Module 6

1st brown stock 2nd brown stock 3rd brown stock

wash press wash press wash press

From de-
knotting Dilution Dilution Dilution
conveyor conveyor conveyor

To oxygen stage
feed MC pump

Pressate Pressate Pressate

tank LC tank tank LC tank tank
Screening
(see module 5)

700 B 2040 938-02

1.0 Introduction
1.1 Objectives

Upon successful completion of this foundations module, you will be

able to:

• State the purpose of the brown stock washing department.

• Identify the main system and equipment components.

• Explain the purpose of the main system and equipment compo-

nents.

• Trace the process flows to, through and from the main pieces of
equipment.

• Identify and explain the function of the main control components.

• Describe the principles of operation of the main pieces of equip-

ment and systems.

800 T 2040 896-02 1

 Module 6

1st brown stock 2nd brown stock 3rd brown stock

wash press wash press wash press

From de-
knotting Dilution Dilution Dilution
conveyor conveyor conveyor

To oxygen stage
feed MC pump

Pressate Pressate Pressate

tank LC tank tank LC tank tank
Screening
(see module 5)

700 B 2040 938-02

1.2 Overview

The brown stock washing system is located between the deknotting

and the oxygen delignification areas.

The brown stock washing system includes three DPA-presses. The

screen room (see module 5), is located between the second and third
press.

Pulp (accept) from the disc knotters flows at about 3 % consistency to

the first brown stock wash press. The press dewaters the pulp,
increasing the consistency to 32 %. The pressate leaving the pulp in the
press flows to the first brown stock pressate tank. The pulp leaving the
press is diluted to 12 % in the first brown stock wash press dilution
conveyor using pressate from the second brown stock pressate tank.
The dilution conveyor is a standard screw conveyor that mixes the
pulp with the liquor. The diluted liquor is discharged to the first brown
stock LC tank.

From the first LC tank, the pulp is pumped by a centrifugal pump to

the second brown stock wash press at about 3.5 % consistency.
Similarly to the first press, the pulp is dewatered, diluted and dis-
charged to the second brown stock LC tank. From the second LC tank,
the pulp is pumped by a centrifugal pump to the screen room (see
module 5).

800 T 2040 896-02 2

 Module 6

Overview (cont´d)

Pulp (accept) from the primary screens flows at about 3 % consistency

to the third brown stock wash press. The press dewaters the pulp,
increasing the consistency to 32 %. The pressate leaving the pulp in the
press, flows to the third brown stock pressate tank. The pulp leaving the
press is diluted to 12 % in the third brown stock wash press dilution
conveyor by using pressate from the first post oxygen pressate tank.
The diluted liquor is discharged to the oxygen stage feed MC pump
standpipe.

800 T 2040 896-02 3

 Module 6

Water

Chips Oxygen
Cooking Washing Washing Bleaching
delignific.

Black liquor
Weak liquor

Evaporation Lime kiln

Lime, Lime mud,

CaO CaCO3
Na2SO4 Recovery
boiler Causticizing
Green Water
liquor
White liquor Dregs

700 B 2040 890-02

1.3 Washing of pulp

After cooking the liquid of semi-chemical and chemical pulps con-

tains organic and inorganic components. These components must be
removed. In most cases, these components will be treated by evapo-
ration or combustion to recover the heat value in the wood substance,
as well as chemicals to be reused. The pulp is separated from the liquid
by using an appropriate washing method.

The washing should be as complete as possible but still be carried out

with the lowest possible dilution factor (a state of opposition compris-
ing different practical solutions for different pulp mills). The figure
above shows the most common washing procedure after the cooked
chips have been defibrated, as well as the circulation of chemicals in
the sulphate process.

The very extensive washing of the pulp in most of today’s mills is not
justified from a chemical recovery point of view, but to minimize the
quantity discharged to the recipient. The total permitted discharge
quantity is stated in the water concession agreement for the mill, often
expressed in chemical oxygen demand, COD, the max. permissible
washing loss may also be stated.

800 T 2040 896-02 4

 Module 6

Water

Chips Oxygen
Cooking Washing Washing Bleaching
delignific.

Black liquor
Weak liquor

Evaporation Lime kiln

Lime, Lime mud,

CaO CaCO3
Na2SO4 Recovery
boiler Causticizing
Green Water
liquor
White liquor Dregs

700 B 2040 890-02

Washing of pulp (cont´d)

In order to achieve the best possible washing to the lowest possible

cost, the liquid system of the entire pulp mill is firmly closed and the
screen room is often enclosed in the wash chain.

The purpose of the washing is thus to:

– clean the pulp of spent liquor with a view to converting to final

product
– reuse of the chemicals in the spent liquor
– make use of the wood substance as fuel (or for further processing)
– decrease the content of polluted mill discharges.

Wash losses have historically been measured as salt cake kg Na2SO4/

ton pulp. This was common in former days when the main purpose of
the wash was to keep the "make-up" of the chemical consumption on
a reasonable level.

