Pulp wood pressure grinders or hoggers reduce the delivered wood to ground pulp or coarse chips. The coarse chips are then conveyed to a digester for "cooking" with boiling agents and steam. It is here that the pulp fibre (approx. 50 % of the wood) is separated from the other wood substances (lignin), leaving a brown pulp. In the follow-up grading plant, the brown pulp is cleaned, of heavily resinified insoluble wood remnants, after which it is washed, bleached and ground in refiners. (Coarse chips can also be grounded in refiners directly to pulp). As a highly diluted fibre mixture it is then fed via a pulp chute onto the dandy rollers of the paper machine.

Here the pulp is drained, compacted into a web (press section), dried on heated rollers (drying section), smoothed on glazing rollers, and finally rolled up at web speeds of 100 km/h and higher. A paper factory usually has its own power station and an effluent treatment plant (for environmental protection, recycling of chemicals from the spent cooking liquor, production of combustion energy from the thermal effluent treatment plants). Nowadays, the majority of pumps and other machines in the pulp and paper industry are sealed by mechanical seals.

Wood pressure grinder

Pumps, e.g. pulp pumps

The shaft stuffing box of a pulp pump can be affected by increases of pulp density and vacuum. If this risk exists under the given operating conditions, it is advisable to protect all single mechanical seals against dry running by means of a quench. Following mechanical seal designs have proven successful:

• up to a pulp density of 1.5% abs.dry (= proportion of dry substances):
  Single mechanical seal, balanced, encapsulated spring, dead-end operation, i. e. no flushing and no circulation, if possible with conically hollowed stuffing box. Face materials hard carbon/ceramic or hard carbon/carbide metal, elastomers Viton.
• up to a pulp density of 3% abs. dry:
  Same as above, but with the face materials carbide metal/carbide metal.
• pulp densities above 3% abs. dry:
  Conventionally double mechanical seals. The trend, however, is toward using more and more single, product-flushed mechanical seals with rotating counter ring and springs in a protected arrangement outside the product chamber.

Pulp pump with single-acting, balanced mechanical seal with encapsulated spring. Pulp density 1.6 to 3%

Single product-flushed mechanical seal for pulp densities of over % abs. dry

Disk refiners

A successful mechanical seal design for disk refiners

Disk refiners are used in the pulpwood grindings and defiberizing prozess