Economics And Pump Selection

The normal economic considerations of any continuous process apply equally well to pulp and paper mills, with a few points of difference. In pump installations, the improved cost figures that are possible from larger units are limited to some extent by the manufacturer's standard size units. Because the industry is capital intensive, the overriding factor in any pump installation is reliability. To achieve this, it does not necessarily follow that it is better to have two pumps installed, with one as a standby. one properly designed and serviced unit may well be better than two unknown units; this is especially true where the pump is in a portion of the process that cannot be interrupted without serious losses, either in raw materials or in quality of the finished product.

A duplication of pumps means complications in extra valves, pipework, connections for steam and viscous liquids, electric motors, cables, and starters. The result is that, in modern mills with good machinery and materials of construction, there is a strong tendency away from the duplication of pumps because of increased costs and questionable reliability. It follows that the important thing is to select the right pump and the right duty point in a particular range.

There may be several hundred pumps in a modern pulp and paper mill, but the cost of these pumps is probably less than 5% of the total equipment cost. It is unwise, therefore, to jeopardize mill reliability by compromising pump quality. Corrosion and erosion are major factors in pump life, and even with the best materials, the life of some components in severe service may be 12 months or less. Moreover, the power used by pumps is usually less than one-third of the mill demand. If one remembers that the cost of total power absorbed in a mill is only around 4%, even a 50% reduction in pump power will still be less than 1% net.

Efficiency The best point at which to operate a pump is, of course, its maximum efficiency, but this is not always possible, particularly in the case of stock pumps because of the wide range of process variables in most pulp and paper mills. open impellers and excess clearances also reduce the efficiency, yet these factors are much more important in stock pumps than efficiency. Another important consideration is speed. Stock pumps should be chosen to run at as low a speed as possible to achieve stable operation, and this speed may not produce an efficient pump. The shape of the pump performance curve is much more important than the best efficiency quoted by a manufacturer. A flat or unstable head curve may produce surging or instability in the pump output. Good pump selection, therefore, must emphasize reliability as the first consideration and efficiency and costs as secondary considerations.

Pump Speeds Most of the pump duties in a mill can be accomplished by single-stage pumps and four-pole motor speeds. For liquids other than water, two-pole motor speeds should be avoided if possible. For special duties, including stock pumping, six- or eight-pole motors may be required unless some indirect or variable speed is used. Although it is true that lower speeds mean larger pumps and more expensive electric motors, lower speeds are justified because of reduced maintenance and greater reliability. Some deviation from these speeds may be necessary for pumps generating heads in excess of 150 ft (46 m), but this can often be taken care of by a larger impeller rather than a higher shaft speed.

Although not an option in every circumstance, the use of variable speed drive systems for pumps to replace control valves should also be considered. Reliable alternating current variable frequency drives are now readily available up to 2300 volts and are easily justifiable based on normal power costs at the 440 volt level. The commensurate speed reduction seen by the pump when replacing a control valve and avoiding its necessary pressure drop will also add to the reliability and life of the pump.

Multistage Pumps Except with boiler feedwater, the use of multistage pumps should be avoided. This is particularly true for stock and viscous liquors. The complicated pump design makes such units unacceptable for these services.

Pipeline Systems With black liquor, green liquor, and similar high-viscosity liquors, adequate provision must be made for steaming out and subsequent liquor drainage. The pumps must also be included in this system. Although the liquor pumps should be designed to pass some solid matter, motorized strainers should be used on the pumps for cyclone evaporators and recovery boiler-fuel pumps because both pumps discharge to spray nozzles. Dead pockets and other areas where liquor can collect should be avoided because solids from the liquor will build up in these areas and possibly break away to block pipelines or pump impellers. For protection at shutdowns, even for short periods, steaming out is essential.

Positive Displacement Pumps The principal use of the positive displacement pump is for consistency control of stocks above 5%. The normal measuring device used is quite satisfactory at low consistencies but is less reliable at the higher values. More satisfactory control may be achieved by using a screw pump, where the power is proportional to the pulp consistency at constant flow. Such pumps have been very reliable on consistency control.

