Top 5 Questions on Boiler and HRSG Cycle Chemistry Basics
Tips to Develop a Reliable Steam Cycle Chemistry Program
Selection and proper application of an appropriate cycle chemistry program are critical in developing short-term availability and long-term reliability of steam generators. The key to minimizing corrosion and deposition is a cycle chemistry program based on the purity of makeup water the system receives, and the maximum operating pressure and temperature of the steam generators.
In a recent webinar, I covered the basics of boiler and heat recovery steam generator cycle chemistry. Fellow industry leaders (and HDR teammates) Dan Sampson and John Schubert joined me at the tail end of the webinar to answer attendee questions. There were many insightful questions, so we’ve pulled together our top five to share.
1. What type of online corrosion product testing would you recommend?
To some extent, choosing the best online analyzer to monitor corrosion product transport depends on the configuration of existing sampling systems. There are two general instrument types: low-level turbidity meters and particle analyzers. Each has advantages and disadvantages.
While a host of manufacturers can provide instruments, determining the best instrument is both application and location specific. An engineering consultant can help determine the best instrument for your specific situation.
2. For HRSGs, what are your thoughts on the use of film-forming products versus a more traditional treatment program?
First, let’s discuss how each works. Traditional condensate treatment programs focus on raising pH and neutralizing carbon dioxide using ammonia, an amine blend, or an ammonia-amine blend. High pH minimizes the corrosion of mild steel exposed to treated water. FFP are intended to establish and maintain a water-repellant barrier, preventing any water contact with the metal.
FFP fill a niche and are particularly effective in minimizing flow-accelerated corrosion. However, FFP aren’t always needed. It's important that a facility has a robust corrosion product monitoring program to provide a baseline against which to judge the success or failure of a filming amine test. Filming amines are often used in conjunction with, rather than replacement of, traditional neutralizing amine chemistries.
3. For oxygenated treatment, would degassed cation conductivity be required instead of just conductivity after cation exchange?
Dissolved oxygen itself has no impact on CACE. DGCC would be preferred if the oxygen for OT is added via regular air. Most OEMs prefer DGCC if OT is used simply because the purity requirements are so stringent. So, even if it is not required, DGCC is a good idea for plants that use OT.
4. What is the best feed location for FFP prior to a long outage for high-pressure systems running all-volatile treatment?
The best feed location will depend on the location of the most at-risk portion of the steam cycle. For example, injection into the crossover from the intermediate pressure to the low pressure steam turbine would provide optimum protection of the LP steam turbine and the condenser. Injection into the condensate pump discharge would provide optimum protection of the condensate, economizer and feedwater piping.
Most FFP are volatile, so they will contact all steam cycle surfaces at some concentration. The optimum dosage and the optimum feed points can best be determined experimentally by examining corrosion product transport without the filming agent for a few startups, then repeat with the filming agent. Similarly, feed locations can be changed and the iron transport data evaluated. Compare the data to determine the net reduction in corrosion product transport based on the product dosage and the product feed location.
5. You mentioned a pH range of 8.0-8.5 for OT systems. Is there a problem with running at a slightly higher pH, say around 9?
No. The OT programs can run at lower pH, but there's no concern with operating at higher pH. We have experience with some plants using OT that operate with a feedwater pH in the 9.6-10.0 range.
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