NEW APPROCH IN TANK GAUGING: REDUCING INVESTMENT COST AND DOWNTIME

Eng. Alexandru-Ştefan Ursache

SIEMENS Romania, Industrial Solution (I&S) Division, Bucharest alexandru.ursache@siemens.com

Abstract: This paper present classic and up-to-date technique for refinery tank farms automation. The modern instrument to accomplish this goal is the automated tank gauging system. We present methods for level, temperature, pressure, mass, volume and water interface measurements adequate to tank farms. This measurement methods are used in operational control, custody transfer, inventory control, quantity assessment, leak detection and overfill protection systems based on advanced software platforms. The use of photovoltaic panel supply systems and radio modems can help to create autonomous tank gauging systems. Copyright © 2006 Alexandru-Stefan Ursache

1. INTRODUCTION TO TANK GAUGING

Tank Gauging is the generic name for the static quantity assessment of liquid products in bulk storage tanks. The Tank Gauging System includes products level, temperature, pressure, mass, volume and water interface detection for a wide range of applications, ranging liquefied petroleum gas (LPG) to asphalt within the oil industry. Tank gauging has a long history. Since each user and every application has its own specific requirements, several measurement techniques and solutions are currently available. 1.1 Manual level gauging Tank gauging started with manual gauging (Fig. 1), using a graduated dip tape or dipstick. This technique is still used worldwide, and is today still the verification for gauge performance calibration and verification. The typical accuracy of a dip tape used for custody transfer measurements is often specified as ± (0.1 + 0.1 x L) mm for the initial calibration of new dip tapes. In this formula L is the level in meters. For tapes in use, the recalibration accuracy applies. This accuracy is twice the uncertainty of a new tape. But the tape uncertainty is not the only cause of error. Accurate hand dipping is a difficult task, particularly with cold weather, during nighttime or when special protection equipment has to be used.

Fig. 1. Manual level gauge. Additionally, a human error, of at least ± 2 mm, has to be added to the tape readings. Another disadvantage of manual tank gauging is that employees are often not allowed to be on a tank because of safety regulations, resulting in costly, long waiting times. 1.2 Float and tape gauges The first float and tape gauges (Fig. 2), also called "Automatic Tank Gauges", were introduced around 1930. These instruments use a large, heavy float in order to obtain sufficient driving force. Initially the float was connected via a cable to a balance weight with a scale and pointer along the tank shell indicating the level. Newer versions had the float connected via a perforated steel tape, to a "constant" torque spring motor. The perforations drive a simple

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Fig. 2. Float and tape level gauge. mechanical counter, which acts as local indicator. Typical accuracy of a mechanical gauge is in the range of 10 mm. Due to the mechanical friction in pulleys, spring motor and indicator, the reliability is poor. Remote indication is possible via an electronic transmitter coupled to the indicator. However, this will not improve their liability or accuracy of the mechanical gauge. One of the major disadvantages with float driven instruments is the continuous sudden movement due to the turbulence of the liquid gauged. These movements, which can be rather violent, cause a continuous acceleration and deceleration of the drive mechanism, resulting in excessive wear and tear of the local indicator, transmitter and other devices coupled to the gauge. The indicating system and transmitter cannot follow the reversing motions and accelerations. Often the gear mechanism, driving the indicator and transmitter shaft, disengages, resulting in erroneous readings and de-synchronization of the transmitter. This leads to a considerable maintenance and lack of measurement reliability. In light of the present worldwide concern to prevent product spills, these gauges should no longer be used. Because of their low price, however, a large share of the world’s tanks is still equipped with these instruments. 1.3 Servo gauges Servo tank gauges (Fig. 3) are a considerable improvement over the float driven instruments. They were developed during the 1950s. In this case, the

Fig. 3. Servo level gauge.

