Medium Consistency Pumps

The feed pump must provide the pressure to overcome the hydrostatic head of the

bleaching vessel, any backpressure controlled at the reactor top, as well as the

pressure loss in piping, control valves and mixing equipment. Modern medium

consistency pumps follow the centrifugal pump design with a specially designed

open impeller. Compared to the previously used positive-displacement-type

pumps, they operate at excellent energy economy and reduced maintenance costs.

Medium consistency pulp suspensions tend to have small gas bubbles

embedded in the fiber network. As the pump feed enters the casing, gas is driven

into the low-pressure zone near the center of the pump impeller. If the gas is not

removed, it accumulates at the impeller up to a point where the pump fails to deliver.

Therefore, medium consistency pumps are equipped with means for extracting

the separated gas from the pump casing, either with an integrated or an external

vacuum pump. The Kvaerner Pulping Duflo pump is an example of a medium

consistency pump with an integrated vacuum system (Fig. 7.5). The air contained in

the pulp feed passes through the impeller and is extracted fromthe back of the impeller

by a liquid ring vacuum pump mounted on the same shaft as the impeller.

Fig. 7.5 The Kvaerner Pulping Duflo pump [9].

7Pulp Bleaching

Centrifugal medium consistency pumps are flanged to the bottom of a standpipe

which is operated at constant level, and thus provides a constant hydrostatic

pressure to the suction side of the pump (Fig. 7.6).

Fig. 7.6 Example of medium consistency (MC®) pumping

system with an external vacuum pump [10] (picture from

Sulzer Pumps Finland Oy).

Figure 7.7 shows a Kvaerner Pulping Duflo impeller as an example of an advanced

medium consistency pump impeller. The arms which extend into the

standpipe fluidize the pulp slurry at the entrance to the pump, thus helping to

avoid clogging.

Fig. 7.7 The Kvaerner Pulping Duflo impeller [11].

Following another design philosophy for medium consistency pumps, Andritz

uses individual systems to provide for gas separation and pumping (see Fig. 7.13).

7.2 Bleaching Operations and Equipment

Suppliers of medium consistency pumps claim maximum operating consistencies

to range up to 18%. In everyday plant operation, such a consistency level is

seldom reached and the consistency limit for continuous, unattended operation is

often in the range of 10–14%.

7.2.4

Дата добавления: 2015-10-21