Archive for the ‘power plant training’ Category

The Make-up Valve in the Power Plant Steam to Water Cycle

Monday, October 28th, 2013

      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.

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      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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Boiler Feed Water, A Special Kind of Condensate

Tuesday, October 22nd, 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.

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      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. 

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How A Power Plant Condenser Works, Part 3

Monday, October 14th, 2013

      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.

Power Plant Condenser

      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.

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How A Power Plant Condenser Works, Part 2

Sunday, October 6th, 2013

      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.

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      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.

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How A Power Plant Condenser Works, Part 1

Wednesday, October 2nd, 2013

      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.

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      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.

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Power Plant Inefficiency

Sunday, September 22nd, 2013

      Last week we identified some inefficiencies in our water to steam power plant energy cycle.   The superheater addressed some of these concerns, but not others.   Our illustration discloses one of these wasteful areas to be coming from the turbine exhaust.   That’s energy laden steam being expelled into the surrounding atmosphere!   It’s the same heat energy that was produced in the boiler when water was transformed into steam.   It came from burning fuels like coal, natural gas, and oil, all expensive and precious natural resources.

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      In its present configuration the power plant will work, but because steam is being continually dispersed into the atmosphere, it must continually be replenished.   The key ingredient, water, must be drawn into the power plant from a nearby source, treated for contaminants, then fed into the boiler to make up for lost steam.   That wastes both water and energy, because the make-up pump, which draws water from the lake for treatment, (thus “making up” for spent water), is continuously operating, resulting in excessive wear and tear and increased operating costs.

      Fortunately, power plant engineers have devised methods to correct these inefficiencies.   They’ve come up with a clever means of recapturing exhaust steam, thus enabling it to recycle within the system.   Next week we’ll see how this is accomplished with a piece of equipment called a condenser.

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Superheater Construction and Function

Sunday, September 15th, 2013

      Power plants produce electrical energy for consumers to use, whether at home or for business, that’s obvious enough, but did you know that in order to produce that electrical energy they must first be supplied with heat energy?   The heat energy that power plants crave comes from a fuel source, such as coal, oil, or natural gas, by way of a burning process.   Once the heat energy is released from the coal through burning, it’s transported into a steam turbine by way of superheated steam, which is supplied to it by a piece of equipment named, appropriately enough, a superheater.

      So what is a superheater and how does it function?   Take a look at the illustration below.

Electric Utility Power Plant Superheater

      The superheater looks like a W.   It’s actually a cascading array of bent steam pipe, situated above a source of open flames which are produced by the burning of a fuel source.   A photo of an actual superheater is shown below.

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      So how many bends are in a superheater?   Enough to fill the needs of the particular power plant it is supplying energy to.   Since all power plants are designed differently, we’ll keep things in general terms.

      The many bends in the superheater’s pipes form a circuitous path for steam to flow as it follows a path from the boiler to the steam turbine.   The superheater’s unique construction gives the steam flowing through it maximum exposure to heat.   In other words, the bends increase the time it takes for the steam to flow through the superheater.   The more bends that are present, the longer the steam will be exposed to the flame’s heat energy, and the longer that exposure, the more heat energy that is absorbed by the steam.

      Superheating routinely results in temperatures in excess of 1000°F.   This superheated steam is laden with abundant heat energy which will keep the steam turbine spinning and the generator operating.   The net result is millions of watts of electrical power.

      As we learned in a previous blog, the superheater is designed to provide the turbine with sensible heat energy to prevent steam from completely desuperheating, which would result in dangerous condensation inside the turbine.

      The newly added superheater is a major improvement to a power plant’s water-to-steam cycle, but there’s still plenty of waste and inefficiency in the system, which we’ll discuss next week.

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Condensation Inside the Steam Turbine

Sunday, September 8th, 2013

      Did you know that water droplets traveling at high velocity can take on the force of bullets?   It can happen, particularly within steam turbines at a power plant during the process of condensation, where steam transforms back into water.

      The last couple of weeks in this blog series we’ve been talking about the steam and water cycle within electric utility power plants, how heat energy is added to water during the boiling process, and how turbines run on the sensible heat energy that lies within the superheated steam vapor supplied by boilers and superheaters.   We learned that without a superheater there is a very real possibility that the steam’s temperature can fall to mere boiling point.

      When steam returns to boiling point temperature an undesirable situation is created.  The steam begins to condense into water within the turbine.   To understand how this happens, let’s return to our graph from last week.   It illustrates the situation when there’s no superheater present in the power plant’s steam cycle.

