Reference Manual

10B −5
chip chute is maintained at a constant level by the
use of a level control valve on the overflow line,
which carries excess liquor to a surge tank known
as a level tank. The source of chip chute liquor is
mainly leakage from around the high pressure
feeder and some steaming vessel condensate. In
order to take sudden flow surges, such as when
the high pressure feeder starts rotating or when
chips fall into the chute, the overflow control valve
response is extremely rapid. The screen plates
prevent chips from being carried with the overflow
to the level tank, and keep the chips in the chip
chute to feed the pockets of the high pressure
feeder.
A pressure switch is connected to the make-up
liquor pump from the same pressure-sensing unit
that stops the cold blow pump. This safety device
is necessary to avoid over-pressurizing the
digester and it is set at approximately 240 psig.
The digester operates at 165 psig, measured at
the bottom heating zone. The transfer of chips and
liquor from a 15-18 psig steaming vessel pressure
to the digester operating pressure needs to be
accomplished via a pressure lock system. This is
accomplished by the high pressure feeder, which
is similar to the low pressure feeder. The high
pressure feeder has a tapered rotor with four
helical type pockets which go from one side of the
rotor to the other and are set at an angle of 45_ to
each other. The feeding of chips is continuous.
Two liquor pumps are used to aid in filling and
discharging the high pressure feeder. They are as
follows:
1. Chip chute pump — Chips falling into the chip
chute pool of liquor tend to float or be drawn to the
side screen plates by the liquor overflowing to the
level tank. In order to counteract this effect, the
chip chute circulating pump is set to pull or suck
the chips downward into the rotating high pressure
feeder with a force greater than the sideways pull
of the overflowing liquor. The discharge of the
pump re-circulates the liquor to a point located
above the pool of liquor in the chip chute. When
severe conditions of sawdust and fines are
prevalent, an in-line grainer may be installed
between the pump discharge and the entry above
the pool of liquor. Its function is to take liquor
around the screen section, thus cutting down on
the sideways pull of overflowing liquor. The
pressure drop across the screens will reduce,
allowing the natural wiping action of moving chips
to keep the screens from plugging. As the pump
will deliver 2,000 - 2,500 GPM, closing the valve
between the in-line drainer and the level tank will
allow the in-line drainer to clear itself by liquor flow
to the chip chute.
2. Top circulation pump — The pump circulates
liquor to the top of the digester and back out. Its
action is to flush the chips out of the feeder pocket
once it has rotated to the discharge point and to
carry the chips into the digester. The liquor is then
separated from the chips via the top separator
screen and returned to the suction of the pump to
form a continuous loop.
When a pocket is in a vertical position, the chips
are fed with the help of the chip chute pump.
When the pocket has rotated 90_ to a horizontal
position, the chips are flushed out into the high
pressure system with the liquor from the top
circulation pump. The whole system is arranged
so that there is always one pocket being filled
while another is being emptied.
As mentioned previously, there is liquor leakage
around the rotor due to the pressure differential
between the operating digester pressures and the
pressure in the chip chute and steaming vessel.
This liquor leakage is an important feature of the
high pressure feeder operation, as it provides
constant lubrication between the feeder plug and
housing, and washes sand and grit from this area.
As the liquor is at a high temperature, it will boil in
the chip chute unless held under higher pressure
than the vapor pressure of the liquor. This is
particularly true following a shutdown when heat
rises to the top of the digester from the cooking
zones due to convection currents. The hot liquor
boils rapidly, or flashes, at the high pressure
feeder and chip chute if the high pressure feeder
is started. Therefore, the high pressure feeder
must never be started unless the top section of
the digester is first cooled below 240_ F by
addition of cold filtrate through the make-up liquor
pump. The plug clearance is adjusted from time to
time as it wears. Excessive wear of gap between
the plug and housing allows too much liquor to
pass to the chip chute and overloads the make-up
liquor pump due to excess flow of liquor from the
level tank to the make-up liquor pump. This can
cause liquor to back up through the level tank into
the chip chute and even back up as far as the chip
bin. The high pressure feeder serves to complete
the transfer of the chips from 15 psig to digester
pressure without subjecting the chips to any harsh
mechanical action, which would damage the fiber
and degrade the resulting pulp quality.
The top circulation line, a part of which is a hole
through the high pressure feeder, is a part of the
digester vessel itself during normal operation. This
means that it is under the influence of the digester