What do the standards recommend?
API 2219, entitled “Safe Operation of Vacuum Trucks in Petroleum Service”, is probably the most relevant standard to address the hazards of static electricity in vacuum truck operations directly, although valuable information and recommendations can be sourced from CLC/TR: 60079-32-1 and NFPA 77. Of the many recommendations outlined in API 2219, the most relevant instruction is to fully ground the truck by connecting it to “a designated, proven ground source”, before commencing with transfer operations.
Is earthing point (rod or buried metal structure) capable of dissipating electrostatic charges?
The “ground source” describes an object with a low resistance connection to true earth (ground). It is this connection to true earth that will guarantee the rapid dissipation of electrostatic charges from the equipment ensuring that personnel and the equipment being used are protected from the risk of fires or explosions. The API standard provides some examples of potentially suitable earthing sources, including large storage tanks or piping that is known to run underground.
The standard also states the importance of confirming that the connection resistance between the truck and the designated earthing point is less than 10 ohms and that this resistance should be verified with the use of an ohmmeter. All conductive and semi-conductive components of the transfer system must be bonded together with a connection resistance not exceeding 10 ohms.
Protecting personnel and equipment in everyday operations:
Most chemical processing sites have designated earthing points to which trucks must be connected prior to transferring product. Normally, these sites follow a strict policy of adhering to the standards whereby electricians regularly check the resistance of the earthing points prior to every transfer process ensuring they have not degraded because of environmental and industrial life-cycle effects.
In hazardous material recovery operations, for example sucking up spillage from road & rail incidents, designated earthing sources are not available to ground the truck. In such events, the recovery team must bury rods in the ground and take resistance readings of the rods to ensure they have a secure connection to true earth.
For retail petroleum and gas distribution and the food and beverage industry, many point-of-delivery destinations do not have static earthing systems installed and the best the transporter can expect is that there is a designated earthing point to which he can connect the delivery truck. However, he will have no means of confirming whether or not the earthing point has a low resistance connection to earth.
Based on what the API standard recommends the two key conditions for protecting personnel and equipment from incendive static spark discharges depends on:
- Verifying that the resistance value of the ground point (the ground source) to true earth is of a known and measured value that is capable of dissipating electrostatic charges.
- Ensuring the connection resistance between the truck and this verified earthing point is less than 10 ohms.
Fall of Potential 3 Point Test Method
Verifying the connection resistance of an object to true earth:
The most common method of measuring the resistance of objects located in the ground to true earth is the “fall of potential” 3 point test method. This method is designed to measure the contact resistance of the surface of the electrode to the soil it is in contact with. Soil resistivity levels can vary greatly ranging from very low values of around 2 ohms for marshy ground to over 1000 ohms for rocky ground.
The 3 point test method is a relatively complicated and time-consuming procedure requiring a certified electrician, with the requisite level of experience and training, to follow the correct procedure and determine when a reliable, low margin-of-error, resistance reading is established.
Essentially, 3 electrodes, including the one to be tested, are inserted into the ground and a range of readings are taken until uniform and consistent values of resistance are obtained. Skilled interpretation and interpolation of the acquired data is required to ensure that the readings are not taken at “face value”.
To illustrate, there are specific depths to which rods should be buried in relation to their respective distances from each other and the electrician must know where to position the rods to ensure the soil resistance between the electrodes do not overlap and distort the readings. The rods must also be buried in a straight line relative to each other.