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.
Archive for October, 2013
Last time we learned how the condenser within a power plant acts as a conservationist by transforming steam from the turbine exhaust back into water. This previously purified water, or condensate, contains valuable residual heat energy from its earlier journey through the power plant, making it perfect for reuse within the boiler, resulting in both water and fuel savings for the plant. Today we’ll take a look at a highly pressurized form of condensate known as boiler feed water and how it helps the power plant save money by recycling residual heat energy in the steam and water cycle.
Let’s begin by integrating the condenser into the big picture, the complete water-to-steam power plant cycle, to see how it fits in. The illustration shows that both the make-up pump and the condenser circulating water pump draw water from the same supply source, in this case a lake. The circulating water pump continuously draws in water to keep the condenser tubes cool, while the make-up pump draws in water only when necessary, such as when initially filling the boiler or to make up for leaks during operation, leaks which typically occur due to worn operating parts.
In a nutshell, the condenser recycles steam from the turbine exhaust for its reuse within the power plant. The journey begins when condensate drains from the hot well located at the bottom of the condenser, then gets siphoned into the boiler feed pump.
If you recall from a previous article, the boiler feed pump is a powerful pump that delivers water to the boiler at high pressures, typically more than 1,500 pounds per square inch in modern power plants. After its pressure has been raised by the pump, the condensate is known as boiler feed water.
The boiler feed water leaves the boiler feed pump and enters the boiler, where it will once again be transformed into steam, and the water-to-steam cycle starts all over again. That is, boiler feed water is turned to steam, it’s superheated to drive the turbine, then condenses back into condensate, and finally it’s returned to the boiler again by the boiler feed pump. Trace its journey along this closed loop by following the yellow arrows in the illustration.
While you were following the arrows you may have noticed a new valve in the illustration. It’s on the pipe leading from the water treatment plant to the boiler feed pump. Next time we’ll see how this small but important item comes into play in the operation of our basic power plant steam and water cycle.
We’ve been discussing various aspects of a power plant’s water-to-steam cycle, from machinery specifics to identifying inefficiencies, and today we’ll do more of the same by introducing the condenser hot well and discussing its importance as a key contributor to the conservation of energy, specifically heat energy. Let’s start by returning our attention to the steam inside the condenser vessel.
Last week we traced the path of the condenser’s tubes and learned that the cool water contained within them serve to regulate the steam’s temperature surrounding them so that temperatures don’t rise dangerously high. To fully understand the important result of this dynamic we have to revisit the concept of latent heat energy explored in a previous article. More specifically, how this energy factors into the transformation of water into steam and vice versa.
Steam entering the condenser from the steam turbine contains latent heat energy that was added earlier in the water/steam cycle by the boiler. This steam enters the condenser just above the boiling point of water, and it will give up all of its latent heat energy due to its attraction to the cool water inside the condenser tubes. This initiates the process of condensation, and water droplets form on the exterior surfaces of the tubes.
The water droplets fall like rain from the tube surfaces into the hot well situated at the bottom of the condenser. This hot well is essentially a large basin that serves as a collection point for the condensed water, otherwise known as condensate.
It’s important to collect the condensate in the hot well and not just empty it back into the lake, because condensate is water that has already undergone the process of purification. It’s been made to pass through a water treatment plant prior to being put to use in the boiler, and that purified water took both time and energy to create. The purified condensate also contains a lot of sensible heat energy which was added by the boiler to raise the water temperature to boiling point, as we learned in another previous article. This heat energy was produced by the burning of expensive fuels, such as coal, oil, or natural gas.
So it’s clear that the condensate collecting in the hot well has already had a lot of energy put into it, energy we don’t want to lose, and that’s why its an integral part of the water-to-steam setup. It acts as a reservoir, and the drain in its bottom allows the condensate to flow from the condenser, then follow a path to the boiler, where it will be recycled and put to renewed use within the power plant.
Next week we’ll follow that path to see how the condensate’s residual heat energy is put to good use.
Winter is fast approaching. Imagine living in a house without insulation. Now imagine your heating bill, which will be high due to the tremendous amount of heat loss. Energy is a precious resource, no matter how it’s produced, and its conservation within a power plant’s steam/water cycle is of vital importance.
Last time we learned about the transfer of heat energy within a power plant’s condenser, where some of the heat energy contained within its steam is absorbed by the cool lake water contained inside its tubes.
Steam is continuously flowing into the condenser from the steam turbine, so it’s essential for the circulating pump to keep a fresh supply of lake water flowing through the condenser’s tubes in an effort to keep temperatures under control.
The compensating action that’s provided by the cool lake water flowing within the tubes, represented by green arrows in the illustration, keeps the temperature inside the tubes from rising and becoming equal to the steam’s temperature outside of them. If the flow of cool water through the tubes were to stop and the temperatures inside and outside the tubes become equal, the water contained inside the tubes would boil off to steam, resulting in the tubes bursting and a wrecked condenser.
After absorbing heat energy from the surrounding steam, the warmed lake water within the tubes follows a circuitous path through the tubing, eventually emptying out into the lake. The orange arrows in the illustration show this path.
Okay, with this warm water entering the lake, doesn’t that harm the eco system? Actually its impact is negligible. You see, the temperature of the lake water leaving the condenser is only about 10°F higher than when it was pumped from the lake. Add this to the fact that the volume of water contained within a lake is huge in comparison to the small amount of warmed water being returned to it.
Next week we’ll see how the loss of heat energy affects the steam, and how an important part of the condenser known as the hot well comes into play.
Last time we began our discussion on power plant inefficiencies and indentified a major contributor, the heat energy dispelled into the atmosphere through the turbine exhaust. Today we’ll see how a piece of equipment known as the condenser comes into play to deal with this problem. Let’s see how it works.
First, water from our plant’s water source, say a nearby lake, is siphoned into the condenser circulating pump, which delivers it to the condenser. This lake water path appears in yellow. You’ll notice that the lake water follows a circuitous path from the lake, through the condenser circulating pump, then the condenser tubes, until finally it is returned to the lake.
Now the cool lake water, denoted by green arrows, is made to pass through the condenser’s many tubes, while steam from the turbine exhaust surrounds them. The tubes keep the lake water segregated from the cloud of steam surrounding them inside the condenser vessel. In other words, the lake water’s path is a closed system, never coming into direct physical contact with the surrounding steam.
What’s happening inside our condenser is demonstrative of a fundamental principle of thermal engineering, that is, that hot will travel in the direction of cold. More specifically, within our condenser the heat energy in the steam cloud surrounding the condenser tubes will be attracted to the cool lake water contained within the tubes. This causes the heat energy contained within the steam to leave it, and get absorbed by the cool lake water flowing within the tubes.
We’ll begin to find out how these dynamics influence what’s happening with our water-to-steam power plant cycle next time.