Is a conductive (PE) system safe?
As has been demonstrated in the Spark Discharge Test, a charged conductive (PE) pipe can generate a spark of around 10 times the minimum ignition energy of petrol, based upon a 50m run of pipe. If you install conductive (PE) pipe, it is vital that you are aware of the risks. Conductive (PE) pipe is only safe, subject to the conditions described below:
WHEN THERE IS SUFFICIENT CONTACT RESISTANCE.
When connecting together two conductor elements, it is good practice to apply a force between the surfaces to be connected to reduce the contact resistance. A typical example of this is a bolted connection between a wire and a rod. But the conductive inserts used to bridge the internal conductive surfaces in hybrid conductive HDPE pipe are simply slid into the pipe bore without any device or method to ensure that a contact force is in place to maximise contact.
WHEN THERE IS SUFFICIENT CONTACT SURFACE
Some conductive inserts show a ribbed design to increase the contact force between the pipe bore and the insert. These ribs, while developing some force at contact points, reduce the contact surface, therefore affecting the electrical contact resistance.
WHEN NO IMPURITIES ARE ABLE TO GET INTO THE CONTACT AREA
In use, a film of insulating liquid (many fuels have insulating properties) can slip into the contact area and reduce or eliminate the conductivity of the electric contact between the insert and the pipe. The insert is exposed to the fluid flow within the pipe and the ribs integrated in the design will facilitate this film penetration.
WHEN A GAP INDUCING SPARK IS AVOIDED
In the event of intermittent electric contact interruption, such as by vibration or fluid turbulence, the very small gap between the conductive insert and the conductive inner bore of the pipe can be a dangerous source of a spark discharge.
BUT ONLY IF THE SYSTEM UNDERGOES A PERIODIC CHECK FOR ELECTRICAL CONDUCTIVITY
It is absolutely vital to undertake a periodic check, for instance EVERY 3 MONTHS, to check the electrical continuity of all the installed lines to ensure that none of the issues mentioned above is affecting the safety of the installation, in other words, that the system has not turned into a capacitor because the contact has become broken somewhere along the line. Be aware though that, to check the continuity of the line and that the installation is safe from static discharge, it will be necessary to disconnect each end of the line, test it and reconnect it. On a typical site, that could involve up to 30 tests and mean the digging up of the forecourt and the associated downtime and loss of income and profit.
For 500 sites, that could add up to:
• 60,000 tests per annum
• A cost which does not bear thinking about
• A cost which needs to be budgeted for when purchasing the system
• Considerable disruption to customers
• Closed forecourt
• Lost sales of fuel and from shops
• Huge cost and loss of profits
• All this for no serious valid benefit!!
"Some concern has been expressed that the electrically conductive liner within plastic pipe systems may be affected by medium or long term exposure to certain fuels. Degradation of the conductive liner may have serious implications from the point of view of electrostatic ignition hazard particularly if the liner is highly conductive. In this situation it is conceivable that discontinuities in the liner may result giving rise to electrically isolated sections which in turn may become charged by the flow of fuel and produce spark discharges."
An extract from research by G L Hearn B.Sc C. Eng M.I.E.E of Wolfson Electrostatics Ltd on the electrical/electrostatic performance of conductive and non-conductive polyethylene fuel pipe.
