Posts Tagged ‘steam turbines’

Work and Energy Share an Interesting Relationship

Thursday, December 10th, 2015

      My work as an engineering expert has often required that I perform calculations to quantify the energy consumed by electric motors and steam turbines, such as when they work together at power plants to generate electricity.   Today we’ll see how work and energy share an interesting relationship that is brought out by examining the units by which they are measured.

     Last time we used de Coriolis’ formula to compute work to calculate the amount of work performed while pushing a loaded wheelbarrow a distance of 3 meters.   We found that in order to move the wheelbarrow that distance, a gardener must exert a force equal to 534 Newton • meters of work.   That relationship is shown here,

Work = 178 Newtons × 3 meters = 534 Newton • meters           (1)

     

Work is force times distance

de Coriolis’ Formula to Compute Work

     

     The Newton, as discussed previously in this blog series, is shorthand notation for metric units of force, and we’ll use those units today to demonstrate the special relationship between work and energy.

We’ll start by supposing that you’re unfamiliar with the Newton as a unit of measurement.   In that case you’d have to employ longhand notation to quantify things, which means you’d be measuring units of force in terms of kilogram • meters per second2.

     Putting equation (1) in longhand notation terms, we arrive at,

Work = 178 kilogram • meters per second2 × 3 meters       (2)

Work = 534 kilogram • meters2 per second2                    (3)

     If you’ve been following along in this blog series, you’ll recognize that the unit of measurement used to compute work, namely, kilogram • meters2 per second2, is the same as was used previously to measure energy.  That unit is the Joule, which is considerably less wordy.

     Equations (2) and (3) bear out the interesting relationship between work and energy — they share the same unit of measure.   This relationship would not be apparent if we only considered the units for work presented in equation (1).

     So following standard engineering convention where work and energy are expressed in the same units, the work required to push the wheelbarrow is expressed as,

Work = 534 Joules

     Yes, work and energy are measured by the same unit, the Joule.   But, energy isn’t the same as work.   Energy is distinguished from work in that it’s the measure of the ability to perform work.    Stated another way, work cannot be performed unless there is energy available to do it, just as when you eat it provides more than mere pleasure, it provides your body with the energy required to perform the work of pushing a wheelbarrow through the garden.

     Next time we’ll see how work factors into the Work Energy Theorem, which mathematically relates work to energy.

Copyright 2015 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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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.

Coal Power Plant Training

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.

Steam Turbine Expert Witness

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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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.

Electric Utility Power Plant

      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.

electric utility power plant boiler and steam turbine

      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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