Horsepower valves

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. 

Do not pinch on the horsepower valves. They must be left completely opened, or firmly closed. Leaving them in a partially open position will cut the valve seats and greatly reduce the efficiency of the turbine. 

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. 

Optimize the number of horsepower valves that are closed It is rather like solving two equations with two unknowns. 

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... 

A horizontal rod extending across the width of the turbine has a half-dozen cams fixed along its length. These cams lift and lower, in sequence, plungers attached to the horsepower valves. Watch these plungers in action. If you slow the compressor, here is what will happen ... 

The steam rack will move, so as to close only one of its multiple horsepower valves. 

One cost-effective means of reducing compression valve losses and enhancing compressor efficiency is to replace valve plates with thermoplastic plates equipped with additional flow ports (i.e., openings in the plates for gas passage). Modifying valve plates in this manner will reduce horsepower valve losses due to the frictional pressure drop. 

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... 

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