FLOW & LEVEL CONTROL
A COST-EFFECTIVE SOLUTION TO PREVENT TANK OVERFILL AnnCharlott Enberg, functional safety manager at Emerson, explains how radar gauges with innovative 2-in-1 technology can provide two independent layers of protection against spills, in compliance with the relevant safety standards.
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hen process vessels or storage tanks contain hazardous, flammable or explosive materials, overfills can have catastrophic consequences, including injuries or even death to personnel, as well as significant asset damage, extensive environmental harm, and the blighting of an organisation’s reputation. For these reasons it is essential to invest in a robust safety instrumented system (SIS) that complies with the relevant industry standards. There are two key global standards for overfill prevention. These are: • The International Electrotechnical Commission’s IEC 61511 standard, which outlines best safety practices for implementing a modern SIS within the process industry. IEC 61511 is an industry-specific adaptation of IEC 61508, which is an industryindependent standard for functional safety. • The American Petroleum Institute’s API 2350 standard. This provides minimum requirements to comply with modern best practices in the specific application of non-pressurised above-ground large petroleum storage tanks. Organisations rely on accurate and reliable level measurement instrumentation in both their SIS and their basic process control systems (BPCS). The technology of choice for SIS sensors has historically been mechanical point-level switches. Although this type of sensor has a lower initial purchasing cost than continuous level measurement technology, it does not provide any
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May 2020
online measurement, so it is virtually impossible to know whether it is functioning correctly. These switches consequently require frequent ontank proof-tests, resulting in tank (and possibly process) downtime and risking the safety of personnel who need to climb tanks to perform the tests. For this reason many organisations now use modern continuous level gauges instead. The dominant level measurement technology for BPCS is non-contacting radar. Radar technology provides good levels of accuracy, which is crucial because even a small inaccuracy in level measurement can equate to thousands of gallons of volume uncertainty. It is also extremely reliable, has minimal maintenance requirements, and provides availability close to 100% during the long lifespan of a device.
Diverse and identical separation There is a common misconception that the key standards for overfill prevention require the level measurement technology used for the SIS to be of a different type to that used for the BPCS. This requirement is typically referred to as diverse separation and is one reason why some organisations still use less sophisticated technologies such as pointlevel sensors, servo gauges and floatand-tape gauges for their SIS. However, it is confirmed within IEC 61511-2 that it is legitimate to use the same technology for both the SIS and the BPCS. This is known as identical separation, and IEC 61511 states that this method ‘may have some advantages
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Bulk liquid storage facilities require accurate and reliable level measurement instrumentation within both their basic process control systems and their safety instrumented systems.
in design and maintenance because it reduces the likelihood of maintenance errors’. Diverse and identical separation are both valid options. However, diverse separation introduces extra complexity and makes human error more likely, because personnel would need to learn about installing, configuring, prooftesting and maintaining two different technologies rather than just one. There is an increasing realisation that reducing maintenance and similar ‘handling errors’ is critical – by some estimates, 75% of industrial accidents are traceable to organisational and human factors. In this context, the Buncefield oil storage terminal fire of 2005 provides a case in point. Buncefield had redundant and diverse technology for overfill prevention, but the level measurement device acting as the high-level alarm was inoperable because of human error. It Control Engineering Europe
