Turbine speed flow-controlled

The control scheme shown in Fig. 17.4 is certainly quite common. But is it the best Figure 17.5 is a copy of the crude charge system in a now-defunct refinery in Port Arthur, Texas. I saw it in operation many years ago. It worked fine. The required flow of crude directly controls the governor. The turbine speed is then always at its optimum. The AP across the process-control valve is always zero, because there is no process-control valve. This design is a direct descendant of the original method of controlling the steam flow to pumps. The steam inlet valve was opened by the operator, so that the desired discharge flow was produced. 

It is quite important not to operate a turbine-driven pump by throttling the steam flow to the turbine. Let s assume that the operators have set the turbine speed at 3500 rpm, by adjusting the steam inlet gate valve upstream of a malfunctioning governor. Suddenly, the discharge flow-control valve cuts back, and the pump s flow decreases from 2000 to 1200 GPM. The pump speed will then increase, because fewer pounds of liquid are being pumped, and less horsepower is required to spin the pump. 

In this particular example turbine speed control was taking place. The closed loop feedback is turbine speed in rpm as measured by an optical pickup on the generator box (nearly instantaneous). In this test, the experimental speed controller and the simulated speed controller (in both models), used a proportional gain of 0.001 and an integral gain of 0.001 x 0.75 with an input of speed error in rpm and output in fuel valve %. For the experiment presented, the fuel flow rate in grams per second... 

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. 

Flow control via pump speed adjustment is less common than the use of throttling with valves, because most AC electric motors are constant-speed devices. If a turbine drive is used, speed control is even more convenient. However the advent of the pulse-width modulated (PWM) adjustable speed drive with sensorless flux-vector control has brought adjustable speed (AS) pumping into the mainstream of everyday applications. 

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. 

The interaction between the pressure and turbine speed controllers (Figures 2.133 and 2.134) can be decoupled. Figure 2.136 illustrates how a drop in the speed of the shaft can open both the inlet and the extraction valves, and an increase in shaft speed can close them both. Therefore, one way to eliminate interaction between flow and pressure loops is to allow the pressure controller to throttle both the supply and the extraction valves. 

Figure 7.2c illustrates how a variable-speed drive (a steam turbine) can be used to control throughput. The turbine is driven by high-pressure steam (600 psia) and discharges into a low-pressure steam header (25 psia). A flow controVspeed control cascade structure is used. The output signal from the flow controller adjusts the setpoint of the turbine speed controller, which manipulates the flow of high-pressure... 

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 gas volumes in the body of the turbine are very small, so that the establishment of flow in the turbine is very fast, with a response time measured in tenths of a second. Since these time constants are an order of magnitude less than the rotor time constant associated with turbine speed, it is permissible to treat the equations of fluid flow as algebraic, simultaneous equations. This simplification receives even greater justification when the model of the turbine and the machine it drives are interfaced with a model of the rest of the process plant, which will be invariably much slower still. It is only when the turbine is being considered in detail on its own, for example in designing a speed-control system, that some of the larger turbine pressure constants, such as those associated with the turbine inlet manifolds, will need to be considered. 

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. 

Steam to the turbine driving the compressor is flow controlled, reset by a speed controller, which is reset by a distillate composition controller. 

Air feed is flow controlled by adjusting the speed of the turbine. 

The experimental system is shown schematically in figure 1. Experiments were performed in a straight, smooth-walled acrylic glass pipe 50 mm in diameter. In order to determine the drag reduction from pressure drop measurements the test pipe was provided with 14 pressure taps positioned at I m intervals down the pipe. A sufficient entrance pipe length was provided to ensure fully developed flow at the injection point. The water was pumped from a reservoir tank by a Mohno pump, the speed of which was controlled by a microcomputer to maintain a constant Reynolds (Re) number, during the experiments (between 10 and 10 ). The microcomputer continously measured the temperature of the water with a resistance thermometer and the discharge with a turbine wheel flow meter. 

Feedwater flow control is achieved by adjusting feed pump speed and the feedwater flow control valves. Feed pump speed is adjusted by modulating steam flow to the feed pump turbines. Extraction steam for the deaerator and high pressure heaters is provided from high pressure turbine extraction points, and the low pressure heaters are supplied from the low pressure turbines. 

If, while under normal load, the turbine speed decreases or the speed-load changer setting is increased, a positive speed-load signal is transmitted to the initial pressure regulator and the master flow controller. The increase in signal causes a momentary decrease in the pressure setting of the initial pressure regulator and causes the master controller to increase the flow demand to the recirculation system flow valve controller. 

The second function of the bypass valve is to control reactor pressure during startup of the turbine. This allows the reactor power level to be held constant while the turbine steam flow is varied as the turbine is brought up to speed under the control of its speed governor. 

Mechanical-drive gas turbines normally operate on speed/load control with the set-point provided by the process control system. Figure 6.64 depicts a typical performance curve for a two-shaft mechanical-drive gas turbine, with the load characteristic of a process compressor system superimposed. A process controller might receive the suction or discharge pressure signal of the driven compressor and generate the appropriate speed/load set-point of the gas turbine. Again the fuel flow is still limited by the maximum-firing-temperature control. 

The flow of main steam entering the high-pressure turbine is controlled by four control valves. The turbine control valves are adjusted automatically by electrohydraulic servo actuators. These actuators control the turbine speed when it is starting up, and for load control after the turbine-generator unit is synchronised. In series with each control valve is a stop valve, whose function is to shut off and isolate the steam flow to the turbine when required. 

The primary coolant system flow control scheme is shown in Figure C-61. Flow control will be accomplished through adjustments to the speed of the primary pump drive turbines. Discharge flow rates from each of the five primary ptmps will be monitored and a constant pressure differential across each flow monitor will be maintained by adjustments to each individual turbine speed control. A total flow controller will be utilized to control the sum of the five cells flow, i.e., total flow, by re-adjueting the five Individual turbine speed controllers. Primary coolant flow control is a vitally important system and steps were taken to enhance systetii reliability and minimize the importance of component failure. Liberal use was made of summing point monitors, servo followers, and selector stations to accomplish these objectives. 

Reactor generated heat is transferred to the circulating raw water system in the dump condensers for ultimate dissipation in the Columbia River. About 400,000 gpm of untreated Columbia River water will be pumped from the river, through the tube side of the diimp condensers and turbine condensers, and back to the river. Flow control is exercised primarily through the number of circulating raw water pumps online however, trimming adjustments are possible at each of the condensers. Since the drives for these pumps do not Incorporate speed control, fluctuations in river level will also effect the flow rates in this loop. 

The turbine speed is then always at its optimum. The AP across the process-control valve is always zero, because there is no process-control valve. This design is a direct descendant of the original method of controlling the steam flow to pumps. The steam inlet valve was opened by the operator, so that the desired discharge flow was produced. 

Control of turbine speed, Sd, through flow bypass valve CVl. 

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