Power Plant Chemical Dosing: How to Configure Each Dosing Unit

Power plant chemical dosing in a thermal or cogeneration plant is not a single machine. It is a set of units separated by chemical and injection point: feedwater ammonia, feedwater oxygen scavenger, drum phosphate and usually closed cooling water and circulating water treatment as well. Each unit has its own solution tank, mixer, metering pumps, strainer, relief valve, pulsation dampener and local panel, and all of them connect to a PLC or the DCS for automatic control from feedwater flow and online chemistry analyzers. Getting the design right means defining injection pressure, flow range, standby philosophy and control interface for every unit. This article is for plant chemists, design institutes and buyers, and it deals with configuration rather than water chemistry.

The usual units in power plant chemical dosing

1. Feedwater ammonia. Controls feedwater and condensate pH. Injected at the deaerator outlet or condensate pump discharge, at medium pressure.
2. Feedwater oxygen scavenger. Removes residual oxygen after the deaerator. Some units no longer use hydrazine; follow the unit's water chemistry program.
3. Drum phosphate. Fed into the steam drum at the highest pressure in the system, so it places the toughest demands on the pumps. Once-through boilers have no drum and no phosphate unit.
4. Closed cooling water. Corrosion inhibitor, usually topped up intermittently at low pressure.
5. Circulating water. Scale inhibitor and biocide at higher flows and low pressure, usually located near the circulating water pump house.

In power plant chemical dosing, the first three are normally grouped in a dosing room in the main building and called thermal system dosing.

Choosing pumps for each power plant chemical dosing unit

Pressure comes from the injection point; flow comes from the unit load range.

• Phosphate: injection pressure close to drum working pressure. Plunger or hydraulic diaphragm metering pumps are common, rated above drum pressure with margin.
• Ammonia and scavenger: medium pressure. Mechanical or hydraulic diaphragm pumps, chosen by pressure class.
• Closed and circulating water: low pressure. Solenoid or mechanical diaphragm pumps.

Size maximum flow at full load and maximum dose rate, check minimum flow at low load, and confirm the pump's adjustment range covers both without long periods below 30% stroke. Where it does not, combine variable speed with stroke adjustment.

Standby philosophy: one plus one or two plus one

For power plant chemical dosing on a single unit, each dosing unit usually has one duty and one standby pump. When two units share a dosing room, a two duty plus one common standby arrangement is common, with valves to switch the middle pump to either unit.

Two plus one saves a pump but complicates valves and piping. Write switching procedures clearly and tag valves by unit so nobody lines up the wrong one. Because phosphate goes straight to the drum, some plants insist on independent one plus one sets per unit to avoid cross-connection risk.

Power plant chemical dosing and the DCS

Automatic power plant chemical dosing lives on signals. Typical loops:

• Ammonia: feedwater flow feed-forward with feedwater pH or specific conductivity feedback, adjusting VFD speed or stroke.
• Phosphate: adjusted on boiler water phosphate and pH; many plants dose intermittently or at fixed rates alongside lab analysis.
• Scavenger: paced to feedwater flow, referenced to feedwater dissolved oxygen.

Signals sent to the DCS usually include pump running, fault, local or remote status, tank level, discharge pressure and stroke or speed feedback. The DCS sends start and stop commands and 4–20 mA set points. List every point and signal type in the technical specification.

What technical specifications often leave out

A power plant chemical dosing specification is usually strong on pumps and weak on everything around them. Items that are often missing:

• Design pressure, flow range and adjustment method for each pump, not just a model number.
• Pump head and piping materials, and closed, vented tanks for ammonia and hydrazine.
• Settings and positions for relief valves, dampeners and back pressure valves.
• Tank level instrument type, low-level pump trip and high-level alarm.
• Local panel ingress rating, hand and remote selector and emergency stop.
• Factory tests: flow calibration, pressure test and simulated control loop test.

With these written down, commissioning and acceptance have something to test against.

Layout of the power plant chemical dosing room

Keep each unit's tank, pumps and panel together and label them clearly by chemical and boiler unit. Leave space in front of every pump to remove the head without dismantling piping, and slope the floor to a drain with a bund around tanks. Route high-pressure phosphate lines with as few joints as possible and support them well against pulsation.

Operating experience

Once a power plant chemical dosing system is in service, the usual problems are analyzer drift pulling automatic control off target, standby pumps that never run and develop sticking check valves, and batch strength that varies from shift to shift. Rotate duty pumps weekly, compare lab results with online analyzers daily and log every batch. Ammonia and hydrazine are volatile, so ventilate the dosing room, wear a face shield and gloves when batching and provide an eyewash station.

Conclusion

Configure power plant chemical dosing unit by unit: feedwater ammonia, scavenger, drum phosphate, closed cooling water and circulating water each stand alone. Choose pumps by injection pressure and load range, decide standby by the number of units and list every DCS signal in the specification. That is what makes automatic dosing actually stay automatic. Further reading: how automatic boiler dosing works, boiler dosing pump selection and hydraulic diaphragm metering pump selection. Equipment: hydraulic diaphragm metering pumps and dosing skids. Send unit data and the injection point list and contact our engineers.

FAQ

Do once-through boilers need phosphate dosing?

No drum and no blowdown means phosphate treatment is generally not used. Thermal system dosing centers on feedwater ammonia and the chosen feedwater treatment.

Should the dosing system be skid-mounted?

Skids are piped and tested in the factory and install quickly where space allows. Where space is tight, components can be installed separately.

Can two units share a solution tank?

Yes, but give each unit its own pumps so feed can be adjusted independently.

What if the online pH analyzer is unreliable?

Switch to flow-paced fixed dosing, increase lab sampling and recalibrate or replace the electrode quickly.

Why do phosphate pumps fail more often in power plant chemical dosing?

High pressure and a crystallizing chemical wear check valves and plunger seals faster. Replace wear parts on schedule and flush with demineralized water at shutdown.