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Secure terminal tanks with floating roof overspill prevention

Published by , Editorial Assistant
Hydrocarbon Engineering,

Floating roof tank overspill situations are dangerous and extremely expensive. Equipment failure is one of the most common causes of overflow. The only thing worse than a failing instrument is not knowing that the instrument is not functioning. SIL2 certified instruments are the solution.

Two common reasons for level measurement instrumentation failure

Level measurement instrumentation that measures petrochemical media is subject to numerous chemicals, creating a harsh environment for the material that the probes are constructed from. But it is not only a matter of simple corrosion. Parafin in the oil may be suspended in the tank, which can coat and stick to probes causing false alarms, eventually even destroying them.

In the long run, this coating is unavoidable and must be accounted for even in high-reliability systems. The introduction of brass probes, supported by a strong flexible cable, has made floating roof alarms significantly easier to clean without the need to demount them, lowering the risk of damage as the need for reinstallation is minimised.

The brass probe is connected to a dual-compartment housing that separates the operating circuits and sensor wiring from the customer’s wiring. The probe does not need further calibration or setpoint adjustment, and an adjustable cable establishes the position of the trip point for the alarm. To prevent false alarms this type of hi-level system will automatically recognise and ignore coatings, and be supplied with continuous self-test monitor circuits and sensing elements to detect eventual faults.

SIL2 certified safety

The ideal solution to ensure the functionality of switch-probes is a SIL2 certified product. A SIL2 certified product is required to have auto-verification of the functionality of the probe. SIL2 certification ensures a very high ‘Safety Integrity Level’ in the system. The SIL2 approval ratings cannot be obtained merely by following a set of rules. Instruments must be examined by an external agency before the SIL2 certificate is issued. A SIL2 certificate will assure you of a certain reliability level as it relates to your installation. Meeting overfill protection requirements also guarantees that the instrument has auto-verify functionality, enabling it to check that the probe and cables are properly connected, and that it will function as expected in a real-world alarm condition.

The floating roof will often expose the probe to a solution of rainwater mixed with the oil or fuel contained in the storage tank. A spill prevention system must therefore be mounted on top of floating roof tanks. The probe will activate the alarm, not only when it comes in contact with water, but also with petroleum, or even with the floating roof itself. If the roof is dry when making contact, the alarm is just as important.

High-level or high-high level measurement systems should ideally meet API 2350 regulations and have a functional safety certified as SIL2 under IEC61508-2, ed2, 2010 (Exida).

Prices for SIL2 certified tank level switches

Level measurement instrumentation prices vary significantly. Quite commonly, the SIL2 certified instruments will be priced higher than the ones without SIL2. The certification process sets stringent demands on the manufacturing process and the reliability of the individual components. SIL2 ensures safety of the entire product.

If safety is a concern at a tank storage facility, there is no doubt that SIL2 certified floating roof spill prevention systems should be a priority.

Instrumentation investments should never be measured solely on the purchase price, but also against expenses incurred by false (or worse yet, no) alarms. Additionally, when comparing prices on instrumentation, one must consider the need for service and calibration, and the longevity of the level measurement instrument. The return on investment will often be very positive when all of these factors are taken into consideration – even with the added cost of SIL2 instrumentation.

More information about SIL2 and floating roof tank instruments

Based on more than 50 years of experience as a supplier for the oil industry, Drexelbrook has a vast knowledge base available with information on hazardous zone level instrumentation. In the company’s Learning Zone, you can also read an article that contains more details on SIL2 certifications.

Drexelbrook offers the Floating Roof Tank Spill Prevention System with SIL2 and many global and local safety certifications. You can learn more about the Floating Roof Tank Spill Prevention System here.

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