Vehicles operating in locations which have less than ideal static earth monitoring systems are at risk for fire or explosion. In such situations the Earth-Rite*MGV is successful in protecting vehicles in such situations.
Vacuum tankers are commonly used in hazardous process industries for many purposes, such as cleaning of storage tanks or recovering flammable materials from spills, and many such situations share the common characteristic of having combustible atmospheres.
A recognized risk of such industries is the presence of static electricity as a source of ignition. Since this factor is invisible, it is more dangerous than ever, because in most situations, the only time that one is aware of its presence is when a static spark is seen or heard. However, this one occurrence may be catastrophic if the spark voltage is sufficient to set off a fire in a combustible atmosphere.
This threat may be effectively neutralized by using earthing on vacuum tankers in such spaces. Earthing refers to connecting the object to the ground or rather the mass of the earth itself, which is termed “true earth”. This connection dissipates induced voltage on the tanker due to the proximity of or contact with charged material, rather than permitting it to build up and discharge as a spark.
A Solution that is Appropriate to the Potential Hazard
The earliest earthing on road tankers used to be basic earth bonding reels but this was replaced two decades ago by specialized static earth monitoring devices and systems suitable for applications such as loading cars or platforms in petrochemical and chemical, pharmaceutical, tank farms, and food and beverage manufacturing environments.
These situations combine large amounts of highly combustible matter with a high potential for charge induction on tankers, which is likely to lead to spontaneous fires due to the atmosphere catching fire. The bonding reels were thus substituted with more sophisticated earth monitoring units to make sure that no electrostatic charge builds up on the road tanker during the product loading.
These are loaded on the gantry and include interlocks which interrupt product transfer instantaneously if the earthing system loses contact with the tanker at any moment.
Vacuum tankers share the same risk of fires and the consequences are no less, but they have not received this kind of safety and protection devices, putting them, as well as their operators, and customer personnel, and customer sites, all at risk.
Vacuum tankers currently use basic earthing devices such as a single clamp connected to a reel of single core braided cable. This is because of their mobility, difficulty in monitoring static earthing points quickly, and lack of technology that the user can understand and put to use.
Such tankers are often used in environments which lack accessible, industry standard-compliant, or verified earthing points, or may have none at all. This is also the case with bulk carriers who often deliver to sites without proper earthing systems.
The disadvantages of single core bonding reels include:
- Lack of indication that the clamp is in contact with underlying metal of the earthing metal object, rather than with overlying resistors such as surface rust, deposits, paints or coatings. These would compromise the grounding connection by preventing the formation of a solid pathway of low resistance to the metal object.
- Lack of monitoring of the earthing connection throughout the product transfer, so that drivers and process operators may be unaware of any loss of continuity between the tanker and the earthing point because they are facilitating the safe and secure material loading.
- If the bonding reel is connected to a secondary earthing point, it cannot verify that this is in turn actually connected to a true earth ground.
- The need for an electrician to read the resistance using a multimeter to make sure the pathway resistance through the bonding reel is 10 ohms or less has other problems, such as having to wait for the electrician to complete other tasks including repair, maintenance and installation work. The measurement may be delayed for up to a few hours which will hold up the work of the vacuum tanker, such as cleaning, recovering material from a spill or leak, or unloading a tanker. This is impossible in urgent situations such as if a spill or leak has occurred and will necessitate bonding to any point which seems suitable but have not been named verified true earth grounding points, in the hope that they do have a true earth connection.
- The bond resistance test checks only the resistance between the points to which the tanker is connected and cannot tell if the reel connection point is connected to true earth. Moreover, since the test is performed only once, loss of connection during product transfer will not be detected, compromising safety.
In short, while road tankers and loading gantries are protected against electrostatic charge accumulation by the gantry-mounted monitors, this is not the case with vacuum tankers.
This is an issue of concern to the vacuum tanker contractors as well as customers because of the use of unverified true earth grounding points. It is in answer to this that the Earth Rite* MGV was developed. This is a vehicle-mounted verification system (Mobile Ground Verification) to test static grounding for vacuum tankers with the same level of security as for gantry-mounted static earth monitors, but with the MGV being a permanent fixture on the tanker it protects.
It is designed to be compliant to API RP 2219 standards, namely, “Safe Operation of Vacuum Trucks in Petroleum Service” which is the benchmark for anti-static precautions related to the use of vacuum tankers in transferring materials which are flammable or are being transferred in combustible atmospheres.
API RP 2219 recommendations in this situation include:
- Earthing the truck before starting any operation using a designated and verified ground source by preference
- Ensuring a resistance of 10 ohms or less between the truck and the grounding point with an ohmmeter after the connection is made and before the operation is initiated
- The first requirement is a proven ground source, which is any object having direct electrical connection with the mass of the earth (true earth ground), and the MGV ensures this by providing “Static Ground Verification” (SGV) to verify the earth status of the earthing point to which it is connected.