float is replaced by a small displacer, suspended by a strong, flexible measuring wire. Instead of a spring-motor, servo gauges use an electrical servomotor to raise and lower the displacer. An ingenious weighing system continuously measures the weight and buoyancy of the integral transmitter. Mechanical friction in the servo system and local indicator has no effect on the sensitivity and accuracy of the gauge. Also, turbulence has no direct effect. An integrator in the serve control system eliminates the effects of sudden product movements. The gauge not only produces an average level measurement under turbulent conditions, but it also eliminates unnecessary movements and reduces wear and tear, greatly extending the operational life of the instrument. The original servo gauge does not look much like today's modern version. The instruments have evolved into highly reliable mature products, and are gradually replacing mechanical float gauges, cutting down on maintenance and improving on inventory results. Modern intelligent servo gauges have very few moving parts, resulting in long-term reliability and accuracy. They also have a high degree of data processing power. The instruments do not merely measure the liquid level but are also capable measuring interface levels and product density. Accurate, programmable level alarms are standard. Accuracy's of better than 1 mm over a 40 m range can be attained. The exceptional accuracy and reliability has resulted in the acceptance of the measurements and remote transmission, by Weights & Measures and Customs & Excise authorities in many countries. 1.4 Radar gauges The use of radar to measure product levels in storage tanks is one of the most recent techniques. Radar level gauges were developed in the mid sixties for crude carriers. The majority of these ships were equipped with mechanical float driven gauges. The level gauges were only used when the ship was ashore, loading or unloading. New safety procedures for tank washing with closed tanks during the return voyage, and the necessity to fill the empty tank space with inert gas, made this technique preferable. Accuracy was less important for the level measurement of the cargo tanks, since custody transfer and fiscal measurements used the certified level gauges or flow meters of the shore installation. Radar level gauges do not have moving parts and only an antenna is required in the tank. This results in very low maintenance cost. Although the investments costs are higher when compared to float gauges, the cost of ownership will be considerably lower. The radar instruments use microwaves, generally in the 10 GHz range, for the measurement of the liquid level. The distance the signal has traveled is calculated from a comparison of transmitted and

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Fig. 4. Radar level gauge for free space measurement reflected signals. With tank gauging, relatively short distances have to be measured. Electromagnetic waves travel with nearly the speed of light. Because of the short distances (1 to 35 m is typical) and the required resolution, a measurement based on time is almost impossible. The solution is to vary the frequency of the transmitted signal and measure the frequency shift between transmitted and reflected signal. The distance can be calculated from this frequency shift. Now radar level gauges are available for product storage tanks found in refineries, terminals, chemical industries and independent storage companies. The absence of moving parts, their compact design and their non-intrusive nature, result in low maintenance costs and make them very attractive. Older radar instruments were equipped with large parabolic or long horn antennas, whereas the modern radar level gauges use planar antenna techniques. These antennas are compact and have a much better efficiency, resulting in an excellent accuracy. Several antenna types are available to suit virtually every tank configuration: • Free space propagation is the most common method and is used if the gauge is installed on top of a fixed roof tank (Fig. 4). • On floating roof tanks, the radar gauge can be installed on still-pipe (Fig. 5). • Radar gauges can be also used on high-pressure storage vessels (Fig. 6). Typical example is an LPG storage tank. An isolation valve can be installed between the vessel and the instrument. Verification and calibration is possible while the instrument remains in service.

Fig. 5. Radar level gauge for mounting on still-pipe

Fig. 6. Radar gauge for LPG storage tank. Radar gauges are also a logical choice for tanks containing highly viscous products, like blown bitumen, contaminating products and liquids that are very turbulent. 1.5 Temperature measurement Accurate temperature measurement is essential for level based tank gauging systems, including custody transfer and accurate inventory measurement in liquid bulk storage tanks. Multiple Spot Thermometer (Fig. 7) measures temperature with a number of PT 100 spot elements placed at different heights to provide a tank temperature profile and an average temperature. Only the fully immersed elements are used to determine product temperature. Installation is simple even if the tank is in service and the reliability is good. In pressurized tanks the Multiple Spot Thermometer can be installed in a closed thermo well so that it can be removed for service or inspection while the tank is in operation. The graph of Fig. 8 shows that single spot measurements are unsuitable to accurately measure the temperature of products that tend to stratify. The effects of temperature stratification can be neglected only for light products, mixed frequently. In general, average temperature measuring elements are used in case of temperature stratification.

