- Last Updated: 28 March 2016 28 March 2016
The secondary loop is the power generating loop, and begins at the outlet of the steam generator. Steam from these outlets is sent through a series of dryers, which remove excess water vapor from the steam. From there, it's through a set of safety and throttle valves and directly into the turbines.
Turbines are like massive, enclosed windmills. Steam is directed against the blades of these windmills, spinning the shaft on which they're mounted. To improve efficiency, there are usually at least two stages to each turbine. A high pressure stage is first, and the steam gives up most of its energy there. Then, after being dried again, it is passed to the low pressure stages to expend the rest of its energy.
The shaft of the turbine is connected to a generator, which turns the mechanical power of the rotating shaft into electrical power. Power then leaves the plant through the switchyard.
Turbines, despite their immense size and imposing power, are quite delicate devices and must be treated with the utmost care. Water droplets, if allowed to strike the blades, will erode them -- wear them away! For this reason, a vacuum is developed in the main condenser and the low pressure turbine. This prevents the steam from condensing into water until it reaches the tubes of the condenser, and it also greatly reduces drag on the low-pressure turbine blades.
Should the turbine ever have to be tripped (shut down), we see another aspect of how delicate it is. The turbine must never be allowed to come to a complete stop when it's hot. If this happens, the turbine shaft will sag between its support bearings, throwing the blades out of alignment and incurring a seven-figure repair bill. So, machines called turning gear are used to slowly rotate the shaft, like a rotisserie, until it cools.
After leaving the turbine, the steam is piped into a condenser, which is actually quite similar to a steam generator -- in reverse. Cold water from the tertiary or "circ" water system runs through tubes in the condenser, and when the steam strikes these tubes, it condenses back into water, in the same way that your breath fogs a cold window. The condensed water is now called condensate, and is sent through a set of heaters to bring it back to a usable temperature.
Next, the water passes through a set of ion-exchange tanks, or polishers as they're often called. These remove excess mineral content and impurities from the water and ensure its purity. The water going into the steam generators needs to be free of minerals to avoid the buildup of "scale" (calcium and other deposits) inside the steam generators which would require a long "outage" to clean out.
After this stage, the water is known as feedwater, and is sent into a group of very powerful feedwater pumps, which force it back into the steam generators to be boiled for another cycle.