Static earthing solutions: convention versus flexibility.

Most earthing solutions provide earthing protection for discrete pieces of equipment at risk of electrostatic charging like road tanker trucks, railcars, IBCs and drums. For example, if a road tanker truck requires static earthing protection, a single static grounding system, featuring earth status indicators and interlocks for the pumping system, will be installed at the loading rack. If the earth connection is lost at any point during the road tanker truck filling operation, the earthing system will detect this and shut down the pump, or close a valve, feeding the road tanker truck.

Road Tanker Grounding System

Fig. 2. A dedicated static earthing system for road tanker trucks

Powder processing equipment presents more of a challenge as there are many metal parts that can make up larger assemblies that are electrically isolated from each other. For example, vibration pads that prevent mechanical destruction may be an inherent design feature, such that electrical isolation, by default, is inherent to the design of the equipment. In other cases, electrical isolation can be an unintended consequence of mechanical designs that have not have taken into consideration, or fully appreciated, electrostatic ignition hazards.

It is therefore important to ensure that multiple components that come into contact with charged powders do have a means of being monitored for static earthing protection purposes. This, however, may be easier said than done, especially if the processing equipment layout, available space and personnel accessibility is restrictive, such that multiple earthing systems may be difficult to install with such limited “real estate” to work with. The only alternative is for plant technicians to take regular measurements with ohm-meters to ensure that none of the plant equipment ever becomes isolated. For many operations this task is very time consuming and results in technicians spending less time on critical maintenance and repair work.

Figure 3. Earth-Rite MULTIPOINT monitoring unit

A more specialised static earthing system, like the Earth-Rite® MULTIPOINT II, provides the benefits of eight discrete static earthing systems rolled up into a single package. This gives installer’s the flexibility of providing monitored static earthing protection for multiple components of powder processing assemblies through a discrete wall mounted monitoring unit with eight earth status indicators for each component being monitored.

In addition, the Earth-Rite MULTIPOINT II will continuously check that all components are connected to a reference earth earthing point, thus ensuring that the ground path resistance between the process equipment and the reference ground never exceeds 10 ohms. A monitored ground path resistance of 10 ohms or less is what is recommended in NFPA 77, “Recommended Practice on Static Electricity” and Cenelec CLCTR: 60079-32-1 “Explosive atmospheres – Part 32-1: Electrostatic hazards, guidance.”

If the Earth-Rite MULTIPOINT II monitoring unit detects that an assembly component is not grounded, it will send a signal to the controller which, if interlocked with the circuit powering the operation, can halt the process, thereby eliminating the electrostatic charging mechanism and potential charging of un-grounded equipment.

If such an event does occur, the plant’s technicians can rapidly identify which connection needs to be investigated. They can do this by referencing the monitoring unit’s ground status indicator panel which will indicate which channel needs to be checked. Once the connection to the equipment is re-established the Earth-Rite MULTIPOINT II controller will provide a permissive condition for the process to start again.

Earth-Rite MULTIPOINT: a flexible solution designed for the powder processing industries.

Installation flexibility is one of the key, and unique, benefits inherent to the Earth-Rite MULTIPOINT II. The Monitoring Unit, which consists of 8 green and 8 red panel mounted LED indicators, fitted inside a stainless steel enclosure, can be positioned in a location that is easy to wire to and from, but is also easily accessible for technicians to observe if the LEDs linked to their corresponding monitoring points are green or red. Should any of the red LEDs be indicating a break in continuity with plant equipment the technician can check the corresponding location to investigate if the connection to the equipment is not correctly made or if there is a fault with the monitoring cable.

Earth-Rite MULTIPOINT II Installation

Fig.4. An example of an Earth-Rite® MULTIPOINT II installation

Connections to plant equipment can either be permanent or temporary. Permanent connections are suitable where the plant equipment remains in place for relatively long periods of time. The most common connection is via two conductor cable with eyelet terminations made by both conductors to the equipment that must be grounded. This ensures the equipment is on a 10 ohm ground path loop back to the Monitoring Unit.

If connections are temporary, i.e. where mobile totes and drums are in use, then two pole earthing clamps can be used to ground the totes or drums when they are being charged or discharged with powder.

The most common set up, from an interlock option perspective, is to halt the entire process via a single relay output from the Earth-Rite MULTIPOINT’s controller so that the isolated equipment can be identified and repaired. Halting the movement of the powder ensures that electrostatic charges are not being generated while a piece of plant equipment does not have earthing protection in place.

However, if the earthing of plant is not required 100% of the time then, depending on the characteristics unique to the process, the number of relays being utilised by the operation may be increased. Referencing the illustration in Figure 4, channels 1 through to 7 could be connected to one relay, while channel 8 could be connected to a separate relay. Only when the earthing of the tote is required would channel 8 need to be permissive such that its corresponding relay may open a valve that will charge powder into the mobile tote. The relay taking inputs from channels 1 through to 7 could be interlocked with the starter circuit or PLC controlling the conveying system or powder drying system.

Although 8 monitoring channels are provided, not all may be required. For example, if only 5 process items require monitoring, only 5 channels need to be utilised. However, the available 3 channels may be used at some point in the future should any modifications to the process require the addition of more plant equipment.

Additionally, the Earth-Rite MULTIPOINT II may be installed to utilise individual channels to monitor multiple items of interconnected equipment, provided the equipment has a dedicated connection back to an earth ground reference and no fortuitous connections to earth are inherent in the design of the structure being monitored. A useful connection option for this method is a single pole plug and socket connector that enables plant technicians to rapidly make and break monitoring cable connections to the plant equipment being monitored. Specialist advice is available to determine the most appropriate method of installation.

Summary

In the powder processing industries, the generation and accumulation of static electricity presents an insidious ignition risk in potentially combustible atmospheres. The electrostatic charging mechanisms and corresponding quantities of charge generated are generally higher than those produced by petrochemical operations handling and processing flammable liquids. As such, it is imperative that all conductive components, be they members of machine assemblies or portable containers, have static earthing protection in place to prevent the accumulation and eventual discharge of static electricity as a high energy spark.

The installation of conventional static earthing systems, designed to monitor discrete equipment, like IBCs and road tankers, are unlikely to be a viable solution for typical powder processing equipment like fluid bed dryers or sieve stacks. The quantity of systems required, the lack of available space and installation difficulty would either be too complicated, too expensive, or a combination of both.

A unique static earthing protection system like the Earth-Rite MULTIPOINT II minimises the site’s use of valuable installation “real-estate” while simultaneously bridging the gap between time-consuming checks with multi-meters and the highest levels of earthing protection provided by dedicated static earth monitoring systems.

Mike O'Brien

Author Details:
Mike O’Brien, Managing Director for Newson Gale

If you have any questions relating to the topics discussed in this article,
please contact Newson Gale.


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