Hoses play an important role in hazardous area operations and owing to their direct interaction with moving liquids and powders are especially at risk of becoming electrostatically charged.
Discharges of static electricity from hoses are known to cause the ignition of combustible atmospheres during the transfer of material to or from vacuum tankers and road tankers. Here we discuss the three main reasons why electrostatic discharge from hoses occurs.
One reason is that non-conductive hoses are used to transfer material. Nonconductive hoses are capable of accumulating and retaining high levels of static charge which can result in incendive brush discharges from the hose itself, or the charging of isolated conductive objects attached to the hose like a nozzle or coupling that can discharge a spark themselves. It is generally accepted practice within the hazardous process industries that non-conductive hoses should not be used to transfer potentially combustible liquids and powders and numerous standards and industry association publications repeat this recommendation.
Another common reason for static spark discharges from hoses results from connecting conductive hose, or interconnected conductive hose sections, to a vacuum tanker or road tanker that does not have a verified static ground connection.
The third most common reason for static spark discharges from hoses is where the conductive components of the hose structure become isolated during normal activity.
Both the second and third modes of electrostatic discharge are the most relevant to the hazardous process industries, and are scenarios where improper use of conductive hoses can lead to the accumulation and discharging of static electricity within a combustible atmosphere.