From tried and true to the newest trends
By Donald Gillum
Few measurements are as common and widespread in terms of application, operation, and variety as is level. This measurement is defined as the determination of the position of an existing interface between two media. These media are usually fluids, but they may be solids or a combination of a solid and a fluid. The interface can exist between a liquid and a gas, a liquid and its vapor, two liquids, or a granular or fluidized solid and a gas.
Many techniques are available for the measurement of these interfaces, each with its own trade-offs of advantages and limitations. The best selection depends on the nature of the specific application, including the process to be measured, the degree of accuracy and dependability desired, and economic considerations and constraints. The design engineer must have a working knowledge of the various types of measuring devices available as a guide for the selection and implementation of a system best suited for a particular application. Following is a list of level-measuring methods in present use:
- Hydraulic head
- Variable displacer
- Sonic and ultrasonic
- Fiber optics
- Weight and load cells
- Rotating and vibrating paddle
- Hydrostatic tank gaging (HTG)
With this significant number of types and devices to choose for a specific application in order to select a viable method, it may be a daunting task, especially for a novice or beginner in the field of measurements. This article will deal with the most common of level technologies, with a focus on head type and radar.
Visual measuring techniques
This method includes familiar applications, such as sight glasses, dip sticks, lead lines, steel tapes with weights, and simply observing the presence of product through a transparent or translucent vessel. While these techniques are reliable, accurate, and simple to perform, the lack of signal generation or transmission may limit such use. An automatic tank gage (ATG) is defined in the API manual as “an instrument which automatically measures and displays liquid level or ullages (empty space in a tank) in one or more tanks continuously, periodically, or on demand.” Generally a signal for transmission and proportional to level is required. Float and cable arrangements with specially designed encoders are used to track the movement of a perforated tape connected to a float. Optical and magnetic encoders with digital signal conditioning for transmission can be used to measure level over a 96-foot span with increments and repeatability of 1/16 of an inch.
Variable-displacement measuring devices
When the weight of an object is always heavier than an equal volume of the fluid into which it is submerged, full immersion results, and the object never floats. Although the object (displacer) never floats on the liquid surface, it does assume a relative position in the liquid, and, as the level moves up and down along the length of the displacer, the displacer undergoes a change in weight caused by the buoyancy of the liquid. A small change in the position of the displacer will be used to indicate or generate a signal proportional to the level.
Many level-measurement techniques are based on the principle of hydrostatic head measurement. From this measurement, a level value can be inferred and is usually calibrated in a linear measurement, such as feet or inches, and the level is read directly, therefore eliminating the need for conversions. Such level-measuring devices are common in a wide variety of industries.
When level is elevated in a tank, the force created by this head pressure is applied to the measurement side of the transducer, resulting in an increase in the instrument output. This instrument response caused by the head pressure is used to infer a level value. Changes in atmospheric pressure will not affect the measurement because these changes are applied to both sides of the pressure transducer.
The specific gravity of a fluid must be considered to establish the relationship between level and pressure. If the fluid is lighter, the pressure exerted by a specific column of liquid is less. The pressure will be greater for heavier liquids.
Air purge or bubble system
An air purge or bubble system consists of a tube usually 1/2-inch tube or 1/4-inch pipe in the liquid level. With the supply air blocked, the water level in the tube will be equal to that in the tank. When the air pressure from the regulator is increased until the water in the tube is displaced by air, the air pressure on the tube is equal to that required to displace the liquid – and also equal to the hydraulic head of the liquid in the tube.
Radar measurement overview
The function of a microwave gage can be described where the gage and its environment are divided into five parts: microwave electronic module, antenna, tank atmosphere, additional sensors (mainly temperature sensors), and a remote (or local) display unit. The display may include some further data processing, such as calculation of the mass. Normally, the transmitter is located at the top of a vessel, and the solid-state oscillator transmits an electronic wave at a selected carrier frequency and waveform that is aimed downward at the surface of the process fluid in the vessel. The standard frequency is 10 GHz. The signal is radiated by a dish or horn-type antenna that can take various forms, depending on the need for a specific application. A portion of the wave is reflected back to the antenna, where it is collected and sent to the receiver, where a microprocessor determines the time of flight for the transmitted and reflected waveform. Knowing the speed of the waveform and travel time, the distance from the transmitter to process fluid surface can be calculated. The detector output is based on this difference.
