Safety Insight: Are You Using the Right Hose for Combustible Dust Vacuuming?
Why is it Important?
Significant electrostatic charge can be generated where nonconductive hoses are used for vacuuming applications. The development of electrostatic charge can lead to various types of electrostatic discharge inside of the hose. If the solid particulate being vacuumed is combustible in nature, the discharge can ignite a dense dust cloud inside of the hose, in situations where thick layers or piles of dust are being vacuumed, causing a deflagration.
What Properties Should my Hose Have?
The National Fire Protection Association (NFPA) standards provide guidance with regard to properties of hoses used for vacuuming applications. A suitably conductive or antistatic hose will demonstrate one of the following properties: (1) An end-to-end resistance, not to exceed 108 ohm (100 megohms); (2) A liner Surface Resistivity less than 109 ohm/square, as measured by [1] ASTM D256 test or (inner liner) charge relaxation time less than one (1) second. Note: the latter two properties must be measured in a laboratory using the appropriate test conditions. Hoses incorporating metal wires for reinforcement must be connected so that the wires are bonded to the conductive components on each side of the hose connection and then grounded.
A Common Misconception
The hose I am using has metal reinforcing wires extruded in the liner and I have bonded the wire on both ends to the conductive tool in the hard pipe or conductive inlet to the portable vacuum cleaner so there is no danger of charge build up. Fact: If the construction material of the hose is nonconductive, charge can build up on the inside surface of the hose and lead to development of a propagating brush discharge of sufficient energy to ignite a combustible dust flowing inside the hose. The presence of the conductive wire may enhance this possibility.