Compressor drivers Steam turbines

Figs. 6.21, 6.22, and 6.23 show only one possible layout of the steam system in large ammonia plants. Other similar systems are described in [31, 615, 718, 723, 724, 949]. Other types of drivers for ammonia plant compressors than steam turbines are discussed in [398] (electrical motors) and [399, 400] (gas turbines). Steam and power production for ammonia plants in cogeneration units is described in [725-727, 921]. Steam and power balances in ammonia plants and ammonia-urea complexes are discussed in [959] with special reference to the use of gas turbines. 

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. 

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]. 

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

Usually, dual oil pumps are included, so that one pump failure will not shut down the compressor-driver unit. The first or main pump may be driven by electric motor, and the standby steam or gas may be driven by turbine. Any combination is acceptable as long as the selection takes into account the specific local conditions and service reliability. Figures 12-50A-C show an overall assembly, including accessories. 

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... 

Compressors are of two general classes positive-displacement or centrifugal. The positive-displacement compressor simply compresses an initial gas volume to a final smaller volume in a closed containment a centrifugal compressor compresses gas by changing its velocity. Each of the common types of compressors has their advantages and applications. Drivers for compressors may be either electric motors, steam turbines or gas turbines. 

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. 

The next need for the compressor package is the driver. Most driver types applied to <500-hp loads are usually electric motors at 440-V, 60-standard. Greater loads generally call for gas-fired or steam turbines. Please see Figs. 8.10 and 8.11. These figures display the cost of adding the driver of choice. 

Figure 8.10 covers gas-fired turbine and cylinder-type engines, and Fig. 8.11 covers steam turbine drivers. Motor drivers and gear assembly cost is also shown in Fig. 8.10. Having a required compressor BHP, the driver cost of choice may be read from one of these two figures. The gear assembly cost is applicable to any of the drivers given in Figs. 8.10 and 8.11.

Figures 8.10 and 8.11 are based on the driver FOB fabrication shop cost only. For the completed driver package, additional items, such as a surface condenser for the condensing steam turbine or a starting motor or air piston starter for the gas engine, are required. Tables 8.29 to 8.32 are each dedicated to providing factoring for these specific costs. Such costs will again be based on the compressor horsepower, BHP.

Steam turbines. In modern plants the centrifugal synthesis gas compressors, including recycle, are almost exclusively driven by a steam turbines. These are generally extraction turbines with a condensing section. Steam is extracted at suitable pressure levels (e.g. 45-55 bar) to provide, for example, the process steam in steam reforming plants, and for other drivers, e.g., air compressor, ammonia compressor, boiler feed water pumps, and blowers. 

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