Turbines, steam back pressure

Boiler bubbling fluidized bed (8 kg/s of steam at 61 bar pressure and 510°C) Steam turbine back pressure steam cycle with district heat produced at one pressure level... 

However, if the turbine back pressure is raised to above atmospheric pressure so that the turbine exhaust steam can be transported to the process heat load then the steam will give up its latent heat usefully rather than reject this to the condenser cooling water. Although the steam turbine output is reduced, the overall efficiency is increased significantly as the generated steam is used to provide both heat and electrical power. 

These turbine units finally exhaust the steam at considerably less than atmospheric pressure to a condenser (in most circumstances a surface condenser is employed). The condenser is designed to raise turbine operating efficiency by reducing the turbine back-pressure to an absolute minimum. This is achieved by condensing the exhaust steam into a smaller volume of condensate, thus creating a substantial vacuum. 

To maximize turbine efficiency (by minimizing turbine back-pressure). This is achieved by condensing steam and creating an adequate vacuum. The level of vacuum created by the reduction in steam-to-water volume is typically on the order of 26 to 29 inches of mercury and is in large part a function of the cooling water inlet temperature. A contribution to the maintenance of the vacuum is obtained through the mechanical pumps and air ejectors, which form part of the condenser system. 

For most chemical plants, process steam is used at pressures of 1.825 MN/m" (250 psig), saturated or lower. When combined heat and power generation is economically justified, the steam may be generated at about 5.96 MN/m" (850 psig) appropriately superheated and used to drive back-pressure steam turbines passing out process steam at the required pressure level. 

Steam needed for process, so that a back-pressure turbine should be considered... 

A back-pressure (noncondensing) turbine may also be used if there is a profitable use for intermediate-pressure steam. In the unlikely event that large quantities of steam are required, additional high-pressure steam from an external source might be necessary. However, while it is theoretically possible that the amount of heat generated in the nitric acid plant will be insufficient to cover the entire demand, this is not usually a valid concern. 

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 turbine has several advantages and disadvantages over the back pressure turbine. The advantages are that it requires less change in the live steam for various turbine loads and is therefore easier to control. It also requires less steam because the enthalpy drop is larger. Finally, only one steam level is affected for a change in power requirements. 

Steam turbine, 53, 146, 282-92, 179 back pressure, 282 blade deposits, 479 condensing, 282 efficiency, 288 extraction, 282 induction-type, 282 paitial admission, 288 rating, 290 reliability, 478 selecuon variable, 275, 285 speed, 278 stage losses, 286 steam temperatures, 284 steam velocity, 288 trip and throttle valve. 479 Step unloading system, 80 Stiffness coefficients, 385 Stodola slip, 153, 155 Stonewall, 186 Straight labyrinth. seal leakage, 532... 

The simple back-pressure turbine provides maximum economy with the simplest installation. An ideal backpressure turbogenerator set relies on the process steam requirements to match the power demand. However, this ideal is seldom realized in practice. In most installations the power and heat demands will fluctuate widely, with a fall in electrical demand when steam flow, for instance, rises. 

This process steam flow will dictate output generated by the turbo-alternator and excess or deficiency is made up by export or import to the supply utility, as appropriate. The alternative to the system in Figure 15.15 is to use a back-pressure turbine with bypass reducing valve and dump condenser, as shown in Figure 15.16. 

Figure 15.23 The effect of back-pressure upon steam turbine performance...

The exhaust pressure of a steam turbine is fixed by the operating pressure of the downstream equipment. Figure 23.9a shows a back-pressure turbine operating between a high-pressure and low-pressure steam mains. The pressure of the low-pressure steam mains will be controlled elsewhere (see later). 

For any load of a given steam turbine with fixed inlet pressure, back-pressure and inlet temperature, the isentropic... 

For back-pressure turbines, the turbine exhausts to a steam main. Therefore, in order to enable estimation of steam main conditions, it is important to predict the condition of the exhausted steam also. With a value of the mechanical efficiency, the enthalpy of the exhaust steam can be calculated from an energy balance9 ... 

Example 23.2 A process heating duty of 25 MW is to be supplied by the exhaust steam of a back-pressure turbine. High-pressure steam at 100 barg with a temperature of 485°C is to be expanded to 20 barg for process heating. The heating duty of the 20 barg steam can be assumed to be the sum of the superheat... 

Figure 23.20 illustrates the features of a typical steam system. It is usual to have at least three levels of steam. On larger sites, steam may also be generated at a very high pressure, which will only be used for power generation in the boiler house. Steam would then be distributed typically at three pressures around the site. Back-pressure turbines let steam down from the high-pressure mains to... 

If a steam turbine generates electricity, then the flow through the turbine can be varied within the minimum and maximum flows allowed by the machine. If a steam turbine is connected directly to a drive (e.g. a back-pressure turbine driving a large pump), then there is likely to be no flexibility to change the flowrate through the steam turbine as this is fixed by the power requirements of the process machine. 

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