A pump seal on pulp stock copes with fiber by keeping the stock off its springs and faces and by matching the seal arrangement to the stock consistency. A plain single seal handles only dilute stock; thicker stock calls for a flushed single seal or a double seal fed with pressurized seal water.

How Fiber Damages a Pump Seal

Fiber damages a seal by cutting off the cooling liquid to its faces. Pulp collects in the low-flow, low-turbulence zone next to the seal gland and dewaters there into a mat. That mat either insulates the seal or blocks the water reaching it, so the faces run close to dry and heat up.

The heat shows first in the elastomers. O-rings harden and lose their sealing force, and the sliding face can hang up on packed fiber and heat-damaged O-rings, so it no longer stays closed against its mating ring.

Fiber also packs between the coils of springs on the rotating side, where the stock wets them. A clogged spring cannot keep pushing the face closed as it wears or moves.

Choosing the Seal Arrangement by Stock Consistency

A plain single seal suits stock up to about 2–3% consistency; above that, use a single seal with a flush or a double seal with pressurized seal water. These limits are seal makers’ ratings for their own seals, so check the rating of the specific seal you are buying.

  • Plain single seal: rated for stock up to 2% consistency; some single seals run without a flush in stock up to 3%.
  • Single seal with quench: a quench of non-pressurized water on the atmospheric side cools the seal and washes away stock that dries there. One maker builds its quench single seal for stock pumps in pulp mill washing and screening, without publishing a consistency limit.
  • Single seal with flush: one maker rates its flushed single seals for fibrous stock up to 8% consistency.
  • Double seal: the same maker rates its double seals for fibrous stock of any consistency.

On large stock pumps where pulling the rotor means a long outage, a split seal goes on around the shaft without dismantling the pump.

Spring Design for Fiber Service

Choose a seal whose springs sit outside the stock: stationary springs in the gland, or springs mounted outside the seal entirely.

If a seal’s springs do sit in the stock, they need a non-clogging design, such as one large coil rather than a ring of small springs on the rotating side.

Seal Face Materials for Stock Pumps

For most stock pumps the face pair is silicon carbide against silicon carbide, or tungsten carbide against silicon carbide, provided the faces get a film of clean water. Where the water film is marginal, ask for a silicon carbide grade loaded with graphite.

Between the two hard pairs, silicon carbide is the harder face with the better abrasion and corrosion resistance, while tungsten carbide is tougher and takes mechanical shock and vibration better. On clean white water and very dilute stock, carbon against silicon carbide still works and tolerates a short loss of water better than a hard pair.

For black, white and green liquor and other caustic duties, specify sintered silicon carbide: reaction-bonded silicon carbide contains free silicon that caustic attacks.

Flush and Seal Water for Fiber Stock

On fiber stock, use clean water from an outside source: a Plan 32 flush for a single seal, or pressurized seal water from an external supply, as in Plan 54, for a double seal. A single seal with a quench takes its non-pressurized water through Plan 62.

Plan 11 takes its flush from the pump discharge, so on stock it feeds fiber through an orifice that plugs.

Set the flush water or seal water 1 to 1.7 bar (15 to 25 psi) above the seal chamber pressure, for a single seal on Plan 32 and a double seal alike. Feed the seals from a steady, filtered supply.

A single seal on Plan 32, and a double seal whose seal water runs to drain, use about as much water as packing; only a closed-loop barrier system cuts a double seal’s water loss to a few fluid ounces a day.

Shaft Movement on Stock Pumps

The shaft movement a stock pump seal must take is the axial travel each time the impeller clearance is reset, plus the misalignment, vibration and bearing play its spring-loaded face absorbs in normal running.

On the Goulds 3175, a pump built for stock service, the shaft is moved 0.38 mm (0.015 in) to reset the impeller-to-sideplate clearance, and the adjustment is repeated whenever head or capacity falls off.

A component seal set on the shaft cannot be moved afterwards, which makes those adjustments difficult. A cartridge seal can: loosen its shaft collar, adjust the impeller, and reset the collar.