Steam turbine drivers

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. 

Close-Coupled Pumps (Fig. 10-38) Pumps equipped with a built-in electric motor or sometimes steam-turbine-driven (i.e., with pump impeller and driver on the same shaft) are known as close-coupled pumps. Such units are extremely compact and are suitable for a variety of services for which standard iron and bronze materials are satisfactory. They are available in capacities up to about 450 mVh (2000 gal/min) for heads up to about 73 m (240 ft). Two-stage units in the smaller sizes are available for heads to around 150 m (500 ft). 

The advent of electric motors, steam turbines, and other drivers has relegated the steam engine to a minor position as an industrial driver. It does have the advantages of reliabihty and operating characteristics that are not obtainable with other drivers but so the disadvantage of bulldness and oily exhaust steam. 

Economics Power-recoveiy units have no operating costs in essence, the energy is available free. Furthermore, there is no incremental capital cost for energy supply. Incremental installed energy-system costs for a steam-turbine driver and supply system amount to about 800 per kilowatt, and the incremental cost of an electric-motor driver plus supply system is about 80 per kilowatt. By contrast, even the highest-inlet-pressure, largest-flow power-recoveiy machines will seldom have an equipment cost of more than 140 per kilowatt, and costs frequently are as low as 64 per kilowatt. However, at bare driver costs (not including power supply) of 64 to 140 per kilowatt for the power-recovery driver versus about 30 to 80 per Idlowatt for... 

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. 

Lube oil units are typieally available in two versions the manu-faeturer s standard or in aeeordanee with API Standard 614. The major eomponents of a unit are the oil tank, auxiliary oil pump, double filter and, seleetively, one or two oil eoolers. All eomponents of the smaller units are mounted on a eommon bedplate, separated from the other eomponents. The oil ean be heated by an eleetrieal or steam-powered heating unit. The neeessary instrumentation is a standard supply item and, if requested, the switehes and motors ean be prewired. The main and auxiliary oil pumps are driven by different types of drivers (e.g., one by an eleetrie motor and the other by either a small steam turbine or by direet eonneetion to the shaft end of a major maehine easing in the turbotrain). 

Step 4. A study of starting characteristics of the train is necessary to select the correct startup driver rating (motor and/or steam turbine). 

The second step in developing the process air train is selecting the air blower main driver. This step includes many alternatives such as power recovery expanders, steam turbines, and electric motors. The following example illustrates these alternatives. 

As a first step in the driver analysis, the eapital required to make eaeh alternative operational is estimated. An orifiee ehamber is required to reduee the flue gas pressure for the steam turbine and motor alternatives. In this partieular ease, it is assumed that a third-stage separator is required for the power reeovery alternatives only and that an eleetrostatie preeipitator is used in all eases. Construetion and engineering are estimated as pereentages of total direet material and total material and eonstruetion, respeetively. An allowanee of 15% is made for eontingeney. Beeause the separator often ineludes a royalty fee, this item is added to the power reeovery alternates. As shown in Table 4-7, the motor alternative will require the least eapital. The power reeovery alternatives require additional eapital amounting to 4.63 and 4.75 per million respeetively. 

Side studies are often required for major decisions, and a good example is a study of gas versus steam turbines for LNG plants (Reference 5). Indications are that higher capital investment is associated with gas turbine drivers than with steam turbines for liquefaction train service, but that the added capital investment for gas turbines shows... 

Instruments and controls checked Refractory dried out Piping Strains on Equipment Electric Motors Rotation Drying out No-load tests Steam-Turbine Drivers... 

Reciprocating compressors are available with a large variety of other drivers, which include the piston engine, steam turbine, or, in rare cases, a gas turbine. Next in popularity to the electric motor is the piston engine. The arrangement lends itself to skid mounting, particularly with the semi-portable units found in the oilfield. The unit is also popular as a lease unit, which may be lifted onto a flat bed trailer and moved from one location to another as needed. The engine is either direct-coupled or. as with smaller compressors, it may be belt-connected. 

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. 

Figure 5-16. A tandem driven multi-body centrifugal compressor train with a steam turbine driver. (Courtesy of Demag Delaval Turtromachinery Corp)...

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. 

One of the first questions the designer must answer concerns which type steam turbine should be used. The back pressure turbine is. selected when process steam demands are greater than the steam required for process drivers such as large compressors. This type turbine is also selected when various steam levels are required by the process. 

The condensing steam turbine has a relatively low thermal efficiency because about two-thirds of the steam enthalpy is lost to cooling water in the condenser. Expensive boiler feedwater treatment is required to remove chlorides, salts, and silicates, which can be deposited on the blades causing premature failure. The blades are already under erosion conditions because of water drops present in the condensing steam. Even with these disadvantages, the condensing turbine is still selected, especially in a process that requires very large compressor drivers and relatively low amounts of process steam. 

A rule of thumb Select an extraction steam turbine when 15 to 20 i of the driver power requirements can be supplied by the extracted steam. 

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]. 

The pumps may be driven by any of several drivers. For many years the favorite arrangement has been a steam turbine for the main pump and... 

Driver. If steam is available, a common choice is a steam turbine driver for the main pump and an electric motor for standby. Some plants prefer two electric motor-driven pumps. Give the minimum steam condition. Give the expected voltage drop on the electric system if more than 10% is expected, especially on large systems. 

Axial or propeller blade fatis are eithet Irelt or gear driven. Some drivers are variable speed tnoiors. and some fatis have variable pitch blades. Iti s iecial circumstances, steam turbine, gas or gasolitie etigine drivers are used. Geais should be carefully specilied to avoid overload and should be special ) sealed to prevent moisture entering the case. 

Compressor centrifugal, screw or reciprocating electric motor, steam turbine, or other driver... 

Impeller mach number at eye and at periphery. Maximum possible speed of compressor, also of driver. Maximum horsepower possible for driver to develop, with any changes necessary to bring up to this maximum (such as changing nozzles, nozzle ring of steam turbine, changing blades or buckets). Paint specifications for exterior of unit. 

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... 

All machines require some form of motive power, which is referred to as a driver. This section includes the monitoring parameters for the two most common drivers electric motors and steam turbines. 

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. 

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