Reference Manual

13−3
operation and efficiency of the lime kiln.
An accurate and reliable control valve is
required for optimum performance.
— The underflow valve must be
appropriately sized to ensure the mud
level in the tank never reaches the filter
socks. The filter cannot operate properly
if this occurs.
D Typical Process Conditions:
— Fluid: Lime mud
—P
1
= 20 - 50 psi
—P
2
= 0 - 5 psi
—T = ∼175 °F
— Q = 100 - 300 USGPM
— SG = 1.36
D Typical Valve Selection:
— NPS 3 - NPS 6 V500 ANSI 150/Reverse
flow/Trim #4 for erosive service
— Plug: VTC Ceramic on alloy 6 Hub
— Seat: Solid VTC Ceramic
— Shaft: Oversized, 17-4PH Stainless Steel
— Retainer: Solid alloy 6 with Ceramic bore
— Bearings: Sealed alloy 6 construction
— Packing: PTFE
— Actuator: 2052, fail closed
— Positioner: FIELDVUE DVC6200 with
Performance Diagnostics
White Liquor and Lime Mud Pressure
Filters
Although white liquor clarifiers and lime mud
washers are predominate in installed base, the
trend in recent years has been to substitute
filtration equipment. This is possible because of
the relatively large size of lime mud particles. A
typical filtration system is shown in figure 13-2.
The white liquor pressure filter performs the same
function as the white liquor clarifier. The vessel is
divided into two compartments by a tube sheet.
The tube sheet supports a number of perforated
tube filter elements. Each perforated tube is
covered with a polypropylene filter sock. The white
liquor slurry must pass through the filters to reach
the upper compartment. As lime mud builds up on
the filters backflushing is required to restore
normal operation. Backflushing is accomplished by
recirculating the feed white liquor back to the
causticizer. This allows the level to drop and the
air cushion at the head of the vessel to expand
and force the liquor into the socks. This knocks
the lime mud from the sock filters. The lower
portion of the pressure filter acts as a settling zone
for the lime mud following backflushing. Clarified
white liquor is removed from the upper
compartment above the filter elements.
The lime mud pressure filter performs the same
function as the lime mud washer and the principle
of operation is the same as the white liquor
pressure filter. The lime mud is removed from the
bottom of the unit and the filtrate known as weak
wash is removed from the upper compartment
above the filter elements.
Valve: FV-17 Lime Mud to Filter
D Typical process conditions:
— Fluid: Lime mud
— T = 176°F
— P = 50 - 60 psig
D Typical valve selection:
— These valves can range from NPS 4 to
NPS 10. A SST body with 316 CRPL
ball, alloy 6 hard faced seat and alloy 6
bearings should be used in this
application.
Valve: FV-18 Lime Mud Recirculation
to Lime Mud Mixer
D Typical process conditions:
— Fluid: Lime mud
— T = 176°F
— P = 50 - 60 psig
D Typical valve selection:
— NPS 2 or NPS 3 SST valves with solid
VTC (ceramic) internals and alloy 6
bearings.
Lime Mud Filter
Lime mud from storage at 45-50% suspended
solids is pumped to a precoat filter for dewatering.
Dilution water is added at the intake of the transfer
pump to dilute the solution to 25% suspended
solids. The lime mud slurry is pumped to a vat
containing a rotating filter. The lime solids build to
a sufficient thickness on the filter and are
dewatered by means of a vacuum maintained on
the inside of the filter with a vacuum pump. A