Main keywords for this article are Accumulation of Static Electricity on Belts, How to Protect Belt Driven Machinery from Static Electricity.
How to Protect Belt Driven Machinery from Static Electricity
Belt-driven machinery operating in a dry atmosphere is a potential source of danger from static discharges. The belt may become electrified by friction of the belt on the pulley, the separation of the belt from the pulley, the bending or flexing of the belt, and the friction of the atmosphere on the belt. The generated static potential has been measured and found to exceed 50,000 volts in some cases. If the machine operates in an area of high humidity, the surface of the belt will become semiconducting and no trouble will be experienced from sparking. In a dry atmosphere, some means shall be used to drain off the charge from the belt before a high potential is built up. A static discharge method, suitable for nonhazardous areas, for belt-driven machinery is shown in Figure 2.
For preventing the accumulation of static electricity on belts and pulleys antistatic belts on metal pulleys or sheaves shall be used. These are also suitable for hazardous areas if sheaves are adequately grounded. Flat or V-belts of this type are available. They have a static conducting element impregnated in the cover. The Rubber Manufacturers Association has established the following test procedure governing the manufacture and sale of static-conducting belts. ‘The standard method of measuring conductivity of endless V belts shall consist of two 18.5 mm (5/8 in) diameter flat contacts, 216 mm (81/2 in) on centers, and moistened with water, and pressed against the belt with 5.68 kg (121/2 lb) pressure per contact. The conductivity shall be measured on a 500 volt megger unit. A 500 volt megger shall not be used in a Division 1 area and only in a Division 2 area after a check that an ignitable mixture does not exist in the area of use. Belts having a resistivity of 6 megohms or less, measured by the above method, can be classified as static conducting’. In general, the less resistance, the greater is the conductivity and effectiveness as a static belt.
Conveyor belts, especially those operating at high speeds, and belt drives shall be treated as above. If roller-type conveyors are used, the rollers shall be made of conductive material and their supports bonded together and grounded.
Most metallic bearings are sufficiently conductive to carry off static charges, but sometimes it has been found that the flow of static electricity across the oil film has resulted in roughening bearing surfaces enough to adversely affect bearing life. Where experience indicates that such conditions are likely or the bearing incorporates a nonconductive material, for example nylon, the shaft shall be bonded to the grounded stationary part of the system.
Main keywords for this article are Accumulation of Static Electricity on Belts, How to Protect Belt Driven Machinery from Static Electricity.
How to reduce Static Electricity
- As indicated in the foregoing sections, generation of static electricity is difficult to prevent, but the danger from static discharges that are likely to cause fire or explosion can be prevented by bonding, grounding, humidification, ionization, or a combination of several of these methods. After a new process has been installed, it may take several investigations and some time to find all of the locations where dangerous static accumulations are developed and provide for their elimination.
- Frequent testing is recommended for static electricity in rooms and areas where flammable liquids, gases, or dusts are processed or handled. Tests made during periods of high humidity are not conclusive and shall be repeated when the humidity is low. Tests in dangerous locations shall be made with care. Testing instruments and the personnel conducting the tests shall be brought to ground potential frequently during the tests, by making contact with a good ground in a nonhazardous area where a possible spark will do no harm.
- Bare or insulated wire may be used as grounding wire, or grounding conductor, for the removal of static. No. 4 minimum size copper wire (or its equivalent) shall be used for connecting the various pieces of apparatus to the grounding electrode or ground bus (size selected for its mechanical strength rather than its current-carrying capacity). The size of conductor, flat strap, and braid used for the main grounding bus shall be equivalent to No. 1 minimum size copper. For connections between stationary and moving parts of machines, extra-flexible copper braid, 60 ampere capacity shall be used. Grounding conductors shall be run in full view so that any damage or breakage may be readily detected. In general, stranded wire shall be used.
- Resistance to earth of any grounded parts shall be tested.
- Static grounding systems shall have their primary connection made to grounded building or support steel, the plant ground system itself, or to ground rods of adequate low resistance.
- All points where ground connections are made shall be filed or sandpapered clean to ensure that the grounding conductor will be in good electrical contact with the grounded object. If the protective finish on the ground object has been removed or marred by filing or sand-papering, all such surfaces shall be suitably painted or otherwise treated to prevent corrosion after the ground connection has been made, and tested for adequacy of installation.
- Grounding connections that are not made by brazing or welding shall be made with solderless connectors.
- Cleaning or ventilating of tanks, mixers, and other equipment introduces an acute static hazard since not only are flammable or combustible liquids involved but also workmen are endangered. All air, inert gas, and gas lines used shall be electrically connected to the tank shell or equipment as well as being grounded. Workmen shall touch some grounded object to equalize static differences prior to entering tanks or hazardous buildings.