Variable-speed drivers

Air-Flow Control Process operating reqmrements and weather conditions are considered in determining the method of controlling air flow. The most common methods include simple on-off control, on-off step control (in the case of multiple-driver units), two-speed-motor control, variable-speed drivers, controllable fan pitch, manually or automatically adjustable louvers, and air recirculation. 

When a variable-speed driver, such as a steam turbine, is used for the compressor, the compressor performance can be varied to meet various operating flows and pressures by moving up or down the rated point locus of Figure 12-61. As... 

Plankenhorn, J. H., Solid-state Variable Speed Drivers, Specifying Engineer, p. 117, Sept. (1981). 

All turbines are variable-speed drivers and operate near or above one of the rotor s critical speeds. Narrowbands should be established that track each of the critical speeds defined for the turbine s rotor. In most applications, steam turbines operate above the first critical speed and in some cases above the second. A movable narrowband window should be established to track the fundamental (1 x), second (2x), and third (3x) harmonics of actual shaft speed. The best method is to use orders analysis and a tachometer to adjust the window location. 

For positive-displacement pumps the only variable is the operating speed. The only ways to change the capacity of these pumps are to use a variable-speed driver, use a variable-speed transmission (not usually recommended), or replace a given rpm motor by another. Since most motors run at 1,750 or 3,500 rpm, the last method may be used only if the speed is to be doubled. 

Variable speed drivers can increase compressors discharge pressures about desired amounts causing a... 

Variable speed drivers can increase pump discharge pressures about desired amounts causing a process upset. With the provisions of constant speed drivers, such of electrical motors the possibility of overspeed is highly remote. 

Pumps connected to variable speed drivers should be equipped with an approved relief valve. The discharge of the relief valve should be directed back to the source where possible or to drains. Piping of a relief valve back to pump suction piping should be avoided. Whenever the relief valve does not discharge to the atmosphere, a sightglass flow indicator should be installed. Constant speed pumps also require relief valves when the shut-in pump discharge pressure exceeds the rated pressure for the system components. 

An obvious method of controlling the capacity of a compressor is to vary the speed. This method is applicable to units driven by variable-speed drivers such as steam pistons, steam turbines, gas engines, diesel engines, etc. In these cases the regulator actuates the steam-admission or fuel-admission valve on the compressor driver and thus controls the speed. 

The system shown in Fig. 22.1 has no provision for controlling the evaporator temperature this is controlled solely by the compressor suction pressure. The lower the compressor suction pressure, the lower the evaporator temperature. This is exactly how our home air conditioner works. If the evaporator temperature is too cold, what can we do Well, if this were a steam turbine compressor, gas engine drive, or any other type of variable-speed driver, we could reduce the compressor s speed. This would reduce the flow of refrigerant, and raise both the evaporator and compressor suction pressure. 

Does this mean that we would be better off driving a large centrifugal compressor with a variable-speed driver Perhaps with a steam turbine or gas-fired turbine. You bet Especially when the molecular weight is highly variable. 

Due to an increasing concern about conserving energy there has become a requirement for variable speed drivers in the oil industry. The most common requirement is for compressors where the gas composition may not be well known or it may vary in composition and desire throughout, which is often the case in the production industry. Occasionally, benefits may be obtained by driving pumps with variable speed motors, especially large oil loading and pipeline pumps. 

Variable-speed driver Water droplets, dust may cause damage. No surge line... 

Restart if accidentally there is a power outage and then we want to restart, some of the issues are if the particles settled, are they densely packed or loose packed How long does it take to settle For most slurries, the restart procedure outlined above can be used if the restart can be done within 1—2 h. However, if longer and if the particles pack densely, then water or liquid injection near the lower impeller is needed to loosen the soUds. From the solids concentration one can estimate how many impellers are immersed in the bed of particles. Perhaps use a two-speed motor or variable speed driver so that we can start at a lower rpm if the particles are loosely packed or only lower impellers are sitting in the bed of particles. 

The pump speed can be controlled by using a variable-speed driver (VFD) especially, when the capacity of the pump varies greatly. VFDs include electric, electromechanical, mechanical and hydraulic power recovery turbines. 

If a relief valves is fitted (for variable speed drivers), verify piping is rated for maximum pump pressure output then isolate relief valve only during testing period, otherwise leave relief valve in service during testing activities. 

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