The passive layer protects hydrocarbon and power plant equipment from corrosion and all the devastating effects that corrosion can cause. Cycle chemistry is critical to maintaining and protecting your passive layer – and your equipment.
All metals will form a protective passive layer. The passive layer protects equipment and piping from corrosion and the devastating effects that come with it. Thermodynamics causes these protective layers to form. Cycle chemistry determines the type of protective layer formed and protects a passive, protective layer on critical surfaces of equipment directly involved with condensate, feedwater and other aspects of the steam/water cycle.
Any time you move away from what’s considered normal water chemistry for your unit, you’re rolling the dice with the fate of your critical equipment.
There are four main factors that could negatively affect your passive layer:
- Mechanical - Any elements that disrupt fluid flow – such as elbows, reducers, bends, expanders – can cause pools or eddies, leading to highly dangerous Flow Accelerated Corrosion (FAC).
- Straining of the substrate material - Differences in thermal expansion caused by rapid heating or cooling can cause thermal stress on your equipment.
- Contamination - Sodium, chlorides and sulfates are destructive contaminants that can enter the cycle and, when coupled with dissolved oxygen, damage the protective layer.
- pH changes - The protective layer will be dissolved when pH decreases to 7.0 or less.
Protect your passive layer 24/7
Cycle water chemistry control helps you develop the proper protective layer on hydrocarbon processing equipment and steam and water piping, while continuing to protect the passive layer during plant operation and shutdown.
1. Install automated systems.
Automated chemistry control systems help control pH over a wide range of plant operating conditions to prevent corrosion, leading to deposition in critical equipment.
2. Install key instrumentation to warn you of conditions that can start damaging the protective layer.
- In Condensate Equipment:
- Cation conductivity to detect contamination
- Sodium analyzer to verify cycle contamination from sodium salts and potentially sodium hydroxide
- Specific conductivity analyzer to control pH
- In Feedwater Equipment:
- Dissolved oxygen analyzer to determine if the deaerator is working properly or to control oxygen feed
- pH analyzer - a pH below 7.0 will immediately start damaging the protective passive layer
- Cation conductivity to ensure water going to boiler is not contaminated and checks the condensate cation conductivity indicators
3. Monitor the top three contaminants that can damage the protective layer:
- Sodium analyzers – Sodium hydroxide can lead to stress corrosion cracking of carbon steel. Sodium analyzer will also confirm in conjunction with cation conductivity if contamination is due to salts like sodium chloride or sodium sulfate have entered the cycle.
- Chloride analyzer – Detects chlorides in the cycle directly.
- Sulfate analyzers – Detects sulfates directly in the cycle.
4. Consider the composition of condensate, feedwater and heater drains.
The metallurgy of the condensate feedwater and heater drains will determine the type of chemistry that will be used on these systems.
- For systems made of carbon steel and/or stainless steel, the chemistry of choice would be oxygenated feedwater treatment (OT) or all volatile treatment oxidizing (AVT O). These will feed a chemical for pH control (typically ammonium hydroxide) and either air or dissolved oxygen to ensure the oxidation reduction potential of the cycle is kept positive.
- For systems with copper or copper alloys, the chemistry of choice is all volatile treatment reducing (AVT R). Typically, ammonia will be added for pH control and an oxygen scavenger will be added to remove trace amounts of dissolved oxygen.
5. Monitor the passive layer on the condensate feedwater and heater drains by determining the corrosion rates using a corrosion product monitor.
6. Add chemicals to elevate pH.
7. Monitor steam contamination.
Sodium, chlorides and sulfates when entering the steam circuits can lead to turbine blade failures due to stress corrosion cracking and corrosion fatigue. These contaminants will cause pitting of turbine blades when the unit is not operating. These pits can turn into cracks as the turbines are operating and stress is applied to turbine blades. In addition, silica contamination will deposit on the back end of the LP turbine, causing efficiency loss of the LP turbine. Silica needs to be monitored with silica analyzers to maintain levels below 10 ppb in steam.
8. Implement Flow Accelerated Corrosion (FAC) Control
To prevent FAC:
- Maintain proper chemistry
- Monitor corrosion product transport
- Conduct regular piping and equipment inspections
- Replace damaged area with chrome containing material
- Pay attention to the equipment to what mother nature is showing you
Don’t roll the dice with the fate of your power generation equipment. Install cycle chemistry instrumentation with proper sample conditioning to help prevent, recognize and minimize damage to the passive layer.