The Importance of Purging A very important step which must not be overlooked by LP-Gas distributors is the importance of properly purging new LP-Gas containers. Attention to this important procedure will promote customer satisfaction and greatly reduce service calls on new installations. Consider the following: Both ASME and DOT specifications require hydrostatic testing of vessels after fabrication. This is usually done with water Before charging with propane, the vessel will contain the normal amount of air Both water and air are contaminants They seriously interfere with proper operation of the system and the connected appliances. If not removed, they will result in costly service calls and needless expense far exceeding the nominal cost of proper purging. Neutralizing Moisture Even if a careful inspection (using a pen flashlight) reveals no visible moisture, the container must still be neutralized, since dew may have formed on the walls; additionally, the contained air may have relative humidity up to 100%. A rule of thumb for neutralizing moisture in an ASME container calls for at least 1 pint of genuine absolute anhydrous methanol* (98.85% pure) for each 100 uswg capacity of the container. On this basis, the minimum volumes (US measurements) for typical containers would be as shown below: Minimum Volume Container Type Methanol Required 100 lb ICC Cylinder 1/8 pt. (2 fl. ozs.) 500 gal. tank 5 pts. (2 1/2 qts.) 1000 gal. tank 10 pts. (1 1/4 gal.) 2000 gal tank 20 pts. (2 1/2 gal.) * IMPORTANT - Avoid substitutes - they will not work. The secret of the effectiveness of methanol over all other alcohols is its high affinity for water plus a boiling point lower than all other alcohols, and most important: a boiling point lower than water. The Importance of Purging Air If the natural volume of atmosphere in the vessel is not removed before the first fill, these problems will result: The air mixture present in the vapour space will be carried to the appliances. This may result in as many as 5 or more service calls from pilot light extinguishment. If a vapour return equalizing hose is not used, the contained air will be compressed above the liquid level, resulting in slow filling. If a vapour equalizing hose is used, the air, and any moisture it contains, will be transferred from the storage tank to transport. Additionally, if atmospheric air is properly purged from the storage tank; transfer will be faster, the pump will not stall, less energy will be required to fill the container. Never Purge with Liquid The wrong way of course is the easiest way. Never purge a container with liquid propane. To do so causes the liquid to flash into vapour, chilling the container, and condensing any moisture vapour on the walls where it remains while the pressure is being blown down. Additionally, less than 50% or as little as 25% of the air will be removed by this easy but wrong method. The correct procedure for purging air is shown below. Install an unloading adapter on the double check filler valve, leaving it in the closed position. Install a gauge adapter assembly on the house service POL outlet connection. Exhaust to atmosphere, any air pressure in the container. Attach a truck vapour equalizing hose to the vapour return valve on the container. Open the valve on the outlet end of the vapour equalizing hose, throttling it to avoid slugging the excess flow valve on the truck. Carefully observe the pressure gauge. When the gauge reading shows 15 psig, shut off the vapour valve on the hose. Switch the lever on the unloading adapter to open the double check filler valve and blow down to exhaustion. Close the unloading adapter lever, allowing the double check filler valve to close. Repeat steps (4), (5), (6) and (7) FOUR MORE TIMES. Total required time is 15 minutes or less. Caution: Never purge the container in this manner on the customer's property. Discharge of the vapour into the atmosphere can seriously contaminate the surrounding area. It should in all cases be done on the bulk plant site. Here's What Happened While performing the operation above, the percent of air in the container was reduced as shown in the table below: % of Air Remaining % of Propane Remaining 1st purging 50 50 2nd purging 25 75 3rd purging 12.5 87.5 4th purging 6.25 93.75 5th purging 3.13 96.87 6th purging 1.56 98.44 Experience indicates that a reduction of the residual air content to 6.25% is adequate. The resulting mixture will have a thermal value of about 2400 BTU. In this case, the serviceman can adjust the burners for a slightly richer product. Moreover, the slight volume of air will to some extent dissolve in the propane if the installation stands unused for a few days. How Much Propane was Consumed If instructions on the preceding page were followed carefully and the vapour was purged five times, a total of 670 cu. ft. (18.4 gal.) would have been used for a 1000 gallon tank. In a 500 gallon tank, a total of 9.2 gallons would have been used. There is no other practical way to extract air from the tank so quickly or so cheaply.