Today, washing is carried out in order to obtain the smallest possible

carry-over of organic dissolved material to the bleach plant. That is
why the wash loss today is measure as kg COD/ton pulp, which is a
more relevant. There is a relation between COD and Na2SO4, but it
varies for different washing equipment.

800 T 2040 896-02 5

 Module 6

Washing liquor 290 m3/h

Pulp in 60 ADT/h Washing Pulp out 32 %

Pressate

700 B 2040 891-02

Washing of pulp (cont´d)

The wash liquor quantity is indicated by the dilution factor DF, which
is expressed as m3 water per ton pulp and is the wash liquor quantity
that dilutes the black liquor. It is calculated as the difference between
the wash liquid quantity used and the liquid discharged out of the wash
system together with the pulp.

Calculation: 1) Wash liquid in =

290 = 4.83 m3/ADT
60

2) Liquid in outgoing pulp =

100 - 1 = 2.13 m3/ADT
32

DF = 4.83 - 2.13 = 2.7 m3/ADT

Modern washing systems are called counter-current washers, as the

wash liquor is conveyed against the pulp flow. Hot water is added in
the last wash stage. The filtrate from this stage is in turn used as wash
liquor in the previous wash stage, etc.

A certain dilution factor should normally be maintained in the last

wash stage. The dilution factor is used to control the flow and DS
content of the liquor to the evaporation plant.

800 T 2040 896-02 6

 Module 6

Ct

Co Cm

CPDU B 2037 194A

Washing of pulp (cont´d)

Modern washers can be divided according to two different washing

principles: displacement and dewatering. There are often various
combinations of them.

Displacement

The degree of displacement (DR = displacement ratio) expresses the

displacement efficiency and is valid for displacement at constant pulp
consistency.

Co – Cm
DR = –––––––
Co – Ct

Co = Conc. of the liquid in the feed pulp

Cm = Conc. of the liquid in the discharged pulp

Ct = Conc. of the wash liquid

Displacement means that the displacement liquid – in this case added

wash liquid – pushes the thicker liquor forward out of the pulp layer,
just like a piston pushes a gas or liquid forward in a cylinder.

800 T 2040 896-02 7

 Module 6

Ct

Co Cm

CPDU B 2037 194A

Washing of pulp (cont´d)

In practice, however, the displacement procedure differs from the

ideal, because some of the liquids mix when adding wash liquid, and
some of pulp layer is not homogeneous. Because of non-uniform
packing, there are differences in the permeability of the layer. This
means that the displacement is not equally efficient all over of the pulp
bed.

When free liquid is displaced from a pulp suspension by pure water or

liquor with a lower dry solids content, this will result in a decreased
content of dissolved material. This principle is applied at washing in
presses and filters.

The efficiency of a wash stage, for example a filter, is thus a function

of the thickening effect as well as the degree of displacement effi-
ciency. An apparently low degree of displacement efficiency can
correspond to a high washing efficiency, if the consistency increase of
the pulp is considerable, i.e., wash presses.

800 T 2040 896-02 8

 Module 6

Washing of pulp (cont´d)

Considering the whole production, the optimal situation for the wash-
ing process is that the costs for the washing system are as low as
possible, both for the washing unit itself and for other connected
departments, without deteriorating the quality of the chemical pulp.
Some limitations should, however, be added to these criteria. The
demands from the environmental protection and the concession
conditions of the authorities are not included in the the above optimi-
zation of the costs.

The washing costs can be divided as follows:

– costs for evaporation

– increased costs for bleach chemicals because of wash losses
– costs for replacement of cooking chemicals
– costs for the hot water
– costs for defoamers

800 T 2040 896-02 9

 Module 6

Diffuser 10 %
10 % Displacement

12 % 12 %
Filter Dewatering
1,5 % Displacement

Displacement 12 % 12 % 32 %
press
3,5 % Dewatering Displacement Dewatering

CPDU B 2037 195-02

1.3.1 Comparison between different washing principles

The figure above shows a comparison between three different types of

washing systems.

A Only displacement (for example, diffuser washer)

B Combination dewatering - displacement (for example, filter

washer)

C Combination dewatering - displacement - dewatering (for ex-

ample, press washer)

800 T 2040 896-02 10

 Module 6

Washing efficiency
1.0
Press

Filter
0.9
Diffuser

0.8

0.7

0.6

0.5
0 0.25 0.5 0.75 1
Displacement ratio
700 B 2040 946-02

Comparison between different washing principles (cont´d)

The figure shows the total washing efficiency over a wash stage vs. the
displacement ratio, DR, for the three washing principles.

A A displacement washing system, e.g. diffuser, is completely

dependent on a satisfactory displacement, i.e. high DR. A slight
disturbance that deteriorates the displacement, will results in a
considerable deterioration of the washing efficiency. Channel-
ling, air entrance or a low dilution can often disturburts displace-
ment.

