Chemical dosing system (boiler water treatment: separate pumps feeding oxygen scavenger, phosphate or caustic and alkali to the feedwater and steam drum)
Boiler water chemical dosing · oxygen scavenger into the feedline, phosphate or caustic into the steam drum, alkali for pH, one metering pump per chemical, dose rate from dissolved oxygen and feedwater flow
A chemical dosing system here is the set of metering pumps, chemical containers and injection lines that treat the water of a steam boiler: an oxygen scavenger into the feedtank or feedline, an alkali to hold the boiler water pH, and on drum boilers phosphate or caustic into the drum, each from its own pump. Ask three things first: which chemicals the water treatment specialist has prescribed and where each must enter, how much of each the feedwater flow and dissolved oxygen call for, and what pressure each injection point is at.
Multi-chemical dosing systems for steam boiler water treatment: oxygen scavenger, alkali, phosphate and caustic feed with their pumps, containers and injection points on the feedtank, feedline and drum. General packaged dosing skids for water and wastewater treatment are covered on the Chemical dosing pump skid page; pH neutralisation systems on the Automated pH monitoring and dosing system page; container sterilisers for beverages on the Beverage container sterilization system page; CIP detergent dosing on its own page.
CNFX editorial research · Sources checked October 12, 2026 · 3 sources
Sulphite dose from dissolved oxygen, and the pH the boiler water must hold
Spirax Sarco's guide to feedtank and feedwater conditioning gives the basis for oxygen scavenging: "the standard dosing rate for sodium sulphite is 8 ppm per 1 ppm of dissolved oxygen", with "an additional 4 ppm to maintain a reserve in the boiler", and notes that liquid catalysed sodium sulphite "contains only 45% sodium sulphite". It explains that heating the feedwater lowers the dissolved oxygen and so the scavenger needed. For pH it gives a typical 1% caustic solution to reach a target of 9 to 11, and cites BS 2486 as recommending pH 10.5-12.0 for shell boilers at 10 bar, with pH 9 only for higher-pressure boilers; deciding the chemical regime is "a matter for a skilled water treatment specialist". [1]
Editorial calculation: with an assumed 2 ppm dissolved oxygen in the feedwater, sulphite needed is 8 × 2 + 4 = 20 ppm, or 20 ÷ 0.45 ≈ 44 ppm of 45% liquid product. For the 7,875 kg/h steam rate in the AMPC case study, treating the same mass of feedwater takes about 44 g/t × 7.875 t/h ≈ 350 g/h of liquid product. The oxygen level is an assumed example. [1][3]
The AMPC case study of a meat processor's water-tube boiler (850-900 kPaG, 7,875 kg/h, about 50% condensate return) describes corrosion pitting in a de-aerator tank that doubled as the feed tank while its steam heating was out of use, and explains that the de-aerator, installed about 10 m above the feed water pumps to avoid cavitation, can run at about 95 °C, where water holds less oxygen. [3]
Give the dosing supplier the boiler type and pressure, steam rate and condensate return, make-up water analysis, the chemical programme from the water treatment specialist (each chemical, strength and target residual), injection points and their pressures, how dosing is paced (fixed, by feedwater flow, by analysis), container sizes and bunding, and alarms.
02
Feedline, feedtank and drum: where each chemical goes in
Spirax Sarco states that conditioning chemicals are added in measured amounts "into either the feedwater tank or the feedwater pipeline prior to its entry into the boiler". [1] For drum boilers, the IAPWS technical guidance on phosphate and NaOH treatment says "the optimum location for injection of either phosphate or NaOH is into the drum and not into other plant locations such as the economizer/preheater of HRSGs", with just upstream of the drum also acceptable, and that these solid-alkali treatments apply only to drum-type boilers and "must not be used for once-through boilers". [2]
IAPWS also warns that phosphate treatment can cause acid phosphate corrosion in higher-pressure boilers above 10.4 MPa and is sometimes difficult to control, and that caustic treatment can cause caustic gouging outside its optimal range, so both must be monitored to avoid carryover into steam. [2] Editorial note: a drum injection point is at boiler pressure, so its pump and check valve have to be rated above drum pressure, while a feedtank point is near atmospheric.
03
One pump per chemical: containers, pumps and injection lines
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
How is this different from a general dosing skid?
This page covers boiler water treatment with more than one chemical fed to the feedwater and drum; general water-treatment dosing skids are on the Chemical dosing pump skid page. [1][2]
How much sodium sulphite does a boiler need?
Spirax Sarco gives 8 ppm per 1 ppm of dissolved oxygen plus 4 ppm reserve, and notes liquid product is only 45% sodium sulphite. [1]
Where should phosphate go in?
IAPWS says into the drum, or just upstream of it, not into the economizer, and only on drum boilers. [2]
Why heat the feedtank?
Hotter feedwater holds less dissolved oxygen, so less scavenger is needed; one case study raised its de-aerator to about 95 °C. [1][3]
Sources and what still needs a direct check
Sources checked 2026-10-12. Spirax Sarco explains feedtank and feedwater conditioning; IAPWS publishes technical guidance on phosphate and NaOH treatment for drum boilers; AMPC published a boiler water chemistry case study.
