Reference Manual

10B −14
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-60C and D Bottom
circulation screen backflush valves
These valves work in conjunction with tags KV-60
A and B to ensure that the bottom circulation
screens remain free of chips. 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 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: PV-16 PV-17 No. 1 Flash tank
relief steam pressure valves
These valves are used to slightly pressurize the
flash tanks in order to reduce foaming of the black
liquor in the tanks.
D Typical process conditions:
— Fluid: Steam
— T = 220_F
— P = 60 psig
— dP = 5 psi
— Q = 15000 – 75000 gpm for PV-16
— Q = 20000 – 100000 gpm for PV-17
D Typical valve selection:
— Both of these valves have been specified
by Kamyr as HPBV valves with PV-16 being
an NPS 8 or NPS 10 valve and PV-17 being
an NPS 16 to NPS 18 valve. Both valves
should be supplied with a stainless steel
body and disc, 17-4 shaft, PEEK bearings,
and a NOVEX metal seal.
Cooking Zone
Below the heating zones, the chips enter the
cooking zone at the full cooking temperature.
Here, the actual cooking takes place. The active
chemicals in the cooking liquor are sodium
hydroxide, NaOH, and sodium sulfide, Na
2
S.
These chemicals react with the lignin in the wood
chips, converting it into chemical compounds
which dissolve in the alkaline cooking liquor. The
lignin, as it exists in wood, is somewhat like a
cementing material and holds the individual fibers
together; however, when it is made soluble during
cooking, the fibers are set free and can be
separated into the fibrous mass called wood pulp.
The chemicals in the cooking liquor also react with
the pulp fibers themselves. This is not desirable
because the fibers are required in their original
condition. Therefore, cooking conditions are used
which result in the highest removal of lignin with
the least attack on the cellulose fibers.
The chips continue to fall and will increase slightly
in temperature until cooking is complete and it has
reached the next stage (hi-heat washing). The
cooking reaction is stopped by cooling the chip
mass down to about 280_ - 300_F. This is done by
extracting the residual cooking liquor through
screens on the side of the digester, and replacing
it with cool liquor, which rises from the wash zone.
The amount of cool liquor used is determined by
the quantity of wash liquor required to wash the
pulp to a suitable low soda content. In order to
control the uniformity of temperature, a portion of
the extracted cooking and washing liquor is
returned to the digester via the quench circulation
pump through a pipe inside the central distribution
chamber. This re-circulation lowers the
temperature of the entire chip mass and permits
washing to be carried out without further
delignification or over-cooking.
Valve: KV-19 A-D A-F Modified cooking
extraction switching valves
These valves extract liquor in the modified cooking
zone of the digester located near the middle of the
digester vessel. The extracted liquor is then sent
to the modified cooking heater. 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