The image of a kid causing the tub to overflow at bath time may make for stock comedy, but when gasoline starts spilling out of a tank thousands of times bigger, it’s can result in a tragedy. Overfill incidents happen at chemical processing plants and tank farms with surprising regularity. Fortunately, they don’t all result in catastrophe, but even a small one can create costly environmental incidents and product loss.
Real-world experience says these incidents often happen because operators try to put liquid into a tank without allowing for the existing contents. Why? Because they don’t have an accurate measurement of the existing contents due to poor level instrumentation. This lack of poor operator awareness is one of the topics addressed in an article by Lydia Miller and AnnCharlott Enberg in the February 2019 issue of Flow Control titled A Systemic Approach to Storage Tank Overfill Protection. The article cites two well-known overfill disasters and explains how they had the same cause.
Operators must have an accurate, complete and real-time picture of the contents of every tank. One common recipe for an overfill incident is when operators begin pumping liquid into a tank without realizing it is closer to being full than they thought. It should be easy to see the contents of any tank from the control room, but this depends on having accurate and reliable level instruments for every situation. In both the Buncefield and CAPECO incidents, level instruments were malfunctioning.
But tank filling shouldn’t depend entirely on the awareness of operators, should it? Shouldn’t there be a mechanism to sound an alarm and turn off the flow when the tank reaches its safe limit? Yes, there should, but again it depends on effective instrumentation.
Automated filling control systems must be equipped with a safety system capable of stopping the filling operation before the tank begins to overflow. Such an SIS involves three functional components: a level instrument, a logic solver and final control elements. The actual design of an SIS is complex, and there are experts to consult for help, but to understand the basic concepts, the following describes each element individually and explores valuable advances. The level instrument can use a variety of technologies to provide a continuous or point level measurement. It must be able to function independently of any other instrument on the tank, and it must have this safety function as its primary duty. Vibrating forks are commonly used in this application because they are simple and highly reliable.
Vibrating fork devices, like the Rosemount 2120 Level Switch, are available with safety certification and can be used in shut-down systems. For continuous level readings, a non-contacting radar instrument, such as the Rosemount 5900S Radar Level Gauge, is also available with safety certification, delivering high-precision measurements over a wide range. Such instruments can be critical to effective safety systems.
Effective tank management involves safety but should encompass far more. It begins with accurate and reliable level measurement instruments using the best technology for the application. Data must be collected, processed and displayed appropriately to provide a high degree of operator awareness. An effective system integrated with sophisticated instruments avoids many of the failure modes common to older technologies and can even perform self-diagnostic routines to verify every element is functioning correctly. In the event of an instrument failure, an alarm is activated to make operators aware of the issue.
All of these elements work together to ensure safe and positive operation for terminals and smaller facilities storing feedstock and finished products. You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Level Group and other specialty areas for suggestions and answers.
Posted by Deanna Johnson, Director Integrated Marketing Communications for Machine Automation Solutions
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