Urea Metering Pump Not Dosing in SCR Systems: Crystals, Temperature and Filters
In SCR and SNCR denitrification systems on boilers, kilns and engines, a urea metering pump delivers urea solution to the injection lances, where it reduces nitrogen oxides in the flue gas. If the urea metering pump stops dosing, emissions can exceed limits quickly. The causes are closely tied to the nature of urea solution: it crystallizes easily, it is sensitive to temperature and it does not tolerate impurities. Keep those three facts in mind and most faults are found fast.
Properties of Urea Solution
Denitrification urea solution is made from urea granules and water, and it has a few properties that matter for dosing.
It crystallizes. When water evaporates or temperature drops, urea comes out of solution as crystals that coat valves, lines, filters and lances.
Dissolving absorbs heat. As urea dissolves, the solution cools, which matters most when mixing in winter.
It is sensitive to impurities. Contaminants in the granules, hardness in the make-up water and sediment in the tank can all reach the lines and block filters and nozzles.
Why a Urea Metering Pump Stops Dosing
Crystals in valves and lines
This is the most common cause. After a shutdown, solution left in the lines and pump head loses water and crystallizes. Balls stick and lines narrow or block. On restart the urea metering pump strokes but nothing comes out.
Remove the valves and soak them in warm water to dissolve the crystals, then refit. Flush blocked lines with warm water. Prevention is simple: flush the pump head, lines and lances with demineralized or clean water before every shutdown.
Low temperature
In winter, outdoor lines and cold pump rooms let urea crystallize in the lines or make the solution thicker and harder to draw. The cooling effect during mixing makes it worse.
Heat trace and insulate tanks, lines and pumps, keep the pump room reasonably warm, watch solution temperature during mixing and heat it if needed. Mix to the design concentration, because stronger solution crystallizes more easily.
Blocked filters
Suction and discharge filters protect the pump and lances. Impurities from the granules, crystals and tank sediment gradually block them, suction resistance rises and the pump cannot draw.
Clean or replace filter elements regularly, watch the pressure difference across each filter, and clean sediment from the tank bottom.
Air in the head
Low tank level, leaking suction fittings or a blocked suction filter that creates high vacuum can all let air into the head. The pump strokes without output. Keep the tank level up, check the suction fittings and open the bleed valve.
Concentration and quality problems
Too much urea in the mix raises concentration and crystallization risk. Poor quality urea with many impurities blocks filters faster. Hard make-up water forms deposits. Use specified urea and demineralized water and mix to the design concentration.
Control interlocks
The urea metering pump is normally interlocked with the control system. Low tank level, insufficient heat trace temperature, discharge overpressure or an unfinished lance purge can stop the pump or prevent it from starting. If the pump does not move, check alarms and interlock conditions first.
Troubleshooting Order for an SCR Urea Pump Fault
Work in this order for any SCR urea pump fault. Check the control system for alarms and interlocks. Check tank level and solution temperature. Inspect the suction filter. Open the bleed valve to look for air. Finally remove the valves and inspect the lines for crystals. On the discharge side, also check the lances and the discharge filter.
Lances and Discharge Lines
If the pump is dosing but a lance does not spray or sprays weakly, the problem is downstream: a blocked discharge filter, crystals in the lance nozzle or not enough atomizing air. Clean lances regularly and purge and flush them according to the procedure at shutdown.
Pump Selection and Accessories
Urea solution places some demands on the pump. The head and valves must resist urea solution and be easy to clean. A flushing connection makes pre-shutdown flushing simple. Fit a suction filter and bleed valve, and a back pressure valve, pulsation dampener and pressure gauge on the discharge. In cold regions, include heat tracing and insulation for the pump and lines. A urea metering pump selected with these points in mind is far less likely to stop dosing.
Routine Maintenance
Flush the pump head, lines and lances before every shutdown. Clean filters and valves regularly. Check heat tracing and insulation. Clean the tank periodically. Mix to the design concentration with specified urea and demineralized water. Record flow, pressure and solution temperature on rounds. With these habits a urea metering pump runs reliably through the winter and an SCR urea pump fault becomes the exception.
Spares and Standby Pumps
Because emissions limits do not wait for repairs, most SCR systems run with a standby urea metering pump. Switch duty and standby pumps on a regular schedule so the standby urea metering pump does not sit full of solution for months and crystallize. Keep valve kits, filter elements and a diaphragm on site. When a urea metering pump does fail, the standby takes over at once and the faulty pump can be cleaned and repaired without pressure on the team.
FAQ
What should I use to clean urea crystals?
Warm water. Soak and flush until the crystals dissolve, then refit the parts.
The urea pump will not dose in winter. What should I check?
Check that heat tracing and insulation are working and that solution temperature is not too low. Heat the solution and lines if needed.
How often should filters be cleaned?
Base it on pressure difference and how quickly they block. Shorten the interval where impurities are high.
Can I mix with tap water?
Demineralized water is recommended to reduce deposits and scale.
The pump is dosing but a lance does not spray. Why?
Check the discharge filter, crystals in the lance nozzle and the atomizing air supply.
Conclusion
A urea metering pump that will not dose is usually suffering from the nature of urea solution itself: crystals in valves and lines, crystallization at low temperature, blocked filters, air in the head, wrong concentration or poor quality, or an unmet control interlock. Check the controls, tank, filters, air and valves in that order, flush before every shutdown and keep heat tracing in good order. For a pump configured for urea solution, contact us.