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Throughout the bulk liquid storage industry it is increasingly being viewed as "good practice" to provide some form of overfill protection on above ground storage tanks. Worldwide government regulations and requirements that companies must follow differ and a few companies put into practice tighter regulations on a voluntary basis. For those companies it is all about safety and environmental issues. In the US a small spill could cost a tank farm or terminal $3-4 million in clean up and fines. For this price, new level alarm devices could be placed on a few hundred tanks for independent overfill protection and ensure safety.
General Overfill Options
It is widely agreed that high and high-high level alarms provide adequate protection. The height of the alarms on the tank will depend largely on company, local, state or federal regulations and are based on the individual application, tank height and product transfer. Some considerations include:
- The measurement of the level for overfill protection is independent of other gauging systems installed on the tank.
- High and high-high level alarms that can bee seen or heard by the operator controlling the tank transfer, such as audible sirens or horns and visual lights.
- A device that when activated can automatically trigger a shut down procedure for the process causing the alarm.
More stringent regulations may be required for facilities with:
- Permeable ground materials that are inside the containment area
- "Old" tanks that can be more susceptible to corrosion.
- Tanks near surface water such as rivers, wetlands or lakes
You may also refer to specific industry guidelines and instruments that provide approvals and guarantees for overfill protection applications. For the oil & gas industry API 2350 defined by the American Petroleum Institute (API) for high level alarms and SIL, SIG or TUV approvals are appropriate.
Measuring for Overfill
There are two defined ways to measure for overfill protection, single point measurement and continuous measurement. Within each group there are a number of technologies that can be utilized successfully.
Single point measurement
For single point measurement, an instrument is installed at the high and/or high-high level and will sense when the product reaches that level. The most common of which are simple stand alone mechanical switches or electronic tuning fork technology. Single point measurement devices tend to be simple, very reliable, low cost units that can be installed easily.
Continuous measurement
For continuous measurement, a standard level gauge is a common solution. Any of the major gauging technologies can be used, float and tape (mechanical), servo, radar or magnetostrictive technologies. Each provides their own benefits based on the application. Continuous measurement is favored by some over single point measurement. Your operators can check the status of the gauge at any time and the gauge's operation can be monitored for deviation against the primary level device. The main benefit - you know if there is a problem with the gauge before it becomes an overfill problem and your operators do not need to climb a tank to check the operation.
Example real-world solutions
Radar Tank Gauge Application
In a small terminal 10 tanks had existing 2500 ATG with transmitters that provided a primary level measurement back to the office for inventory control. Each tank also had an independent mechanical float switch. After many reliable years of service the terminal manager could no longer rely on these old switch devices for overfill protection.
The Varec solution - install a 7240 Radar Tank Gauge on each tank to provide independent and continuous level measurement for overfill protection. The loop-powered radar utilized the same two wires that where used by the mechanical switch. The gauge was also installed on a small, 1.5" NPT fitting on the top of the tank. This resulted in a very low cost installation.
The existing Float and Tape gauges continue to provide inventory control grade measurements that are appropriate for the terminal operations. The two systems, both of which are continuous measurements, can be check against one another for operation and accuracy of measurement. Finally, the SIG/TUV approval for this application gives the terminal owner extra peace of mind knowing the local environment is well protected.
Mechanical Level Switch Application
A major oil company applies risk assessments values to processes in their terminals and sets minimum guidelines. After assessment, one facility in Latin America, without any tank gauging or measurement of any kind was required to meet their minimum rating. In this case, each of the eight, Mogas cone roof tanks would require a level gauge and high level alarm switch. This switch would have to be independent of the gauge and also follow API 2350 guidelines for high level alarms.
Based on the tank types and measurement accuracy needed for the refined products stored and distributed at the terminal, float and tape level gauges (2500 ATG) along with a mechanical (displacement) level switch were selected. The level switch provided independent high-high measurement and testing. A lever could be pulled to perform a routine manual test required by the site on monthly bases. The installation offered a non-obstructive approach to the day-to-day operations of the facility and provided the most economic, reliable and user friendly solution for this particular terminal and its employees The terminal is now in a position to upgrade its tank gauging system and company safety level by installing transmitters with inputs for temperature as the need arrives in the future.
Tuning Fork Application
The same oil company applied the risk assessment to a facility in Florida, USA. The facility currently met their internal requirements but the existing 26 high level alarm devices (2 devices per tank) were deemed obsolete technology. Based on age of the in-fared technology, the manufacturer was unable to support the facility for maintenance and repairs. Each switch, high and high-high on each tank, was completely independent from one-another was feed into an electrical panel that provided audible and visual indication to the operators. They were also located at grade level for easy access and the sensor was positioned in an overfill pipe design as some of the tanks contained internal floating roofs. Located in Florida , the site also needed to maintain state regulations for overfill protection.
Varec proposed a direct replacement device that utilized tuning fork technology. This allowed the customer to use the existing wire, conduit and mounting arrangement which meet the original internal risk assessment guidelines. The new level switch also requires no calibration and is tested with a compact device that carries a SIL rating of II, as a complete solution a distinct advantage over the older system. During the upgrade a new, compact electrical panel was also installed that allowed each tank to be out-fit one at a time, with no disturbance to the daily operations of the terminal.
Moving forward
Before a particular solution is chosen and a system installed into your facility your application and operating requirements should be clearly defined and any regulations known. Understanding the implications of each factor will allow selection of the most appropriate system for your needs. For tanks with no gauging or systems the choice may be clear and simple based on the alarm requirement, the tank and the application. If you have existing gauging it may be possible to use that system for standard daily inventory tank gauging measurements and install a new, independent measuring device for overfill protection, or visa-versa. |
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