Steam turbines horsepower 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). 

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. 

If we close off one of the two horsepower valves, steam flow into the turbine will drop—initially by 20%. The turbine will slow. This will cause the governor valve to open. The pressure drop across the governor will decrease. The pressure in the steam chest will rise. The flow of steam through the 60 percent port nozzle and the remaining 20 percent port nozzle will increase. The velocity of the steam striking the buckets will also increase. The turbine wheel will now come back up to its set point. 

If we now attempt to shut the second horsepower valve, we may be able to save a second 10 percent increment of steam. But, if the governor goes wide open, we will have to open this horsepower valve again. You see, once the governor is 100 percent open, we will have lost our ability to control the turbine s speed. This is no good, because we want the turbine to run at a constant speed. 

Slowing a turbine closes the governor valve. This may now permit us to close an additional horsepower valve, without losing our flexibility to control the speed of the turbine. Closing that final horsepower valve will save us another 10 percent of steam. 

On larger steam turbines, we have automatically, rather than manually, operated horsepower valves. The mechanism that controls the movement of the horsepower valves, is called a steam rack. If you have... 

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 ... 

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. 

If reduced driver steam pressure is to be a long-term condition, a larger nozzle block on the turbine steam inlet may be available at a relatively small price. Sometimes the turbine governor can be adjusted to open wider. Turbine hand valves, used to reduce nozzle block cross-sectional area, should be wide open when trying to maximize horsepower. 

If we close off one of the two horsepower valves, steam flow into the turbine will drop— initially by 20 percent. The turbine will slow. This will cause the governor valve to open. The pressure drop across... 

What I wish to achieve is to maintain the same horsepower output from the turbine. But at the same time, I want to force open the governor speed-control valve, raise the pressure in the steam chest, but decrease the steam flow through the steam nozzle. The only way this can be done is to make the nozzle smaller. 

This technique was used daily on an alkylation unit that was refrigeration-compressor limited. During periods of cold weather, the compressor would change from horsepower-limited to speed-limited. Propane was then added to the refrigerant accumulator drum. The turbine governor control valve (i.e., the driver steam inlet valve) would open to maintain compressor speed. When the governor valve was fully open, the compressor was using maximum available horsepower. The optimum propane concentration had been reached. 

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