Fig. 7. Multiple Spot Thermometer

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Fig. 8. Temperature stratification in a storage tank The latest development is the Multiple Spot Thermometer shown in Fig. 7 which utilizes up to 16 thermo sensors evenly distributed over the maximum possible liquid height. A very accurate class A Pt100 element at the bottom is the reference. Accuracies of better than 0.05 °C are possible. The elements can also be individually measured to obtain temperature profiles and vapor temperatures. Multiple Spot Thermometers are available with both nylon and stainless steel protection tubes. It provides a rugged construction suitable for the harsh environments of a bulk storage tank. Another type of average temperature measuring element is the Multi-Resistance Thermometer. Its operation is based on a number of copper-wire temperature sensing elements of different lengths. Average temperature measurement is achieved by measuring the longest fully immersed resistance thermometer chosen by a solid-state element selector. A drawback of Multi-Resistance Thermometers is the delicate construction of the elements. The very thin copper wire used makes the device susceptible for damage, especially during transport and installation. 1.6 Pressure (density, vapour pressure) measurement When the radar gauge is connected with a pressure transmitter (Fig. 9) near the bottom/top of the tank, the density/vapour pressure of the product can be calculated and presented on-line. The accuracy of the density calculation largely depends on the accuracy of the pressure transmitter.

Fig. 9. Pressure transmitter

1.7 Water level measurement The Water Level Sensor (Fig. 10) continuously measures free water level below the oil surface and provides input for on-line net inventory. The sensor can be integrated with the Multiple Spot Thermometer. The Water Level Sensor is welded to the Multiple Spot Thermometer to get a hermetic

Fig. 10. Water Level Sensor integrated with Multiple

Spot Thermometer design and it has no moving parts, because it uses a capacitive sensor. The output signal is connected directly to a radar gauge. There is a PT100 temperature sensor inside the probe, allowing measurements at low levels. The Water Level Sensor is designed for difficult applications in corrosive environments and is detachable for repair. It can be used for crude oil applications and for lighter fuels (such as gasoline).

2. TANK GAUGING – THE ULTIMATE TOOL FOR THE OIL STORAGE INDUSTRY

Each user and system requirements depend on the type of installation and operation. The following types of operation, each having its own specific requirements, can be categorized: • Operational control • Custody transfer • Inventory control • Quantity assessment • Leak detection • Overfill protection 2.1 Operational control Generally tank content measurements for day-to-day operational use, for scheduling purposes and for blending programs do not require the same accuracy as custody transfer operations. However, measurement reliability and repeatability are important. Reliable level alarms are also a must to operate safely. A high degree of accuracy and reliability will allow operations to safely use the maximum tank capacity. Past experience indicates that a 5 % storage capacity gain can be achieved. Oil movements generally have very strict equipment

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requirements. They specify compatibility with their supervisory control and management systems. For oil movement and operations, either mass or volume measurement techniques can be used. Volume can be derived from level only; mass can be measured directly by means of pressure transmitters. Additional information can be obtained by measuring vapor temperature and pressure. Density measurement can also be added, with accuracy's from 0.5 % up to 0.1 %. Whichever technique is selected, it should be compatible with the operations of all parties using the data from the tank gauging system. As stated earlier, plant management and control systems can facilitate oil movement and operations. Maintaining data integrity from the field to the receiving system is essential. A high degree of integration of the transmission of field-instruments is a pre-requisite. However, as long as a worldwide standard for digital communication is missing, different protocols will be in use. 2.2 Custody transfer Many installations use their tank gauging system for custody transfer measurements of oil products. A tank gauging system is a very cost effective and accurate solution compared to flow metering systems, especially when high flow rates are present and large quantities are transferred. When flow-measuring systems are used, however, the tank gauging system offers a perfect verification tool. Where custody transfer or assessment of taxes, duties or royalties is involved, the gauging instruments and inventory control system are required to be officially approved and certified for this purpose. In countries where such legal certification does not yet apply, surveying companies often carries out verification of the measurements. They generally use dip tapes, portable thermometers and sampling cans to measure level, temperature and density prior to, and after the product transfers. This is labor intensive and requires considerable time. Surveyors use the same procedures to calculate volumes or mass as do modern tank gauging systems. Hence, the presence of a reliable certified accurate tank gauging system facilitates their surveys and will reduce the turn around time. 2.3 Inventory control Inventory control is one of the most important management tools for any refinery, terminal or storage company. Inventory represents a large amount of assets for each company. Tank inventory control is either based on volume or mass. However, neither volume nor mass is the sole solution for accurate and complete inventory control. Products received internal product transfers and delivered products of refineries, chemical plants and terminals are quite commonly measured in often incompatible volumetric or mass based units. Conversions from volume to mass and vice versa have to be frequently