Contact and non-contact operation
Non-contact radar detectors operate by using pulsed radar waves or frequency-modulated continuous waves (FMCW). In pulsed wave operation, short-duration radar pulses are transmitted, and the target distance is calculated using the transit time. The FMCW sensor sends out continuous frequency-modulated signals, usually in successive (linear) ramps. The frequency difference – caused by the time delay between transmit and reception – indicates the distance, which directly infers the level.
The low power of the beam permits safe installation for both metallic and non-metallic vessels. Radar sensors can be used when the process material is flammable and when the composition or temperature of the material in the vapor space varies.
Contact radar measuring devices send a pulse down a wire to a vapor-liquid interface, where a sudden change in the dielectric of the materials causes the signal to be partially reflected. The time of flight is measured, and the distance traversed by the signal is calculated. The non-reflected portion of the signal travels to the end of the probe and gives a signal for a zero reference point. Contact radar can be used for liquids and small granular bulk solids. In radar applications, the reflective properties of the process material will affect the transmitted signal strength. Liquids have good reflective qualities, but solids usually do not. When heavy concentrations of dust particles or other such foreign materials are present, these materials will be measured instead of the liquid.
The radar signal is reflected directly on the liquid surface to obtain an accurate level measurement. Any dust or mist particles present must have no significant influence, as the diameters of such particles are much smaller than the 3-cm. radar wavelength. For optical systems with shorter wavelengths, this is not the case. There can be slight measurement errors for a few specific products in the vapor space of the tank; this is especially true when the composition may vary between no vapor and fully saturated conditions. For these specific products, pressure and temperature measurement may be required for compensation. Such compensation is made by the software incorporated in the tank intelligence system provided by the equipment manufacturer.
End-of-the-probe algorithm can be used in guided-wave radar when there is no reflection coming back from the product. This new technology innovation provides a downward-looking time of flight situation, which allows the guided-wave radar system to measure the distance from the probe mounting to the material level. An electromagnetic pulse is transmitted and guided down a metal cable or rod, which acts as a surface wave transmission line. When the surface wave meets a discontinuity in the surrounding medium, such as a sudden change in dielectric constant, some of the signal is reflected back to the source, where it is detected and timed. The portion of the signal that is not reflected travels on and is reflected at the end of the probe.
Interface detection by time domain reflectometry
Liquid/liquid measurement can be provided by guided wave radar, also called time domain reflectometry (TDR), which is similar to pulse radar but is conductor-based and used with electrical pulses without carrier frequency. This technology is particularly suited for interface measurement because there is less signal weakening and reduced interference from tank geometry situations.
Radar level measurement
The radar level gaging technique used on tanker ships for many years has been used in refineries and tank farms in recent years. Its high degree of integrity against virtually all environmental influences has resulted in high level-measurement accuracy; one example is the approval for radar automatic tank gage (ATG) for 1/16-in. accuracy. Nearly all level gaging in a petroleum storage environment can be done with radar level gages adopted for that purpose.
Although radar level technology is a relatively recent introduction in the process and manufacturing industries, it is gaining respect for its reliability and accuracy. While the time-of-flight principle of sonic and ultrasonic level-measurement systems is similar to radar, there are distinct differences. The primary difference is that sound waves produced by ultrasonic units are mechanical and transmit sound by expansion of a material medium. Since the transmission of sonic waves requires a medium, changes in the medium can affect the propagation. The resulting change in velocity will affect the level measurement. Other factors can also affect the transmitted or reflected signal, including dust, vapors, foam, mist, and turbulence. Radar waves do not require a medium for propagation and are inherently immune to the factors that confuse sonic-type devices.
Advantages and other characteristics of radar level measurement are:
- Generally immune to vapor space material and conditions
- Immune to obstructions in the tank
- Improved accuracy and stability by microprocessor generation and computation of algorithms – can be used for custody transfer applications
- Two wire designs
- Little effect of temperature and pressure, and this can be compensated
- Immune to errors caused by foam
- Relatively easy to install and configure
- Up to 300 ft. measured span
- Used with guide wires to measure liquid-liquid interface
- Some units are battery powered
- Some units are wireless
- Used extensively in the greater petroleum-based industries
- Limited use in the pharmaceutical and food industries
Level measurement summary
In specifying level-measuring instruments, one usually examines the data furnished by the manufacturer or supplier. This data, when used to evaluate equipment from various manufacturers, can be difficult to use for comparison of actual performance. Such issues as instrument location and mounting, differences in the expression of terms, the amount of detail and completeness given, and the methods used to obtain instrument data should be considered.
