What is the effect of moisture in steam on steam systems ?
Steam from the boiler carries moisture alongwith it, this could be because the demand for steam is higher than the generating capacity of the boiler, steam is being generated at lower than the rated pressure or water carry over takes place due to improper TDS control within the boiler which results in foaming. As steam travels through the distribution network additional condensation is continuously taking place . This moisture/condensate has to be removed immediately as steam is carrying it forward at velocities in excess of 25m/s. This could cause severe waterhamering conditions & errosion in the pipeline damaging all equipments installed on line. The result is a decreased overall plant efficiency with reduced performance and production rate from the process equipment, particularly in the applications such as steam operated vacuum ejectors used for vacuum evaporating and drying of the product.
Much of the water in the steam distribution system will originate from the boiler, so install a Separator with a steam trap. (sized on at least 10% of the boiler evaporation) on the main steam outlet from the boiler.
• What is the correct steam pressure for distribution and why ? What is the correct location for installation of Pressure Reducing System (PRS) ?
Steam has to be distributed at rated boiler steam generation pressures & reduced near the point of consumption. Most steam boilers are designed to work at high rated pressures and should not be run at lower pressures, since carryover of water is liable to occur. Distributing steam at higher pressures has the advantage of lower specific volumes & the fact that any pressure dropsin the distribution line do not effect the performance parameters of the process in which steam is to be utilized.Reducing pressure improves the dryness fraction of steam & also the latent heat of evaporation.
So the Pressure Reducing System should be fitted right before the items requiring steam at a lower pressure. Such an arrangement has the advantage that small bore distribution pipes can be used, due to the relatively small volume occupied by steam at high pressure leading to lower capital costs in pipelines and related accessories.
• Is air present in the steam system ? What are its effects and how to remove it ?
When the steam is turned off during shutdown, it condenses in the pipework and therefore a vacuum situation develops, causing air to be drawn into the steam distribution pipes and heat exchange spaces. In addition, nitrogen, oxygen and carbon dioxides are readily absorbed by water exposed to the atmosphere in the boiler feed tank, and when that water is heated in the boiler, these gases are released into the steam distribution system. Because of this there is air in the steam system.
Due to presence of air first the enthalpy of evaporation is reduced, because in a mixture of gases, each of them exerts a partial pressure and the sum of these partial pressures makes up the total pressure. According to this the steam mixed with air will have a lower pressure than without air in the pipes. The pressure of the steam determines its temperature, so also the temperature will be less and the air will reduce the usable heat in the system.
Secondly air may lead to steam trap malfunction. In some cases, air can temporarily isolate a steam trap from condensate which needs to be discharged, simply because the trap is unable to release air quickly. A large volume of air may need to be handled in such a short time that it can form a compressed column in the condensation pipe leading to the trap. Temporary air binding then occurs and the trap cannot function.
And lastly, air occupies the steam heating space and acts as an insulator to heating. Thus heat transfer rates are drastically reduced.
In order to remove the gases, Thermostatic Air Vents should be fitted to process equipment and to the steam distribution pipework, particularly at the steam trapping points at the terminal ends of steam mains and steam branch lines.