Introduction
A wet scrubber can be perfectly designed with the correct column diameter, packed height, and L/G ratio, yet still fail to meet its permit limit if wet scrubber operation procedures are not followed. The difference between a scrubber that consistently achieves 99 percent removal and one that drifts between 92 and 96 percent is rarely the equipment. It is the quality of the startup sequence, the frequency of parameter monitoring, the rigor of the maintenance schedule, and the correctness of the shutdown procedure. A scrubber operated according to a written procedure with logged parameters and scheduled maintenance will outlast and outperform an identical scrubber operated by shift preference with no written standards.
This guide covers the complete wet scrubber operation cycle including pre-start checks and startup sequence, normal operation monitoring and chemical dosing adjustment, normal and emergency shutdown procedures, routine maintenance with cleaning methods for exhaust scrubber system cleaning, and seasonal considerations for cold-weather operation and extended shutdowns. Each section includes specific parameters, frequencies, and step-by-step procedures that a plant engineer or operator can apply directly.
Key Takeaways
- The correct startup sequence for a wet scrubber is pump first, then chemical, then fan never the reverse. Starting the fan before the recirculation pump is established can dry out the packing and damage the mist eliminator within minutes.
- Five parameters must be monitored at defined intervals: pH and recirculation flow every 30 minutes, pressure drop and gas temperature every 2 hours, and chemical tank level every shift. A trend of 0.1 pH units per day for 5 consecutive days indicates a sensor problem, not a process change.
- The normal shutdown sequence runs the recirculation pump for 5 minutes after the fan stops to flush accumulated solids from the packing. Skipping this flush allows dissolved solids to precipitate on the packing during idle periods, accelerating scale buildup.
- Three conditions require immediate emergency shutdown: inlet temperature exceeding the material limit, sump level below pump suction minimum, and pH excursion below 4.0 or above 12.0 from chemical feed failure. Distinct alarms for each condition should be posted on the control panel.
- Winterization of outdoor scrubbers requires maintaining recirculation flow at 20 to 30 percent of design during freezing weather. If shutdown is unavoidable, the sump must be drained, piping blown out with compressed air, and the pH sensor removed and stored in buffer solution.
Startup Procedures
Pre-Start Checklist
Before starting a wet scrubber, verify seven items in sequence. First, check the sump liquid level. The level should be at 50 to 60 percent of the sump height, which provides sufficient volume above the pump suction to prevent vortex formation and enough freeboard to contain the normal level rise when the recirculation pump starts. If the level is below 40 percent, add makeup water before startup. If the level is above 70 percent, check whether the automatic makeup valve is stuck open. Second, confirm that the scrubbing liquid chemistry is within the required range. For an HCl scrubber using NaOH, the sump pH should be at or above 8.5 before startup. If the pH is below 7.0, add chemical manually until the pH reaches 8.5 before starting the recirculation pump. For SO2 scrubbers using lime slurry, the target pH is 5.5 to 6.5.
Third, verify that all manual valves in the recirculation line are open and the chemical feed valve is in the correct position. A closed valve on the pump discharge is the most common cause of startup recirculation failure. Fourth, check the chemical tank level. There should be sufficient inventory for at least 24 hours of operation at the expected dosing rate, and standby drums should be available for refill. Fifth, confirm that the mist eliminator wash water supply is available if the system includes automatic wash. Sixth, verify that the fan inlet damper is closed to prevent motor overload during startup. Starting a fan with the damper open draws maximum power and can trip the motor overload protection. Seventh, confirm that the exhaust stack is clear and any outlet damper is in the open position before the fan starts. These seven checks take 10 minutes and prevent the most common startup problems including pump cavitation, fan overload, and chemical overfeed.
Startup Sequence
The correct startup sequence for wet scrubber operation is pump first, then chemical, then fan never the reverse. Starting the fan before the recirculation pump is established can dry out the packing and damage the mist eliminator within minutes. Step one: start the recirculation pump and verify that the discharge pressure reaches the design value within 10 seconds. A discharge pressure reading below the design value indicates a blocked suction strainer, a closed valve, or a pump air lock. If the pressure is low, stop the pump immediately, check the suction conditions, and restart. Step two: verify recirculation flow through the sight glass or flow meter. The flow should stabilize at the design rate within 30 seconds. Step three: start the chemical dosing pump in manual mode and set the stroke rate to the minimum expected dosing rate based on the projected inlet load. For a 500 ppm HCl inlet at 20,000 CFM, the initial NaOH dosing rate should be approximately 50 to 70 percent of the calculated stoichiometric rate to allow the pH controller room to adjust upward.