B Normally, a filter has a DR of 0.6–0.8. Even with a less efficient

displacement on the filter, a satisfactory washing can be achieved
via the dewatering stage.

C A displacement press has a lower DR (ca. 0.5) than other washing

devices, but is more insensitive to disturbances in the systems.
Also, at DR = 0, the washing efficiency is about 80 %, as result
of the high washing efficiency obtained at the final dewatering.

800 T 2040 896-02 11

 Module 6

1.3.2 Factors influencing the function of the washing

equipment

A modern washing system is very complex. Changes in a process stage

can cause disturbances both in the preceeding and subsequent stages.
Any disturbance in this stage will be prolonged and magnified in the
washing system.

Wood quality

The quality of the wood can influence the drain properties of the pulp
as well as the contents of extractive substances. Many extractive
substances can cause foaming problems in the press.

Digester room

Besides the quality variations in the chips, there are primarily three
factors in the digester room that influence the pulp washing. These
factors are the composition of the white liquor and the black liquor and
variations in kappa number.

The black liquor can contain varying dry solids content, Sodium,
organic substance and soap going with the pulp to the washer room.
Residual Alkaline variations, caused by the kappa number distribu-
tion, can also be found in the washer room. The drainage capacity
decreases linearly with lower kappa numbers. Mixing bark into the
chips increases the zero fiber quantity, which considerably deterio-
rates the drainage capacity.

800 T 2040 896-02 12

 Module 6

Washing efficiency (kg COD/BDT)

25

20

15

10

0
1.0 2.0 3.0
DF (m3/BDT)

CPDU I 2025 912-02A

Dilution factor

It is important to control the dilution factor in the system. To a large

extent, the displacement efficiency is dependent on the dilution factor.
Reducing the dilution factor from 2.0 to 1.0 m3/t can in certain
washing plants, increase the wash losses from 6-8 kg COD/t to more
than 20 kg COD/t. If a washing plant does not work evenly the dilution
factor can vary strongly between the individual washing stages. The
impact on the whole washing unit will also be significant, i.e., the
wash loss will be considerably higher than it should be according to
DF on the last press.

800 T 2040 896-02 13

 Module 6

DF=1.4 DF= 0.4 DF=3.6 DF=2.0

11.5 kg BDT/t

O2

Messurement 1

DF=2.0 DF=1.8 DF=2.4 DF=2.2

6.1 kg BDT/t

O2

Messurement 2

CPDU B 2037 196-02A

Dynamic factors

A common reason for the problems is that the washing equipment, for
different reasons, is run at different capacities in different departments
of the mill. The more closed the process is, the more sensitive. The
buffer volumes are usually not dimensioned to manage varying
production conditions.

The wash liquid should move strictly countercurrently with smooth

flows without the so called “by-pass valves” being unnecessarily
opened. Clean wash water should be added, only to the last washing
stage.

This can be difficult in mills with frequent campaign changes, where

there will never be a balance. An objective for all mills should be to
run as smoothly as possible, without by-passing washing liquid, and
strictly countercurrently.

800 T 2040 896-02 14

 Module 6

DF=1.4 DF= 0.4 DF=3.6 DF=2.0

11.5 kg BDT/t

O2

Messurement 1

DF=2.0 DF=1.8 DF=2.4 DF=2.2

6.1 kg BDT/t

O2

Messurement 2

CPDU B 2037 196-02A

Dynamic factors (cont´d)

The figure shows how imbalance in a washing line can influence the
wash loss. The results emanate from two different measurements in a
mill. At one measurement, DF is comparatively equally distributed
over the whole washing line, thereby causing a small wash loss. The
other measurement shows a very large distribution of DF over all
machines with a considerably higher wash loss.

A balanced washing system can become in balanced as a result of one

mistake for a short period of time. Once this occurs, it will taken 1-2 days
for the system to regain balance.

800 T 2040 896-02 15

 Module 6

Temperature

The temperature for the whole washing system should be kept as high
as possible. A high temperature provides better drainage properties
and less problems with resin and soap. The temperature is controlled
by the wash liquid in the last washing stage.

Dry content of presses

The dry content on the presses should be kept as high as possible, to

obtain optimal washing efficiency.

Wash nozzles

It is important that all wash pipes on the filters are open and not partly
plugged in order to obtain the best washing performance.

800 T 2040 896-02 16

 Module 6

1st brown stock 2nd brown stock 3rd brown stock

wash press wash press wash press

From de-
knotting Dilution Dilution Dilution
conveyor conveyor conveyor

To oxygen stage
feed MC pump

Pressate Pressate Pressate

tank LC tank tank LC tank tank
Screening
(see module 5)

700 B 2040 905-02

2.0 1st, 2nd and 3rd brown stock wash

press
2.1 Purpose

The brown stock wash presses serve two purposes. The presses
remove solids from the pulp dissolved in the cooking stage. This
minimizes carry over into the oxygen delignification stage.

