Boiler Chemical Feed: Continuous vs Intermittent (Slug) Dosing

Is boiler chemical feed supposed to run continuously? It depends on the chemical and the boiler. Oxygen scavenger and ammonia or amine for feedwater pH are almost always fed continuously, because dissolved oxygen and contaminants arrive with the feedwater all the time. Phosphate on medium and high-pressure drum boilers is normally continuous too. On small low-pressure boilers, internal treatment chemicals can often be slug fed once per shift or once per day, if the operating instructions allow it. This article is for boiler house operators and engineers deciding between the two approaches.

Start with what each boiler chemical feed is doing

One question settles most cases: is the thing this chemical treats coming in all the time?

Oxygen scavenger removes residual dissolved oxygen in the feedwater. As long as the feed pump runs, oxygen keeps arriving, so any gap in feed leaves the boiler unprotected for that period. That is why oxygen scavenger is a continuous boiler chemical feed in practice. Ammonia or neutralizing amine holds feedwater pH up, and the logic is the same: stop the feed and pH starts dropping quickly.

Phosphate treats residual hardness in the boiler water, and it stays in the drum for some time, so there is room for intermittent addition. The higher the pressure, the more sensitive the boiler is to swings in boiler water chemistry, which is why medium and high-pressure drum units normally keep phosphate on continuous feed. Low-pressure boilers have looser limits and relatively large water volume, so periodic additions are common there.

How continuous and slug boiler chemical feed differ in the water

The difference is the shape of the residual curve. With continuous boiler chemical feed, the residual is close to a flat line. Lab results are steady and blowdown is easy to manage. Slug feeding produces a sawtooth: a spike right after the addition, a slow decline as chemical is consumed and blown down, and a low point just before the next dose.

A high spike raises dissolved solids and can cause foaming and carryover. A deep valley means a period of weak protection. The longer the interval and the bigger each slug, the steeper the sawtooth. Even if you choose intermittent feed, small frequent additions are steadier than one large daily charge.

Where boiler chemical feed goes in

Injection points differ by chemical, and so does the pressure the pump must beat:

• ammonia, amine and oxygen scavenger usually go into the condensate or feedwater line, so the pump works against line pressure at that point;
• phosphate goes straight into the steam drum, so the pump must beat drum pressure;
• on some small low-pressure boilers, slug doses are added through a pressurized pot feeder on the feedwater side, letting the feedwater carry the chemical in.

Never let one pump feed two points at different pressures, because the chemical will take the path of least resistance. For matching pump ratings to injection pressure, see our boiler dosing pump selection guide. For the wider system, read boiler water treatment dosing essentials.

Pump sizing is not the same for both methods

A continuous boiler chemical feed pump is sized for the maximum continuous demand and spends its life at mid stroke, low flow and steady pressure. For small points a single solenoid metering pump is often enough. The more common field problem is the opposite: an oversized pump that has to run at a very short stroke, losing accuracy and letting crystals stick the check valves. Diluting the solution brings the pump back into a healthy stroke range, and our article on why stroke length should stay above roughly 30% explains the reason.

Slug feeding has to push a full dose in a short time, so the pump is much larger and then sits idle for long periods. Chemical left standing in the head can settle or crystallize, especially phosphate, and the pump may not prime next time. Flush the head and line with demineralized water after each slug.

A simple decision path for boiler chemical feed

1. Chemical: oxygen scavenger and amine default to continuous; phosphate depends on boiler pressure.
2. Boiler: medium and high-pressure or continuously fired drum boilers go continuous; small low-pressure units that only run day shifts can accept slug feed.
3. People and instruments: unattended plants are easier to run on continuous automatic boiler chemical feed; a site with a dedicated shift chemist can manage manual slug dosing.
4. Rules: if the boiler maker or the water treatment specification sets a method, follow it.

Common mistakes with slug boiler chemical feed

The first is blowing down right after the dose. Part of the chemical leaves before it does any work, residuals read low, and operators respond by feeding more. Schedule bottom blowdown between doses instead.

The second is timing the slug against load. Dumping a full day of chemical in at night when load is low creates an even taller spike.

The third is mixing too far ahead. Solution that sits for days settles or crystallizes and plugs the pump valves. Slug-feed tanks need agitation too, and fresh batches are better.

Converting from slug to continuous boiler chemical feed is usually straightforward: the existing tank and piping can often stay, and you add a correctly sized pump plus a control signal. See our packaged chemical dosing skids for complete setups.

Safety note: dosing lines connect to pressurized systems. Close the isolation valve and release pressure before opening any fitting, and wear gloves and eye protection when mixing chemicals.

Conclusion

Oxygen scavenger and amine should be fed continuously, and phosphate on medium and high-pressure boilers mostly is too. Small low-pressure boilers can use slug feeding for internal treatment, as long as doses are small, frequent and kept away from blowdown. Continuous boiler chemical feed gives steady residuals and smaller pumps; slug feeding is simpler hardware with bigger swings. Decide the injection point and pressure for each chemical first, then the pump and control method. Send us the boiler pressure, chemicals and daily consumption and contact our engineers for a recommendation.

FAQ

Should boiler chemical feed continue while the boiler is shut down?

No. Interlock the dosing pump to stop with the boiler. Lay-up protection during long outages is a separate procedure, not normal running feed.

How many slug doses of boiler chemical feed per day are right?

There is no universal number. Base it on boiler water test results and the operating instructions, aiming for small, frequent doses that avoid big swings.

Does continuous feed waste chemical?

Usually not. Feed matches demand, residuals stay steady, and blowdown losses tend to be lower.

Can one boiler have some chemicals continuous and others intermittent?

Yes, it is common. Amine might run continuously while phosphate is topped up each shift, each with its own pump and injection point.

Can we keep the old pump after switching to continuous feed?

Often it is oversized and would run at a very short stroke, hurting accuracy and valve life. Choose a smaller pump or dilute the solution.