Sulfur Hexafluoride (SF6) Gas Handling

Description Sulfur hexafluoride (SF6) in its pure state is inert, nontoxic, odorless, nonflammable and colorless. It has excellent arc quenching properties and exhibits exceptional thermal stability. SF6 has exceptionally good insulating properties and at atmospheric pressure has 2.5 times the dielectric strength of air. SF6 gas at kPa (42 psig) has the same dielectric strength as transformer oil. Sulfur hexafluoride remains a gas without liquefying down to -34.4°C (-30°F) at the gas pressure normally used in the puffer type breaker. The density of SF6 is about five times that of air (molecular weight equal to 146). Heat transfer by free convection is 1.6 times that of air at atmospheric pressure and 2.5 times the value of air at kPa (30 psig).

Arcing, which will occur during the normal operation of the circuit breaker, does not materially affect the insulating properties of the gas. The solid arc decomposition products, observed as a fine gray powder, have equally good dielectric properties, however, the powder will absorb moisture and become conducting if exposed to high humidity. Under normal conditions the arced by-products are not sufficient to require any restriction in operation. However, if the compartment is opened for any long period of time for maintenance, the powder should be removed with a vacuum cleaner.

Precautions Handling Arced SF6 A chemical breakdown of sulfur hexafluoride (SF6) occurs when it is exposed to a very high temperature or electrical arc as in the circuit breaker. The decomposition products, gas and fine powder may be toxic, strong irritants and attack the respiratory system. CAUTION Arced SF6 gas is accompanied by a strong and irritating odor indicating toxic decomposition products. The arc products may be injurious and exposure to them should be avoided. The arc powders are reactive with moisture and if inhaled could cause irritation in the nose, throat and lungs. Contact with body perspiration could cause skin irritation.

A strong irritating odor is sufficient warning of the presence of the decomposition products. This would normally be present only after several major fault interruptions or an unusual arcing condition within the gas enclosure. If this condition occurs, the area should be thoroughly ventilated. If work is necessary within this environment prior to the area being adequately ventilated, a self-contained breathing mask should be used. Gas which has been subjected to heavy arcing and containing considerable decomposition products can be cleaned and reused. The gas should be circulated through molecular sieve filters to remove the active products. Most gas service trailers are equipped with molecular sieve filters and with provisions for circulating and cleaning the gas.

CAUTION All freshly activated absorbents should be cooled to ambient temperature before introducing SF6 to avoid exothermic reaction. Large accumulations of powder (solid decomposition products), resulting from abnormal arcing conditions, can be neutralized by mixing the powder in a bucket containing a solution of water and bicarbonate of soda and safely disposed.

Handling of Sulfur Hexafluoride WARNING Pure SF6 gas without oxygen (20%) will not support life. Do not enter any tank previously containing SF6 without thorough ventilation. SF6 gas is heavier than air and will accumulate in or be trapped in low “pockets.” Precautions should be observed in nearby “trenches” and “depressions” where the SF6 gas can accumulate for short periods of time before dispersing into the atmosphere. Sulfur hexafluoride is furnished in standard industrial type cylinders color which are usually coded green at the top end and the balance silver for easy identification. The gas is stored in the cylinders at its vaporization pressure which at 24°C (75°F) is 2241 kPa (325 psig).

CAUTION Any connection made to the cylinders must be suitable for 4137 kPa (600 psig). A pressure regulator is required when connecting a cylinder to a gas compartment fitting. The gas compartment may be filled with gas directly from the cylinders or from a gas service trailer. When transferring the gas from the cylinders the process can be accelerated by keeping the cylinders warm to increase the rate of vaporization of the liquid SF6 stored in the cylinders. This can be done by setting the cylinder in a tank of warm water maintained at a maximum temperature of 51.7°C (125°F).

WARNING Do not allow the temperature of the SF6 cylinders either in processing as above or in storage to exceed 51.7°C (125°F). Excessive temperature can result in overpressure and potential failure of the cylinder. Greenhouse Gas SF6 gas has been identified by the Intergovernmental Panel on Climate Change as a "greenhouse gas“ with the potential to contribute to global warming. Therefore, SF6 gas should not be deliberately released to the atmosphere. The gas handling and leak detection procedures described in this instruction book should be carefully followed to limit the amount of SF6 gas released over the life of the circuit breaker. Used SF6 gas, even after internal arcing, can be effectively filtered for recycling. Mitsubishi Electric Power Products' circuit breakers are designed, manufactured, and factory tested to be free of leaks. If leaks develop while in service, appropriate action to locate and eliminate the leaks should be taken.

Material Safety Data Sheet For additional information concerning the composition and characteristics of sulfur hexafluoride gas, review the product safety sheet supplied by the manufacturer of the SF6 gas. Pole Unit Each pole unit consists of a SF6 gas filled enclosure (Main Enclosure) which contains the interrupter assembly supported by insulators at each end of the interrupter. Two porcelain bushing assemblies extend from the top of the main enclosure. Conductors inside the bushings connect the interrupter assembly inside the metal main enclosures to the overhead line.

The SF6 gas in the pole unit is common to the metal main enclosure and the porcelain bushings. An inspection cover is located at the end of the main enclosure to inspect and maintain the interrupters. An absorbent assembly is located on the inside of the inspection cover to maintain the dry atmosphere required inside the SF6 gas equipment. The rupture disc (*) is provided only when specified. The SF6 gas is sealed in the main enclosure by O-rings at each end of the enclosure, O-rings in the slide seal assembly through which the operating rod passes, and O-rings in the bushing assemblies.