So, what is Internal Treatment for in High-Pressure Boilers?
By the time water enters the boiler, it has passed through a series of unit operations. Clarification, demineralization and deaeration have removed suspended solids, dissolved solids and residual oxygen. Deposition, fouling and oxygen corrosion are already under control before the water enters the boiler. Microbiology isn't a risk at this stage, and conditioning of the steam and condensate lines takes care of the other end of the cycle.
If the main problems have already been solved before the water reaches the boiler, what's still left to do?
In this type of boiler, with ultrapure water, Internal Treatment serves to control corrosion. It adjusts the boiler's chemistry so it operates in the range of lowest corrosion rate. The ASME consensus for high-pressure industrial boilers, across the three pressure ranges (901 to 1000 psig, 1001 to 1500 psig and 1501 to 2000 psig), specifies a feedwater pH range of 8.8 to 9.6.
The strategies to reach this objective vary from program to program and depend on the specific risks of each boiler. The process of developing these programs is in constant evolution.
From Caustic Corrosion to Equilibrium Phosphate
The coordinated phosphate program was created by Purcell and Whirl in 1942 to solve caustic corrosion. Even so, this type of corrosion still occurred under certain conditions, and the program evolved into congruent phosphate, created by Marcy and Halstead in 1964. This aimed to reduce the risk of free OH in boiler water.
With caustic corrosion solved, and with the emergence of higher-pressure boilers, the phenomenon of Hideout came to light, leading to the development of equilibrium phosphate, or EPT.
There are other Internal Treatment programs for high-pressure boilers, with variable efficiency in controlling scaling under contingency conditions. What matters is that each step in boiler water treatment has a specific objective, and the failure of one can sometimes be compensated by the next step, but the cost always increases.
If your plant is operating below expected results and the root cause still isn't clear, that's usually a sign that a specific treatment step is out of specification. Identifying which step is the source is often faster and cheaper than it seems.
In the coming parts of this series, we cover the Coordinated Phosphate program and its evolution.