Liquids Pumped In A Mill

There are, broadly, three categories of liquids to be pumped in a paper mill:

1. Water and similar fluids

2. Liquors and slurries—mainly chemicals and solids in solution or suspension

3. Stock—suspension of cellulose fibers in water

Water Apart from the quantities involved, there are no special requirements concerning the water in pulp and paper mills because operating conditions are well within normal limits. However, iron and carbon steel piping should not be used in bleach pulp mills because of iron pickup.

Process water treatment is frequently used to purify process water for the mill and to remove undesirable elements such as iron. Higher-quality water is required for chemical reparation in the bleach plant and for boiler feedwater where demineralizer plants are used. Rubber- or epoxy-resin-lined pumps are used for those components in contact with the demineralized water.

pumps FOR mill water For the majority of pumps, standard cast iron or stainless steel fittings are used except as noted for demineralized water. In many mills, however, stainless-steel-fitted pumps are standard because this permits a minimum number of spares to be held in stock for other duties.

In the paper mill, water used to form the sheet on the paper machine has a very low fiber content—1 to 1% consistency—and is known as white water. Fiber contents this low usually do not cause any pumping problems except in wear ring areas where flashing or slotting is used to keep leakage paths open and free from binding.

Much of this water is recirculated, and where bleached products are produced, pumps must be constructed an austenitic stainless steel.

Liquors and Slurries Depending on the process and the particular point in that process, the liquor characteristics may require special pumps or special materials. Although the liquor cycle is a difficult one as far as the pumps are concerned, standard designs should be used whenever possible because this reduces the number of different types of pumps in the mill. In some cases, it may be necessary to use a higher material specification than necessary to achieve interchangeability. Liquor and slurry pumps may be grouped as follows:

Group A—Standard designs suitable for most process uses where corrosion or erosion is not a major factor. Impellers are typically stainless steel with casings of cast iron.

Group B—Standard end-suction designs suitable for corrosive liquors. All liquid end components are typically 316 stainless steel. Duplex stainless steels may be used where erosion may be a factor.

Group C—Standard or nonstandard designs suitable for special services. Pumps are similar to group B for most applications but are of 317 or 317L stainless steel. For most corrosive services, glass-reinforced epoxy, resin, titanium, super austenitic stainless steels are used for both impeller and casing. Mechanical seals in place of packed boxes or dynamic seals are usually fitted to these pumps.

Recommendations for liquor and slurry pumps are

1. All liquor pumps should be classified as slurry type with open nonshrouded impellers of the end-suction and back pull-out type. Simply supported, double suction pumps are also used for fibrous slurries (stock)—particularly 2% to 3% consistency stock on the paper machine. This includes most fan and cleaner pump applications.

2. On group A and B pumps, sealing is accomplished with dynamic seals, mechanical seals, or packed stuffing boxes.

3. For group C pumps, in particular, it may be necessary to depart from a standard design or type of centrifugal pump. For example, if a positive displacement characteristic is required, a screw-type pump may be used with confidence. In addition, all pumps handling stock with consistency above 6% must be regarded as nonstandard types.

After the pumps are grouped, it becomes necessary to decide which pump may be used for specific liquors. Requirements for individual mills will differ in detail, but the following may be taken as an indication of current practice, particularly in modern sulfate (kraft) mills. In every case, manufacturers should be made aware of the liquor characteristics and of the location of the pump in the process.

cooking liquor (white LIQUOR—sulfate process) This is essentially an alkaline solution made by causticizing green liquor. The liquor is prepared at concentrations over the range of 50 to 100 g/liter depending on the wood species, and the amount of active alkali (expressed as Na2O) may be from 14 to 30% of the dry wood weight. White liquor is mainly sodium hydroxide, with a small percentage of sodium sulfide which depends on the mill sulfidity. Higher values of active chemical are used in bleached pulp mills. The term sul-fidity is used to denote the ratio of chemicals present; it is frequently expressed as Na2O and calculated from the expression

Na2S

NaOH + Na2S

The sulfidity value commonly used is from 20 to 30%; the higher values usually denote better chemical recovery. The specific gravity of the liquor will be approximately 1.2, and after clarification only small quantities of grit should be present. The liquor must be con sidered an abrasive that produces a high rate of wear on pump rotating elements. White liquor has a tendency to crystallize on internal surfaces of pipes and pumps, but there are no special viscosity problems and a pump head loss allowance of about 10% above that of water should be adequate. Group B pumps are recommended.

blow tank discharge As the liquor introduced with the chips into the digester combines with the noncellulose and hemicellulose fractions of the wood, it changes from white liquor to black liquor before reaching the blow tank. In addition, the sudden release of pressure frees the cellulose fibers from the other matter, so the blow tank contains both raw stock (pulp) and black liquor. Pulp from the blow tank is often entrained with air, sand, and other contaminants. A stock pump, therefore, is required for this duty because the stock concentration is quite high.

black liquor For convenience these pumps are divided into three groups.

Weak Black Liquor (Total Solids Up to 20%) During washing, hot water is used to dissolve away the surplus organic matter from the pulp, and the liquor produced is termed black liquor. This liquor is a mixture of the lignins and carbohydrates in the original wood plus the cooking chemicals: it is alkaline with a solids content of 14 to 16% in a sulfate mill. The temperature will be about 180 to 190°F (82 to 87°C), and the specific gravity about 1.08. Washing is usually carried out with a minimum of three countercurrent stages, and the solids content given previously is representative of the liquor leaving the stages nearest the inlet; that is, where it is most concentrated. The quantity of recirculated liquor is quite high, and many mills have found group A pumps with stainless trim to be satisfactory and economical. With the low solids content, there are no special viscosity problems. This may be seen from Figure 2.

Black Liquor with Total Solids of 20 to 50%c This liquor is formed by the evaporation of water from weak black liquor. The concentration is accomplished in multiple-effect evaporators, which usually discharge liquor with about 50% total solids at close to 200°F (93°C). In some odor-free installations, the solids concentration is much higher. Because of the nature of the evaporation, special pumps are usually required.

TEMPERATURE, °C 40 50 60 70 80 90

TEMPERATURE, °C 40 50 60 70 80 90

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