Pumps/pumping pump selection

The pumping energy that is invested to overcome the valve differential is wasted energy. The amount of this waste is the difference between the pressure required to "push" (transport) the fluid into the process (see AP in Figure 2.60) and the pump curve of the constant-speed pump. Pumps are selected to be able to meet the maximum possible flow demand, and therefore, most of the time they operate at partial loads. Consequently, using control valves to manipulate the flow of constant-speed pumps wastes energy and thereby increases operating cost. 

Apparatuses are devices which can perform mechanical operations but not make measurements. Many are essential elements of flow assembhes, e.g. syringe pumps, peristaltic pumps, selection valves, micropumps, external solenoid valves. Although commercially available in a wide variety of forms and models, many apparatuses lack an interface for connection to a computer. This should be an important consideration at the time of purchase. However, since they rarely need to feed back information to the computer, they can operate one-way (to receive commands from the computer). Table 6.2 lists available DLLs for apparatuses. 

The need for redundancy in the culture system needs to be assessed. Failure of a well pump that brings up water to supply a static pond system may not be a serious problem in countries where new pumps can be purchased in a nearby town. However, it can be disastrous in developing countries where new pumps and pump parts are often not available, but must be ordered from another country. Several weeks or months may pass before the situation can be remedied unless the culturist maintains a selection of spares. 

Selection of pump for a given appHcation is not a trivial task. Often more than one pump type can accomplish the required job. Thus a final choice on a pump type is often a result of personal experience and usage history. As a rule of thumb, the choice of a kinetic, such as centrifugal, or a positive displacement pump is made on the basis of the specific speed. Whereas specific speed is appHcable primarily for centrifugal but not positive displacement pumps, the US value can be used as a guide. Generally, for calculated values of specific speed, eg, nS > 10 [NS > 500), kinetic-type pumps are usually selected. For nS < 10 [NS < 500), positive displacement pumps are typically appHed. 

From the definition of specific speed (eqs. 9 and 10), it follows that reciprocating pumps operate at high pressures and low flow rates. Conversely, centrifugal pumps are appHed at lower pressures and higher flow rates. Many rotary pumps are selected for viscous Hquids having pressures equal to or less than, and capacities lower than, centrifugal pumps. However, these limits are relative and a gray area exists as some pump types cross boundaries into the domain of other types. 

Presence of Solids When a pump is required to pump a hquid containing suspended solids, there are unique requirements which must be considered. Adequate clear-liquid hydraiilic performance and the use of carefully selected materials of construction may not be all that is required for satisfacdoiy pump selection. Dimensions of all internal passages are critical. Pockets and dead spots, areas where solids can accumulate, must be avoided. Close internal clearances are undesirable because of abrasion. Flushing connections for continuous or intermittent use should be provided. 

Pump Selection One of the parameters that is extremely useful in selecting a pump for a particular apphcatiou is specific speed N,. Specific speea of a pump can be evaluated based on its design speed, flow, and head. 

Windmills (induction generators) as an unconventional energy source, vertical hollow shaft motors and submersible pump sets, selection of belts for transmission of load, the phenomenon of shaft currents are discussed. 

A Statistical Analysis of Nuclear Power Plant (Pump and Valve) Failure Rale Variability me Preliminary Results Nuclear All IPRDS data base records for the pumps and valves selected for analysis The set of valves and pumps selected for analysis from the IPRDS data base 104. 

Pumps, compressors, turbines, drivers, and auxiliary machinery should be designed to provide reliable, rugged performance. Pump selection and performance depend on the capacity required and tlie nature of Uie fluids involved. Remotely controlled power switches and shutoff valves are necessary to control fluid flow during an emergency. The inlets for air compressors should be strategically located to prevent the intake of hazardous materials. 

Compressors and Expanders Selection and Application for the Process Industry, Heinz P. Bloch, Joseph A. Cameron, Frank M. Danowski, Jr., Ralph James, Jr., Judson S. Swearingen, and Marilyn E. Weightman Metering Pumps Selection and Application, James P. Poynton Hydrocarbons from Methanol, Clarence D. Chang Form Flotation Theory and Applications, Ann N. Clarke and David J. Wilson... 

In general, the final pump selection and performance details are recommended by the manufacturers to meet the conditions specified by the process design engineer. It is important that the designer of the process system be completely familiar with the action of each pump offered for a service in order that such items as control instruments and valves may be properly evaluated in the full knowledge of the system. 

Pumps are normally selected to operate in the region of high efficiency, and particular attention should be given to avoiding the extreme right side of the characteristic curve where capacity and head may change abrupdy. 

Total Suction Lift (as water) = 15 — 33 = —18 feet. Therefore, a pump must be selected which has a lift of at least 18 feet. The pump of Figure 3-36A is satisfactory using an interpolated Suction Lift line between the dotted curves for 16 feet and 21 feet of water. The performance of the pump tvill be satisfactory in the region to the left of the new interpolated 18-foot line. Proper performance should not be expected near the line. 

The pump selected for this application (water boiling at 0.98 psia) must have a required NPSH less than 4.7 ft, preferably about 3 to 3.5 ft. This is a difficult condition. If possible the vessel should be elevated to make more head (S) available, which wll raise the available NPSH. 

Now the pump selected reads NPSHr on its pump performance curve of 12 feet for cold water ser dce. 

Assume that a boiler feed water is being pumped at 180 °F. Read the chart in Figure 3-46 and the water vapor pressure curve, and follow over to read NPSH reduction = 0.45 feet. A pump selected for the sertice requires 6 feet cold water service NPSHr ... 

Determine proper pump selection and specifications when pumping oil with SpGr of 0.9 and viscosity of 25 cen-tipoise at the pumping temperature, if the pump must deliver 125 GPM at 86 feet total head (calculated using the viscous liquid). 

The pump specified identifies the design data, key portions of the construction materials and driver data as required informadon for the pump manufacturer. If the pump is to be inquiried to several manufacturers this is all that is necessary. The individual manufacturers will identity their particular pump selection and details of construction materials and driver data. From this informadon a pump can be selected with performance, materials of construction, and driver requirements specified. 

In the example the manufacturer has been specified from available performance curves, and the details of construction must be obtained. The pump is selected to operate at 22 GPM and 196 to 200 feet head of fluid, and must also perform at good efficiency at 18 GPM and a head which has not been calculated, but w hich will be close to 196 to 200 feet, say about 185 feet. Ordinarily the pump is rated as shown on the specification sheet. This insures adequate capacity and head at conditions somewhat in excess of normal. In this case the design GPM w as determined by adding 10 percent to the capacity and allowing for operation at 90 percent of the rated efficiency. Often this latter condition is not considered, although factors of safety of 20 percent are not unusual. However, the efficiency must be noted and the increase in horsepower recognized as factors w hich are mounted onto normal operating conditions. 

If solids are carried in the fluid, this can present a difficult problem if they are not properly flushed from the pump on shutdown. Some spare or second pumps are selected for 100 percent spare others are selected so that each of two pumps operate in parallel on 50 percent of the flow, with each being capable of handling 67 to 75 percent of total load if one pump should fall off the line. This tlten only reduces production by about 25 percent for a short period, and is acceptable in many situations. These pumps are usually somewhat smaller than the full size spares. 

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