Step four: after the liquid chemistry has stabilized for 5 to 10 minutes, slowly open the fan inlet damper to 25 percent of full travel. Monitor the fan motor current: if it exceeds the nameplate rated current, reduce the damper opening and investigate the cause. Step five: gradually increase the fan damper to the full design position over 2 to 3 minutes, monitoring the pressure drop across the scrubber. The pressure drop should match the baseline value within plus or minus 10 percent. If the pressure drop is more than 20 percent above baseline at full flow, the packing may be scaled or the mist eliminator may be fouled. Switch the pH controller from manual to automatic mode after the gas flow has stabilized at the design rate and the pH has been at the setpoint for at least 5 minutes. The total startup sequence takes 15 to 20 minutes from the first pump start to full automatic operation.
Normal Operation and Monitoring
Parameter Monitoring Frequency
During normal wet scrubber operation, five parameters must be monitored and recorded at defined intervals. The sump pH and the recirculation flow rate should be checked every 30 minutes and recorded on an operator log sheet. If the pH has drifted more than 0.3 units from the setpoint, the operator should verify that the chemical dosing pump is operating at the correct stroke rate and that the pH sensor has not been fouled by scale buildup. The pressure drop across the packed bed and the gas inlet temperature should be checked every 2 hours and compared with the baseline values recorded at the most recent startup. A pressure drop increase of more than 15 percent above the baseline triggers a packing inspection at the next scheduled shutdown. The chemical tank level should be checked every shift to ensure sufficient inventory for the next 8 hours of operation.
A daily log sheet with entries every 30 minutes for pH and flow, every 2 hours for pressure drop and temperature, and every shift for chemical inventory provides the data needed to detect trends before they become problems. A pH trend that drifts 0.1 units per day over 5 consecutive days indicates a sensor that needs cleaning or calibration, not a process change. A pressure drop trend that increases by 2 percent per week over 4 weeks indicates progressive packing fouling that will require cleaning or replacement. Without the log, these trends are invisible until the scrubber fails a stack test. The log also serves as a legal record during regulatory inspections. An operator who logs every 30-minute pH reading demonstrates that the scrubber was operated within its permitted parameters continuously, even if the continuous monitoring system has a data gap.
Chemical Dosing Adjustment
The chemical dosing rate must be adjusted when the inlet load changes, not held constant at the rate set during the previous shift. A scrubber treating exhaust from a batch chemical reactor experiences inlet HCl concentrations that vary from 100 ppm during idle periods to 800 ppm during active batch cycles. The pH controller must be allowed to respond to these changes automatically through its PID control logic. If the controller is in manual mode, the operator must adjust the dosing pump stroke rate whenever the inlet load changes significantly. The correct adjustment is to increase the stroke rate in direct proportion to the inlet concentration increase. If the inlet concentration doubles from 300 to 600 ppm, the stroke rate should be doubled. A common operator error is to increase the dosing rate by only 20 to 30 percent when the load doubles, which leaves the pH below setpoint and efficiency below design.
The pH reading on the HMI trend display shows whether the adjustment was sufficient within 1 to 2 minutes of the change. If the pH continues to drift downward after the adjustment, the dosing rate was insufficient and should be increased further. If the pH overshoots the setpoint by more than 0.5 units, the dosing increase was too large and should be reduced. The correct dosing rate produces a pH that stabilizes within plus or minus 0.2 units of the setpoint within 3 to 5 minutes after the adjustment. For scrubbers with automatic pH control, the operator should verify that the controller output tracks the load changes correctly and that the dosing pump responds to the controller signal within 5 seconds. A dosing pump that responds sluggishly or sticks at a fixed output regardless of the controller signal needs maintenance before the next batch cycle.
Record Keeping and Logs
An operator log that records pH, pressure drop, flow rate, temperature, and chemical consumption every 30 minutes serves three distinct purposes. It provides the data needed to detect gradual performance decline before it becomes a compliance problem. It creates an auditable record for regulatory inspections. And it documents the relationship between process conditions and scrubber performance, which helps the engineering team diagnose problems and optimize operating setpoints. The log should be reviewed by the shift supervisor at the end of each shift and by the plant engineer weekly. Any parameter that shows a consistent trend in one direction increasing pressure drop, declining pH, rising outlet temperature over three consecutive shifts should be investigated. The log is also the primary data source for the monthly compliance report. A scrubber with complete operator logs can demonstrate continuous compliance even if the continuous emissions monitor has a data gap due to calibration or maintenance. The log forms should be kept on file for a minimum of 3 years or as required by the facility air permit.
