Boiler Dosing Pump Selection: Phosphate and Ammonia, Pressure First

Boiler chemical feed rates are usually small, from a few liters to a few dozen liters per hour, so many buyers pick a small solenoid pump on flow alone and then find it can't push chemical into the boiler. When you choose a boiler dosing pump, pressure is the first gate and flow comes second. Below we cover the two most common chemicals, phosphate and ammonia, how to work back from injection pressure to a model, and what a dosing package specification should list.

Where the Chemical Goes Sets the Pressure

Phosphate is usually dosed straight into the steam drum, so the boiler dosing pump has to overcome drum pressure. Ammonia is typically injected into the feedwater or condensate line, so the pump works against line pressure at that point. On the same boiler the two points can be very different, and one pressure figure can't be used for both pumps.

Before selecting, take the maximum working pressure at each dosing point from the boiler nameplate and operating procedures. The NEWDOSE mechanical pump manual is explicit that the maximum working pressure at the pump outlet must not exceed the rated pressure on the nameplate. So the boiler dosing pump must be rated above the injection pressure, with headroom, rather than running at its limit all the time.

Boiler Dosing Pump Options by Pressure

Small flow, up to 20 bar

Only a few solenoid models can serve as a boiler dosing pump at these pressures:

• DFD-02-20-HX: 2 L/h at 20 bar;
• DFD-02-16-X: 2 L/h at 16 bar;
• DFD-06-16-GX: 6 L/h at 16 bar;
• DFD-05-12-HX: 5 L/h at 12 bar.

Flows are very small, suiting small boilers with low chemical demand. The DP and DM ranges also include the 02-16-X and 06-16-GX sizes, so switch series if you need a pulse or 4 to 20 mA input. The 20 bar HX model is only available in the manual DFD range.

More flow, around 16 bar

At high pressure a solenoid pump can't deliver much, and a few liters per hour is already its ceiling. For larger feed rates, go mechanical. The small NDWS sizes are rated 16 bar: NDWS-8/1.6, 12/1.6, 18/1.6 and 24/1.6, covering 8 to 24 L/h. Heads are available in PVC, PVDF, PTFE or SS316. Motor power is 180 W, three-phase as standard, with a 220V single-phase motor available.

Higher pressure: the HND series

HND is the NEWDOSE high-pressure mechanical diaphragm boiler dosing pump range, with SS316 heads only:

1. HND 47/3.0: 47 L/h at 30 bar;
2. HND 70/2.0: 70 L/h at 20 bar;
3. HND 115/1.8: 115 L/h at 18 bar;
4. HND 135/1.6: 135 L/h at 16 bar;
5. HND 160/1.2: 160 L/h at 12 bar.

It uses mechanical drive with spring return and has no hydraulic oil chamber, so it's simpler to maintain than a hydraulic diaphragm pump. Above 30 bar injection pressure you're beyond the NEWDOSE mechanical range and need a hydraulic diaphragm or plunger pump.

Boiler Dosing Pump Head Materials and Chemicals

Phosphate solution can crystallize when cold and lodge in the check valves. The mechanical pump manual lists crystals or debris in the upper and lower check valves as a cause of unstable discharge pressure; the fix is to clean or replace the valves. Dissolve the chemical fully when making up and fit a strainer on the suction side.

Ammonia is volatile and gas tends to collect in the head. As with hypochlorite, use flooded suction and a head with a bleed valve. Phosphate and ammonia can generally be handled with a stainless head, so the SS316-only HND fits these duties, but send the actual concentration and temperature to the manufacturer for confirmation.

What a Boiler Chemical Dosing System Spec Should List

For an inquiry or technical specification, list at least:

• Chemical, concentration, maximum feed rate and maximum working pressure at each dosing point;
• Number of pumps, and whether you want duty and standby;
• Boiler dosing pump model, head material and power supply (380V three-phase standard on mechanical pumps, 220V optional);
• Control: manual, 4 to 20 mA, or an electric stroke actuator;
• Tank size and mixing: for small packages, NEWDOSE integrated tanks hold one solenoid pump in the 60 L size or one or two in the 120 L size; where mixing is needed, NDM agitators cover tank sizes from 100 L to 2000 L;
• Line accessories: Y strainer, safety valve, back pressure valve, pulsation damper, diaphragm pressure gauge and calibration column.

Never skip the safety valve

Boiler dosing means pumping into a pressurized system. If an outlet valve is closed by mistake, a mechanical pump will build very high pressure. The manual requires a safety valve on the discharge line between the pump and the nearest shut-off valve, with its outlet piped back to the tank or drain and the end visible so leaks are noticed. Never run the pump against a closed outlet valve; that can cause irreversible damage.

Commissioning and Calibration

On first start of a mechanical boiler dosing pump, set the stroke to 0%, jog the motor to confirm rotation and run it unloaded for 5 minutes, which matters most in cold weather. Then bring flow up slowly to 30% and bleed the air out of the lines. As the gauge approaches the target pressure, adjust the back pressure valve slowly without exceeding the rated pressure. After 12 hours of running, calibrate at 100%, 50% and 20%, which is enough to characterize the full adjustment range.

Conclusion

Selecting a boiler dosing pump goes pressure first, then flow, then materials. For small flows up to 20 bar, look at high-pressure solenoid models like the DFD-02-20-HX. Around 16 bar with a little more flow, use the small NDWS sizes. Higher still, HND. Above 30 bar, move to hydraulic diaphragm or plunger pumps. List the dosing package specs point by point and fit the safety valve, back pressure valve, damper and calibration column for stable dosing.

FAQ

Can a standard solenoid pump work on a low-pressure boiler?

Yes, as long as its rated pressure is above the maximum working pressure at the dosing point with some headroom. A model like the DFD-02-16-X is often enough.

Can phosphate and ammonia share one boiler dosing pump?

Not recommended. They go to different points at different pressures, and with one pump on two points the chemical flows toward the lower-pressure side. Give each chemical its own pump.

Do I need a standby boiler dosing pump?

For boilers in continuous service, yes. Rotate the two pumps regularly so you can switch over immediately if one fails.

The gauge on my boiler dosing pump shakes hard. Is that normal?

Diaphragm pumps pulse by nature, but heavy shaking usually means there's no pulsation damper, or the check valves are crystallized or worn. Check the valves first, then consider adding a damper.

What should I do with the boiler dosing pump after a shutdown?

If a mechanical pump has stood for more than a week, set the stroke to 0% and run it unloaded for a few minutes before returning to normal service.

Tell us the chemical and pressure at each dosing point, and contact us to settle the pump models and package specs together.