Reference Manual

10B −13
and flows to the outer shell equally in all
directions.
Chips continue down the digester for a short
distance into the lower cooking zone, where a
second heating process occurs. In this case, the
chips are heated to the desired cooking
temperature. To accomplish this, liquor is again
extracted radially through screen plates to a
second heater. It is returned through a pipe, which
is located inside the central distribution chamber,
and is discharged at a point just above the lower
cooking screen plates. A spare heater is provided
and may be valved as either the upper or lower
heater so that the heat exchangers, which
periodically become fouled by liquor scale, can be
acid-cleaned while the digester is still in operation.
Normally, the impregnation zone is subject to
considerable scaling and the upper heater will
require more cleaning than the lower. The time a
heater may be run before it requires cleaning must
be determined by operating experience on your
liquor, wood, and cooking conditions. Once
determined, heaters may be cleaned before they
become fouled so severely that the tubes plug.
The function of all circulations is two-fold: first, to
carry heat and chemicals into the digester;
second, to homogenize the conditions in the
digester cross-section. If the circulation is to work
properly, the screen plates must be kept clean.
This will not be possible if the chip column stands
still and a large quantity of liquor is drawn through
them. Therefore, the screen sections are built in
two sections, one above the other. These two sets
operate alternately, one set sucking while the
other rests and is cleaned by the wiping action of
the downward moving chip column. The change is
accomplished by digester switching valves
mounted in the circulation suction lines. The chip
column is not too dense at this point, and alternate
automatic switching time is fairly short (typically 90
seconds).
Temperature recorders on the upper and lower
cooking zone heaters show a cycle of 5_ to 10_F.
The temperature cycle is due to the temperature
differential across the heating zone screens and
follows the timing of the switching valves. The
recorded temperatures are averages for the entire
screen section, and provided the liquor circulation
is adequate, the end temperature is in control. The
inlet and outlet of each heater should be within
10_F of each other.
The temperature recorders serve as an indication
of the chip column movement. Interruptions of
movement are reflected by temperature changes.
Failure of the chip column to move is reflected by
the coming together of the inlet and outlet heater
temperatures, whereas a sudden drop of the chip
column is shown by a sharp drop in the heater
inlet temperature.
Valve: KV-8A and B Trim liquor
switching valves
These valves extract liquor through screens
located in the upper part of the digester. The
extracted liquor is then sent to the bottom
circulation heaters. The KV tagged valves are
required to fully stroke approximately every 90
seconds. This causes a flow reversal through the
extraction screens preventing the screens from
plugging with chips and fiber.
D Typical process conditions:
— Fluid: Black liquor
— T = 325_F
— P = 130 psig
— dP = 130 psi
— Q = 1500 gpm
D Typical valve selection:
— NPS 6 to NPS 8 size range. The DSV
valve is suitable for this application. This is a
modified 8510 body with a strengthened
shaft and no seal. Used in conjunction with
the 1061 actuator with a quad seal option,
this assembly is capable of a relatively long
life in this service.
Valve: KV-60A and B Bottom
circulation return switching valves
These valves extract liquor from the digester (in
the upper region of the digester vessel) and send
liquor to the bottom circulation heaters. These
valves are only found on dual vessel digesters.
The KV tagged valves are required to fully stroke
approximately every 90 seconds. This causes a
flow reversal through the extraction screens
preventing the screens from plugging with chips
and fiber.
D Typical process conditions:
— Fluid: Black liquor
— T = 325_F
— P = 120 psig
— dP = 120 psi
D Typical valve selection:
— NPS 12 to NPS 14 size range. The DSV
valve is suitable for this application. This is a
modified 8510 body with a strengthened