made, so that all measuring parameters like product level, water interface, density and temperature measurements are equally important. The combination of volume and mass as realized in hybrid systems provides the most attractive solution. In-plant accuracy requirements for inventory control are often non-critical. The measurement uncertainties do not result in direct financial losses. Reliability and repeatability are much more important. Independent storage companies and terminals that strictly store and distribute products, owned by their customers, cannot operate without an accurate inventory control system. Such system should be very reliable, very accurate and should provide all inventory data. 2.4 Quantity assessment The uncertainties of quantity assessment of a tank gauging system depend on the measuring uncertainties of the installed instruments, tank capacity table and installation. Level gauging instruments measure the liquid level in the tank. Pressure transmitters measure the hydrostatic pressure of the liquid column. Both level and pressure are primary functions for the calculation of volume and mass respectively. Conversions from volume to mass or vice versa are made using density and temperature as secondary inputs. The density input may be obtained from an outside source, such as a laboratory, or may be measured in the tank by using pressure transmitters or servo density. The temperature input is obtained from a temperature measuring system in the tank. 2.5 Leak detection For many decades the oil industry has been concerned with the financial consequences of oil losses. In recent years, there has also been an increased awareness of the industry's environmental impact. Pollution, caused both by liquid spills and atmospheric emissions, is an area of increased concern, caused the industry to initiate programs to reduce the risks of environmental damage. Maintaining an accurate leak detection program is a necessity for any environmentally conscious tank farm owner. The basis for reliable leakage detection in a storage tank is to have an accurate level measurement. A high accuracy leakage detection system can be based on level measurement only if the product temperature is reasonably stable. Changes in product temperature will change the product volume, which can be observed as an increasing or decreasing level. For these tanks, the leakage detection system should operate on standardized volume and therefore the product temperature also should be measured. Factors which normally are important for accurate level and volume measurement, such as changes of tank height due to tank bulging effects, accuracy of

tank capacity tables etc., have little influence on a leakage detection system, since these factors are negligible in static leak alarm conditions. Accuracy in level and temperature measurement, system reliability, and the lack of measurement drift, are factors with significant influence for a leakage detection system. 2.6 Overfill protection With radar gauge it is possible to have both overfill protection and simultaneously use the gauge for continuous level measurements. This radar gauges have to be approved as overfills protection devices by an independent certification organism (like TÜV, for example). This leads to cost savings as there is no need for two independent systems. The gauge replaces a traditional Hi-Hi switch. At the same time, environmental and economical advantages are gained through spill prevention. A traditional Hi-Hi switch (e.g. mechanical, capacitive, vibration, ultrasonic) is activated only when the product level reaches a certain point and requires manual testing.

3. NEW APPROCH IN TANK GAUGING: REDUCING PROJECT COST AND DOWNTIME

In order to present this new approach, we wheel examine a case study: Installation of tank gauging system within Nord-Benzina Tank Farm of VEGA Refinery, Ploiesti (The Rompetrol Group). The main contractor of The Rompetrol Group, Rominserv S.A., was in charge with the management of this new project. The project started with only two tanks: A1 and A8. The goal of the project was the online measurement of level, temperature and density in each tank and the transmission of theses parameters to control room. This was accomplished by installing on each tank a radar gauge, a multi-spot thermometer and a pressure transmitter near the bottom and by installing Operator Interface Software in control room. All this equipment was already in stock. We only needed to install it and transmit the signals to control room. Our client did not approve the burying of cables. Electrical power was available from the electric panel located 350 m away from the tanks.

