How a Urea Dosing System Works: Metering, Dilution and Injection

A urea dosing system takes urea solution from a storage tank, meters an exact amount with a positive displacement pump, blends it with dilution water, and splits the mix between injection lances that spray it into the flue gas. The metering pump only decides how much urea goes in. Dilution water, the distribution module and atomizing air decide whether that urea actually reaches the NOx and reacts. If you are specifying a urea dosing system for an SCR or SNCR retrofit, or taking over one that never quite performs, it helps to see the whole chain rather than just the pump.

This article is for plant engineers, EPC designers and buyers putting together a urea dosing system scope for a boiler, kiln or engine DeNOx project.

The heart of a urea dosing system: the metering pump

Most urea metering pumps are reciprocating diaphragm pumps. A motor drives an eccentric or cam, the diaphragm moves back and forth, and two check valves control direction. On the suction stroke the inlet valve opens and the chamber fills. On the discharge stroke the outlet valve opens and that fixed volume is pushed out.

Flow equals volume per stroke times strokes per minute. Stroke length sets the first figure, motor speed or stroke rate sets the second. As long as the valves seat cleanly and there is no gas in the head, output is very repeatable.

Why diaphragm pumps rather than plunger pumps

Urea solution that leaks dries into hard white crystals. A plunger pump relies on packing, and even a small weep crystallizes and scores the plunger. A diaphragm pump has no dynamic seal in contact with the liquid, so for low and medium pressure urea service a mechanical diaphragm pump is the usual first choice. Hydraulic diaphragm or plunger designs come in mainly when pressure or flow is high.

Inside a urea dosing system, step by step

A typical urea dosing system layout looks like this:

1. Storage tank with heat tracing and insulation, holding urea solution at the design concentration.
2. Suction strainer to keep undissolved granules, rust and crystals out of the pump.
3. Metering pump, usually duty and standby, taking a speed signal from the control system.
4. Discharge accessories: pulsation dampener, back pressure valve, pressure gauge and flow meter.
5. Dilution water skid with its own pump and flow meter, normally demineralized water.
6. Mixer, where urea and dilution water combine.
7. Distribution module, splitting the mix into one branch per lance, each with a control valve, flow meter and pressure gauge.
8. Injection lances using compressed air to atomize the liquid into the furnace or duct.

Why dilution water is metered separately

Storing a stronger solution keeps the tank small and cuts down on batching. Lances, though, need a minimum total flow to atomize well and cover the cross section. Separating "how much urea" from "how much liquid" solves both: the urea pump follows the NOx demand, and the dilution water keeps the lance flow in its good spray range. Premixing everything in the tank works on a small urea dosing system, but at low load the lance flow drops with the urea flow, the spray coarsens and ammonia slip (unreacted ammonia leaving with the flue gas) tends to rise.

How the distribution module balances lances

Each branch is a small parallel circuit. During commissioning the branches are trimmed against the temperature and NOx profile across the duct or furnace. Expect several passes: opening one branch changes the pressure, and therefore the flow, in all the others. A urea dosing system with no per-lance flow meters is very hard to balance properly.

Does a urea dosing system differ between SCR and SNCR?

The pump principle is the same. What changes is downstream.

In SNCR, the solution is sprayed straight into the furnace at the right temperature window, so there are more lances and distribution matters a lot. In SCR, the reaction happens on a catalyst, and on larger plants urea is often converted to ammonia first by hydrolysis or decomposition before injection. Smaller SCR units may inject atomized urea solution directly upstream of the catalyst. Because the downstream resistance differs, size pump pressure and flow for each urea dosing system on its own rather than copying another project's numbers.

Control signals in a urea dosing system

The DCS or PLC works out the required urea flow from outlet NOx, unit load or flue gas flow, then sends a 4-20 mA signal to a VFD or the pump's own controller. The pump changes its stroke rate to follow. Size the pump for maximum load plus a margin, or it will run flat out and still fall short on peak demand. For wiring options, see 4-20 mA versus pulse control for metering pumps.

Three details that cause most field problems

Do not skip the back pressure valve. Lance inlet pressure can be low. If the pump discharge sees too little pressure, liquid siphons through, and actual flow runs above the setpoint. The reasoning is covered in why metering pumps need a back pressure valve.

Avoid dead legs in the urea dosing system. Unused branches and long blind ends are where still solution crystallizes first. Design a recirculation line back to the tank where you can, and flush the pump head and lines with demineralized water before a long shutdown.

Heat-trace right up to the pump head. Plenty of sites trace the pipework and leave the pump head and valves bare, and that is exactly where winter blockages start. If an existing urea dosing system has already stopped delivering, work through why a urea metering pump stops dosing in SCR systems.

One more: a mechanical pump delivers pulsed flow, so a flow meter downstream will flicker. Operators sometimes chase that by retuning the pump. A pulsation dampener settles both the reading and the lance pressure.

Safety: stop the pump, isolate power and relieve pressure before opening any part of the urea line, and isolate heat tracing circuits before working on them.

Conclusion

A urea dosing system works by separating jobs. The diaphragm metering pump delivers a fixed volume per stroke and sets how much urea goes in. Dilution water sets total lance flow. The distribution module and atomizing air decide where the spray goes and how fine it is. Size the pump for peak load, fit a back pressure valve and dampener, keep dead legs out of the layout and trace heat all the way to the pump. Our mechanical diaphragm metering pumps and packaged dosing skids cover the full chain. Send us your urea concentration, peak demand, lance count and discharge pressure, and contact our engineers.

FAQ

What keeps a urea metering pump accurate?

A fixed volume per stroke and clean check valves. Crystals on a valve seat or gas in the head will reduce output, so strainers and venting matter.

Can I premix dilution water in the storage tank?

Yes, on a small urea dosing system. You lose turndown, though, because lance flow drops with load and spray quality suffers.

How many lances can one pump feed?

Several, through a distribution module. The more lances you have, the more each branch needs its own flow meter and control valve.

Why is flow higher than the setpoint when pressure looks normal?

Usually siphoning caused by low back pressure or a back pressure valve set too low. Check the valve setting, then run a drawdown calibration.

What should I send for a urea dosing system quote?

Urea concentration, maximum urea flow, number of lances, discharge pressure and site minimum temperature. The last two drive material, pump type and heat tracing.