Shutdown Procedures
Normal Shutdown Sequence
A planned wet scrubber shutdown during normal wet scrubber operation follows a five-step sequence that prevents chemical accumulation, solid precipitation, and damage to internal components. Step one: switch the pH controller from automatic to manual mode 10 minutes before shutdown. This prevents the controller from adding fresh chemical after the recirculation pump stops, which would leave unreacted chemical in the sump and could cause a pH excursion during the next startup when the pump restarts and mixes the concentrated chemical. Step two: close the fan inlet damper to minimum position and stop the fan. The damper should be closed slowly over 10 to 15 seconds to prevent a pressure surge that can force liquid out of the mist eliminator drain channels.
Step three: allow the recirculation pump to continue running for 5 minutes after the fan stops. This flush cycle removes any suspended solids that have settled in the sump bottom or are clinging to the packing surface, carrying them to the filter or drain. The flush water flow should be at the normal recirculation rate. Step four: if the shutdown will last longer than 24 hours, perform a blowdown cycle to reduce the dissolved solids concentration in the sump to below 50 percent of the normal blowdown setpoint. This prevents salt precipitation in the sump, pump casing, and piping during the idle period. For an HCl-NaOH scrubber, the conductivity should be reduced from the normal 80,000 to 100,000 microsiemens per centimeter to below 40,000. Step five: stop the recirculation pump and verify that the chemical feed valve is closed. For scrubbers equipped with automatic mist eliminator wash, run one complete wash cycle after the fan stops and before the pump stops. This final wash removes any acid droplets or particulate that have accumulated on the mist eliminator vanes, preventing corrosion during the idle period.
Emergency Shutdown Conditions
Three conditions require an immediate emergency shutdown of the wet scrubber. First, if the gas inlet temperature exceeds the maximum safe operating temperature of the vessel material 185 degF for polypropylene, 220 degF for FRP, and 400 degF for SS316L the fan must be stopped immediately and the inlet damper closed to prevent thermal damage to the shell. Continued operation above the material temperature limit can cause the shell to soften, warp, or fail catastrophically within minutes. Second, if the sump level drops below the pump suction minimum, the recirculation pump must be stopped within 30 seconds to prevent pump damage from dry running. A pump that runs dry for more than 60 seconds can destroy the mechanical seal and damage the impeller. Third, if a chemical feed system failure such as a ruptured caustic line or a chemical pump that runs away at maximum speed causes the sump pH to drop below 4.0 or rise above 12.0, the scrubber should be shut down and the sump contents diluted or neutralized before restart.
Each of these conditions should trigger a dedicated alarm on the scrubber control panel that is distinct from the normal process deviation alarms. The alarm should be audible and visible, and the alarm message should state the condition and the required operator action. The emergency shutdown procedure should be posted on the control panel and included in the operator training manual. A drill of the emergency shutdown procedure should be conducted quarterly with all operators who work on the scrubber. The drill should simulate each of the three conditions and verify that the operator can complete the required steps within 2 minutes of the alarm. After any actual emergency shutdown, the cause should be investigated, the corrective action documented, and the incident reviewed with all operators before the scrubber is restarted.
Routine Maintenance and Cleaning
Daily, Weekly, and Monthly Maintenance Tasks
A well-planned wet scrubber operation includes maintenance tasks at three intervals. Daily tasks take approximately 15 minutes and include checking the sump pH with a handheld meter to cross-check the continuous pH sensor, inspecting the sight glass for proper recirculation flow, listening for unusual noises or vibrations from the pump and fan, and verifying that the chemical tank level is adequate for the next 24 hours. A handheld pH reading that differs from the continuous reading by more than 0.3 pH units indicates that the continuous sensor needs cleaning or calibration. The daily check also includes a visual inspection of the area around the scrubber for leaks, puddles, or chemical odors.
