Turndown

Standard catalog hydrokinetic and hydroviscous units offer the regulation described above over a 5-to-1 turndown on centrifugal machines and 4-to-1 turndown when driving positive displacement pumps on constant-pressure systems. Specially designed fluid drives have been sold that give stable control at 10-to-1 turndown. Hydrodynamic units are limited primarily by heat dissipation capabilities and range from turndown values of 100 to 1 to 1.2 to 1.

Figure 10 is typical of a boiler-feed pump where a high percentage of the developed head is relatively constant. In this case, this fixed head is the boiler pressure. The figure demonstrates the savings in pressure and power realized when this system is used rather than a feedwater regulating valve.

Figure 11 assumes that a positive displacement pump is working on a system where pressure is constant. Although this type of system is seldom found, it is shown here to demonstrate that the rapid reduction in fluid drive efficiency does not require overmotor-ing the pump. Although system efficiency is much poorer than that of a bypass valve, fluid drives are used to provide no-load starting, isolation of torsional vibrations in reciprocating pumps, and elimination of the bypass valve in slurry systems where erosion is severe.

Response It must be recognized that all fluid couplings being discussed here are slip devices. Thus, any demand speed change cannot be accomplished in microseconds or milliseconds. However, the time required to change the torque applied varies from one type of unit to another.

Hydrokinetic Drive In the scoop-trimming fluid drive, response speed is affected by many factors. The speed with which oil can be added to the working circuit (a factor of the size of the oil pumps) or removed from it (a factor of the size of the scoop tube) influences response capability.

In the leakoff unit, the size of the leakoff ports determines how quickly the unit will empty. However, the oil pumps must be sized to replace this oil and have additional capacity to fill the coupling in a reasonably short time.

Scoop-control units are limited by the ability of the scoop tube to pump oil from the reservoir into the working circuit and by the ability of the leakoff ports to return it to the

FIGURE 10 Comparison of adjustable-speed and constant-speed pressure and power characteristics for a typical centrifugal boiler-feed pump

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Survival Treasure

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