load cell.
A very poor turndown to the net weight is caused by the total weight, implying that the net weight is only a small percent of the total weight. In addition to this, the growth of the supporting structure due to uneven heating, side load, rigid piping, wind load and binding from the overturn-prevention hardware could also be shown as level. Therefore, the requirements of the load cell weighting systems need to be maintained all through the initial vessel support and piping system, failing which performance can be quickly degraded.
Magnetic Level Gauges
Magnetic level gauges are suitable replacements for sight glasses. Although they are similar to float devices, the communication of the liquid surface level occurs magnetically. The float in this case is a set of strong permanent magnets, which move in an auxiliary column that is attached to a vessel by two process connections.
The float is laterally confined by the column so it remains close to the side wall of the chamber. The position of the float moves up and down according to the fluid level, which is indicated by a magnetized shuttle or a bar graph that moves along with it, showing the float’s position and thereby indicating the level.
For this technology to work the chamber walls and the auxiliary column should be made of non-magnetic materials. The float designs provided by most manufacturers are optimized for a large selection of float materials and the specific gravity of the fluid that is measured, be it propane, oil, acid, water, butane or interfaces between two fluids.
Figure 5 shows magnetic level gauges.
Figure 5. Magnetic level gauges use a magnetically coupled shuttle to locate a float’s position in the chamber.
Magnetic level gauges are capable of withstanding high pressures, high temperatures and corrosive fluids. For applications that expect buildup oversized float chambers and high-buoyancy floats may be used. Plastic materials like Kynar or special alloys like Hastealloy C-276 can be used to make the process connections, flanges and chambers. Extreme conditions like steam jacketing for liquid asphalt, temperature designs for liquid nitrogen and refrigerants or oversized chambers for flashing applications require special configurations of chambers.
When handling high-temperature, high-pressure, corrosive-fluid and low-specific-gravity applications, metals and alloys like Incoloy and Monel may be used. Additionally, these gauges may be fitted with guided-wave radar and magetostrictive transmitters to facilitate the conversion of the local indication to 4-20 mA outputs and corresponding digital communication that can be sent to a control system or controller.
Capacitance transmitters are based on the difference in the dielectric constants (ᶓ) of process fluids and air. The dielectric constant of oils ranges from 1.8 to 5, whilst for pure glycol it is 37. And for aqueous solutions, it varies from 50 to 80.
The basic operating principle is based on the variance in capacitance which itself is based on the variation in the liquid level. The change in capacitance is induced by an insulated rod coupled to the transmitter and the process fluid, or by an un- insulated rod coupled to the transmitter and the reference probe or the vessel.
There is a proportional rise in the capacitance as the fluid level rises and fills the apace between the plates. Using a capacitance bridge the overall capacitance is measured, which provides a continuous level measurement.
Capacitance transmitters are illustrated in Figure 6.
Figure 6. Capacitive level sensors measure the change in capacitance between two plates produced by changes in level. Two versions are available, one for fluids with high dielectric constants (A) and another for those with low dielectric constants (B).
Time-of-flight (TOF) measurements are used by current liquid-level measurement techniques. In these devices, the distance between the liquid level and a reference point at a sensor or a transmitter placed near the top of the vessel is measured. A pulse wave at a reference point is generated by the system, which is transmitted through the conductor or the vapor space. This wave is then reflected off the liquid surface and re-transmitted to a pickup at the reference point.
The total travel time is measured by the electronic timing circuit. The distance to the surface of the liquid is obtained by dividing the travel time by twice the wave’s speed. The main difference in all these technologies is the type of pulse that is used for measurement. Technologies like microwaves, ultrasound and light are extremely effective for such measurements.
Magnetostrictive Level Transmitters
The benefits of using a magnet to restrict the float in order to determine the liquid level has been proven already. Moreover, magnetostriction is an established technology to enable precise recording of the location of the float. Magnetostrictive transmitters may be used to replace mechanical links to determine0 the speed of a torsional wave using a wire to locate the float and report its position.
The float in a magnetostrictive system (Figure 7) contains a series of permanent magnets.
Figure 7. Magnetostrictive level transmitters use the speed of a torsional wave in a wire to produce a level measurement
At the transmitter, a sensor wire is connected to a piezoceramic sensor, and at the opposite end of a sensor tube, a tension fixture is attached. There are two ways of placing the sensor tube: one is through a hole in the float’s center, and another is placing it next to the float outside a non-magnetic float chamber.
A short current pulse is sent by the transmitter through the sensor wire to locate the float, which sets up a magnetic field across its length. A timing circuit is switched ON at the same time.
