3.4 Feedwater Systems, Deaerators, Condensate Return & Boiler Water Treatment

Feedwater Systems, Deaerators, Condensate Return & Boiler Water Treatment

Core Trade Concept: Module 34201 covers boiler systems, not just the pressure parts. Nearly every boiler tube failure a boilermaker is called to repair — oxygen pitting, caustic gouging, hydrogen damage, scale-driven overheating — is a water chemistry failure that arrived through the feedwater train. Understanding the train tells you why the tube failed, which is the difference between replacing a tube and fixing a problem.

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1. The Feedwater Train, End to End

   STEAM to process/turbine
        |
   [ CONDENSER / PROCESS ]
        |  condensate
   [ CONDENSATE RECEIVER ] --- makeup water (softened / demineralized)
        |
   [ DEAERATOR (DA) ]  <-- pegging steam; removes O2 and CO2
        | DA storage section (elevated for NPSH)
   [ BOILER FEED PUMPS ]  multistage centrifugal, min-flow recirc
        |
   [ FEEDWATER REGULATOR / CONTROL VALVE ]
        |
   [ ECONOMIZER ]  <-- final flue-gas heat pickup
        |
   [ STEAM DRUM ]  via feedwater distribution pipe

Two arrangement details are commonly tested:

• The deaerator storage tank is mounted high — typically well above the boiler feed pump suction — because DA water sits at saturation temperature. Any drop in suction pressure flashes it to steam and the pump cavitates. That elevation is the net positive suction head (NPSH) margin.
• A minimum-flow recirculation line runs from the pump discharge back to the DA. A boiler feed pump operating at or near shutoff head with no flow will overheat and destroy itself in minutes.

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2. The Deaerator: Mechanical Oxygen Removal

Dissolved oxygen is the single most aggressive corrodent in a boiler system. The deaerator removes it physically, using Henry's Law: the solubility of a gas in water falls toward zero as the water approaches its saturation temperature at the operating pressure.

• Typical operation is around 5 psig, which corresponds to roughly 227°F saturation temperature.
• A properly operating deaerator delivers residual dissolved oxygen at or below 0.005 cc/L (about 7 parts per billion) — the standard performance guarantee for this equipment.
• The vent must stay open. A deaerator with the vent throttled shut cannot expel the liberated gas, and it silently stops deaerating while all pressure and temperature indications look normal. A visible plume at the vent is a working DA.

Chemical scavenging finishes the job

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3. Internal Treatment and Blowdown Control

Once water is in the boiler, treatment has three jobs: keep hardness from forming hard scale, hold pH in the passive range, and keep dissolved solids below carryover limits.

• Phosphate treatment precipitates residual calcium as a soft, non-adherent sludge that leaves with the bottom blowdown, and buffers pH. Coordinated and congruent phosphate programs constrain the sodium-to-phosphate ratio specifically to prevent free caustic from concentrating under deposits.
• All-volatile treatment (AVT) — ammonia plus a volatile scavenger, no solids — is used on high-pressure drum units and is mandatory on once-through supercritical boilers, which have no drum and therefore no blowdown path to remove solids at all.
• Cycles of concentration links blowdown rate to water quality:

$\text{Cycles} = \frac{\text{Boiler water TDS}}{\text{Feedwater TDS}} \qquad \%\ \text{Blowdown} = \frac{100}{\text{Cycles}}$

Worked example. Feedwater at 40 ppm TDS, boiler water limit 2,000 ppm TDS:

$\text{Cycles} = \frac{2000}{40} = 50 \qquad \%\ \text{Blowdown} = \frac{100}{50} = 2\%$

Two percent of steaming rate must leave as blowdown. Reducing blowdown below that point drives boiler water TDS above the limit and starts carryover; raising it far above wastes fuel with every pound of hot water dumped.

• Silica gets its own limit because silica is volatile at high pressure: it leaves the drum with the steam rather than staying in the water, and deposits as glass-hard scale on turbine blades. Permitted boiler-water silica falls steeply as drum pressure rises.

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4. Reading a Failure Back to the Chemistry

This table is why a boilermaker on an outage should always ask what the water chemistry logs show before assuming the tube simply "wore out."

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5. What the Boilermaker Actually Builds and Repairs

• Deaerator vessels crack. DA storage tanks and the head/shell weld seams are a well-known cracking location because of corrosion fatigue in a hot, cycling, two-phase environment. Wet fluorescent magnetic particle inspection of DA welds during outages, and weld repair to NBIC rules, is standard boilermaker scope.
• Economizer tube and header replacement, including cold-end corrosion damage covered in Section 3.3.
• Feedwater piping and the drum feedwater distribution pipe, which must be reinstalled with the correct orientation so cold feedwater is dispersed rather than jetted at the hot drum wall.
• Condensate receiver and DA piping, where oxygen in-leakage on the vacuum side of the system is a chronic problem.

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6. Realistic Trade Scenario: Pitting in a Reinstalled Economizer

A plant replaces an economizer bank during a two-week outage. Six months later boroscope inspection finds fresh oxygen pitting on the inlet rows.

The crew leader walks it back through the train rather than blaming the tube supplier:

1. Was the bank wet-layup or dry-layup during the remaining outage? An economizer left full of untreated, aerated water for ten days will pit before it is ever fired.
2. Is the DA vent open? Plant operations had throttled the DA vent to reduce steam loss during the low-load restart period.
3. Is the sulfite residual being maintained at the boiler feed pump suction, not just dosed at the DA?
4. Are there air in-leaks on the condensate return? Sub-atmospheric condensate lines pull air in through packing and joints.

The physical repair is straightforward; the value the crew adds is identifying that the throttled DA vent, not the tube material, put the pits there.