Boiler Efficiency: The measure of how much combustion energy is converted to steam energy.
Steam Quality: The measure of how much liquid is present in the steam produced.
There are huge benefits to using steam to transfer heat. First, there is a large amount of heat released when steam condenses into water. With condensation as high as 1,000 BTU per pound, it doesn’t take much steam to carry a significant amount of energy. In addition, steam is safe, nontoxic and nonflammable, and it has the ability to deliver heat at a constant, controlled temperature.
Steam is also beneficial because it can be delivered by conventional piping and valve equipment that is readily available, inexpensive, long lasting and easy to maintain. Other heat delivery and distribution systems cannot compare in price and are not 100-percent recyclable like steam.
Despite these advantages, steam users also have to deal with system safety problems, including premature equipment failures and low steam-system efficiency. Some examples of specific problems include repeated boiler shutdowns which occur from:
- Low-water level
- Damaged steam pipes
- Damaged valves due to hammer vibration
- Overloaded steam traps
What Are the Risks of Low Steam Quality?
Low steam quality is usually the cause of problems associated with repeated boiler shutdowns. Steam quality is determined by the amount of liquid in the steam. High-velocity steam that contains water droplets can be as grainy as sand particles, which can wear away at valve seats. Water allowed to collect in steam pipes will eventually be picked up by the high-velocity steam and sped up to near-steam velocity, which will increase the chances of it running into elbows, tees and valves. These collisions can lead to erosion, vibration and water hammer.
How to Improve Steam Quality
Again, steam quality does not depend on the efficiency of the boiler, but on the ability of the steam to separate from the boiling water without carrying liquid particles with it throughout the entire scope of boiler operations. Studies of internal boiler operations, using video cameras, show that there are several recommendations for preventing poor quality steam:
- Make sure that steam demand does not exceed boiler capacity by controlling steam usage.
- Regulate changes in steam usage to make sure that quick changes in steam demand will not decrease steam quality.
- Use modulating instead of on/off valves at steam use points to modify steam demand and steam usage.
- Add boiler feedwater with modulating instead of on/off controls.
- Use TDS controls in place of time-based blowdown.
- Operate the boiler close to its maximum design pressure.
Reductions in steam quality can be dramatic if any of these recommendations are not followed. A reduction in steam quality can damage steam equipment, control valves and heat exchangers, resulting in shortened equipment service life, steam loss, low operating efficiency and even safety problems.
Information for this article was taken from The National Board of Boiler and Pressure Vessel Inspectors. More information can be found at www.nationalboard.org.