Urea Injection Control: Load Following and Pump Speed Adjustment

Reliable urea injection control on an SCR or SNCR system usually comes down to a simple split. Set the metering pump stroke length once during commissioning, somewhere in the middle to upper range, then let a variable frequency drive change pump speed to follow the process. The control logic uses unit load or flue gas flow as feedforward and outlet NOx as a slow feedback trim. Most of the trouble we see on site comes from skipping one of those pieces.

This guide to urea injection control is for operators, controls engineers and buyers specifying a diaphragm metering pump for a DeNOx retrofit on a boiler, kiln or engine.

Stroke length versus speed in urea injection control

Pump output equals volume per stroke times strokes per minute. Stroke length changes the first part, motor speed changes the second.

Both work, but they suit different jobs. Stroke length is normally set with a handwheel at the pump. An electric stroke actuator makes it remote, but it is slow. Speed control through a VFD responds quickly, takes a signal straight from the DCS, and output is close to proportional to speed. That makes speed the natural tool for urea injection control during normal operation.

Do not run the stroke very short for long periods. At short stroke, the time the check valves take to open and seat becomes a bigger share of each stroke, so accuracy and repeatability suffer. See why stroke length should stay above about 30%.

How pump speed control works

The VFD changes supply frequency, the motor speed follows, and the gearbox and eccentric turn that into stroke rate. With clean valves and no gas in the head, flow tracks speed closely.

Two limits on speed-based urea injection control are worth checking before you buy:

1. At very low speed, a standard motor's shaft-mounted fan cools poorly. If the unit will spend long periods at low load, specify an inverter-duty motor or a separate cooling fan.
2. If the speed range alone cannot cover minimum to maximum demand, adjust stroke to shift the range, rather than pushing frequency ever lower. Test both ends during commissioning.

Where there is no VFD, an electric stroke actuator can still provide automatic urea injection control, just with slower response. See how electric stroke control motors work.

Urea injection control logic: feedforward plus feedback

Feedback from outlet NOx alone rarely works well. The delays stack up. Urea takes time to travel from pump to lance, flue gas takes time to reach the analyzer probe, and the analyzer adds its own sampling and response lag. By the time feedback sees high NOx and adds urea, the load may already have changed. With urea injection control built on feedback only, the result is a loop that swings between NOx excursions and high ammonia slip, meaning unreacted ammonia leaving with the flue gas.

A steadier approach has two layers:

• Feedforward: calculate a base urea flow from unit load or flue gas flow and inlet NOx. When load moves, the pump moves straight away.
• Feedback: trim that base using the error between outlet NOx and its setpoint, with limited gain and rate.

Keep the feedback slow. Strong integral action in a long-delay process overshoots easily. Many plants describe the target as a molar ratio of reagent to NOx and let the feedforward calculate from that, but whichever form you use, the principle is the same: the model does the heavy lifting and feedback fine-tunes it.

Dilution water has to follow

When urea flow changes, adjust dilution water so the lances still receive enough total flow to atomize. Urea injection control that ignores dilution water gives coarse spray at low load.

Calibrate after tuning

Even with urea injection control tuned well, the percentage on the VFD is theoretical. Back pressure, solution temperature and valve condition all shift real output. During commissioning, run a drawdown test at minimum, normal and maximum speed and enter the measured speed-to-flow relationship in the DCS scaling. The method is in metering pump calibration with a calibration column.

Field mistakes that upset urea injection control

Duty and standby pumps set differently. Both have VFDs, but only one had its stroke set and calibrated. After a changeover the flow jumps and the control loop gets blamed. Calibrate each pump and tag the stroke position.

Stroke set too short for the minimum load. Someone winds the stroke right down to cover the lowest load, and accuracy at normal load suffers. Work out the full load range first and pick a stroke that covers it.

Thick solution in winter. Cold solution raises line resistance, output at a given speed drops, and feedback keeps pushing up. Include heat tracing and solution temperature in routine rounds.

A bump when switching from manual to auto. In manual the pump sits at one speed; the moment auto takes over, the controller output is different and the pump lurches. Set up bumpless transfer in the DCS so the auto output tracks the current speed first.

Safety: adjust the stroke handwheel only while the pump is running, as forcing it at standstill can damage the mechanism, and relieve pressure before opening any fitting.

Conclusion

Effective urea injection control is about giving each part one job. Stroke length sets the working range at commissioning, the VFD follows load day to day, feedforward moves first and feedback corrects slowly. Calibrate at several speeds and keep separate records for duty and standby pumps. For new projects, a VFD-driven mechanical diaphragm metering pump with a flow meter for live checks is a sound starting point. Send us your load range, urea concentration and discharge pressure, and contact our engineers.

FAQ

Is VFD speed control or stroke adjustment better for urea injection control?

Use the VFD for daily load following because it is fast and remote. Set the stroke once at commissioning and touch it again only when the duty changes.

The VFD is at a very low setting and the pump moves but delivers nothing. Why?

Bleed the head first, then check the stroke before blaming urea injection control. Low speed combined with a very short stroke may not lift the valve balls at all.

Outlet NOx keeps swinging up and down. What should I change?

Usually the feedback is too fast and there is no feedforward. Slow the integral action, add load feedforward, and watch whether the swings narrow.

Flow changed after switching to the standby pump. Is the control faulty?

Normally not. The two pumps have different stroke settings or scaling. Calibrate each one and store its own factor.

Does the pump need manual adjustment every day?

No. In auto it should not. Recheck calibration on a schedule, at season changes and when the urea supply changes.