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.
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.
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.