Steam quality affects the energy available for heat transfer. That is why controlling quality is critical for process and industrial efficiency.
In this article, we will explore which factors influence the condition of steam, the challenges poorly controlled steam can cause and how to improve the quality of steam you generate.
What is the ideal condition of steam?
The ideal steam to use in your industrial processes is clean, dry and free from air and other non-condensable gases. Steam in this condition is the most efficient type for heat transfer1. The steam tables that help you manage the generation and distribution of steam also assume you produce steam with these qualities.
To optimise the quality of steam2, you need to monitor the conditions in your boiler house, ensure regular maintenance of steam distribution pipes and implement controls around your plant. Without these good working practices, you will likely face engineering and maintenance problems.
What are the problems of wet steam?
While steam should be completely dry, there will inevitably be a level of moisture in the steam produced by the boiler house. The moisture within steam is measured by its dryness fraction, the proportion of steam that is dry.
For example, in the steam phase diagram, a dryness fraction of 50% (A) means the steam is 50% dry and 50% wet.
When you produce steam without controlling its quality, you can have too much moisture or a poor dryness fraction. That is when problems occur, as the steam you produce and distribute is wetter than your plant has been engineered for.
Process inefficiency
Wet steam reduces the efficiency of heat transfer in several ways. Firstly, the wetter the steam, the less useful energy is available within the steam. There is a reduced mass of steam arriving at the process due to the low dryness fraction.
In addition, the heat exchangers in your processes are sized to operate with an expected dryness of steam. However, if you produce steam that is wetter than planned, the heat exchangers will not work as efficiently. When steam reaches the heat exchangers, it will take longer to get the necessary heat energy from the steam. The process time is likely to extend, you may not meet the required process temperature, and the product could be spoiled.
Wet steam is also more likely to condense and give up energy to the distribution pipework. For example, you could produce 10,000kg/hr of steam but lose 500kg/hr to the pipework. As a result, you have to generate more steam to deliver the required level of heat energy to the processes.
In other words, wet steam is less efficient for heat transfer.
Erosion and corrosion
Moisture within steam can erode pipes. As the pressure of steam carries moisture droplets along the pipes at high speeds, the friction of the droplets against the pipes causes damage over time.
The presence of wet steam also increases the risk of corrosion. The chemical reaction of the moisture on the metal pipes degrades the pipes and reduces their usable life.
Erosion and corrosion both increase the required maintenance for the steam distribution pipes.
Water slug and water hammer
When wet steam condensates in the steam pipes, the liquid falls to the bottom of the pipe. As the droplets of condensate increase, they form a slug of water, which is pushed through the pipes by the movement of the steam.
However, when there is a bend in the pipe, the impact of water hitting the pipe can cause noise and vibration, known as water hammer. The damage caused by the water hammer means extra maintenance work is necessary and could limit the life of pipework.
How to control the dryness of steam
There are different controls you can use to reduce the moisture within steam and minimise the dryness fraction.
Steam separators
Steam separators remove moisture droplets from within the steam flow and are used before steam enters critical equipment in the plant.
The separators use baffles to prevent water droplets from passing through while dry steam is allowed to continue flowing through the pipe. The water droplets and any condensate then drain from the system.
Steam traps
Steam traps remove condensate from steam pipes without releasing steam. Using steam traps improves safety, reduces maintenance requirements and improves steam quality.
There are multiple types and sizes of steam traps and it is important to choose the most suitable traps for your plant. They need to be sized accurately so they work efficiently. If steam traps are not working correctly, you will lose a significant amount of energy.
Why does steam need to be clean?
Scale that builds up on pipe walls and within the heat exchanger affects the quality of steam and reduces its efficiency for heat transfer. Dirt can also be left behind from excess jointing or welding material, which leads to erosion within pipes, steam traps and valves.
The scale can occur due to incorrect water treatment or boiler operation. These can cause impurities in the boiler to be transferred through moisture droplets.
How to keep steam clean
Fitting pipeline strainers removes dirt and prevents it from contaminating the steam. Good engineering practice is to install a strainer before steam traps, valves and other critical equipment.
Boiler controls are also essential to remove chemicals and minerals in water that build up as solids within the boiler. A bottom blowdown valve within the boiler improves the quality of the water and the steam. Opening the valve allows the pressure of the steam in the boiler to blow the sediment away.
What problems does air within steam cause?
Steam supply pipes contain air when started up, as air is drawn in by the vacuum created when the system was previously shut down and steam condensed. Air can also be present in the water used to feed the boiler. Unless you remove the air and other non-condensable gases though, they will lower the temperature of the steam.
As air is a poor conductor of heat energy, a mass of air within the steam will also act as a barrier in the heat exchanger, reducing the efficiency of the heat transfer process.
What problems does air within steam cause?
Steam supply pipes contain air when started up, as air is drawn in by the vacuum created when the system was previously shut down and steam condensed. Air can also be present in the water used to feed the boiler. Unless you remove the air and other non-condensable gases though, they will lower the temperature of the steam.
As air is a poor conductor of heat energy, a mass of air within the steam will also act as a barrier in the heat exchanger, reducing the efficiency of the heat transfer process.
How to remove air from steam
When you fill the system with steam, it will force air towards the drain points and the remote points, those furthest away from the steam inlet. Adding steam traps to the drain points and air vents to remote points allows you to remove air from the system.
Generating the best quality steam
To maximise the efficiency of using steam for heat transfer, you need to produce steam in an ideal condition. That means ensuring it is as dry as possible to optimise the useful energy available for heat transfer. Keeping the steam clean and minimising moisture will reduce maintenance problems in the steam pipes. Removing air from the steam pipes also helps to improve the efficiency of heat transfer.
Learn more about steam
1 Heat transfer - https://www.spiraxsarco.com/learn-about-steam/steam-engineering-principles-and-heat-transfer/heat-transfer
2 Steam quality - https://www.spiraxsarco.com/learn-about-steam/steam-engineering-principles-and-heat-transfer/steam-quality