When flowing high purity hydrogen through an insulative tube, could a charge develop in the tube? The application in question is a system for filling stratospheric balloons with hydrogen lift gas. The system will be flowing > 99.2% hydrogen through a polyethylene film (insulative) pipe. Would a significant charge develop solely due to the gas flow? Does this apply both to conductive and non-conductive piping?
The HSP discourages the use of this type of piping for hydrogen despite the low likelihood of ignition within the pipe itself. The polyethylene film can indeed develop a charge and, depending on the film thickness and substrate wall material, will eventually produce an electrostatic discharge. Since the nominal hydrogen concentration is so high (and assuming the purge-in and purge-out procedures preclude flammable mixture formation), these electrostatic discharges should not produce an ignition in the pipe itself. However, besides this type of pipe not being a good practice for flammable gas flow, there is concern about an electrostatic discharge possibly occurring at the pipe exit and at any junctions between pipe sections.
One particularly energetic type of electrostatic discharge is a propagating brush discharge, which occurs when a thin film of an insulating material with a high charge retention capacity is attached to a conductive material. The propagating brush discharge occurs over a large section of the insulating film and might extend beyond the pipe exit if the polyethylene film extends to the exit. If the hydrogen is flowing into an air atmosphere, an electrostatic discharge at the pipe exit could ignite the flammable mixture formed near the exit. If the pipe wall itself is combustible, a local ignition from leaking hydrogen could lead to pipe degradation and eventual containment failure of the pipe.
As a good practice, hydrogen piping should be conductive and bonded/grounded to avoid electrostatic charge generation from a variety of mechanisms including atmospheric discharges. The hydrogen gas itself should not develop an electrostatic charge during pipe flow, but the gas flow can entrain pipe scale or other contaminant particulates that develop electrostatic charges that can ignite the hydrogen vented into air. Autoignition during emergency venting of hydrogen at pressures greater than 200 bar can occur due to shock wave compression heating near the vent exit, but it’s doubtful this would occur during controlled filling of balloons because of the lower velocities expected in that application.