The 900 psig sulfite ceiling is a TDS budget limit, not a thermodynamic one

At what boiler pressure does sodium sulfite stop working as an oxygen scavenger?

The U.S. Department of Defense’s UFC 3-230-13 (March 2023) draws the line at 900 psig (6,205 kPa). Above that pressure, Section 4-2.6.2.2 lists only specialty volatile oxygen scavengers: hydroxylamine, hydroquinone, carbohydrazide (CAS 497-18-7), hydrazine sulfate and erythorbic acid for oxygen control. Sodium sulfite is absent from the high-pressure list. Between 600 and 900 psig, the same manual (Section 4-3.8) permits either sulfite or the volatile alternatives, making that bracket a judgment call.

The number everyone quotes, 900 psig, is real, but the mechanism behind it is dissolved solids, not a sudden failure of sulfite’s chemistry.

Why sulfite adds solids and why that matters more at high pressure

Every mole of sodium sulfite that reacts with dissolved oxygen becomes sodium sulfate. The sulfate is non-volatile. It stays in the drum, adding to total dissolved solids. At low pressures and generous blowdown budgets, the extra TDS is manageable. As operating pressure rises, two things tighten simultaneously: ASME and ABMA feedwater guidelines shrink allowable TDS in the drum, and the energy penalty per unit of blowdown grows because each kilogram of water vented carries more enthalpy.

The result is a squeeze. A 400 psig firetube boiler can blow down freely enough to flush sulfate without meaningful cost. A 900 psig watertube boiler running the same sulfite program may find its blowdown rate climbing to hold TDS inside the guideline, wasting treated water and fuel.

There is a second pathway that catches buyers who think they cleared the pressure gate. If the boiler feeds a desuperheater (attemperator), any sulfite residual or sulfate in the spray water carries non-volatile solids directly into superheated steam. That contaminates turbine blading and process heat exchangers in ways that drum blowdown cannot fix. A deeper treatment of attemperation water as the real decision driver is worth reading alongside this piece.

NPDES permits can make the sulfate problem a compliance issue, not only an efficiency issue. Boiler blowdown discharged to surface water falls under 40 CFR Part 136 monitoring requirements. The 2021 CWA Methods Update Rule (86 FR 27226) updated the analytical methods used for discharge monitoring under those permits. If a facility’s NPDES permit carries a sulfate limit, every pound of sulfite fed to the boiler eventually reports as sulfate in the blowdown stream.

How does carbohydrazide differ from sulfite in the boiler?

The functional difference is volatility. Carbohydrazide (CH₆N₄O, molecular weight 90.09 g/mol, CAS 497-18-7) decomposes at boiler temperatures into hydrazine and CO₂. The hydrazine then scavenges oxygen and breaks down further into nitrogen and water. Every end product is volatile, so nothing stays behind to inflate TDS.

A patent background statement reports that carbohydrazide decomposes to form hydrazine and carbon dioxide at temperatures above 360 °F (US 4,681,737, 1987, background section). A separate 2024 study (Sipilä et al., Corrosion Science 240, 112476) reports that above 200 °C carbohydrazide decomposes in aqueous solution to ammonia, nitrogen, hydrogen and CO₂. The ammonia is operationally significant: it raises condensate pH, which benefits carbon-steel return lines but can accelerate attack on copper alloys.

The practical takeaway: at high-pressure boiler temperatures, carbohydrazide does not persist as itself. It works through its decomposition products, and those products leave with the steam.

UFC 3-230-13 states that for boilers over 900 psig, these volatile scavengers are maintained in the parts-per-billion range in boiler water (Section 4-2.6.2.2, p. 65). The 2019 CWA Methods Update Rule (84 FR 56750) established the prior-cycle analytical framework that facilities use to benchmark discharge monitoring for boiler blowdown constituents.

In a field trial at a 67 barg boiler, carbohydrazide was injected at 0.5 ppm at the deaerator outlet, targeting a feedwater residual of 20–40 ppb. Residual hydrazine was specified at around 20 ppb in feed water. Measured values were inconsistent and most frequently zero within limits of detection (Rahman et al., Water Resources and Industry 29, 100212, 2023, Section 2 and Section 3.2). That result is orders of magnitude below typical sulfite feed rates, which reinforces the cost comparison: carbohydrazide is more expensive per pound, but the feed rate is far lower, and the avoided blowdown recovers energy and water.

Scavenger comparison by pressure bracket

The 600–900 psig bracket is where the real trade-off lives. A boiler with high-purity makeup, excellent condensate return and no attemperation duty may run sulfite to the upper end of that range without TDS trouble. A boiler with poor condensate return, high makeup rates or a desuperheater may need to switch well below 900 psig. The pressure number is a proxy. The variable it proxies for is the dissolved-solids budget.

What should a buyer evaluate when switching scavengers?

Start with the CoA. Spec carbohydrazide by active-ingredient percentage, not by trade name or total weight, to compare quotes on an equivalent basis. A quote priced per kilogram of solution at one active-ingredient concentration is not comparable to one at a different concentration without adjusting.

Verify that the supplier’s CoA states assay by a method you can reproduce or cross-check. Request at least three consecutive lot CoAs before qualifying. If assay drifts outside the tolerance band stated on the initial sample, flag it before the material enters the feed system.

For import or REACH-jurisdiction purchases, confirm the CAS number matches your filing. Buyers should verify the current REACH registration dossier for carbohydrazide (EC 207-837-2) on the ECHA website to confirm that boiler treatment is a registered use under their jurisdiction. A CAS mismatch between the CoA and an import filing will hold the lot at documentation review.

One limitation worth naming: carbohydrazide decomposes to hydrazine in the boiler. Hydrazine is a suspected carcinogen with strict handling requirements. The concentrations involved in a properly run feedwater program are in the parts-per-billion range, but the safety data sheet and site EHS review must account for the hydrazine intermediate, not only the parent compound. Plants that eliminated hydrazine for safety reasons and switched to carbohydrazide should understand that the chemistry still passes through hydrazine inside the boiler. The benefit is that the operator never handles hydrazine directly.

Whether the 600–900 psig bracket justifies a scavenger switch at your plant depends on one ratio: the cost of incremental blowdown (water, fuel, discharge treatment) against the cost of a volatile scavenger program at the feed rate the application requires. That ratio is site-specific, and the number that sets it is your current blowdown percentage, not the drum nameplate.

Request pricing and specification

Request current pricing, availability, a specification sheet and lot-sample CoA for carbohydrazide or sodium sulfite anhydrous technical granular through the RawSource product pages linked here. Specify the active-ingredient percentage and packaging format your water treatment program requires.

Related on RawSource

• Carbohydrazide product page: sourcing, grades and CoA request for the primary volatile oxygen scavenger discussed in this article.
• Sodium sulfite anhydrous technical granular: pricing and specification for the low-to-mid-pressure scavenger covered here.
• Sodium sulfate product page: relevant as the non-volatile oxidation product that drives TDS accumulation in sulfite-treated boilers.
• Water treatment industry page: broader context for boiler chemistry sourcing through RawSource.
• Attemperation water: not pressure alone decides when to switch scavengers: companion article on the desuperheater pathway that makes the switch necessary below 900 psig.