Engineering Expert Witness Blog

      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.

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.

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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