Module 2.4 — Feedwater train chemistry for once-through and supercritical units: amine selection, distribution ratios, and why AVT is a feedwater-train story more than a boiler-drum story.
Click any stage to see why amine behavior matters here. With no drum to buffer or blow down, every gram of amine fed at the front end has to do its job all the way to the turbine.
Module 1.2 introduced AVT as the mandatory choice for once-through and supercritical units because there's no drum to blow down accumulated solids. That structural fact reshapes the whole chemistry conversation: where phosphate and caustic treatment (Modules 2.1–2.3) are fundamentally about managing conditions in a boiler drum, AVT is about managing conditions continuously along the entire feedwater and steam path, since that whole path is a single, uninterrupted flow with no vessel to concentrate or remove anything.
AVT programs use ammonia or an amine to control pH. Ammonia is the simplest and most common choice, but different amines (morpholine, cyclohexylamine, and blended proprietary amine products among others) are selected based on a property called the distribution ratio — how much of the chemical stays in the liquid phase versus how much partitions into steam as water flashes to vapor at different points in the cycle. An amine with a distribution ratio close to 1 stays proportionally represented in both phases; one with a very different ratio concentrates unevenly, potentially leaving one part of the cycle under-protected while another runs excess.
| Amine | Distribution Behavior | Typical Use Case |
|---|---|---|
| Ammonia | Volatile, distributes broadly through liquid and vapor phases | Most common baseline choice, simple and inexpensive |
| Morpholine | More liquid-favoring distribution than ammonia | Where more consistent liquid-phase pH control is prioritized |
| Cyclohexylamine | More vapor-favoring distribution | Where protecting condensate/steam-side piping pH is prioritized |
Module 1.2 introduced the distinction: AVT(O) allows a small, deliberate amount of dissolved oxygen to promote a stable oxide layer on all-ferrous feedwater systems, while AVT(R) uses a reducing agent to eliminate oxygen entirely, typically because copper alloys are present somewhere in the feedwater train and are more vulnerable to oxidizing conditions. The practical operating consequence: an AVT(O) program requires tighter control of exactly how much oxygen is present — too little forfeits the protective oxide benefit, too much becomes corrosive in its own right — while AVT(R) requires reliable, continuous reducing agent feed, since any lapse reintroduces the oxygen the whole program is designed around excluding.
Because AVT relies on tight control with no drum to catch problems, cation conductivity (introduced in Module 1.1, expanded on in Module 1.4) becomes the primary early-warning indicator for AVT units specifically. A target of under roughly 0.2 µS/cm is common, and because there's no blowdown to remove contamination once it enters the cycle, any ingress — a condenser leak, a demin upset — has a more direct path to fouling downstream surfaces including the turbine, covered in Track 6.