Boiler Water Treatment Chemicals: What Phosphate, Ammonia and Hydrazine Do
Boiler water treatment chemicals fall into three main groups. Phosphate goes into the drum to turn hardness into removable sludge instead of hard scale. Ammonia, or a neutralising amine, goes into the feedwater to raise pH and slow corrosion. Hydrazine or another oxygen scavenger goes into the feedwater to remove the dissolved oxygen the deaerator leaves behind. Smaller industrial boilers follow the same logic with caustic soda, sodium sulfite or a blended product. Put simply: scale control in the boiler, corrosion control in the feedwater, and oxygen removal before the water reaches the boiler.
This guide to boiler water treatment chemicals is for boiler operators, water treatment technicians and buyers putting together a dosing package.
Boiler water treatment chemicals: boiler water versus feedwater
Before asking which chemical to buy, separate the two places it can go.
Boiler water is the water evaporating in the drum. Its problem is scale. However well the makeup is treated, traces of calcium and magnesium get in, concentrate as water evaporates, and deposit on heated surfaces. Heat transfer suffers, and in bad cases tubes overheat.
Feedwater is the water between the deaerator and the boiler. Its problem is corrosion. Dissolved oxygen and carbon dioxide attack feed lines and the economiser, and the corrosion products are carried into the boiler, where they become deposits.
That is why boiler water treatment chemicals are usually dosed by two separate systems: one skid for drum phosphate, another for feedwater ammonia and oxygen scavenger.
Phosphate: scale control and pH buffering in the drum
Phosphate, often trisodium phosphate, reacts with calcium in the boiler water to form a loose sludge that leaves with continuous and intermittent blowdown rather than baking onto the tubes. It also helps hold boiler water alkalinity steady.
The injection point is the drum, so the pump must beat full boiler pressure. Of all boiler water treatment chemicals, phosphate places the highest pressure demand on the pump. How to set the phosphate residual and match it to boiler water pH is a topic of its own; here we only cover what it does.
Feedwater boiler water treatment chemicals: ammonia and amines
Ammonia added to the feedwater or condensate raises pH, neutralises carbon dioxide and reduces acidic attack on carbon steel. Some plants use neutralising amines for the same purpose.
There is a trap. If the system contains copper alloys, for example in some feedwater heater tubes, too much ammonia speeds up copper corrosion. Dose to the plant's water chemistry limits and materials, not "the more the better".
Hydrazine and other oxygen scavengers among boiler water treatment chemicals
The deaerator removes most dissolved oxygen, but never all of it. Hydrazine is a reducing agent that reacts with the remaining oxygen and helps form a protective oxide layer on steel.
Hydrazine is toxic, and that is why many plants have moved to lower-toxicity scavengers, or to oxygenated treatment on once-through units. Which approach is right depends on the plant's chemistry guidelines and design, so oxygen scavenger dosing should always follow the site chemistry programme.
Safety: hydrazine and concentrated ammonia are toxic and irritating. Wear gloves, goggles and suitable respiratory protection when mixing, keep the chemical room ventilated, and stop the pump, isolate and depressurise before opening any high-pressure dosing line.
Boiler water treatment chemicals for low-pressure industrial boilers
Smaller industrial boilers rarely have a full steam cycle chemistry programme. Typical choices are:
- caustic soda or phosphate to hold alkalinity and control scale;
- sodium sulfite as the oxygen scavenger instead of hydrazine, because it is safer and simpler to handle;
- a blended boiler treatment product from a chemical supplier that covers scale and corrosion in one drum.
The right mix depends on boiler pressure, makeup treatment and the supplier's programme. For complete schemes, see our boiler chemical dosing guide and power plant boiler water treatment.
What each chemical means for the dosing pump
The boiler water treatment chemicals you use decide the pump:
Drum phosphate means high pressure. A plunger metering pump or a hydraulic diaphragm metering pump is the usual choice, rated above drum working pressure with a margin.
Ammonia and oxygen scavengers injected at the feed pump suction or into the condensate system see lower pressure, but flows are small and must be steady. Because hydrazine is toxic, leak-free diaphragm designs are preferred.
Give each of your boiler water treatment chemicals its own tank, mixer, strainer and relief valve. Never share a tank between chemicals. More detail on sizing is in boiler dosing pump selection.
How to tell if the chemistry is working
Knowing which boiler water treatment chemicals to use is only half the job. Boiler water is usually checked for pH and phosphate, and feedwater for pH, dissolved oxygen and scavenger residual, at intervals set by the plant's chemistry routine. When a reading drifts, check the pump before you change the dose: has someone moved the stroke or speed, is the tank level falling as it should, is the head gas-locked? A surprising number of "the chemical isn't working" complaints end with a pump that was not delivering.
Conclusion
Boiler water treatment chemicals come down to three jobs: phosphate in the drum for scale control, ammonia in the feedwater for pH, and hydrazine or another scavenger in the feedwater for oxygen. Low-pressure boilers often use caustic, sulfite or blended products instead. Each chemical has its own injection point and pressure, so give each its own pump. Send us your boiler pressure, steam output and treatment programme, and contact our engineers for a matched dosing package.
FAQ
Can boiler water treatment chemicals like phosphate and ammonia share one tank?
No. They go to different injection points and are controlled to different targets, so each needs its own tank and pump.
Can hydrazine be replaced?
Yes. Sodium sulfite is common on low-pressure boilers, and power plants use other lower-toxicity scavengers. Confirm any change against your chemistry guidelines.
Does a small boiler really need chemical dosing?
Usually yes. Even with softened makeup, alkalinity and oxygen still need control, or scale and corrosion follow quickly.
Why does a boiler dosing pump need such high pressure?
Phosphate is injected straight into the drum, so the pump has to overcome full boiler pressure.
Is a higher dose of boiler water treatment chemicals always safer?
No. Excess phosphate can push boiler water out of limits, and excess ammonia attacks copper alloys. Dose to lab results.