A second purpose of the 3rd brown stock wash press is to increase the
consistency of the pulp, going from the screen room to the oxygen
delignification stage, to about 32 %.

All three presses are Valmet displacement press model DPA-1555.

800 T 2040 896-02 17

 Module 6

3
1

5 4 To 2nd brown
stock wash press

From disc
knotters

To knot tank

To digester plant

700 B 2040 906-02

2.2 System components (1st brown stock wash

press)

Component Purpose
1 1st brown stock Dewaters and washes the pulp coming from
wash press the deknotting. The counter-rotating press
422 E 001 rolls are driven by separate hydraulic mo-
tors.

2 1st brown stock Receives the dewatered pulp, shreds it,

wash press and conveys it to the outlet by means of a
shredder screw screw

3 1st brown stock Supplies oil to drive the press roll hydraulic
wash press motors, to control the damping cylinders
hydraulic drive on the movable roll, and to raise or lower the
unit vat.

4 1st brown stock Receives pressate from the 1st brown stock
wash press wash press and supplies it as shower and
pressate tank dilution water.
422 T 005

5 Level pipe Avoids air in the pressate by maintaining

922 T 003 the pressate level in the pressate outlet pipe.

800 T 2040 896-02 18

 Module 6

10 To 2nd brown
stock wash press

From disc
knotters

8
To knot tank

To digester plant

700 B 2040 907-02

System components (cont´d)

Component Purpose

6 1st brown stock Pumps pressate as shower water to the 1st

wash press roll brown stock wash press roll showers.
cleaning shower
pump
422 P 035

7 Knot tank Pumps pressate to the knot tank.

dilution pump
422 P 036

8 Dilution pump Pumps pressate to the digester plant.

422 P 037

9 1st brown stock Dilutes the pulp coming from the 1st brown
wash press stock washing press.
dilution conveyor
422 C 001

10 LC tank Receives pulp from the dilution conveyor

422 T 006 and supplies it to the 2nd brown stock wash
press.

800 T 2040 896-02 19

 Module 6

To 2nd brown
stock wash press

From disc
knotters

To knot tank

To digester plant 11 12

700 B 2041 100-02

System components (cont´d)

Component Purpose

11 Tank agitator Circulates and blends the pulp in the LC

422 A 006/007 tank. Valmet flow jet agitators (FJA) are
used.

12 Press feed pump Pumps pulp to the 2nd brown stock wash
422 P 038 press.

800 T 2040 896-02 20

 Module 6

2
3

512 B 2040 646-02

2.3 Equipment components – press

Component Purpose

1 Pulp inlet Admits pulp and allows even distribution of

pulp into the press vat. Orifices at each of the
entrances to the press vat provide the dif-
ferential pressure for this distribution.

2 Press vat Contains the pulp to be dewatered. The

center section of the vat can be raised or
lowered using hydraulic cylinders for
cleaning or clearing a plug.

3 Pressate outlets Discharges the pressate from the press rolls

(2) to the pressate tank.

4 Hydraulic motors Drives the press rolls, in opposite directions.

(2)

5 Shredder Shreds and discharges the washed and pres-

screw sed pulp from the washer.

800 T 2040 896-02 21

 Module 6

6 7

520 B 2040 647-02

Equipment components – press (cont’d)

Component Purpose
6 Fixed press roll Compresses pulp against the vat wall in the
press zone, squeezing black liquor from the
pulp into the roll. This roll rotates on a fixed
axis.

7 Movable press Compresses pulp against the vat wall in the

roll press zone, squeezing black liquor from the
pulp into the roll. This roll rotates and moves
to allow adjustment of the distance between
the rolls.

8 A nip This is the distance between the rolls, and it

is one of the factors determining the final
pulp consistency.

800 T 2040 896-02 22

 Module 6

10 9 9 10

11

520 B 2040 661-02

Equipment components – press (cont’d)

Component Purpose
9 Roll doctors (2) Scrap the dewatered pulp mat from the press
roll surface. The doctor blades are made of
plastic. The doctor on the moveable roll
follows the roll as it moves towards or away
from the fixed roll.

10 Roll showers (2) Supply filtrate to clean the surfaces of the

rolls and the longitudinal seals. The
shower pipes are equipped with internal
cleaning brushes, so that filtrate containing
fibers can be used as shower water.

11 Press hood Encloses the press to contain the vapors and

off gases. The hood has inspection covers
and ventilation connections.

800 T 2040 896-02 23

 Module 6

12

14

13

15

711 B 2034 942-02

Equipment components – press (cont’d)

Component Purpose
12 Longitudinal seals Seal along the length of each roll, keeping
(2) the pulp from filling the space under the
hood. The seal strips are held in place by a
seal water system.

