Rotative Speed

The rotative speed of a pump is dependent upon the impeller characteristics, type fluid, NPSH available and other factors for its final determination. The most direct method is by reference to manufacturer s performance curves. When a seemingly reasonable selection has been made, the effecL of this selected speed on the facLors such as NPSH required, suction head or lift, fluid erosion and corrosion, eLc., must be evaluated. For many systems these factors are of no concern or consequence. 

Normal electric motor speeds run from the standard induction speeds for direct connection of 3600, 1800 and 1200 rpm to Lite lower speed standards of Lhe synchronous motors, and then lo the somewhat arbitrary speeds established by V-belt or gear drives. For some cases, the pump speed is set by the type of drivers available, such as a gasoline engine. 

Electric motors in pump application never run at the standard rotative design speeds noted above, but rotate at about (with some deviation) 3450, 1750, and 1150 rpm, which are the speeds that most pump manufacturers use for their performance curves. If the higher numbers were used (motor designated or name plate) for pump performance rating, the pumps would not meet the expected performance, because the motors would not be actually rotating fast enough to provide the characteristic performance curves for lhe specific size of impeller. 

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FIG. 10 36 Relationships lietween specific speed, rotative speed, and impeller proportions (Woiihim lon Pmiij) Itu ., Pump World voj. 4, no. 2, 197-Sj. 

Screw-Type This type of rotary compressor, as shown in Fig. 10-80, is capable of hancmng capacities up to about 4.248 X lO" m m (25,000 fF/min) at pressure ratios of 4 1 and higher. Relatively small-diameter rotors allow rotative speeds of several thousand rev/min. Unhke the straight-lobe rotary machine, it has male and female rotors whose rotation causes the axial progression of successive sealed cavities. These machines are staged with intercoolers when such an... 

In operation seal oil pressure is about 30-50 psi over the process gas pressure. The high-pressure oil enters the top and completely fills the seal cavity. A small percentage is forced across the carbon ring seal faces. The rotative speed of the carbon ring can be anywhere between zero and full rotational speed. Oil crossing the seal faces contacts the process gas and therefore contaminated oil. The contaminated oil... 

The small-spiral-large-sbaft type (Fig. ll-60b) is inserted in a solids-product line as pipe banks are in a fluid line, solely as a heat-transfer device. It features a thin burden ring carried at a high rotative speed and subjected to two-sided conductance to yield an estimated heat-transfer coefficient of 285 W/(m °C) [50 Btu/(h fU °F)], thereby ranking thermally next to the sheU-fluidizer type. This device for powdered solids is comparable with the Votator ol the fluid field. 

Operating Costs Power cost for a continuous thickener is an almost insignificant item. For example, a unit thickener 60 m (200 ft) in diameter with a torque rating of 1.0 MN-m (8.8 Mlbf in) will normally require 12 kW (16 hp). The low power consumption is due to the very slow rotative speeds. Normally, a mechanism vi l be designed for a peripheral speed of about 9 m/min (0.5 ft/s), which corresponds to only 3 r/h for a 60-m (200-ft) unit. This low speed also means veiy low maintenance costs. Operating labor is low because little attention is normally required after initial operation has balanced the feed and underflow. If chemicals are required for flocculation, the chemical cost frequently dwarfs all other operating costs. 

Another factor to consider is the compressor rotative speed relative to valve wear. The lower the speed, the fewer the valve cycles, which contribute to longer valve life. A desirable speed range is 300 to 600 rprn... 

Type specific speed is defined as that operating specific speed that gives the maximum efficiency for a specific pump and is the number that identifies the pump type [25]. This index number is independent of the rotative speed at which the pump is operating, because any change in speed creates a change in capacity in direct proportion and a change in head that varies as the square of the speed [25]. Practice is to true type the specific speed of the pump reasonably close to the conditions of maximum effi-... 

Rotative speed, revolutions per minute = RPM = rpm P = Positive external pressure on surface of liquid ( + ) or partial vacuum on surface of liquid ( —)... 

Head per section is another important parameter, as the smaller the number of impellers per section, the higher the head required per compressor stage. This leads to higher rotative speeds and operation at higher Mach numbers, which in turn, tends to limit the operating range, flatten the head rise characteristic, and reduce efflciency. ... 

N = harmonic number, number of cylinders, or = rotative speed, rpm... 

The horizontal pumps are available for capacities up to 900 m /h (4000 gahmin) the vertical in-line pumps, for capacities up to 320 m /h (1400 gahmin). Both horizontal and vertical in-line pumps are available for heads up to 120 m (400 ft). The intent of each ANSI specification is that pumps from all vendors for a given nominal capacity and total dynamic head at a given rotative speed shall be dimensionally interchangeable with respect to mounting, size, and location of suction and discharge nozzles, input shaft, base plate, and foundation bolts. 

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