Steam turbines speed

Whenever the process machine operates at the same speed as its driver, the two can be directly coupled. This direct couphng stiU allows for a variable speed, through acuustments of the speed or the driver. Steam turbine speed can be easily adjusted, and electric motor speed can also be varied by the use of special drives that vaiw the frequency of the power applied to the motor. Wdiether the speed is fixed or variable, direci coupling of two machine shafts presents the problem of accommodation of misalignment. To this purpose, machines are coupled through a.flexible coupling. 

The electrohydraulic control system automatically and continuously monitors and sets the steam turbine speed to satisfy customers specific requirements. Low maintenance operation typifies this type of system, which also features solid-state electronics, rediuidant speed pickups, and signal processing channels to reduce problems ordinarily associated with mechanical controls. The result is improved reliability, accuracy, and overall enhanced steam turbine performance. 

FIG. 29-11 Steam-turbine capability versus speed, To convert horsepower to kilowatts, multiply by 0,7457,... 

In contrast to steam turbines, in which runaway overspeediug is always a problem, pump-turbines operating at design head go to zero torque at about 130 to 140 percent of design speed. Thus, overspeed protection may not be necessary if the pump-turbine can withstand 140 to 150 percent of design speed and it is the sole driver. When a steam-turbine helper is used, it should be provided with the usual overspeed trip-out mechanism. 

When used to eompress air, the axial eompressor may be equipped with adjustable stator blades, either partially (Figure 4-16) or on all stages. In the ease of a motor serving as the assoeiated driver or with a generator eoupled to the train, the adjustable stator blade feature allows more effieient eontrol than suetion throttling. With a steam turbine as the assoeiated driver, stator blade adjustment may be eombined with the obvious shaft speed eontrol. This eombination allows for speeial operating eonditions, sueh as startup, or extreme part-load operation. 

Beeause there is eonsiderable design flexibility in aeeommodating optimum turbine speed, standard meehanieal drive steam turbines are often ehosen. The offset aiTangement (Figure 4-41) provides additional freedom for the turboset eomponent layout. 

Figure 4-70 shows a four-body TPG train (string). As before, the expander supplies power to the generator. The steam turbine supplies power to the generator, provides startup power, and provides control for synchronization. The generator provides electricity, and the gear is used to allow the expander and steam turbine to operate at near optimum efficiency with the generator at its desired speed. 

Steam turbine performance is modeled using a standard steam flow versus horsepower map and valve position versus steam flow. The turbine inlet valve(s) is positioned by the governor system to maintain constant speed (or another parameter when synchronized). 

Steam turbine considerations. The speed at whieh the turbine must operate and the steam eonditions available influenee the operating effieieney of the unit. Double-ended steam turbines require speeial eonsideration of thrust bearing sizing, overspeed proteetion, and governing systems. 

Expander-compressor shafts are preferably designed to operate below the first lateral critical speed and torsional resonance. A flame-plated band of aluminum alloy or similarly suitable material is generally applied to the shaft in the area sensed by the vibration probes to preclude erroneous electrical runout readings. This technique has been used on hundreds of expanders, steam turbines, and other turbomachines with complete success. Unless integral with the shaft, expander wheels (disks) are often attached to the shaft on a special tapered profile, with dowel-type keys and keyways. The latter design attempts to avoid the stress concentrations occasionally associated with splines and conventional keyways. It also reduces the cost of manufacture. When used, wheels are sometimes secured to the tapered ends of the shaft by a common center stretch rod which is pre-stressed during assembly. This results in a constant preload on each wheel to ensure proper contact between wheels and shaft at the anticipated extremes of temperature and speed. 

For Main Air Blower serviee without a steam turbine in the string, the unit shall be started by first applying windage steam and then aeeelerating the train to rated speed using the eleetrie motor. After aehieving stable rated speed, the expander inlet valve will be... 

High-frequency, destructive vibrations. Unaffected by operating speed. Steam turbine valve vibration Rare occniTence change valve ping, seat shape, and/or increase valve gear rigidity... 

Some designers limit the speed of the cheaper small steam turbines to 3,600 rpm. 

For most of the rotary compressors in process service, the driver is an electric motor. Compressors in portable service, however, particularly the helical-lobe compressor, use internal combustion engines. Many of the rotary compressors require the high speed that can be obtained from a direct-connected motor. The dry type helical-lobe compressor is probably the main exception as the smaller units operate above motor speed and require a speed increasing gear which may be either internal or external (see Figure 4-1). The helical-lobe compressor is the most likely candidate for a driver other than the electric motor. Aside from the portables already mentioned, engines are used extensively as drivers for rotaries located in the field in gas-gathering service. Steam turbines, while not common, probably comprise most of process service alternate drive applications. 

Historically, the most popular driver for the centrifugal compressor has been the steam turbine. Steam turbines can readily be speed matched to the compressor. Prior to the upsurge in energy costs, reliability, simplicity, and operational convenience were the primary factors in driver selection. The steam turbine, with its ability to operate over a relatively wide speed range, was ideal for the centrifugal compressor, which could be matched to the process load by speed modulation. 

The merits of a variable-speed motor would appear to be obvious, as many compressors in the past have benefited from the variable speed available in a steam turbine. A compressor may be adjusted as required to meet the process needs. The advent of the variable-frequency drive returns some of the benefits to the process operator that were lost when the more favor able electric energy caused motors to replace steam turbines. 

A Campbell diagram is frequently used to determine the effect of multiple excitation frequencies in high-speed steam turbines. Figure 7-11 shows a Campbell diagram for a condensing steam turbine. If this partic uiar turbine operates at a speed of 8,750 rpm, the turbine blades would not be excited. But, if the turbine speed is reduced to 7,500 rpm, the turbine blades would be excited at four times running speed. If the turbine were operated at 10,000 rpm, a three-times running speed excitation would be encountered. What this means is that any vibration in the... 

As with the motor driver, the steam turbine must be matched to the compressor. Also, a turbine rating of 110% of the maximum power required by the compressor should be specified. This should be at the compressor s normal speed point. The turbine speed should include a maximum continuous speed 105% of the normal compressor speed. API Standards 611 and 612 cover general purpose and special purpose steam turbines 7, 8]. 

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