Reference Manual

10B −6
Figure 10B-3. Cooking Flow Diagram
pressure control valve. Two large piston-operated
valves are designed to isolate the top circulation
line from the digester. These valves cannot be
opened unless there is equal pressure on each
side of the valve. This prevents damage to piping
and valves by a sudden surge of liquor from the
digester flowing into the lines. A pressure switch is
mounted on the return leg and is set to hold the
valves closed until the pressure in the line is
almost equal to that in the digester. Only then can
these valves be opened by hand switches on the
instrument panel. Chips only enter the digester
through the top separator. This unit consists of a
cylindrical screen with continuous vertical slots
that separate the liquor from the chips so the
liquor may be re-circulated through the high
pressure feeder back to the top separator again. A
slow-moving vertical screw conveyor inside the
screen pushes the chips downward into the
digester, keeping the screen clear. The chips fall
onto the top of the column of chips. This is
continuous throughout the entire digester. In other
words, there is a solid body of chips from the top
to the bottom of the digester undergoing various
stages of treatment.
The sluicing liquor is extracted through the top
separator screen and is returned to the suction of
the top circulation pump to be recycled with chips
to the top of the digester. Built into the top
separator is a level-indicating device, which allows
the operator to have an indication of the level of
chips inside the digester. This level device is a
small paddle which turns with the top separator
down in the chip mass. The resistance of the chips
against the paddle is measured by a torque
indicator on the top of the top separator. This
measurement is transmitted to the control panel,
which has a low level light (green), a normal
(yellow) and a high (red). The digester is normally
run at a yellow-red level indication. If the level gets
high enough so that the chips ride against the
screw conveyor, an increase in amperage of the
motor will be seen on the control panel. When the
load becomes severe, the top separator alarm will
go off, warning the operator to take corrective
measures. The drive shaft of the top separator is
sealed by packing. Digester pressure compresses
the packing, forming a seal. When the pressure is
radically changed, the packing may not properly
seal for some time.
Valve: FV-3A High pressure feeder
purge
Note: In older systems, this is HV-3A or HV-35.
D Typical process conditions:
— Fluid = White liquor