13 Seal holder plate Holds the seal in place against the roll
deck plate.

14 Seal water cavity Contains water under pressure to hold the

seal in place. Water pressure is regulated in
the cavity to allow the roll movement.

15 End seals (2) Seal along the lower circumference at the

ends of the press rolls, to prevent leakage of
the pulp into the filtrate outlets.

800 T 2040 896-02 24

 Module 6

3 4 2

1
5

Pressate Pressate

711 B 2040 666-02

Equipment components – showers

Component Purpose

1 Shower pipe Supplies pre-bleach pressate to the shower

nozzles. The rolls are cleaned on a timed
cycle.

2 Nozzle Sprays the pressate onto the roll surface to

re-move pulp fibers remaining on the roll
surface.

3 Brush rod Holds the cleaning brushes against the

inside surface of the shower pipe. The
brush rod is located in the center of the
shower pipe.

4 Cleaning brushes Oscillate with the brush rod to sweep

fibers from the nozzles and the inside
surface of the shower pipe.

5 Air cylinder Moves in and out to move the brush rod and
cleaning brushes.

800 T 2040 896-02 25

 Module 6

1 3 2

700 B 2039 518

Equipment components - agitator

Component Purpose

1 Flow jet agitator Mixes the pulp inside the LC tanks with the
incoming filtrate, to produce a uniform con-
sistency flow out of the tanks.

2 Motor Drives the agitator screw.

3 Drive Regulates the speed of the agitator screw.

800 T 2040 896-02 26

 Module 6

Feed Pump Main Pump Motor Feed 1

3

2
Cooling Water
4

Pressure
Relief Valve Damping
Reservoir Cylinders

Cooler

Rolls
Apart
Pump Switch

2
Vat Center
Section Controls

700 B 2041 483-02

Equipment components – hydraulic drive unit

Component Purpose
1 Hydraulic motors Rotates the press rolls, in opposite direc-
(2) tions. One motor drives each press roll.

2 Hydraulic unit Provides hydraulic pressure to drive the hy-

pumps (4) draulic press roll motors, operate the damp-
ing cylinders on the moveable roll, and raise
or lower the press vat.

3 Damping Holds the moveable press roll in place dur-

cylinders (4) ing normal operation.

4 Reservoir Contains the hydraulic oil.

800 T 2040 896-02 27

 Module 6

To pre oxygen exhaust fan

1
To dilution conveyor

5 4 From press feed pump

2 From washing liquor pump
Pressate squeezed from pulp in
3 press to press pressate tank
Pressate and fibers from press
6 roll to press pressate tank
Pressate to press roll showers
from press pressate tank
Pressate from press roll
showers to press pressate tank

512 B 2040 909-02

2.4 Process flows – press (principle is the same

for all three brown stock wash presses)

1 Pulp from the press feed pump at 3 to 4% consistency is pumped

to the press inlet distributors.

2 Washing liquor flows to the press wash liquor inlets (2).

3 Pressate from the brown stock wash press flows to the brown
stock wash press pressate tank.

4 Pressate from the press pressate tank is pumped to the press roll
cleaning showers.

5 Pressate from the press roll showers is recirculated back to the

press pressate tank, when the showers are not operating

6 Pressate and fibers rinsed from the press rolls flow to the pressate
tank.

800 T 2040 896-02 28

 Module 6

8 To pre oxygen exhaust fan

To dilution conveyor

From press feed pump

From washing liquor pump
Pressate squeezed from pulp in
press to press pressate tank
Pressate and fibers from press
roll to press pressate tank
Pressate to press roll showers
from press pressate tank
Pressate from press roll
showers to press pressate tank

512 B 2040 910-02

Process flows – press (cont’d)

7 Gases from the press are vented to the pre-oxygen exhaust fan.
8 Thickened pulp from the press, at 32% consistency, flows into the
dilution conveyor.

800 T 2040 896-02 29

 Module 6

Feed Pump Main Pump Motor Feed

1

Cooling Water

4
Pressure
Relief Valve Damping
Reservoir Cylinders
7
Cooler
3
2

5 Rolls
6 Apart
Pump Switch

Vat Center
Section Controls

512 B 2040 744-02rA

Process flows – hydraulic drive unit

1 Hydraulic pressure is supplied to the press roll hydraulic motors.

2 Hydraulic oil from the press roll hydraulic motors is returned to
the hydraulic drive unit reservoir.

3 Hydraulic pressure is supplied to the damping cylinders on the

moveable roll.

4 Cooling water is supplied to the hydraulic oil cooler.

5 Hydraulic pressure is supplied to the vat raise and lower cylin-

ders.

6 Hydraulic oil from the raise and lower cylinders returns to the
hydraulic drive unit reservoir.

7 Hydraulic fluid is returned from the raise/lower cylinder to the

power unit.