- The MGV also constantly monitors the connection between the tanker and the verified earthing point for continuity, making sure the resistance is always at or below 10 ohms, and also ensuring its own connection to the tanker is intact. This is vital in keeping both tank and chassis earthed via the MGV, so that electrostatic charge can constantly dissipate to the ground. If the clamp connecting the tanker to the earth loses contact with the circuit at any point it is detected by the “Continuous Ground Loop Monitoring” (CGLM) function of the MGV.
The Earth-Rite* MGV must show both SGV and CGLM checks to be positive before the static ground connection is made, and at this point three LED indicators will pulse bright green as a message to the operator that the tanker is safely earthed and that material transfer may be initiated. The generation of static during this process is thus safely discharged through the MGV and obviates the possibility of fire or electrocution by static charge buildup.
The system is simple for the driver to operate, because a red LED indicator shows the absence of an earthing connection prompting the drive to connect the earthing clamp to any object which is to be verified as a true earth grounding point.
If it is verifiable, and if the pathway has 10 ohms or less of electrical resistance, the LEDs will start to pulse green, showing the presence of continuous static earthing for the tanker and that both SGV and CGLM checks have been passed.
The vacuum tanker can also be interlocked with the pump by two volt-free contacts if required, so that if an earthing circuit becomes loose or is disrupted while the product is being transferred the process automatically shuts down at once. The cessation of transfer ends the charge generation and buildup.
A strobe light interlocked indicator may also be mounted high on the tanker to act as a visual indicator for the recovery process operators who may have to work some distance away or in any position facing away from the MGV system, making it difficult for them to see the earth status indicators.
Primary Earthing Points and Secondary Earthing Points
The earthing point is any metal object which has solid contact with the ground, whether it is a formal earthing pipe dug into the ground or just several interconnected metal rods jutting out of concrete.
The maximum resistance in the earthing pathway is typically due to the ground around the earth electrode (the earthing point), and this varies with the soil type because of differences in soil resistivity as well as moisture content, both of which may vary from season to season.
Soils which retain a lot of moisture may offer very little resistance to a true earth ground, but in winter, for instance, the freezing of water results in a sudden increase in resistance. The outcome could quite possibly be impedance to the flow of electrostatic charge into the ground.
Thus the soil itself is one of the resistors in the circuit and this property must be measured accurately in order to assess whether it is low enough that the circuit can dissipate static from the tanker to the earth in a safe and predictable manner.
Any work site that contains areas that fall within the category of hazardous areas will be supplied with electrical fault and lightning protection. These protection units are linked to engineer-tested structures called designated earthing points, which may also be utilized to connect earthing to any equipment or vehicles in the plant that may accumulate static charge. These are the primary earthing points, and it is vital to keep them on a regular inspection and testing schedule so that they will operate reliably to conduct stray currents as well as lightning bolts to earth as well as prevent static charge buildup.
Electrostatic charge is a different type of risk from lightning or stray currents and in the context of the former, the true earth ground may have higher resistivity than would be permitted for the latter. The reason is that the amplitude of static charge accumulation leading to a current is lower than that of the current of a lightning bolt or electrical leakage, even though the voltages in this situation are very high.
The low amplitude of static charging current means that secondary earthing points such as underground or basement pipes or beams, or storage tanks and temporary earthing pipes, may be verified by the MGV in order to test if they have a low enough resistance to allow static charge to flow to the true earth ground. These points cannot be used and are not tested for fault current or lightning protection, but may be tested for suitable resistance values to true earth to dissipate static safely as they have an inbuilt and permanent contact with the subsurface soil.
Vacuum tanker service operators and customers have long been denied the same level of static protection that most companies provide to operate loading gantries for tankers equipped with their own interlocking static earth monitoring units. Bulk carriers also often carry flammable products to sites which have been allowed to run obsolete earthing protection units or have none at all.
The Earth-Rite MGV system brings earthing protection against electrostatic charge accumulation to this segment of service providers and bulk carriers, with comparable safety and reliability as well as control of the process. This is a huge step towards providing complete security the tankers as well as personnel and the customer sites, equipment and personnel against static-induced ignition hazards.
In case primary earthing points are not determined to be valid or if secondary earthing points are found necessary, the MGV or a similar system is used to test the latter points before they are introduced into the system.
Only a verified resistance can allow the rapid leakage of static charging currents from the tanker or hoses or other equipment used in this process into the true earthing in order to prevent fires starting from the static spark within a potentially flammable atmosphere.
This information has been sourced, reviewed and adapted from materials provided by Newson Gale.
For more information on this source, please visit Newson Gale.