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I understand in residential plumbing, that a thermal expansion tank is needed before the hot water tank. From my understanding this is to allow for the water in the hot water tank to expand, instead of creating large amounts of pressure, when the water is being heated while the system is closed.

However, I'm most curious about on large scale, industry projects. I am in a power generation course at the moment and we are discussing steam power plants. Is thermal expansion not a factor in this situation? It hasn't been mentioned in class (this course is largely applied thermodynamics, not power plant design). Is this because the fluid is actually flowing and not stationary, or are there expansion tanks that we just haven't discussed in class?

It is my understanding that pumps provided the pressurization for the system, and not the steam.

Thanks!

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No need for a thermal expansion tank in the water steam cycle of a steam power plant.

A thermal expansion tank is added to residential plumping, because your plumping system only contains water and no steam. As water is approximately incompressible, it generates such a high pressure through thermal expansion that it would burst the whole pipe if there was no thermal expansion tank.

The thermal expansion tank technically adds a gas to the plumping system, but keeps it contained to the thermal expansion tank by separating it from the water with a flexible bladder. The gas is compressible. Therefore it can buffer the thermal expansion of the water and keeps the overall pressure of the pipe system within the specification.

The water steam cycle in a steam power plant already contains two phases namely water and steam. Any fluctuation of the average water temperature would indeed change the total water volume, but the change is small and buffered by the compressible steam in the system. Nobody needs to worry about it.

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The expansion of water is always considered and catered for.

The diagrams you study on your course are to show the major components that are needed to calculate the conditions of water or vapour at relevant points, density, specific volume etc.

These diagrams are not detailed enough to be used to design and build a power plant. This is the case for many circuits shown in electrical textbooks for example and the same applies to many other situations and disciplines.

In mechanics text books a pin joint will be shown where the diameter of the pin is calculated along with the dimensions of the part it goes in to, however, when it is actually built a bearing will be added to the assembly.

Going back to water, the diagram I was given to show the layout of a "heat main" (two large 8" insulated pipes across site to carry hot water between buildings) was sufficient to calculate the total flow rates etc. When the build was completed I had added an expansion tank based on the delta T between cold and hottest points and the expansion of the total amount of water held in the system - this also was at the highest point to get air out of the system.

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  • $\begingroup$ Mike, nobody puts an expansion tank on a steam plant. $\endgroup$
    – Tiger Guy
    Apr 9, 2023 at 6:28
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No one cares about this while making steam except nuclear plant operators in pressurized water plants (& natural circulation submarine power plants).

Water is heated in a steam plant in the boiler. Steam draw, feedwater inlet, and thus boiler water level are what matters. No one cares that feedwater coming out of the economizer is marginally less dense.

Pressurized water nuclear plants need to manage primary coolant expansion using a pressurizer, which maintains a water level. Some nuclear submarine plants use natural circulation to move water through the core, which is driven by density changes. But no one cares on the secondary steam side.

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