800 T 2040 896-02 30

 Module 6

SSL
1213
To pre oxygen exhaust fan

LSH
1219

HS
1226 SI
1210

PI PI
ZSX ZSX 1209 1216
1217 1218
To dilution conveyor

1 From press feed pump

3 From washing liquor pump

Pressate squeezed from pulp in
press to press pressate tank
Pressate and fibers from press
roll to press pressate tank
Pressate to press roll showers
from press pressate tank
Pressate from press roll
showers to press pressate tank

512 B 2040 913-02

2.5 Controls - 1st brown stock wash press (The

principle is the same for all three brown stock
wash press)

Control Purpose

1 Pressure indicators Indicates the pressure in the press vat.

PI 1209/PI 1216 Alarms warn the operator when the vat
pressure reaches the preset high value.
At a very high pressure, the feed pump will
stop, the feed valve will close and the
shower water pump will stop.

2 Speed indicator Indicates the speed of the press rolls.

SI 1210

3 Position switch Warns the operator when the distance

alarm between the press rolls reaches the preset
ZSX 1217/ maximum value. The feed pump will stop,
ZSX 1218 the feed valve will close and the press rolls
will stop, if the roll moves to far out.

800 T 2040 896-02 31

 Module 6
SSL To pre oxygen exhaust fan
5 1213

LSH
1219

6 4

HS
1226 SI
1210

PI PI
ZSX ZSX 1209 1216
1217 1218
To dilution conveyor

From press feed pump

From washing liquor pump
Pressate squeezed from pulp in
press to press pressate tank
Pressate and fibers from press
roll to press pressate tank
Pressate to press roll showers
from press pressate tank
Pressate from press roll
showers to press pressate tank

512 B 2040 914-02

Controls (cont’d)

Control Purpose

4 Level switch Indicates the pressate level with vat. Alarm

high warns the operator when the level reaches
LSH 1219 the preset high value.

5 Speed switch Warns the operator when the shredder screw

limit speed reaches the preset low value.
SSL 1213

6 Hand switch Switches the roll cleaning showers between

HS 1226 Auto and Off.

800 T 2040 896-02 32

 Module 6

From 1st brown stock wash press Mill water To gas treatment

PIC
1214
2

LIC
1227

Pressate to 1st brown

stock wash press

To knot tank

To cooking plant

700 B 2040 915-02

Controls - first brown stock wash press pressate

tank

Control Purpose

1 Pressure Indicates and controls the pressure of air and

indicating foam in the level pipe by regulating the
controller discharge to the pressate tank. Controlling
PIC 1214 this pressure controls the pressate level in
the end chambers of the press.

2 Level indicating Indicates and controls the level of pressate in

controller the level pipe by regulating the pressate
LIC 1227 discharge to the pressate tank. The level is
compensated for by the pressure of the foam
as measured by PIC 2115.

800 T 2040 896-02 33

 Module 6

To pre oxygen exhaust fan

To dilution conveyor

From press feed pump

From wash liquor pump
Pressate squeezed from pulp in
press to press pressate tank
Pressate and fibers from press
roll to press pressate tank
Pressate to press roll showers
from press pressate tank
Pressate from press roll
showers to press pressate tank

512 B 2041 104-02

2.6 Principles of operation

The first brown stock wash press is the first of three presses in the
brown stock washing. The pulp is dewatered in the press to raise the
consistency from 3 % to 32%, and to remove dissolved solids.

The pulp at 3-4 % consistency is pumped to the press and distributed

into the vat through the inlet box. The pulp dewaters, or drains, against
the roll surfaces, forming a mat on each roll. The consistency at this
point is about 8%. The mats are dragged along with the rolls and merge
as they enter the fixed nip between the rolls. Filtrate is squeezed from
the mat as the mat is forced through the nip, increasing the consistency
to the desired 32%.

The dewatered pulp is doctored from the upper roll surfaces into the
press shredder screw area. The discharger consists of a toothed screw
flight which shreds the pulp, and a smooth screw flight which directs
the shredded pulp to the outlet chute and into the dilution conveyor.

800 T 2040 896-02 34

 Module 6

Fixed Movable
Fixed Press Roll Press Roll

Slides

Wash Perforated
Filtrate Plate

711 B 2034 957-02

Principles of operation (cont’d)

The rolls are identical, except for the bearing arrangement. Each roll
is built up from a hollow shaft with stub shafts welded at each end.
Longitudinal ribs and an outer shell made of sets of rings are attached
to the hollow shaft.

The outer surface of the shell is covered by a perforated deck plate. The
perforations in the deck plate are small enough to allow liquor from the
pulp to pass into the roll, but not the fibers. The filtrate is pushed
through the roll deck surface into the drainage channels which drain
to filtrate collection chambers at either end of the press. From here, the
filtrate flows by gravity to the filtrate tank.

The rolls are supported by spherical roller bearings. One roll rotates
on a fixed axis while the other roll is supported in a sliding bearing
assembly, allowing it to slide a small distance towards or away from
the “fixed” (but rotating) roll.

