=Releasing Pressure.=--When the tank is cooked, shut off the steam and open the pet-cock for about ten minutes, then the escape valve, being careful not to give it too much escape at first, or the tank will “roll.” By this is meant that the original heat in the oil contents will generate steam in the contents beneath which, when the pressure is relieved, will cause the contents to boil violently, but if the pressure is reduced slowly, this condition will not arise. When a tank has “rolled” it takes a great deal more time to settle it, the grease being thoroughly mixed up with the residue of the tank. As the pressure decreases the operator can increase the rapidity of release. [Illustration: FIG. 71.--DETAIL OF PIPING CONNECTIONS. TOP OF RENDERING TANK.] The pressure being relieved from the tank, take out the head, use about a water-pail full of fine salt to settle the tank. This salt is scattered over the contents in the tank and has the effect of settling sediment and tissue which may be held in suspension. The tank should stand about two hours to thoroughly settle before drawing the oil. Draw the oil from the side valves of the tank into coolers. It is often necessary, in doing this, to raise the contents of the tank, as there may be considerable tallow or lard below the lowest draw-off valve. Such being the case run water into the bottom of the tank sufficient to raise its contents so that all the rendered oil can be drawn through the lowest valve. [Illustration: FIG. 72.--DETAIL OF PIPING CONNECTIONS, BOTTOM OF RENDERING TANK] =Operating Tank Blow-Off.=--Descriptive of the connections for operating rendering tank connections is the following subject matter. The exhaust connections are illustrated in drawings, Figs. 71 and 72, and essentially consist of, beginning at tank neck: (1) Spring pop safety valve set at 45 pound pressure to automatically release in case of excessive pressure arising and failure of weighted pop valve to relieve. (2) Pressure gauge on each riser to indicate pressure on the individual tank to insure its being under pressure and under heat. The siphon bend between gauge and pipe is preferably filled with glycerine. (3) Try cock on bottom of drip line to open occasionally as test for “gas.” (4) Globe valve, ³⁄₈ inch, operated slightly open to allow gas to release from tank and create very small circulation. (5) Globe valve, 1¹⁄₂ inch; blow down valve to be used when releasing all pressure from tanks. (6) Angle type pop safety valve to relieve into the discharge line. (7) Check valve attached to the line to prevent any foaming or steam from an adjacent tank discharging into this unit via the exhaust system. (8) Exhaust header used in conjunction with the system to collect all gases and steam discharged from the battery of tanks.