Steam turbines speed valves

Steam turbine performance is modeled using a standard steam flow versus horsepower map and valve position versus steam flow. The turbine inlet valve(s) is positioned by the governor system to maintain constant speed (or another parameter when synchronized). 

For Main Air Blower serviee without a steam turbine in the string, the unit shall be started by first applying windage steam and then aeeelerating the train to rated speed using the eleetrie motor. After aehieving stable rated speed, the expander inlet valve will be... 

High-frequency, destructive vibrations. Unaffected by operating speed. Steam turbine valve vibration Rare occniTence change valve ping, seat shape, and/or increase valve gear rigidity... 

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... 

A governor valve controls the flow of steam to the turbine. This valve is actuated by the governor, which is operated by the speed of the machine. When the speed exceeds the set value of the governor, a trip-valve is actuated to completely shut off the steam supply. The trip valve may... 

Turbine speed is controlled by two split-range valves, one on the 10 psig inlet to the turbine and the other on the 100 psig steam that can also be used to drive the turbine. Your instrumentation system should be designed so that the valve on the 10 psig steam is wide open before any 100 psig steam is used. 

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. 

Figure 17.3 shows a steam turbine with three, rather than one, nozzles. The single, largest, left-hand valve is called the main nozzle. It handles 60 percent of the motive-steam flow. Each of the two smaller nozzles handles 20 percent of the steam. These 20 percent nozzles can be plugged off by a device sometimes called either a horsepower valve, jet valve, speed valve, star (for the handle shape) valve, or port valve. 

Figure 17.4 shows a centrifugal pump, driven by a steam turbine. The correct operating speed for the pump and turbine is that speed that puts the process-control valve in a mostly open, but still controllable, position. As we slow the turbine to force open the process-control valve, the turbine s governor valve will close. Steam flow to the turbine will decline in accordance with fan laws ... 

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. 

During the evening, my operators decided to increase the combustion airflow from the blower or air compressor. This is done by opening the suction valve to the blower. Naturally, it requires more work to compress more air. But the turbine could not produce any more work or horsepower because the steam turbine s governor speed-control valve was already 100 percent open ... 

Even if the governor-steam inlet control valve is 100 percent open, it may still be possible to increase the supply of motive steam into the turbine. The position of the governor when it is wide open can often be reset to admit more steam flow. I once increased the flow through a crude tower-bottom pump by pushing hard against the base of the governor, and forced it to actually open to its maximum position. The pump speed came up 300 rpm, and the flow increased by about 15 percent. 

Another possibility is to open the speed or hand valve, as described in Chap. 17, Steam Turbines. This will introduce more steam into the... 

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. 

Example 7.2 is of cases with control valve throttling and pump speed control. In large systems, the value of power saved can easily overbalance the extra cost of variable speed drives, either motor or steam turbine. 

Steam turbines are most often used in processes to provide power to compressors or electric generators as shown in Figure 3.28. Multistage turbines may also admit or extract steam between stages. Turbine speed is a fast loop, controlled by manipulating the supply steam valve or valves, as often there will be a rack of parallel steam valves supplied as part of the turbine system. 

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. 

Avoid control valves by controlling pump/compressor by adjusting driver speed (for example, use of VSD for a motor and use of steam turbine governor control valve)... 

A fast response, modulating-type valve, controlled by the steam bypass pressure regulator system, is used to perform three basic functions. The primary function is to reduce the rate of rise of reactor pressure when the turbine admission valves are moved rapidly in the closing direction. To perform this function, the bypass valve needs about the same speed of response as the turbine admission valves to prevent a pressure-induced reactor scram from high neutron flirx when the turbine load is suddenly reduced by partial or complete closure of the turbine admission valves. 

Mechanical torques of the prime movers, steam or hydrauhc, for large generators depend on rotor speed. In unregulated machines the torque speed characteristic is linear over the rated range of speeds. The prime mover speed of a machine will drop in response to an increased load, and the valve position must be opened to increase the speed of the machine. In a regulated machine (governor controlled) the speed control mechanism controls the throttle valves to the steam turbine or the gate position for a water turbine. 

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