Four hydraulic damping cylinders hold the movable roll in its working
position. These cylinders (shock absorbers) allow the press roll to
move slightly so that thick wads of pulp or undesired material may
pass through with minimum damage to the roll deck plate. If the rolls
are forced too far apart, a limit switch will shut down the press.

800 T 2040 896-02 35

 Module 6

Principles of operation (cont’d)

The roll surfaces are rinsed with filtrate from high pressure showers.
The showers operate intermittently, supplying 0.6 MPa pressure
filtrate. There are cleaning brushes in the shower pipes, which operate
on a timed basis, to sweep away fibers which may build up in the
shower nozzles and on the inside surface of the shower pipes. The
cleaning brushes operate using pneumatic cylinders. When the show-
ers are not operating, the filtrate is recirculated to the filtrate tank. This
allows the press roll cleaning shower pump to run continuously during
the actives sequence.

800 T 2040 896-02 36

 Module 6

Feed Pump Main Pump Motor Feed

Cooling Water

Pressure
Relief Valve Damping
Reservoir Cylinders

Cooler

Rolls
Apart
Pump Switch

Vat Center
Section Controls

512 B 2040 557-02

Principles of operation (cont’d)

Press hydraulic system

With the exception of the press discharger, the press is driven
hydraulically. The hydraulic circuits are supplied by the pre-bleach
hydraulic drive unit.

The main pump supplies the necessary hydraulic oil to the drive
motors that are attached to one end of each press roll. This pump is
capable of variable displacement, which means it can vary the volume
of hydraulic oil supplied to the drive motors. The rolls turn faster as
the oil flow is increased to the motors.

A gear is located on the other end of each press roll. The two gears
mesh together so that the rotation of the rolls is synchronized.

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 Module 6

Principles of operation (cont’d)

Press roll speed can be indirectly controlled by a pressure indicating

controller that senses pressure in the hydraulic supply lines to the drive
motors. If the pressure controller is running automatically, the speed
of the rolls will adjust to the fluctuating production rate and drainage
characteristics. The consistency of the outgoing stock can be con-
trolled by adjusting the speed of the rolls. For example, if the inlet
consistency decreases, decreasing the production rate, the rolls will
turn easier due to less resistance (less stock). This reduces the back
pressure in the hydraulic lines momentarily. The pressure controller
sends a signal to the variable displacement pump, which reduces the
volume of hydraulic oil being pumped. This reduces the roll speed,
which raises the hydraulic pressure back to the setpoint pressure.

The effort required to turn the press rolls is called torque, and is
indicated by measuring the pressure of the oil flowing from the
hydraulic drive pumps to the hydraulic motors, located on the ends of
the press rolls.

The pulp consistency leaving the press is directly related to the torque.
It requires more pressure to turn the rolls slowly, producing drier pulp,
than it does to turn them fast, producing wetter pulp.

For example, as the feed consistency decreases (less pulp in the same
flow of water), the rolls are easier to turn. The torque controller senses
this as a slight drop in pressure and slows down the roll speed by
sending less oil to the motors. As the roll speed decreases, the effort
to turn the rolls (torque) returns to its previous level and the pulp
discharge consistency is kept constant.

Torque is created by the press rolls’ resistance to turn, as well as

overcoming all friction of the pulp seals and friction in the hydraulic
motors and lines. If the press is completely empty of pulp, a certain
pressure is required to turn the rolls. This is typically about 500 - 650
psi, for a given oil temperature and speed. If the press has been flushed,
yet stagnant pulp persists, the pulp will act as a brake shoe. This brake
shoe effect can cause a rise in the torque, which cannot be over come
by the hydraulic drive system, resulting in a trip out.

800 T 2040 896-02 38

 Module 6

Limit switch

Damping cylinder

711 B 2034 925-02

Principles of operation (cont’d)

Damping cylinders
The high pressure pump supplies hydraulic oil to the damping cylin-
ders. There are two cylinders attached to each end of the moveable
press roll. These act as shock absorbers, allowing the roll to move.
When excessive pressure between the two rolls overcomes the hy-
draulic pressure in the damping cylinders, the moveable roll backs
away from the fixed roll. This allows any thick pulp wads or other
foreign material to pass between the rolls. The damping force then
returns the roll to its working position.

If the nip becomes excessively large, the press rolls will stop. Travel
limit switches will shut down the press, if the moveable roll moves
outward to its maximum limit.

800 T 2040 896-02 39

 Module 6

1st brown stock 2nd brown stock 3rd brown stock

wash press wash press wash press

From de-
knotting Dilution Dilution Dilution
conveyor conveyor conveyor

To oxygen stage
feed MC pump

Pressate Pressate Pressate

tank LC tank tank LC tank tank
Screening
(see module 5)

700 B 2040 900-02

3.0 First brown stock washing dilution

conveyor (principle is the same for
all three dilution conveyors)
3.1 Purpose

The purpose of the first brown stock washing press dilution conveyor
is to provide mixing and dilution of the dewatered pulp from the press,
while directing it to the LC tank ahead of the second brown stock wash
press.

