Warning about dangerous hoses (in view of electrostatics)

There is a type of hoses that has a metal helix coiled around the outside and also one on its inside of. It is very common in petrol and chemical handling.

With this hose an ignition has recently happened in a chemical depot when loading a road tanker with toluene:
After loading the tank the hose was disconnected from the piping on the depot. After a few seconds an ignition happened inside the hose. The fire stopped by itself. A few seconds later, another ignition happened but this time more powerful. The operator suffocated the fire when he pushed his glove into the hose end. Both ignitions happened when the operator held the hose with gloves. He did not touch the metal coupling or touch any installations with the coupling when the ignitions occurred.
After the accident we measured the hose and found that it did not show any electric contact between the inner helix and the end fittings. When contacting the supplier of the hoses it was found out that the inner helix was coated with transparent polypropylene that insulated the helix from the end fittings. The inner helix and the inner walls of the hose will become electrostatically charged when the liquid flows thru it. When the hose is disconnected from the terminals later, piping air may enter the hose. If there is a spark discharge in the gap between the metal helix and the metal end fitting it may ignite the solvent gases.
Control of the internal conductivity in hoses may be done by inserting a dry sponge into the hose, well past the coupling. The sponge is then wetted with a conductive fluid, e.g. tap water. Then the resistance between the sponge and the end fitting is measured. The result should be less than 1 Mohm (1 x 106 ohm). Keep in mind, if there may be combustible fumes inside the hose an Ex-proof instrument must be used.
Note: it is not possible to control these hoses in view of their inner helix by measuring the resistance from one end fitting to the other one as the helix on the outside of the hose may give an erroneous result. Details on how to perform the control is stated in the standard ISO 8031, under
Original article: http://www.ind-ex.info/download/publications/Warning-about-dangerous-hoses.pdf?phpMyAdmin=f705c4f025fc044988a31f9c38a80980

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