A cost-effective solution to prevent tank overfill

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.

When 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 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 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

Bulk liquid storage facilities require accurate and reliable level measurement instrumentation within both their basic process control systems and their safety instrumented systems.

had been taken offline for testing and had been reinstalled incorrectly, and was therefore not functioning.

2-in-1 technology

Although many new installations use two radar level gauges for the BPCS and the SIS, some existing tanks have practical limitations that make it costprohibitive to install two separate level gauges. These include instances where only one tank opening is available, and where modifications would involve taking the tank out of service, resulting in additional costs.

One solution to this problem is Emerson’s non-contacting Rosemount 5900S 2-in-1 radar level gauge, which has been verified by thirdparty assessor Exida as fulfilling the requirements of IEC 61511 to be used simultaneously as a BPCS sensor and as an independent SIS sensor. This device – which is also certified according to IEC 61508 – consists of two galvanically separated and independent electrical units and a common antenna. The gauge utilises frequency modulated continuous wave (FMCW) technology, and a shift in transmission frequency during measurements. Signals from the two units, which are transmitted via the common antenna, will remain independent of each other. Therefore, when connected with its cables separated in different cable trays and with separate power sources, a single level gauge can be used for both BPCS and separate SIS sensor measurements.

The most obvious benefit of this configuration is that it requires only a single tank opening which allows for cost-efficient upgrades of existing tanks by replacing a single existing BPCS or SIS sensor with two continuous level measurements with a minimum of tank modifications. Often, a radar level gauge with 2-in-1 technology fits the antenna of earlier generations of devices and therefore requires no tank modifications.

The Rosemount 5900S consists of two galvanically separated and independent electrical units and a common antenna, and therefore requires only a single tank opening.

FLOW & LEVEL CONTROL

Conclusion

When selecting level measurement technology for use in the BPCS and SIS on bulk liquid storage tanks, technology diversification is not a requirement. Mixing technologies may even make the system less safe. It is more important to select a reliable technology with minimal maintenance requirements. This has led to many organisations installing two separate radar gauges to provide level measurement and independent overfill prevention on new tanks. For existing tanks, where an upgrade with two separate level gauges can be cost-prohibitive, 2-in-1 technology is a viable solution, enabling the simultaneous use of a single radar gauge as both a BPCS sensor and a separate and independent SIS sensor.

AnnCharlott Enberg is functional safety manager at Emerson.

80 GHz radar transmitters

Siemens has introduced the Sitrans LR100 series of 80 GHz radar transmitters with a narrow beam for flexible installation into existing vessel openings, or non-intrusive installation through plastic vessels.

The 80 GHz high frequency delivers robust and reliable measurements even in challenging environments such as those with vapours, condensation, turbulence, or solids.

The IP68-rated series consists of three products – Sitrans LR100 for basic measurement to 8m, Sitrans LR110 with communication and hazardous approvals options and range to 15m, and Sitrans LR120 with communication and a range of up to 30m and optional submergence shield for flooding protection. 2-wire loop powered with HART or optional Modbus RTU connectivity consumes very low energy and fast start up.

The dependable readings of these transmitters can help reduce engineers exposure to hazardous situations as there is no need to climb tanks, lean out over sumps, or crawl into confined spaces to maintain the instruments. Zero-meter blanking distance allows measurement right up to the sensor, avoiding costly overfilling. And 2mm accuracy enhances operational safety through precise measurement through the full range of the application.

Simple commissioning is achieved via the Bluetooth interface and the Sitrans mobile IQ App or the Sitrans RD150 remote display. In remote areas connected to the Sitrans RTU3030C remote terminal unit, critical data can be transmitted and the units can be remotely serviced.