Water scale on any heat transfer surface reduces the effectiveness of that heat transfer, resulting in reduced equipment efficiency and increased energy consumption, costs and even plant downtime. Some equipment found in power plants, such as cooling towers, heat exchangers and vacuum pumps, are especially susceptible to water scale.
Because of downtime costs, this “buildup” problem often is either ignored or relegated to repair either at the last minute or when a mechanical failure occurs. This is based on the premise that descaling is time consuming.
Descaling can be done either mechanically or with chemicals. Mechanical cleaning obviously takes a lot of time because this approach cleaning requires equipment to be dismantled. Gaskets and seals must then be replaced, adding to the cost. In addition, mechanical cleaning does not always reach all the scale.
Chemical cleaning can reach scale buildup that isn’t always reachable in mechanical cleaning. However, five major concerns are associated with chemical cleaning:
- Potential corrosive effect on equipment material (the metal, seals and gaskets) in the system being cleaned
- Flushing out the material after use
- Making sure all the scale deposit is actually removed
- Cleaning speed
- Material disposal after use
Dozens of descaling chemicals are on the market; however, some can be prohibitively expensive to use in large applications and others can be weak and slow, leaving the problem of downtime as a major remaining issue. A descaler must be more than just an inhibited acid-many companies supply only that. Inhibited acids tend to be slow and often do not remove all the scale deposits because deposits can consist of calcium combined with rust, silica, oil and whatever else can be in a water system. A descaling chemical should have adequate and high quality detergents capable of removing such deposits. Inhibited acid alone will often not even touch some deposits.
Many products claim low or negligible corrosion rates but often meet that claim simply by supplying a slow and weak, heavily diluted acid. At the other extreme, a relatively good descaler that makes claims of no corrosion at all may be purchased.
These concerns have not prohibited some good products from being offered-several good descalers are on the market. A good chemical descaler should include these attributes:
- Low corrosion rates. The descaler removes a good amount of scale, but with low corrosion rates that are verifiable and it does not harm seals and gaskets.
- Easily rinsed out. The descaler is free rinsing and does not need to be neutralized in the system after use because a quick flush with water will get rid of the product, leaving no residual. (The pH levels should still always be checked after use to make sure the residue meets disposal standards prior to disposal in a sewer system.)
- Cleans more than just scale. The descaler should contain ingredients to remove oil rust and loosen silica, and anything else that often is mixed with the scale deposits.
- Fast to use and fast cleaning. The major descaler criterion for many companies is minimal downtime. Speed is important, so the descaler must have the right mix of acids and detergents to clean scale, and anything else mixed with it, quickly.
- Easy disposal. Any spent material should be easy to dispose of in a sewer system, meaning all ingredients must be biodegradable, corrosion rates must be low enough to avoid excess metal loss after use and the pH after use should be above the standard for disposal or if not, easily neutralized.