Digester Circulating Pumps Digester circulating pumps are used with indirectly heated batch digesters to circulate the liquor at the digester pressure and temperature. Maintenance problems are common on these pumps because heads can be as high as 150 lb/in2 (1034 kPa) and temperatures as high as 350°F (177°C). In addition, the circulating liquor contains some raw pulp even though screens are fitted to the digester outlets. Pumps for this service, therefore, should be centerline mounted, of very heavy construction, have closed impellers, and mechanical seals. In addition, there is often considerable pipework involved, for a digester may easily be 60 ft (18 m) high and pipe loads are often imposed on the pumps. Expansion joints and long-radius bends are used, but it is desirable to support the pumps on springs or slide bases.

Pumps for Heavy Black Liquor Above 60% Solids A typical pump for this service is shown in Figure 11. An open impeller in a 316 stainless steel casing is recommended. A heavy sleeved shaft of 316 stainless steel with ample clearance between the rotating parts is also required for satisfactory operation. These pumps may be required to handle black liquor up to 2000 centistokes viscosity and should be provided with water cooling. Steam

FIGURE 11 Black liquor pump (Courtesy ITT/Goulds Pumps, Inc.)

jacketing is not always satisfactory because the liquor may tend to bake on the walls of the casing. Pump speeds should be below 1800 rpm, if possible.

Multiple-Effect Evaporator Pumps Pumps in this service often operate in cavitation owing to problems in regulating the flow between evaporation stages. Level control valves in the suction line to the pump can alleviate this problem, but cavitation can still be expected in the pump. A self-priming pump may give longer life of the rotating elements than the condensate pump usually used on this service.

Diaphragm Pumps Diaphragm pumps are used in pumping lime mud slurries of high concentration. They consist of a rubber or neoprene diaphragm with a pulsating air supply on one side, controlled by a timer, and the slurry on the other.

Medium Consistency Stock Pumps Pumping of medium consistency stock (from about 8% to about 16% consistency) can be accomplished with positive displacement screw pumps or centrifugal pumps that have been specially adapted. Medium consistency stock is a thick viscoelastic material made up of strong networks of fibers called flocs. A high shear rate is necessary to render the stock capable of flow. Therefore all centrifugal medium consistency pumps are equipped with an inducer or feeder vane device in front of the pump suction to provide the shear necessary for the stock to flow into the impeller eye. There are also large amounts of entrained air in medium consistency stock and most centrifugal medium consistency pumps are equipped as well with an auxiliary vacuum pump to remove the air allowing for stable pump operation and high efficiencies. An example of a medium consistency centrifugal pump is shown in Figure 4.

Fan Pump Another specialized pump application in the paper mill is the pump that feeds the headbox or nozzle that spreads white paper onto the moving wire sheet that

FIGURE 12 A typical low flow pump used for shower services. (new) (Courtesy ITT / Goulds Pumps, Inc.)

forms the paper. The fan pump is usually a large horizontally split double suction pump in austenitic (AISI 316) trim or all austenitic (AISI 316) stainless steel construction. To form an evenly distributed sheet, the headbox must be provided with flow that is free of pressure pulsations and flow disturbances. The fan pump rotating element (impeller, shaft, sleeves, and so on) is designed and built to special tolerances to guarantee a typical maximum pressure pulse of 0.5% of pump total dynamic head, peak-to-peak, at any frequency. A typical fan pump is shown in Figure 5.

Shower Services Shower services require pumps to operate at low flows and high pressures. Operation at low flows requires special pump designs to ensure good pump reliability. These pumps are usually of single stage design with special, circular volute construction to minimize radial loads on the rotor and bearings. A typical low flow pump is shown in Figure 12. Some high pressure shower services require multistage or high speed (greater than 3600 rpm) pumps to reach the desired pressure. Generally, multistage and high speed pumps require more maintenance than single stage units because of mechanical complexity.

