3.2.20 Pulp pumps

Pulp pumps

Pulp pumps are used for pumping a mixture of liquid and fibrous materials. The impellers for pulp pumps are semi open with back vanes for simultaneous balancing of axial thrust and cleaning (flush) behind the impeller, see figure 3.2.20. Pulp pumps are usually horizontal, centrifugal pumps in back pull-out design with large inspection covers facilitate a quick and easy removal of any clogging. Special impeller types are fitted which match the type of pulp and the pump application. These include open impellers, which can be combined with a side channel impeller if the pulp is hard to pump, and free-flow impellers.

The choice of material is normally acid resistant steel and for simpler instances even grey cast iron. Due to the presence of entrained air in pulp suspensions it is necessary to design the blades of the impeller intake so as to prevent air locks. The concentration of air also causes harsh running with increased levels of vibration and mechanical stress. Compared to a water pump, the mechanical construction of a pulp pump requires an extra safety factor of approximately 2 in order to achieve satisfactory reliability. A wear plate, which is easy to replace, protects the pump casing against wear on the suction side. The pump shaft is protected against corrosion and wear at the wetted end by means of a shaft seal which prevents the shaft from any contact with the fluid pumped. The shaft seal is either a gland packing or a mechanical seal, both of which are supplied with a barrier fluid.

For handling paper pulp it is necessary to use specially designed pumps depending on the concentration of fibre in suspension, as follows:

• Up to 0.5% normal pumps
• 0.5 to approx. 2% pulp pumps or normal pumps fitted with special semi-open impellers
• Approx 2% to 5% pulp pumps
• Approx 5 to 6-7% pulp pumps with specially shaped inlet blading
• Above 6-7% dense pulp pumps, positive displacement type

Consistencies of 7% corresponds to a content of 70 kg dry solid pulp (no liquid) in one tonne of pulp/water mixture. Pulp suspensions with a pulp consistency of 0,5% can be considered as pure water as far as their hydraulic (flow and pressure) characteristics are concerned, in respect of the design of pumps, valves, piping and fittings. For unscreened pulp of concentrations above 3% the pump flow passage should be at least 40 mm and for suspensions of up to maximum 6 % slurry pumps must be used.

Non-clogging free-flow pumps are used if the pulp is contaminated e.g. pumping twigs and waste from the macerator, non-clogging free-flow pumps are also used to advantage creating a vortex for particular processes.

Properties of pulp that make pumping more difficult

• Content of solid particles, i.e. concentration. The pulp fibers build up a webb like bond that becomes denser with increased concentration. It is relatively difficult to put pulp with high concentration in motion, because when high energy impulses are transmitted locally in the piping system, it is likely that the bond is broken and that the movement of pulp occurs locally.
• Air Content. Air in the form of bubbles in pulp is very unfavorable from a pumping point of view. An air content of 1-2% is enough to change the pump characteristics.
• Pressure drop. At low velocity flows, the pressure drop for pulp is much higher than for water. Generally, pulp that is hot and contains additives easier to pump than cold and pure pulp.
• Re-introducing flow of pulp in a pipe system after a temporary stop (break down) can be problematic, especially with regard to highly concentrated pulp. This is because the “static pressure drop” is larger than the pressure drop at low flow rates. Dilution with water in the pump makes it easier to start.
• Disintegrating properties, i.e. flocking in pulp, wood splinter, twigs and synthetic fiber can cause flocking in the pump. A total clogging of both pump and pipe is even possible.
• Tendency of pulp to thicken in tapered ducts. Strongly tapered conical parts on the suction side of the pump, e.g. reduction of suction line from a pipe with dimension 400 mm down to 150 mm can cause flocking already at concentrations of 3-4%.
• Flow losses in pipelines.