S. Sivaraman
Exxon Research & Engineering Co.
Florham Park, N.J.
Radar tank gauging technology was recently field-tested by Exxon Research & Engineering Co. on an asphalt tank at Exxon's Bayonne marketing terminal in Bayonne, N.J.
Results of the 3-month test demonstrated that the technology is comparable to, and most likely better than, manual gauging methods.
Radar tank gauging technology provides a noncontact, noninvasive method of tank gauging. It lends itself for application to vertical, cylindrical, atmospheric storage tanks in asphalt, acid, wax, and heavy, viscous product service or other corrosive and high-temperature service.
Radar gauging offers the advantages of reliability, essentially trouble-free operation, and the potential to reduce manpower requirements for inventory management. Tests also show that radar tank gauging provides the accuracy and repeatability that make it suitable for consideration for custody-transfer measurements.
Radar technology merits consideration for custody transfer applications, along with other automatic gauging systems.
GAUGING TECHNIQUES
Manual tank gauging is still the most widely applied method for gauging vertical, cylindrical storage tanks in custody-transfer service despite its limitations. Rightly or wrongly, it is also the reference method for evaluating any new technology, no matter how superior the new technology may be.
The manual method of tank gauging has been in use since the turn of the century. It is currently covered by American Petroleum Institute (API) Standard 2545, issued in 1965.
Recently, automatic tank gauging systems (ATG's) have gained some degree of acceptance for custody transfer. These automatic devices, such as float and servo gauges, have been in use for a long time for operational control only.
The performance of these systems has been significantly affected by maintenance problems and, thus, the track record for reliability was not conducive to promote applications for custody transfer. With improved preventive maintenance programs and emphasis on periodic verification and calibration, conventional ATG's have attained a better level of acceptability within the petroleum industry. However, use of the conventional ATG's for special applications, such as asphalt or acid, still presents a major problem.
Radar technology has been in use for more than 25 years in marine environments. Use of the technology in shore-tank applications has been a more recent trend, but the advantages of the technology have not yet been realized.
One of the major factors is the lack of availability of any test data as to the performance of radar technology compared to manual methods. Even the limited data that exist within the petroleum industry, for some reason or another, have not been disseminated to the public at large.
For those reasons, a field test and evaluation of radar technology was undertaken on an asphalt storage tank. The main objective of the test was to evaluate the variability of the radar level gauge with the reference manual method.
RADAR TECHNOLOGY
The main radar gauging device consists of a radar transmitter which, when powered by a standard power supply, emits a microwave signal in the range of 9-10 ghz.
This is a frequency-modulated continuous wave.
At this frequency, the microwave is essentially immune to, and free from, any external interferences such as moisture, vapors, and atmospheric attenuations. The microwave strikes the surface of the liquid in the tank and is echoed back to a receiving antenna.
The difference in the frequency between the forward signal and the return or reflected signal is proportional to the distance from the antenna to the liquid surface. Once the unit is calibrated for a specific service, the ullage (innage) from the radar gauge can be read directly.
At any given level, the response time of the system is generally in fractions of a second.
The microwave frequency signal will require a free space for transmission within the tank with a cone angle of about 12 at the source (Fig. 1). In this conical zone, the space should be free of any metal parts or deadwood to prevent echo distortion, and to ensure the integrity of the reflected signal.
There is no direct physical contact of the transmitter with the product. The radar transmitter is simply mounted on top of the tank at a convenient location.
The transmitter output is sent to a microprocessor that can interface with any standard control or display unit.
TESTS AND EVALUATION
Field tests were conducted on a fixed-roof tank in asphalt service with the product temperature at about 300 F. The radar unit was flange-mounted close to the center of the tank.
This location was chosen simply because a flange opening was readily available and no additional tank modification was required to mount the radar unit. Fig. 1 illustrates the general arrangement of the installation.
The calibration factor was first estimated and computed based on the radar gauge reference height (the height of the antenna from the tank bottom). This was then revised and fine-tuned by physical verification at the maximum and minimum product levels in the tank.
The tank was loaded and unloaded in close increments. At each increment of liquid level, the manual level (by manual gauge) and the radar level were recorded. Manual readings were instantaneous readings taken by an experienced operator using a certified, temperature-stabilized tape.
The same tape and the same operator were used for the duration of the tests. Table 1 shows the field data collected.
The maximum fill height was restricted to 25 ft ullage due to structural limitations of the tank. Also, the maximum displacement within the tank was limited to 30 ft because of operational constraints. Despite the limitations, a reasonable number of samples (levels) was obtained for both loading and discharge operations.
Measurements were taken at each level several times and at different time intervals to ensure repeatability of manual gauging and to minimize the effects of settling time. Radar ullage measurements were also repeated by switching the unit on and off at various levels.
Field tests were carried out over a period of 3 months. During this time, no adjustments of any kind were made to the system.
A comparative analysis of the data from Table 1 clearly indicates that the radar level tracked the manual level very closely to within 2.5 mm mean overall variation. The population spread is about 3 mm.
This variability should be considered excellent and well within the generally acceptable criteria for asphalt, acid, and heavy viscous fluids. On the assumption that the manual gauge is generally accurate to within 3 mm, the contribution of radar to the overall population spread (2 sigma) is estimated to be about 1 mm.
It can be concluded from the test that the accuracy and performance of the radar technology is comparable to, and directionally better than, manual methods, based on a statistically significant confidence level.
The tests were of limited nature, both in terms of duration and frequency of loading and unloading. But the results of these tests are indeed promising and point toward long-term reliability and performance of the technology for the aforementioned services.
OTHER CONSIDERATIONS
The principal feature of radar technology is that it is noninvasive and has no mechanical moving parts or linkages. Therefore, it can be expected to be essentially trouble free and free from routine maintenance problems normally experienced with devices that contact the liquid surface.
However, over a long period of time, the radar antenna assembly may get coated with dust particles from the product depending on temperature fluctuations. It is, therefore, desirable to periodically clean the assembly.
The parabolic design of the antenna can be expected to minimize the condensation and particle buildup, but periodic cleaning is still desirable. This can be accomplished in situ without problem.
Like any other measurement system, proper installation of the transmitter unit is important to ensure long-term reliability and accuracy. A minimum distance of 3-4 ft is recommended between the antenna and the maximum liquid level to prevent echo distortion and loss of accuracy. Also, temperature limitations of the transmitter, if any, in relation to the product temperature should be considered.
Radar technology offers improved plant inventory management and facilitates better utilization of manpower by minimizing the large amount of time spent on month-end inventory measurements by manual methods.
Radar technology can be applied to fixed-roof and floating-roof tanks, as well as to other tank designs, such as spheres. In addition to asphalt, acids, and heavy, viscous fluids, the technology can also be used for conventional products, such as gasoline and fuels.
ACKNOWLEDGMENT
The author expresses his thanks to the management and staff at the Exxon Bayonne marketing terminal for providing the tank facilities and manpower for conducting the tests, and to Johan Sanberg, technical manager, SAAB Tank Control for providing the radar system and technical advice in conducting the tests.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.