Is Your Loading Arm Fully Conductive?
While doing some preparation to speak at the Fuel Ethanol Workshop in Milwaukie, WI on loading rack and skid safety, I started looking closely again at NFPA 77 as related to rail car loading. I was specifically looking at loading arms and what has to be done to ensure the arms, with their swivel joints, are properly grounded. Looking at NFPA 77: 4.3 Tank Car Loading, Section 4.3.2 states that many tank cars are equipped with non-conductive bearings and non-conductive wear pads between the rail car and the chassis. It states the resistance from the tank car compartment to ground through the rails may not be low enough to prevent the accumulation of an electrostatic charge on the tank car body.
It goes on further to say bonding of the tank car body to the fill system piping (the loading arm as shown in the illustration below- Figure 6-Tank Car Bonding) is necessary for protection against static accumulation. It also states, because of the possibility of stray currents and to prevent an ignition hazard as a result of such currents, loading lines should be bonded to the rails. The illustration shows an example of how to properly bond the tank car and loading arm (the fill system piping) as shown by the NFPA. But something is missing. Not all loading arm swivels are made the same and the diagram makes an assumption that the loading arm can fully conduct electricity over the entire length of the arm. Typical swivel joint design will not allow conductivity with their seals and ball bearing design, with grease for lubrication that allows the swivel to rotate. A typical swivel is two parts held together by ball bearings, and the path for any current is more or less blocked because the ball bearings may not be in contact with the metal around them being covered in grease. To get around this, some loading arm companies use jumper cables that attached to the main arm piping and go around the swivel joint to ensure full conductivity. I have seen many companies write standards on how to do this, so there is no doubt it’s a concern and has to be addressed when working around flammables where stray current can ignite vapors or the product itself. There is a downside to this method and that is the cables can wear and break since a loading arm is dynamic, moving up/down, left/right and it becomes yet another item to inspect before usage and maintain and commonly falls into disrepair.
Emco Wheaton Loading Arm
There is an alternative. In talking to a representative with Emco Wheaton who manufactures loading arms and swivel joints, I was made aware that their dual, split-flanged D-2000 swivel joint, which utilizes a bearing module design, will conduct electricity through the swivel joint, allowing a complete loading arm with its piping and 3 to 6 swivels, to be fully conductive. The design, with the flanges on either side being in direct contact with the main bearing, allow a clear path for any current to pass through the swivel. NFPA 77 states the resistance between the tank truck or rail car and the verified grounding point at the loading rack should never exceeds 10 Ohms. Emco Wheaton has verified, according to DIN VDE 0303 testing standards, that the resistance of the entire loading arm between its inlet flange and outlet flange have never reached the limit value of 10 Ohms.
When using the Emco Wheaton design loading arm with the D-2000 swivel, the NFPA illustration becomes more accurate as you only need to bond the loading arm to the rail car at one point and don’t need jumper cables over the swivel joints for full arm conductivity. This eliminates the need to worry about jumper cables being broken and yet another item to inspect before operating. To go a step further, to ensure the loading arm is properly grounded while loading or unloading, Newson Gale manufactures their new Multipoint II ground verification system that can monitor up to 8 points with a single unit. This new system will allow you ensure proper ground at multiple points (car, rail, arm…) as per the NFPA 4.3.2 Figure 6 Illustration. In addition, the system can be connected to your pump and meter skid controls, giving permissives to start pumping, or shut down the system in the event grounding is lost during loading or unloading, ensuring your operation remains safe at all times.
If you’re designing a loading rack that includes a loading arm, and you have concerns about how to mitigate the risk of fire or explosion when handling flammable products and stray currents, it’s a good idea to turn to NFPA 77 for guidance. Today’s technology can help remove the risk from your operation. So be sure to engage a supplier that truly understands your operation, its potential risk and is there to advise you and provide insights and not just “sell” you a widget.
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