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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 ‘steam leaks’
The Make-up Valve in the Power Plant Steam to Water Cycle
Monday, October 28th, 2013
Coal Power Plant Efficiency
Sunday, July 11th, 2010|
Is there any price a man dying of thirst in the desert would not pay for a tall glass of cold water? What is the point at which Americans will decide they can do without heat, refrigerators, electric lights? My neighbor refuses to run the air conditioner, even when it’s 90 degrees and 90 percent humidity. They have obviously made the choice to sweat and be uncomfortable in their homes rather than pay high utility bills. Most of us are concerned with the environment, but when times are hard like they are now many of us become more concerned with our pocketbooks. Just as we need to make our financial ends meet, so do energy suppliers. Without a certain level of profit, their service to us will decline, and regular, dependable delivery of their precious commodities to us will suffer. If they were to go out of business, what then? Reading by candlelight may be romantic for a night or two, but nights on end? Let’s consider the energy provided by coal-fired power plants, for example. They’re in the electric utility business, and they provide us with the lion’s share of our energy. To keep a handle on operating costs, power plant engineers monitor how many British Thermal Units (BTUs) of heat energy are going into the power generation process versus how many kilowatt-hours of electricity are coming out. What’s a BTU and what does it matter to us? Well, it’s the amount of heat energy your kitchen stove uses to raise the temperature of one pint of water by one degree Fahrenheit. As for a kilowatt-hour, that’s a thousand watts of power produced over the space of an hour– enough to light ten 100 watt light bulbs. Now that we’ve explained the key term, we can explore the notion of heat rate, terminology very important to efficient power plant operation. Heat rate is simply the ratio of BTUs to kilowatt-hours. So what’s the importance of monitoring heat rate? For one thing, in order to get the most bang for your buck you want to keep the heat rate as low as possible. When the heat rate is high, you’re burning more coal than you have to because you’re wasting heat energy. This results in higher electricity costs to the consumer. This is exactly the situation at play when low sulfur coals are used as compared to the better burning coals of yester-year. So where does the wasted heat energy go if it isn’t being converted into electrical energy? For one thing, it can be lost through steam and water leaks in the power plant piping system. There are other losses too. Another way to lose heat energy is when thermal insulation is missing from pipes, causing heat to escape into the atmosphere. The opposite side of inefficiency is presented by the problem of too much heat energy building up, unable to be transferred to the steam. This is the result if ash is allowed to accumulate inside the boiler, acting as a thermal insulator. The heat has nowhere to go except up the smoke stack and into the atmosphere. Needless to say it’s important to keep heat rate as low as possible by keeping power plant equipment insulated and in good repair. But there are some things that affect heat rate that we just can’t do anything about, they’re known as “uncontrollable factors,” and we’ll learn about them next week. _____________________________________________
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