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| Spirax Marshall>> Complete Steam Solutions>> Steam Distribution>> Best Engineering Practices |
There
is no external motive energy required for making steam flow.
The
biggest advantage of steam is that it flows by virtue of pressure drop.
Steam should be free from air and other
gases.
There will always be some incondensable gases or air in the steam due to frequent plant start-ups and shut-downs, but every measure should be undertaken to remove this entirely from the system.
If air or incondensable gases are mixed with steam, the pressure gauge will show the correct pressure, but the temperature of the air/gas/steam mixture will be less than that shown in the steam tables for the corresponding pressure and furthermore,the total heat in steam will be less than expected. This is because the measured pressure shows the sum of the partial pressures of the different gases, so steam itself is not at the pressure indicatedby the pressure gauge - but is actually at a lower pressure & hence lower temperature.
To get rid of these gases, balanced pressure thermostatic air vents should be fitted on the steam line dead ends and on all jacketed vessels. The air vent at the end of any steam distribution system should be installed in parallel to the steam trap, so that particularly on a plant start up condition when the air inside the steam pipes needs to be removed, the air vent can rapidly expel it. The air vent obviously has its use during a running condition and that is to expel the incondensable gases which will be to a greater or lesser extent entrained with the steam.
Steam should be free from dirt and particulate matter.
Dirt in the steam will cause fouling up of heat exchange surfaces, even in excess of any fouling factor designed into the equipment. Dirt also deposits to build up within steam control valves and pressure reducing valves and affects their performance, to the detriment of the process being controlled and the energy efficiency of the plant.
Therefore the application
of a pipeline strainer is essential for removal
of dirt from the steam system.
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