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Types of Boiler Corrosion

First, there are two groups of boiler oxygen scavengers, organic based and sulfite based. Your boiler steam system will determine the type of boiler oxygen scavenger you use. Regardless of the type of boiler scavenger you require, the purpose and goal of the chemical is the same which is the removal of dissolved oxygen within the boiler water. Dissolved water is extremely corrosive to iron boiler tubes at elevated temperature and pressure. If you have a steam boiler, you must be using a boiler oxygen scavenger.

Types of Boiler Corrosion
If you have recently opened your boiler and have observed corrosion, you will see one of two types of corrosion:
  1. Uniform corrosion on one tube area or over a very large area.
  2. Large corrosive pits.
Uniform corrosion is a sign your pH is low within the steam boiler. You should boast your boiler pH levels to above 10.5. If you have a hot water recirculating boiler, you need to passivate the boiler with a nitrite based chemical. Steam boilers and hot water recirculating boilers are chemically treated much different in regards to oxygen scavengers. Typically, you do not require a oxygen scavenger within a hot water boiler system. In steam boiler systems you always require an oxygen scavenger.

Large corrosive pits in steam boilers is a sign you are not properly removing dissolved oxygen within your boiler system. You must remove your water treatment program.

Troubleshooting
  • Are you using an oxygen scavenger? 95% of low pressure boilers use a sulfite based oxygen scavenger chemical. Check your SDS to ensure you see this chemical on your product formula.
  • Is your pump primed? If you are using a diaphragm designed pump, make sure you are checking that the pump is primed weekly. These pumps lose the ability to pump chemicals if ignored. Also, sulfite will crystalize and may plug the discharge side of your sulfite chemical pump. An easy way to keep your pump primed is to keep the pump below the top levels of the chemical tank.
  • Where and how are you feeding the chemical? The oxygen scavenger should be feed as far back in the boiler system as possible, such as the feed water tank or storage section of the deaerator tank. Also, the chemical should be feed towards the tank center and below the water surface. This can be accomplished using a stainless steel injection quill.
  • Are you doing residual tests? Performing routine boiler tests is important when trouble shooting boiler corrosion. You need to be performing a few basic daily or weekly boiler chemical residual tests. In regards to boiler corrosion, you should be testing your boiler water sulfite residual and pH. For a more complex discussion you can refer to this AWT article.
  • Are you monitoring your chemical inventory? One of the simplest way to double check if your boiler chemical is getting into your system is to verify your chemical levels weekly. Monitoring inventory is just as important as performing chemical testing.
  • Is your boiler water "cycling"? The purpose of a steam boiler is to reuse water. The "cycling" of boiler water refers to the number of times the water is reused. General Example: If your makeup water is 100 umhos, your boiler water should be roughly 2,000 to 3,000 umhos. That is your boiler water is being reused 20 or 30 times (or your blowdown rate is 5% to 3.3%). Blowdown rate is calculated as ((1/cycles) x 100). When you reuse or recycle your boiler water, your oxygen scavenger also "cycles up". If you are not properly operating your steam boiler and for example blowing down at 200 umhos (50% blowdown rate) you are flushing out the chemical. If this is the case, you need to automate your boiler blowdown with a boiler conductivity controller.
  • Do you maintain enough chemical residual when your boiler is off? Dissolved oxygen exists the boiler very rapidly once entering the steam boiler during operation. Residual of the oxygen scavenger in the boiler water means you have no oxygen presence in the boiler water. If you have a low duty boiler, it is very important to check the chemical residual during idle times and well as when the boiler is firing. Also, if you have dual or multiple boilers and you are not dry laying up your backup boiler, make sure your boiler water levels are maintained to keep the top tubes fully submerged in the boiler water.
  • Maintaining good inventory logsheets? You should be using some form of boiler chemical logsheet for testing and monitoring your inventory.
Addressing these simple questions should solve your boiler corrosion problems.
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