Fig. 11. Approximate layout of Nord-Benzina Tank

Farm (tanks A1, A8 and control room).

3.1 The classic approach The classic approach consists in laying the cables on trestle bridge (Fig. 11). The client requested that this trestle bridge to be sized to support the total number of 16 cables, because Nord-Benzina Tank Farm has a total number of 8 tanks. For each tank, it is necessary to use two cables: one for radar gauge power supply and one for signal transmission. The total length of this trestle bridge is around 400 m. The classic approach leads to an amazing cost of around 50 000 USD (procurement and installation work for trestle bridge and tank gauging installation work) and a 1÷1.5 month downtime. 3.2 The new approach The new approach consists in using stand-alone photovoltaic (solar) power systems and radio modems instead of cables (for power supply and signal transmission) and trestle bridge. Tank gauging system mounted on every tank has about 100 W power rating. Cables are still needed, but only to locally interconnect radar gauge with multi-spot thermometer, pressure transmitter, solar panel supply system and radio modem to create an autonomous tank gauging system (Fig. 12).

Fig. 12. Autonomous tank gauging system The solar panel supply system (Fig. 13) has the following components: • Solar panels (1) are used for converting solar energy into electricity; • Solar charge controller (2) is used to regulate the power flowing from a solar panel into a rechargeable battery; • Inverter 12V DC/220V AC (3) is used for converting direct current (DC) into alternating current (AC). • Rechargeable batteries (4) are used for electrical energy storage over the night.

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Fig. 13. Solar (photovoltaic) panel supply system Radio modems (Fig. 14) are used instead of cables to make wireless connection between Operator Interface Software PC and the radar gauge. Depending on topographical conditions as well as gain and installation height of the antenna, the operating range is 2÷40 km. In case there is a need to extend the coverage, a radio modem can be used as a repeater equipment and field. This new approach leads to an acceptable cost of less than 25 000 USD (procurement and installation work for 2 pcs. solar panel supply systems and radio modems and tank gauging installation work) and less than 1 month downtime.

Fig. 14. Radio Modem.

4. CONCLUSION This paper has presented an overview of tank gauging measurement techniques and equipment. In cases where electric energy, needed for tank gauging equipment, is not available or to costly to install we can always use solar (photovoltaic) panel supply systems. In cases where the distance between storage tanks and control room is long and requires trestle bridge, we can always use radio modems. In our project analysis we concluded that producing electric power using photovoltaic panel supply system (for 100 W power rating equipment) is cheaper than transporting electric power to tanks locations, on trestle bridge. This is because of low procurement cost for 2 pcs. photovoltaic panel supply systems (8 000 USD) and installation work compared to building trestle bridge. The advantages of solar energy are: renewable energy, free energy, it needs no fuel, produces no pollution, equipment has no moving parts and does not require much maintenance. Complementary to using solar energy, in order to eliminate trestle bridge, we used wireless connection (radio modems) to transmit all the signals to control room. This technique is becoming a standard for today applications in industrial environment due to low cost of investment and very low maintenance. In this way, we can obtain a reliable autonomous tank gauging system. Most of tank gauging manufactures provide support for wireless connection. This means that we can procure the radio modems from the same manufacturers of tank gauging systems. These manufacturers do not supply the solar panel supply systems, in most cases. In this situation care should be taken in order to assure compatibility between tank gauging power requirements and solar panel supply systems capabilities. This compatibility can be assured only by judicious solar energy estimation for every location and sizing. The sizing of solar systems is affected by nebulous sky. This is a parameter that cannot be very accurately estimated unless statistic meteorological data are available. The result of sky nebulosity estimation is over-sizing of solar panel supply system. This must be correlated with the fact that the solar panel produces in summer about 5 times more electric energy than in winter. This way, in sunny days we obtain an excess of electric power that can be sold to the local electric company. Because of these arguments, solar energy has a very high potential in countries that get enough solar radiation. Romania has a high potential using solar energy, compared to other European countries like Germany, Austria, Belgium and Nederland.

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