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Last time we learned how the condenser recycles steam from the turbine exhaust by condensing it back into water for its reuse within the power plant steam-water cycle. This water is known as condensate, and after leaving the boiler feed pump at high pressure, it’s known as boiler feed water. Today we’ll introduce a special valve into the system, whose job it is to perform the important function of compensating for lost water. It’s known as the make-up valve.
The illustration shows the flow of steam and water within the cycle. Tracing the path of orange arrows will reveal it as a closed system. Under ideal operating conditions recycled condensate from the condenser would provide enough water to keep the boiler indefinitely supplied. In reality water and steam leaks are a chronic problem within power plants, even when well maintained. Leaks typically occur due to worn parts on equipment, a condition which is commonly present due to the demanding operating conditions they must endure. First, there is the strain of continuous operation, then there are the high temperatures, typically greater than 1000°F, and high pressures that pipes, valves, pumps, and the boiler itself must endure. We’re talking about pressure higher than 2000 psi, that is, pounds per square inch. As a result, water levels within the boiler must periodically be replenished. While tracing the arrows through the diagram, you would have come across the new make-up valve under discussion. It’s located on the pipe leading from the power plant’s water treatment system to the boiler feed pump. It’s normally kept closed, except under two circumstances, when the boiler is initially filled at startup, or when water replenishment needs to take place. Due to water loss and difficult operating conditions, maintenance within the water-to-steam system of a power plant is a never ending task. There are miles of pipe connected to hundreds of pieces of equipment, all of which are distributed through a huge power plant structure. So the reality is that power plants operate with a continuous eye on leakage. To contend with the leaks, human intervention is often required in the way of a boiler operator. Their job is to manually open the make-up valve to admit a fresh supply of water from the treatment plant to the boiler via the boiler feed pump. Once the system’s water requirements are replenished, the valve is once again closed. Next time we’ll continue this series by discussing how the condenser enables the steam turbine to run more efficiently by creating a vacuum at the turbine’s exhaust.
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Posts Tagged ‘water treatment system’
The Make-up Valve in the Power Plant Steam to Water Cycle
Monday, October 28th, 2013
Heat Energy Within the Power Plant— Water and Steam Cycle, Part 2
Wednesday, August 14th, 2013|
Last time we learned that electric utility power plants must have water treatment systems in place to remove contaminants from incoming feed water before it can be used. This clarified water is then fed to a boiler by the boiler feed pump as shown below.
As it stands this setup will work to provide electricity, however in this state it’s both inefficient and wasteful. We’ll see why in a minute. Boilers, as their name implies, do a great job of heating water to boiling point to produce steam. They do this by adding the heat energy produced by burning fuel, such as coal, to water, then steam. We learned in earlier blogs in this series that the energy used to heat water to boiling point temperature is known as sensible heat, whereas the heat energy used to produce steam is known as latent heat. The key distinction between these two phases is that during sensible heating there is a rise in temperature, during latent heating there is not. For a review on this, see this blog article. When water starts to heat inside the boiler, sensible heat energy is said to be added. This is represented by phase A of the graph below.
During A, heat energy will raise the temperature of the water to boiling point. As the water continues to boil in phase B, water is transforming into steam. During this phase latent heat energy is said to be added, and the temperature will remain at boiling point. In phase C something new takes place. The temperature rises beyond boiling point and only steam is present. This is known as superheated steam. For example, if the boiler pressure is at 1,500 pounds per square inch, steam becomes superheated at temperatures greater than 600°F. Unfortunately, boilers alone do a poor job of superheating steam, that is, continuing to raise the temperature of the steam present in phase C. This is evident by the fact that phase C is quite small in comparison to phases A and B before it. This inefficiency in producing ample amounts of superheated steam results in a small amount of useful energy being provided to the turbine down the line, which is bad, because steam turbines require exclusively superheated steam to run the generator. Next time we’ll see how to provide our steam turbine with more of what it needs to run the generator, more superheated steam. ___________________________________________
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Heat Energy Within the Power Plant – Water and Steam Cycle, Part 1
Monday, August 5th, 2013|
Last time we learned that electric utility power plant boilers are vessels that are reinforced with thick steel and are closed off from the surrounding atmosphere so as to facilitate the building up of highly pressurized steam. This steam is laden with sensible heat energy, meaning it’s a useful energy, and it’s used to run steam turbines, which in turn drive electrical generators. The end result is power to consumers. Let’s now revisit our basic electric utility boiler diagram to see how water and steam flow.
Water is fed into the boiler, heat is applied externally, and steam exits through a pipe leading to the steam turbine. You’ll notice that after the steam passes through the turbine, some of it is expelled into the surrounding atmosphere. Since water is being continuously boiled off to produce steam, the boiler must be continuously replenished with a fresh supply. This is typically supplied by a nearby body of water, hence one reason that power plants are often situated on a lake or river. Since water contains both minerals and organic matter, including algae, a treatment system to remove these contaminants must be added to the water’s inlet area before it can be used. This will keep operating parts such as the boiler and turbine free of damaging deposits.
The treatment system operates much like the water softener in your home, but on a larger scale. Lake water is drawn into the system by a make-up pump, so named because it makes up, or replenishes spent water with a fresh supply. The result is clean, mineral-free water that’s delivered to the boiler by a boiler feed pump, so named because its specific function is to feed water to the boiler. Feeding water to the boiler on a continuous basis is no easy task because of the steam straining to break free, and boiler feed pumps are massively powerful devices built to accomplish this. They effectively force water into the boiler even as high internal pressures try to force the water out. This pressure is often greater than 1,500 pounds per square inch (PSI) in modern power plants. So at this point we’ve discussed the fact that the boiler requires a continuous supply of fresh water, which is converted into high pressure steam, which is then sent on to spin a steam turbine. The turbine powers an electrical generator, resulting in usable energy. If you’ve been reading along closely, you will have identified that as things stand now it’s a rather inefficient and wasteful system, a point which we’ll address in next week’s blog. ___________________________________________
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