The presence of steam at sufficient pressure in a facility does not mean that the process receives steam of the right quality. Water droplets in the steam, non-condensable gases, dissolved substances carried from the boiler water and pressure fluctuations can affect heat transfer and product quality.
Steam quality; It is evaluated by the pressure and temperature of the steam, as well as its dryness fraction, purity and stability at the point of use. This issue is particularly important in food, pharmaceutical, textile, sterilization and precision heating processes.
What is saturated steam?
Saturated steam is steam at the boiling temperature corresponding to its pressure. When it loses heat, it begins to condense and transfers its latent heat to the process. It is preferred in many industrial heating applications because it provides high heat transfer at close to constant temperature.
When the pressure changes, the saturation temperature also changes. It is therefore possible to control the process temperature via vapor pressure; However, line pressure losses should be taken into account.
What is superheated steam?
Superheated steam is steam heated above its saturation temperature at the same pressure. It does not contain a liquid phase and its temperature can vary within a certain range, regardless of pressure.
Superheated steam is advantageous in turbine and some high temperature processes. On the other hand, in exchangers where condensing heat transfer is desired, the heat transfer behavior during desuperheating may differ from saturated steam. The approach of “it is better in every process because it is hotter” is not correct.
Wet steam and dryness ratio
Wet steam contains liquid water droplets that are carried along with the steam. Dryness ratio expresses the ratio of the vapor mass in the mixture to the total mass. Completely dry saturated steam theoretically has a dryness value of 1 or 100 percent.
Main consequences of wet steam:
- The usable energy carried by one kilogram of fluid decreases,
- Heat transfer and process time may vary,
- Erosion may occur in control valves and pipes,
- The risk of water hammer may increase,
- The measured flow rate and the actual amount of energy may not match,
- Quality problems may occur in products with direct steam contact.
Why does steam get wet?
Boiler water drift
Sudden load increase, high water level, high total dissolved solids, foaming, or insufficient steam separation space may cause boiler water to carry over with steam.
Line heat loss
As steam moves through an uninsulated or long line, it loses heat and condensate forms. If drainage is inadequate, this water is carried to the process with the flow.
Incorrect piping
The branch from the lower elevation, insufficient slope, small condensate pocket or defective trap may re-wet the dry steam.
Pressure changes
Pressure reduction and load fluctuations change the thermodynamic state of the steam. Station design should not only consist of valve selection.
What does a steam separator do?
The separator separates the water droplets in the steam flow by changing direction and speed or by centrifugal effect. The separated condensate is discharged with a suitable trap. The separator alone does not solve all quality problems; Boiler water drift and line drainage should also be corrected.
The separator can be used especially at the main line entrance, before the pressure reducing valve and in processes requiring dry steam. Diameter and type should be selected according to flow rate and working range.
Effect of non-condensable gases
Gases such as air and carbon dioxide can form a film on the heat transfer surface, making it difficult for steam to reach the surface. Dissolving carbon dioxide in condensate can increase corrosion. Deaerator helps reduce dissolved gases in the feed water; The air in the lines should be removed with appropriate air discharge elements.
Are clean steam and plant steam the same?
No. Plant steam is often produced with standard boiler water chemistry and used in indirect heating. In clean or pure steam applications, production material, feed water, additive chemicals and hygiene conditions are more stringent. If there is direct contact with food or medicine, the legislation and quality standards of the process should be evaluated separately.
How to improve steam quality?
- Boiler capacity and loading rate are selected correctly.
- Water level and total dissolved matter are kept under control.
- Water conditioning is managed by regular analysis.
- Lines are insulated, slope and drainage points are corrected.
- Traps are tested regularly.
- Separators and air discharge are used where necessary.
- Pressure reducing stations and branches are designed correctly.
- Dryness is verified by measuring flow rate and process performance.
How to choose the right type of steam for the process?
Saturated steam is a powerful heat carrier in most heat exchanger, cooking, drying and heating applications. Superheated steam can be used in turbines and in certain processes where drying or condensation is not desired. In selection, target temperature, pressure, heat transfer surface, product sensitivity and control method should be considered together.
When choosing a high pressure steam boiler, not only the pressure but also the quality of the steam to be delivered to the process and the distribution system should be defined.
Frequently asked questions
Are dry steam and superheated steam the same?
No. Dry saturated vapor is in a state of saturation with no liquid droplets. Superheated steam is above the saturation temperature at the same pressure.
If the vapor pressure is high, is the quality also high?
No. High pressure steam; It may be of poor quality due to water entrainment, poor drainage or contamination.
Does using a separator completely prevent water hammer?
No. The separator separates the droplets; Line slope, condensate pockets, traps and controlled commissioning must also be correct.
How is steam dryness measured?
Calorimetric or special measurement methods appropriate to the application can be used. The measurement point and sampling order directly affect the result; Expert evaluation is required.
Technical resources
- T.R. Ministry of Energy and Natural Resources, Steam and Condensate Systems training materials.
- International steam systems technical resources, steam dryness and separator application notes.