The pulp is diluted with pressate from the second brown stock wash
press pressate tank. The dilution conveyor provides adequate mixing
to dilute the pulp.

800 T 2040 896-02 40

 Module 6

From 1st brown

stock wash press
4 1 2 3

5 6
To 2nd brown stock
wash press LC tank

520 B 2040 901-02

3.2 Equipment components

Component Purpose

1 Pulp inlet Admits the pulp from the first brown stock
wash press discharger.

2 Trough Houses the screw and provides a chamber

for the dilution to occur.

3 Conveyor screw Rotates to mix the shredded pulp and

dilution liquor together, and conveys the
diluted pulp to the outlet.

4 Conveyor motor Turns the conveyor screw.

5 Dilution Introduce the dilution liquor into the dilu-

inlet nozzle tion conveyor.

6 Pulp outlet Discharges the pulp to the 2nd brown tock

wash press LC tank.

800 T 2040 896-02 41

 Module 6

From 1st brown Vents to pre oxygen

stock wash press exhaust fan

1 4

3
To 2nd brown stock
wash press LC tank

From 2nd brown stock

2 wash press pressate tank

520 B 2040 902-02

3.3 Process flows

1 Pulp from the 1st brown stock wash press discharger flows into
the dilution conveyor.

2 Liquor from the 2nd brown stock wash press pressate tank is
pumped to the dilution conveyor for dilution.

3 Pulp from the dilution conveyor flows to the 2nd brown stock
wash press LC tank.

4 Gases from the dilution conveyor are vented to the pre-oxygen

exhaust fan.

800 T 2040 896-02 42

 Module 6

From 1st brown Vents to pre oxygen

stock wash press exhaust fan

SSL
1223

FIC
1220
To 2nd brown stock
wash press LC tank

From 2nd brown stock

wash press pressate tank

520 B 2040 903-02

3.4 Controls

Control Purpose

1 Flow indicating Indicates and controls the flow of liquor

Controller from the 2nd brown stock wash press pressate
FIC 1220 tank to the dilution conveyor.

2 Speed switch Warns the operator when the dilution con-

limit veyor speed reaches the preset low value.
SSL 1223

800 T 2040 896-02 43

 Module 6

From 1st brown Vents to pre oxygen

stock wash press exhaust fan

To 2nd brown stock

wash press LC tank

From 2nd brown stock

wash press pressate tank

520 B 2040 904-02

3.5 Principles of operation

In the pulp leaving the 1st brown stock wash press, part of the pressate
has been removed, raising the consistency to 32%. Cleaner pressate
from the 2nd brown stock wash press pressate tank is added to the pulp
while it is being conveyed to the 2nd brown tock wash press LC tank.
This lowers the consistency of the pulp to 12%.

The dilution conveyor consists of a rotating screw inside the conveyor

trough. The conveyor screw is driven by a motor. A filtrate inlet nozzle
admits the dilution into the trough, and distributes the filtrate, mixing
it evenly with the pulp.

800 T 2040 896-02 44

Brown stock washing
Module 6

RiauPulp
 Module 6

Machine: Brown stock washing

Type:

Manufacturer: Valmet Fibertech AB

851 94 Sundsvall
Sweden
Tel +46-60-165000
Fax +46-60-165500

Editor/Copyright © 2000 Valmet Fibertech AB

Reg.number 700 T 2040 896-02

Revision

800 T 2040 896-02

I
 Module 6 - Contents

Contents

1.0 Introduction ................................................. 1

1.1 Objectives ........................................................................ 1
1.2 Overview .......................................................................... 2
1.3 Washing of pulp ............................................................... 4

2.0 1st, 2nd and 3rd brown stock wash

press ........................................................... 17
2.1 Purpose........................................................................... 17
2.2 System components (1st brown stock wash press) ........ 18
2.3 Equipment components – press ..................................... 21
Equipment components – showers ................................ 25
Equipment components - agitator .................................. 26
Equipment components – hydraulic drive unit .............. 27
2.4 Process flows – press (principle is the same for all
three brown stock wash presses) .................................... 28
Process flows – hydraulic drive unit .............................. 30
2.5 Controls - 1st brown stock wash press (The principle
is the same for all three brown stock wash press) .......... 31
Controls - first brown stock wash press pressate tank ... 33
2.6 Principles of operation ................................................... 34

3.0 First brown stock washing dilution

conveyor (principle is the same for all
three dilution conveyors) .......................... 40
3.1 Purpose........................................................................... 40
3.2 Equipment components.................................................. 41
3.3 Process flows ................................................................. 42
3.4 Controls .......................................................................... 43
3.5 Principles of operation ................................................... 44

800 T 2040 896-02

II