The movement of volatile liquids through pipes or hoses can create static electricity. When the voltage reaches a certain potential, it can jump across to any grounded object or other object at a different potential. Sparks produced can possibly ignite or explode flammable vapors provided, at the moment, there is a suitable air/vapor mixture and that humidity conditions are suitable for such ignition.
Sparks from static electricity cannot occur if the equipment in any flammable liquid handling operation is firmly bonded or connected to a ground. This action dissipates static voltages as fast as they are generated.
If dispensing of flammable liquids is limited to only a few drums, the upper sketch shows how to create effective grounding. Note that all 3 drums are grounded to a No. 4 copper conductor that goes to a cold-water pipe ground (or an equivalent). The cable is in conduit merely to insure that no mechanical injury can happed to it and impair the grounding. If the container being filled is on the floor, it too must be connected to the same ground and the metal hose nozzle dispensing the liquid must touch the metal container being filled. If the receiving container is small enough to fit under the pump’s outlet, it must be attached to a ground wire. The fact that a metal container sits on a grounded metal drum is not a sufficiently sure contact to form an effective low-resistance ground.
When flammable liquids are dispensed from drums in racks, each drum must be individually grounded with a conductor / cable. The lower sketch shows a copper bus-bar attached to the drum racks. If desired, it could be replaced with a cable-in-conduit setup as shown in the upper sketch. Several extra grounding cables are provided to make contact with any containers being filled (as shown in the lower right hand sketch).
As liquids are withdrawn from drums, provisions must be made to admit air to the inside of the drum, otherwise a vacuum is created as the liquid flows out. Vacuum relief occurs in the emerging liquid stream, creating a turbulent and splashing flow. Also, in the summertime, liquids expand as room temperatures increases, creating high internal pressures in tightly closed drums. Some method of pressure relief is needed to prevent pressure leakage at bungs or weak drum joints.
All drums have either end bungs or one end and one side bung. Most manufacturers make pressure relief (spring loaded) devices that can replace the solid shipping bungs in either the end or side of a drum. Then, as pressure builds up, or as liquid is withdrawn, these relief devices can allow air to move back and forth, yet prevent the undue escape of vapors. Pressure relief devices should be listed by a recognized testing laboratory, and can be obtained from almost any major plumbing, drum and pump manufacturer or distributor.