When and why to use steam vs. air blowing is not always easy to tell. Both are good methods and have their use cases. Generally, air blowing is a less stressful method of cleaning than steam blowing, but it doesn’t do quite as thorough a job. Usually, the level of contamination and your target ISO cleanliness will determine which is most appropriate. Let’s dive into specifics though:
Steam blowing, one of the first phases of start-up, uses high-temperature and high-velocity steam to create a high drag force on piping surfaces. This force removes debris, grease and mill scale that forms during the milling, fabrication, and construction of piping and equipment. To remove mill scale the steam must be at a high pressure (usually 300 PSI) and temperature (usually at least 900 degrees Fahrenheit). At lower temperatures, mill scale will stay in place and can eventually break free and contaminate plant equipment during startup. Some best practices include:
- Keep steam turbine’s turning gear in operation during steam blowing
- Reduce steam temperature by injecting treated water at high velocities into the steam, creating a thermal shock effect that speeds the process — keep steam velocities higher than maximum continuous rating (MCR) at all times
There are two types of steam blowing, the puffing method, and the continuous blowing method.
Puffs of steam are injected into your system, beginning at low pressure and slowly escalating to higher pressures, typically about 150 percent of normal operating procedure. It’s performed in stages, across different parts of the system, clearing out debris and mill scale from each area.
Continuous Blowing Method:
The Continuous blowing method uses a similar process, but pressure is maintained continuously during one long blow to reduce force and stress on your system. It puts less stress on your system, but it delivers a slightly lower standard of cleanliness than the puffing method. Depending on the sturdiness and cleanliness requirements of your system, either method can be a good fit.
Air blowing is a similar method in which high-velocity air is pumped throughout your system. In terms of when to use it, air blowing works in great combination with chemical cleaning, such as in power plant systems. The chemical cleaning solution will dissolve mill scale, and then the non-soluble particles that have been dislodged during cleaning can be removed via air blowing. This process puts less direct stress on your system than steam blowing does, so if a chemical clean will be sufficient to remove mill scale, it’s easier on your system to combine it with air blowing.
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With over 35 years of experience performing steam and air blowing in plants, we are always happy to help you determine the best methods of cleaning or pre-commissioning your systems.