Weekly tasks take 1 to 2 hours and include cleaning the pH sensor glass bulb with a soft cloth and dilute hydrochloric acid to remove scale buildup, inspecting the spray nozzles through the access port for visible clogging or wear, checking the fan belt tension and alignment, and lubricating pump bearings according to the manufacturer schedule. The pH sensor cleaning alone restores sensor accuracy by 0.1 to 0.3 pH units in most cases and should not be skipped. Monthly tasks take 4 to 8 hours and include a full visual inspection of the packed bed surface through the manway for levelness and scale accumulation, an inspection of the mist eliminator for fouling or damage, a check of the chemical dosing pump calibration by measuring the actual flow rate against the setpoint, and a review of the operator log for trends. The monthly review should identify any parameter that has changed by more than 10 percent from the previous month and initiate a corrective action if needed.
Exhaust Scrubber System Cleaning Procedures
Exhaust scrubber system cleaning is the most demanding maintenance task and typically coincides with a scheduled process shutdown. The frequency depends on the scaling rate specific to the application. For scrubbers treating relatively clean gas from natural gas combustion or clean room exhaust, annual cleaning is sufficient. For scrubbers treating dirty gas from chemical reactors, waste incineration, or metal finishing with high particulate loading, semi-annual cleaning is recommended. The cleaning procedure begins with a full system electrical lockout and tagout. The sump is drained and the sludge is tested for hazardous characteristics before disposal according to the facility waste management plan. The packing is inspected through the manway using a strong flashlight and a mirror to assess scale coverage on the underside of the packing elements.
If scale accumulation is less than 1/8 inch thick and covers less than 30 percent of the visible packing surface, the packing can be cleaned in place. The cleaning method is high-pressure water washing at 5,000 to 8,000 PSI using a rotating nozzle inserted through the manway. The wash water flows from top to bottom through the bed, carrying loosened scale to the sump drain. The wash should continue until the water exiting the sump runs clear, typically 20 to 40 minutes depending on the column diameter and bed depth. If the scale is thicker than 1/8 inch or covers more than 30 percent of the surface, the packing should be removed and replaced. The labor and water cost of extended in-place cleaning exceeds the material cost of new packing in most cases.
The mist eliminator should be cleaned during the same shutdown. Chevron vane mist eliminators are removed from the vessel and washed with a pressure washer at 3,000 to 5,000 PSI from both sides. Mesh pads that are heavily fouled with scale or particulate are replaced rather than cleaned. The cleaning process rarely restores the original open area of a mesh pad, and the labor cost of cleaning a heavily fouled pad exceeds the material cost of a new one. The recirculation pump mechanical seal should be inspected and replaced if there is any sign of fluid leakage at the seal face. Spray nozzles should be removed, inspected, and replaced if the orifice diameter has worn by more than 10 percent of the original dimension. After completion of all cleaning and replacement tasks, the scrubber manways are closed, the sump is filled with fresh water, and the recirculation system is leak-tested at operating pressure before chemicals are added and the scrubber is returned to service.
Seasonal and Extended Shutdown Considerations
Winterization of Outdoor Scrubbers
Wet scrubber operation in freezing climates requires winterization measures before the first frost. The scrubbing liquid in the sump, recirculation piping, and pump casing can freeze when the scrubber is shut down during cold weather. Water expands by approximately 9 percent when it freezes, and this expansion can crack an FRP or PP sump, burst schedule 80 PVC piping, and destroy the pump casing. The cost of repairing freeze damage to a 10-foot diameter FRP scrubber can exceed $15,000. The first line of defense is to maintain recirculation flow continuously during freezing weather, even when the scrubber is not treating gas. A minimum recirculation flow of 20 to 30 percent of the design rate through a bypass line back to the sump keeps the liquid moving and prevents ice formation. The recirculation pump generates enough heat through friction to keep the sump temperature a few degrees above ambient.
If continuous recirculation is not possible because of energy cost or pump wear concerns, the scrubber must be fully winterized before each shutdown during freezing weather. The sump is drained completely and any residual sludge is removed. The recirculation piping is blown out with compressed air at 30 to 50 PSI through a drain port at the lowest point in each pipe run. All low-point drains are opened and left open. The pH sensor is removed from the flow tee and stored in pH 4 buffer solution to prevent the glass bulb from freezing and cracking. The chemical feed line is flushed with water and drained. The fan inlet and outlet dampers are closed to prevent wind-driven snow or rain from entering the vessel. A sign is posted on the control panel indicating that the scrubber is winterized and must be re-commissioned before startup. The winterization procedure takes 2 to 3 hours for a typical skid-mounted system.