Installation considerations are very important in non-contact level measurement applications, and the following recommendations are listed to help the overall performance of measuring systems:
- Material characteristics can have adverse effects on the measurement quality of the measurement system operation.
- Turbulent surface conditions should be avoided, and stilling basins should be considered in severe turbulent situations.
- Dust and dirt build-up at the sensor can reduce the transmitted and/or received signal strength.
- The conditions at the liquid surface that affect reflectivity of the energy beams can result in error. For radar systems, this is dependent on the dielectric constant, which should be above 10. Levels below 1.5 can preclude the use of radar applications.
- The vapor space is considered to be transparent to the energy of the transmitted and reflected energy beam. Materials in the vapor portion of the vessel that are translucent to the traveling waveform can affect the energy level and the speed of the waveform.
- The velocity of an ultrasonic wave is dependent on the pressure, temperature, and composition of the vapor. Compensation or changes in the vapor is at times necessary. The condition of the vapor should be constant. For example, when the conditions change during start-up, shutdown, and normal operation, and appropriate compensation is not made, error can result.
The vessel geometric design and other conditions can affect the performance of a measuring system. The configuration of the transponders and material entering or leaving the vessel can result in measurement error. The formation of solids at various points along the beam path should be avoided. Parasitic echoes and erratic energy reflection caused by internal tank structures should be discounted by the measuring system. Moving parts like agitators can produce false reflections that can be sensed as level measurement. The wetted portion of the sensor should be mounted perpendicular to the liquid surface. Condensation forming on the sensor should be avoided. Multiple reflections caused by cones, nozzles, and other geometric shapes of the vessel should be avoided by proper mounting and compensation of false echoes.
Selecting a measurement technology for a given application based upon industry trends
A recent survey of instrument users in the greater oil, chemical, and power industries, with regard to level measurement methods for specific applications, is summarized with the following comments:
The major technologies employed for level measurement for our facilities are differential pressure transmitters being the most common.
A great number of displacers are still used.
For applications when liquid density changes in the process are prevalent, both free space and guided-wave radar are starting to be used.
Although differential pressure transmitters have always been used for steam drum level measurement, a guided wave radar system is utilized so that compensation for temperature difference between steam drum and the lead lines are not required.
Guided-wave radar measurements are accurate and are rated for many types of safety systems.
HTG applications to compensate for water, steam, and reference leg density are being studied.
Throughout our facilities, we use ultrasonic, capacitance, and differential pressure transmitters, with and without remote seals and capillary tubing. Guided-wave and through air radar systems are becoming more prominent.
Many capacitance level-based measurement systems are being replaced with guided-wave radar systems.
For level and interface measurement applications, when possible, bubble tube systems are being replaced with guided-wave radar systems. This is due in part to inherent problems with rotometer issues and air leaks in the lines.
Most of the issues with guided-wave radar systems are traced to improper commissioning, start-up procedures, and obstructions inside the tank, such as thermowells, pipe brackets, and other related items, which cause parasitic echoes.
Although capillary seals are utilized to mitigate other issues associated with wet leg applications (i.e., loss of wet-leg fluid caused by leaks and sedimentary contamination inside the wet-leg), specific gravity changes in the process fluid is not compensated. Guided-wave radar technology is frequently replacing systems that require wet-legs.
ABOUT THE AUTHOR
Donald Gillum has more than 40 years of experience as master instructor, department chair, and division director of instrumentation control systems and automation at Texas State Technical College in Waco, Texas. Prior to this, he spent 10 years at a petrochemical facility as an instrumentation and analyzer engineering technician. Gillum spent two terms on the ISA Executive Board, served as program evaluator and commissioner for ABET, and currently sits on the board of directors for ABET. He wrote the ISA book, Industrial Pressure, Level and Density Measurement, Second Edition. He obtained a B.S. from The University of Houston and is a registered professional engineer in control systems engineering. Gillum has considerable experience in industrial training and development.