This magnetic field instantly interacts with the magnetic field created by the float magnets, resulting in the creation of a torsional force in the wire when current flows through it. This torsion is transmitted to the piezoceramic sensor at a particular speed. An electrical signal is produced by the sensor when it detects the tensional wave. This signal intimates the timing circuit of the arrival of the wave and hence, the timer circuit is stopped. The time interval (TOF) between the beginning of the current pulse and the arrival of the wave is measured by the timing circuit.
The float location can be accurately determined based on this information and the transmitter presents this information as a level signal. The main advantage of this measurement method is that the signal speed is known and is constant with process variables like pressure and temperature. Additionally, the signal is unaffected by beam divergence, or false echoes and foam. Another advantage is the absence of many movable parts with the float being the only moving component.
Ultrasonic Level Transmitters
Figure 8 shows an ultrasonic level sensor.
Figure 8. Ultrasonic level transmitters use the speed of sound to calculate level
Ultrasonic level transmitters are capable of measuring the distance between the transducer and the surface based on the time taken by the ultrasound pulse to travel from the fluid surface to the transducer and back (TOF).
The operational frequency of these transmitters is tens of kilohertz and the transit times are approximately 6 ms/m. The composition of the gas mixture in the headspace and its temperature affect the speed of sound (340 m/s in air at 15°C). Even though the sensor compensates for temperature, it is limited to atmospheric measurements in nitrogen or air.
Laser Level Transmitters.
Laser level transmitters have been designed for level measurements of slurries, opaque liquids and bulk solids. The operating principle is similar to ultrasonic level sensors but these sensors measure the speed of light for level measurement, instead of the speed of sound (Figure 9).
Figure 9. A laser transmitter uses a short burst of laser energy to measure level
A short pulse is transmitted by the laser sensor placed at the top of the vessel to the process liquid surface located below; this pulse is then reflected back to the detector.
The time of flight of the signal is measured by a timing circuit to calculate the distance. Since lasers are virtually beamless and devoid of false echoes, they can be directed through spaces as small as 2 inches. Another advantage is that lasers offer precise measurements even in vapor and foam.
Applications that use vessels with number of obstructions can use laser transmitters for accurate measurements up to distances of 1500 ft. Lasers must be combined with specialized sight windows to isolate the transmitter from the process in high temperature or high-pressure applications like reactor vessels. The glass windows must allow the laser to pass through with minimum attenuation and diffusion whilst also containing the process conditions.
Radar Level Transmitters
In air-radar systems, the microwave beam is directed downward from a horn or a rod antenna placed at the top of a vessel. The fluid surface reflects the signal back to the antenna, and the distance is calculated by the timing circuit which measures the round trip time (TOP).
In radar technology, the critical factor is the dielectric constant of the liquid because the amount of energy reflected at microwave frequencies varies with the dielectric constant of the fluid. If the Er is low, then the liquid will allow most of the radar energy to pass through. For high values of Er the reflection at the change in Er is high.
Another type of transmitters is guided wave radar (GWR) transmitters (Figure 10), which provide highly accurate and reliable measurements.
In these transmitters, a flexible cable antenna or a rigid probe channelizes the microwave from the top of the vessel down to the liquid level and then back to the transmitter. The change from a lower to higher Er causes the wave to be reflected.
The efficiency of this method is 20 times greater than that of air-radar since the guided transmission enables a focused path for the energy. Liquids with Er values of 1.4 and lower can be measured by this method. Furthermore, these systems allow both vertical and horizontal installation by bending the guide up to 90° angle.
Figure 10. Guided wave radar uses a waveguide to conduct microwave energy and from the fluid surface.
Most of the advantages of the aforementioned techniques, including ultrasound, laser and radar, and a few limitations are present in the GWR transmitters. The composition, pressure and temperature of the vapor space gas affects the speed of the radar.
It can even work in a vacuum without any calibration. Even foam layers can be measured using GWR. Issues like beam-spread or false echoes from tank walls and structures can be avoided by confining the wave so that it follows a probe or a cable.
The factors that drive the market are based on the trends across various measurement technologies. The use of sophisticated digital electronics is increasing the usefulness of level measurement and other sensors.
Modern level sensors produce reliable results and are easy and less expensive to setup. Level measurement can be integrated into existing control systems thanks to the enhanced communication.
Modern level measurement systems make use of a number of materials and alloys in order to survive harsh environments like acids, oils or high pressure and temperature. Use of new materials enables them to meet specialized requirements, for example, assemblies made up of PTFE jacketed material can be used in corrosive applications; and electro polished 316 stainless steel adheres to cleanliness requirements.
Contact transmitters can be used in a variety of applications by using probes made of these special materials.
This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.
For more information on this source, please visit ABB Measurement & Analytics.