Extended Shutdown Storage
When a scrubber will be out of service for more than 30 days, additional preservation steps prevent corrosion, biological growth, and seal degradation. The sump is drained, cleaned of all sludge, and allowed to dry completely. All manways are left open and covered with mesh screening to provide ventilation and prevent birds or debris from entering the vessel. The open manways allow moisture to evaporate rather than condense on the internal surfaces, which prevents corrosion of metal components and biological growth on the packing. The recirculation pump shaft is rotated manually by one full turn every 30 days to prevent the mechanical seal faces from sticking together. If the pump has been idle for more than 90 days, the mechanical seal should be replaced before restart because the elastomer seal faces can harden and develop leaks when left dry for extended periods.
The fan bearings are greased before shutdown and the fan shaft is rotated quarterly to prevent bearing flat spots. All instrument sensors are removed, cleaned, and stored in a climate-controlled area. The pH sensor is stored in pH 4 buffer solution in a sealed container, never dry. The conductivity sensor is stored dry with the sensing surfaces protected from physical damage. Pressure transmitter impulse lines are blown out with compressed air to remove any trapped liquid that could freeze or corrode the tubing. Before restarting after an extended shutdown, the full pre-start checklist must be completed and the recirculation system must be leak-tested with fresh water at operating pressure for 30 minutes before any chemical is added. The first 24 hours of operation after an extended shutdown should be monitored at increased frequency with readings every 15 minutes to catch any startup issues before they cause damage or emission exceedances.
Frequently Asked Questions About Wet Scrubber Operation
What is the most important daily check on a wet scrubber?
The most important daily check is the sump pH compared with the setpoint. A pH deviation of more than 0.3 units from setpoint indicates either a chemical dosing problem or a sensor that needs cleaning. Cross-check the continuous pH reading with a handheld meter once per shift. If the two readings differ by more than 0.3 pH units, the continuous sensor needs cleaning or calibration.
How often should the pH sensor be cleaned?
The pH sensor should be cleaned weekly by removing it from the flow tee and wiping the glass bulb with a soft cloth and dilute hydrochloric acid. A sensor that has not been cleaned for one month can drift by 0.2 pH units, enough to reduce removal efficiency from 99 percent to approximately 96 percent. Regular cleaning is the single most cost-effective maintenance task for any wet scrubber.
What is the correct startup sequence for a wet scrubber?
Start the recirculation pump first, then the chemical dosing pump in manual mode, allow 5 to 10 minutes for the liquid chemistry to stabilize, then start the fan with the inlet damper at 25 percent and gradually open to full design position over 2 to 3 minutes. See the Startup Procedures section above for the complete seven-item pre-start checklist and five-step sequence.
When should a scrubber be shut down immediately?
Shut down immediately if the gas inlet temperature exceeds the vessel material limit 185 degF for PP, 220 degF for FRP, 400 degF for SS316L. Shut down if the sump level drops below the pump suction minimum. Shut down if a chemical feed failure causes the sump pH to drop below 4.0 or rise above 12.0. See the Emergency Shutdown section above for the detailed procedure for each condition.
How do I winterize an outdoor scrubber?
The preferred method is to maintain recirculation flow at 20 to 30 percent of the design rate continuously during freezing weather. If continuous recirculation is not possible, drain the sump completely, blow out the piping with compressed air at 30 to 50 PSI, remove and store the pH sensor in pH 4 buffer solution, and flush the chemical feed line. See the Seasonal Considerations section above for the complete procedure.
How long should operator logs be kept?
Operator logs should be kept on file for a minimum of 3 years or as required by the facility air permit. The logs serve as the primary record for demonstrating continuous compliance during regulatory inspections and are also the key data source for detecting efficiency trends. For wet scrubber inspection guidance and record keeping requirements, see the EPA wet scrubber monitoring guide.
Conclusion
Proper wet scrubber operation is the difference between a system that consistently achieves its design removal efficiency and one that shows declining performance over time. The seven-item pre-start checklist, the five-step startup sequence with the pump-first-then-chemical-then-fan order, the 30-minute monitoring interval for pH and recirculation flow, the normal and emergency shutdown procedures, and the daily-weekly-monthly maintenance schedule together form a complete operating discipline that protects the equipment investment and maintains compliance. For design methodology and sizing, see our wet scrubber design guide. For control system configuration, see our scrubber control system guide. The seasonal winterization and extended shutdown procedures prevent damage that can take a scrubber out of service for weeks at a time. If your scrubber needs a control panel upgrade to support better operator monitoring, or if you are evaluating a replacement scrubber, contact our engineering team or browse our wet scrubber product range.
