Turbines, steam pressure control

On this system, the turbine is speed controlled and passes steam, depending on the electrical demand. The bypass-reducing valve with integral desuperheater makes up any deficiency in the steam requirements and creates an exhaust steam pressure control. Alternatively, any surplus steam can be bypassed to a dump condenser, either water or air cooled, and returned to the boiler as clear condensate. 

The high-pressure steam from the separator is sent to the steam turbine under speed control, and the generated electricity is sent to the grid or to other users. The low-pressure steam is condensed by cooling it with cooling water. The flow of the cooling water is modulated by the turbine exhaust pressure controller. 

In most utibty boilers, steam pressure regulation is achieved by the throttling of turbine control values where steam generated by the boiler is admitted into the steam turbine. Some modem steam generators have been designed to operate at pressures above the critical point where the phase change between Hquid and vapor does not occur. 

These operating problems must be overcome by selecting the correct system. Figure 15.15 shows an arrangement that balances the process steam and electrical demands by running the turbo-alternator in parallel with the electrical supply utility. The turbine inlet control valve maintains a constant steam pressure on the turbine exhaust, irrespective of the fluctuation in process steam demand. 

If you look on the local control panel of such a turbine, you will probably see four steam pressures displayed ... 

If the plant s demand for low-pressure steam is variable, it is desirable to send that variable amount of steam through a let-down turbine and to recover its energy content in the form of electricity (Figure 2.133). On the other hand, one should not send more high-pressure steam to the turbine than the amount of low-pressure steam demanded by the process. These two goals can be converted into two control loops a pressure controller... 

This control configuration can only be used when the excess steam energy is utilized for the cogeneration of electricity, which can vary. Although energy conservation dictates that the flow through pressure let-down line be minimized, control dynamics require its existence. This is because the speed of response of a let-down valve is much faster than that of a turbine. Therefore, the sensitive control of the LP steam pressure is provided by the let-down pressure controller, while the bulk of the steam passes through the turbine and is used to make electricity. 

In extraction turbines, in addition to the governor valve, a second "valve" is required (Figure 2.134), which controls the steam flow rate that is extracted from the first stage of the turbine and is sent to the second stage. The extraction rate can be controlled either to keep the shaft speed or the pressure of the LP header constant, or a combination of the two. If the turbine incorporates the controls as a built-in feature, the turbine is referred to as an "automatic-extraction" type. Such turbines are generally designed to deliver 100% shaft power and to provide extraction steam only if the load requirements permit. This is the most common type of extraction machine. 

An intriguing aspect of this control configuration is the possibility of completely eliminating the need to throttle steam if the turbine capacity is sufficient to meet the demand for LPS. In that case the supply valve is kept fully open, as was the case in Figure 2.133, and the pressure controls of the LPS header determine the distribution of the extracted steam between the turbine s second stage and the LPS header. 

In chemical plants, in steam and gas turbine powered compressor and power stations and pressure control facilities for natural gas, combustible fluids and gases need to be preheated in order to comply with the process parameters,... 

When the compressor is driven by a variable-speed turbine, the temperature controller adjusts the setpoint of the turbine speed controller, which manipulates the flowrate of high-pressure steam to the turbine. 

Figure 7.7 shows a typical furnace/boiler configuration and its control system. Steam pressure in the 1000-psia header is maintained by furnace firing (the air controls are not shown). Steam pressure in the 300-psia header is controlled by either removing more steam from the turbine if the pressure is too low or dropping steam into the 150-psia header if the pressure is too high. Similar strategies are used for the... 

Steam enters the turbine at two or more pressures through separate inlet openings with means for controlling tne inlet-steam pressures. 

Steam turbines with electrical generators are used to recover the power from high pressure steam when the plant steam balance requires additional low pressure steam. As such, they provide more energy-efficient pressure regulation than a simple pressure control valve. Admission or extraction of steam to a header between stages, and/or condensation of exhaust steam fulfill additional steam balance and energy recovery needs. 

Steam turbines with compressors are used for providing process gas flow at a required pressure in high throughput processes. The process demand is determined by a pressure controller, which adjusts the setpoint on the turbine speed controller. In smaller processes, fixed speed compressors may be used by adjusting either an inlet or discharge valve to achieve pressure control. It is more energy efficient to adjust an inlet valve, or better yet to adjust inlet vanes which provide a pre-rotation to the gas. However, adjustment of speed is the most energy-efficient method control. 

The diverter valve operation is controlled by the waste heat recovery boiler (WHRB) and directs the exhaust gas to the WHRB boiler or out of the bypass stack to maintain the required steam pressure. The ABCO WHRB is rated at 30,000 Ib/hr and has two firing modes. In the turbine-firing mode, a 5.8MMBtu duct burner is available to supplement the turbine exhaust stream. In the direct-fire mode, used when the turbine is offline, the direct fresh-air Are at 41.5MMBtu is available. 

Since we have just derived the two partial derivatives, dW/dy and dW/dAp, we have in equation (23.23) recast the problem in terms of the total derivative of flow to valve opening, dWjdy, when the differential pressure controller is switched out. This last condition implies that the steam throttle valve is held constant, and so, to a good approximation, that the turbine power remains constant. (The power may not stay quite constant because the turbine efficiency depends partly on the ratio of blade speed to steam speed, as described in Chapter 15, and the speed of the turbine and pump combination will undergo a degree of change as the liquid control valve is moved the effect will, however, be second-order.)... 

Column pressure is controlled by changing the setpoint of a speed controller on the compressor turbine. The speed controller output sets a flow controller on the high-pressure steam to the turbine. 

AUTOMATIC EXTRACTION UNIT TURBINE - Bleeds off part of the main steam flow at one, two, or three points. Valved partitions between selected stages control the extracted steam pressure at the desired level. When extracted steam flowing through the unit does not produce enough shaft power to meet the demand, more steam flows through the turbine to exhaust. Located between steam supply and process steam headers. 

Equipment which required no operator intervention. The following events were noted the insertion of the control rods the tripping of the turbine and gas circulators the venting of the boilers to a lower steam pressure the starting of four gas circulators sequentially from the 11 kV Station Board and the starting of the standby boiler feed pmnps. 

Modification of operating conditions of the condenser and boiler Lowering the operating pressure of the condenser reduces the temperature of the saturated steam within the condenser. Therefore, the heat flow from the condenser to the environment will be at a lower temperature. At the same time, we have to control the quality of the discharged steam from the turbine steam with a high level of liquid water and of low quality lowers the efficiency of the turbine and may corrode the turbine blades. 

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