Boiler Feedwater Chemical Treatment: Oxygen Scavengers, pH Control and Where Phosphate Fits

Boiler feedwater chemical treatment is often lumped together with boiler water treatment, and that leads to wrong equipment choices. Three chemical families are usually involved: oxygen scavengers, alkalizing agents and phosphate. Strictly speaking only the first two are feedwater chemicals, injected into the line after the deaerator. Phosphate is a boiler water chemical, fed straight into the steam drum. Specify all three as one system and you can end up with injection points and pump pressures that do not match. Oxygen scavengers remove the dissolved oxygen left after thermal deaeration. Alkalizing agents raise feedwater pH to slow corrosion of steel and copper alloys. Phosphate controls scale and boiler water pH in the drum. This article is for engineers in industrial boiler houses and power plant chemistry departments.

Boiler feedwater chemical treatment versus boiler water treatment

Feedwater is the water leaving the deaerator and pumped into the boiler by the feed pump. Boiler water is what sits in the drum and evaporates. Pressure, temperature and quality limits differ between them:

• Feedwater injection points are usually at the deaerator outlet downcomer or near the feed pump suction, at relatively low pressure.
• The boiler water injection point is on the drum, close to boiler operating pressure, so the pump must overcome that pressure.

That difference decides the pumps. Feedwater dosing pumps need modest pressure. Phosphate pumps must be rated for drum pressure and are often high-pressure metering pumps.

Oxygen scavengers: removing residual oxygen

A thermal deaerator removes most dissolved oxygen, but some always remains, and that residual causes oxygen pitting in feed lines and economizers. In boiler feedwater chemical treatment, chemical scavengers finish the job.

Common choices:

1. Hydrazine. Scavenges oxygen and helps form a protective oxide layer; used on medium and high-pressure boilers. It is toxic, and many plants have cut back or switched to alternatives.
2. Sodium sulfite. Easy to handle and widely used on low and medium-pressure industrial boilers. It adds dissolved solids to boiler water, so high-pressure units generally avoid it.
3. Organic scavengers. Lower toxicity replacements for hydrazine; check the supplier's data for performance.

Inject scavengers after the deaerator, as early as possible, so they mix with the feedwater quickly.

Alkalizing agents: feedwater pH control

In boiler feedwater chemical treatment, pH matters as much as oxygen. Low feedwater pH speeds corrosion of steel piping, and the iron carried into the boiler forms deposits. Ammonia solution is the most common alkalizing agent, with neutralizing amines as an alternative.

Ammonia is volatile and travels with the steam, which also protects the condensate system. Where copper alloys are present, ammonia concentration must not run too high or copper corrosion accelerates. That balance is set by the plant's chemistry control limits.

Alkalizing agents are normally dosed in proportion to feedwater flow and trimmed by online pH or conductivity.

Phosphate: a boiler water chemical

Phosphate, typically trisodium phosphate, is fed to the drum. It converts calcium and magnesium into a loose sludge that leaves with blowdown instead of forming hard scale on heating surfaces, and it buffers boiler water pH. It is not part of boiler feedwater chemical treatment, but it is often built into the same dosing skid, so it belongs in this discussion.

Phosphate dose calculation and continuous versus intermittent feed are covered in separate articles.

Configuring a boiler feedwater chemical treatment skid

A complete skid is usually split into independent units per chemical. Each has a solution tank, mixer, duty and standby metering pumps, strainer, relief valve, pressure gauge, pulsation dampener and local control panel. Key points:

• One unit per chemical. Scavenger, ammonia and phosphate each get their own tank and pumps. Hydrazine and ammonia are volatile, so tanks must be closed.
• Pumps sized by injection point. Feedwater pumps are rated for feed line pressure plus margin; phosphate pumps for drum pressure plus margin.
• Materials. Stainless steel heads and piping suit ammonia and hydrazine, and are commonly used for phosphate as well.
• Control. Feedwater chemicals follow feedwater flow; phosphate follows boiler water phosphate and pH.

Monitoring boiler feedwater chemical treatment

Dosing is only half the job. Boiler feedwater chemical treatment is judged at the sample panel: dissolved oxygen and pH in feedwater, conductivity after cation exchange, and iron or copper on a routine lab schedule. If dissolved oxygen stays high while the scavenger pump is running, check the deaerator first, because no chemical program can make up for a deaerator that is not venting properly. Keep pump stroke settings and analyzer readings in the same shift log so trends are easy to read.

Common boiler feedwater chemical treatment mistakes

• Sizing the phosphate pump to the same pressure as the feedwater pumps, so it cannot inject into the drum.
• Placing the scavenger injection ahead of the deaerator, where much of it is consumed or vented.
• Open hydrazine tanks, which expose staff and leave the room smelling. Hydrazine is toxic: wear a face shield and gloves, and ventilate the room.

Conclusion

Boiler feedwater chemical treatment uses oxygen scavengers to remove residual oxygen and alkalizing agents to raise feedwater pH. Phosphate goes to the drum to control scale and boiler water pH. Because the injection points and pressures differ, build the skid in one unit per chemical and size each pump for its own injection point. Related reading: what chemicals a boiler dosing system adds, boiler phosphate treatment and boiler dosing pump selection. Equipment: dosing skids and plunger metering pumps. Send boiler pressure, steam output and water quality limits and contact our engineers.

FAQ

Do small low-pressure boilers need an oxygen scavenger?

With a deaerator, residual oxygen usually still needs chemical treatment. Without one, small boilers rely on scavengers even more. Follow the applicable boiler water standard.

Can ammonia and hydrazine share a tank?

Avoid it. They do different jobs and are controlled on different parameters, so separate batching and dosing give more accurate control.

Is a solenoid pump acceptable for boiler feedwater chemical treatment?

Yes, where feed line pressure and flow are low. At higher pressure use a mechanical diaphragm or plunger pump.

How is feedwater pH monitored?

Online pH and conductivity at the feedwater sample point, checked regularly against laboratory samples.

Does the dosing room need explosion protection?

Hydrazine and ammonia are volatile, so ventilation is essential. Whether electrical equipment must be explosion-proof depends on the project's safety assessment.