SF6 is an effective electrical insulator in high voltage equipment, such as gas insulated substation (GIS) structures, circuit breakers and circuit switchers, however, it is the most potent greenhouse gas. SF6 emissions to the atmosphere from electrical equipment are due to gas leakage from failed gaskets, manufacturing defects and improper bushing installations. A short term solution to gas leakage is to use SF6 bottle inventory to refill the equipment, thereby, avoiding a long term, equipment outage. This use of SF6 inventory would be captured in the annual accounting report to demonstrate the amount of SF6 that was lost to the atmosphere. A long term solution would involve taking an outage of the equipment and have the manufacturer work with the partner to find and repair the gas leak.
SF6 tracking is done by weighing SF6 bottles, before and after its use, with scales that are capable of a +/- 2 lbs. of true weight tolerance. When a SF6 bottle is shipped by a supplier such as Airgas or Concorde, the product is highly compressed and the bulk of it is in liquid form. At the top of the bottle though, SF6 is in a vapor form, at a pressure of 312 pounds per square inch (PSI). Since gas has negligible weight, when a bottle is weighed, the measurement in lbs. is mostly due to the liquid product. When a SF6 bottle is used to fill equipment, the gas naturally flows from the bottle to the equipment because of the difference in pressure, as gas will always flow from a high pressure atmosphere to a low pressure one. As SF6 flows from bottle to equipment, the liquid volume would diminish, however, the vapor at the top of the bottle would remain at 312 PSI up to a certain threshold point when the SF6 liquid volume becomes very small and the vapor pressure begins to drop. Eventually, the vapor pressure in the bottle approaches the same pressure inside the equipment. When pressure is equalized between the bottle and the equipment, SF6 can no longer flow from the bottle to the equipment, making the bottle practically empty. The remaining weight of the SF6 product left over in the bottle is referred to as a "heel" and the weight of just the bottle alone is referred to as a "tare."
SF6 in equipment is considered an ideal insulator when its pressure is generally around 90 PSI on a 68F degree day. As various equipment have different volumes of tank enclosures to hold the gas, manufacturers specify on its nameplate the required amount of SF6 gas in lbs. that is required to fill the tank enclosure, based on a 68F degree day. According to the Ideal Gas Law, if you maintain constant temperature and constant volume for an enclosure of gas, then the pressure will vary proportionally to the amount of molecules in the enclosure. Therefore, if you inspected equipment on a 68F degree day and found the pressure gauge reading 70 pounds per square inch on the gauge (PSIG), you can determine that a 20 PSIG drop relates to a particular amount of SF6 product lost, based off of information from the nameplate. And if the temperature of the day is different than 68F degrees, manufacturers provide charts to show how nominal SF6 pressure varies proportionally to ambient temperature. Knowing how much SF6 was lost is important when determining how much bottle inventory should be dispatched to the site for refilling.
SF6 tracking is basically verifying the annual replenishment necessary to maintain nominal SF6 pressure for all equipment that uses SF6. If your system has a total nameplate capacity of 17,820 lbs. of SF6 product and 1,782 lbs. of SF6 product from bottle inventory was used to re-gas equipment that had leaks, then 10% of your total nameplate capacity was lost to the atmosphere and the EPA would fine for that loss.
Article by Dan Scrobe III