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Figure 1

 

      After consuming all the sensible heat energy in phase C in Figure 1, the only heat energy which remains available to the turbine is the latent heat energy in phase B.   If you recall from past blog articles, latent heat energy is the energy added to the boiler water to initiate the building of steam.   As the turbine consumes this final source of heat energy, the steam begins a process of condensation while it flows through the turbine.   You can think of condensing as a process that is opposite to boiling.   During condensation, steam changes back into water as latent heat energy is consumed by the turbine.

      When the condensing process is in progress, the temperature in phase B remains at boiling point, but instead of pure steam flowing through the turbine, the steam will now include water droplets, a dangerous mixture.   As steam flows through the progressive chambers of turbine blades, more of its latent heat energy is consumed and increasingly more steam turns back into water as the number of water droplets increases.

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Figure 2 – Water Droplets Forming in the Turbine

 

      The danger comes in when you consider that the steam/water droplet mixture is flying through the turbine at hundreds of miles per hour.   At these high speeds water droplets take on the force of machine gun bullets.  That’s because they act more like a solid than a liquid due to their incompressible state.   In other words, under great pressure and at high speed water droplets don’t just harmlessly splash around.   They hit hard and cause damage to rapidly spinning turbine blades.   Without a working turbine, the generator will grind to a halt.

      So how do we supply the energy hungry turbine with the energy contained within high temperature superheated steam in sufficient quantities to keep things going?   We’ll talk more about the superheater, its function and construction, next week.

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Desuperheating in the Steam Turbine

Monday, September 2nd, 2013

      Last time we learned that the addition of a superheater to the electric utility power plant steam cycle provides a ready supply of high temperature steam, laden with heat energy, to the turbine, which in turn powers the generator.   But this isn’t its only job.   One of the superheater’s most important functions is to regulate the ongoing process of desuperheating that takes place as the turbine consumes heat energy.   To understand this, let’s see what takes place if the superheater were to be removed from its position between the boiler and turbine.

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Figure 1

 

      Without the superheater, the only available remaining source of sensible heat energy to the turbine would come from the meager amount present in phase C steam as shown in Figure 1.   If you’ll recall from a past blog, the sensible heat energy contained in superheated steam is the best source of energy for a steam turbine, because it’s able to keep it operating most efficiently.

      As the turbine consumes the heat energy in phase C, starting at point 3 and continuing to point 2, the steam it’s consuming is in the process of desuperheating, as evidenced by the downward slope between the two points.   Desuperheating is an engineering term which means that as sensible heat energy is removed from the steam due to its use by the turbine, there will be a resulting drop in steam temperature.   And if this process were to continue without the compensatory function provided by the addition of a superheater to the steam cycle, the steam’s temperature would eventually return to mere boiling point, at point 2.   This is an undesirable thing.

      With the steam’s temperature at boiling point, the only remaining source of heat energy to the turbine is the latent heat energy of phase B.   This heat energy will lead to an undesirable circumstance for the operation of our power hungry turbine as we will see next week.

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Superheating, Part 2

Sunday, August 25th, 2013

      Last time we added a piece of equipment called a superheater, positioned between the boiler and steam turbine, to our basic electric utility power plant steam and water cycle.   Its addition enables a greater and more consistent supply of heat energy to the steam which powers the turbine.   How much more?   Let’s look at Figure 1 to get an idea.

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Figure 1

 

      You may have noticed that our illustration lacks numerical representation.   That’s because power plants are designed differently, depending on fuels used and power output required.   So unless we’re talking about a particular power plant, number values would be impractical.   For example, I could specify a boiling point of 596°F at 1,500 pounds per square inch (PSI), and a superheater outlet temperature of 1,050°F at 1,200PSI, and I could make note of esoteric things like enthalpy (British Thermal Units per pound mass) values on the Heat Energy axis.    But to facilitate our discussion we’ll keep things simple and focus on the general process.

      Figure 1 shows in phase D the additional heat energy being added to the steam, thanks to the superheater.   This is significantly more than had been added by the boiler alone, as represented by phase C.   The turbine consumes heat energy added in phases C and D and converts it into mechanical energy to drive the generator, resulting in electrical energy being provided to consumers in the most energy efficient way possible.

      But increasing power output and efficiency isn’t the superheater’s only job.   The heat it adds during phase D ensures the turbine’s safe operation when it’s cranking at full capacity, as represented by the superheated steam zones of phases C and D.

      Next week we’ll discover how the superheater prevents a destructive process known as condensing from occurring inside the turbine.

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