2.3.3 Static Electricity

The uncontrolled discharge of accumulated static electricity may generate sufficient energy to ignite ethylbenzene vapours in air.

Electrical charges can build up on poorly conductive surfaces of equipment until the dielectric strength is exceeded and a spark develops. Countermeasures to minimize this electrostatic hazard are to bond all metal objects preventing possible potential differences and to ground the entire system. Liquids in tanks with a non-conductive coating (e.g. epoxy coating) should be earthed.

When handling a static accumulator cargo, such as ethylbenzene, the product may pick up sufficient charge during transfer to constitute a hazard. The charge may arise from flowing through pipelines, and especially through filters (small mesh), and also from splashing. Submerged filling is recommended to avoid splashing. The inlet transfer line should end at or near the bottom of the tank to make electrical contact with the tank, hence to eliminate uncontrolled electrical discharge. The initial loading pipeline velocity should be kept below 1 m/sec. until the liquid level guarantees submerged filling.

In order to prevent charge accumulation, it is advised to include sufficient fluid residence time downstream of pumps and filters. The electrical resistance of the complete loading system between tank and loading arm should not exceed 10 ohms. For ships, electrically insulating flanges on the loading line as close as possible to the ship’s manifold (liquid and vapour line), having a minimum resistance of 1000 ohms protect against stray currents. In the absence of in-line sampling, a waiting period of 30 minutes is sufficient between loading and sampling.

Operators wearing rubber-soled shoes, especially on floors made of insulating materials, can pick up considerable static electricity, and should exercise appropriate caution.

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Patrick Detournay
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