Solid Handling Services There are many pump applications that require handling of various size solids, particularly in recycle services. Recycle services may contain tramp metal and plastics that can clog stock pumps. Recessed impeller pumps, sometimes referred to as vortex pumps, are particularly useful for these types of services because the impeller is recessed from the main flow allowing large solids to pass through the pump.

Sealless Pump Services In recent years, some of the severely corrosive chemical applications have been served well by sealless pumps. Magnetic drive pumps are particularly useful for tank car unloading or sodium hydroxide and other chemicals where mechanical seals have required high levels of maintenance (See Section 2.7).

Vacuum Pumps These are used to extract water from the sheet on the fourdrinier wire and at the suction presses by means of a vacuum up to a maximum of about 25 inHg (635 mmHg). Approximately 40,000 lb (18,100 kg) of water is extracted by this means for every ton (1000 kg) of paper produced, and this water is removed by entrainment with the air handled by the vacuum pumps. Frequently water separators are used to remove water; their use is a matter of economics, as a reduction of up to 10% in power may be achieved.

Vacuum pumps are of three basic types:

1. Water ring

2. Positive displacement

3. Centrifugal or axial-flow

Many engineers prefer the water-ring type, probably because of its simplicity. In general, however, this type uses more power, mainly because of the heating of the circulating water, which is then discharged to a drain.

Centrifugal and axial-flow machines must be provided with water separators, but they are more efficient overall, particularly when the heat of compression is used in the machine room ventilating system. The machines run at high speed and are usually driven by a steam turbine.

The paper machine system requires vacuums at different levels, from a few inches (millimeters) of mercury to the maximum possible. Often pumps are connected to a common header, and orifice plates are used to divide the flow to ensure some measure of standby capacity. This involves throttling, however, and may create flow problems unless quantities are carefully estimated. The axial-flow machine permits extraction at any point along the rotor within fairly close limits and requires an accurate estimation of the quantities and specific vacuum required. A standby for axial-flow machines cannot usually be justified.

Stock and Liquor Pump Standardization Throughout the mill the duties of many pumps are similar, but different materials of construction may be used. If, at a slightly extra initial cost, the rotating elements of the pump can be standardized, this will reduce spare-parts inventories. This is also an advantage when purchasing pumps for a new mill. For example, if a standard arrangement consists of a complete rotating element, including bearings, only the impeller size and material need be different. Standardization is an additional reason for recommending that all pumps be stainless-fitted. Obviously, large pumps need individual evaluation. Standardization of stock pumps is less feasible, but up to about 6% stock, similar pumps can usually be specified.

Pump Selection Guidelines In many cases, an excess margin on head and capacity is specified. Where margins are excessive, mechanical troubles and cavitation often occur. The following guidelines will help to properly size a pump and motor in order to avoid these problems:

1. Carefully calculate the pump duty, using the TAPPI technical information sheet (TIS 408-4) for stock friction and the proper viscosity corrections for liquor friction. Always use a schematic of the actual piping system to be installed.

2. Properly size control valves to minimize the required pressure drop. Consider the use of variable speed drives.

3. Make the proper corrections to head and power for the presence of entrained gas in the fluid being pumped, per pump manufacturers recommendations.

4. Select a pump from the manufacturer's curve and note the impeller diameter and range for the pump. If the duty point falls near the end of the curve or if the impeller diameter is greater than 95% of the maximum impeller diameter, it is advisable to select the next size larger pump. For maximum reliability, select an impeller diameter equal to or nearly equal to: {(maximum impeller diameter—minimum impeller diameter) X 0.75} + minimum impeller diameter.

5. Allow 10% or 3 ft (whichever is larger) between NPSHA and NPSHR. Systems should be designed for a maximum available S value (suction specific speed) of 8,000.

6. Motors should be sized to be non-overloading for 105% of the impeller diameter chosen. Baseplates should allow for the installation of the next larger frame size.

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Survival Treasure

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