The wet scrubber is comprised of two important operational elements. Water flow and air flow, in balance. If either of these is not set up correctly, a plug may develop.
The water flow to the spray nozzles is counter-current to the air flow and needs to be adjusted to wash down any lime or powder laden vapours.
- Too much water flow and unacceptable process dilution might result.
- Too little water flow will result in the venturi blower pulling in airborne powder that has not been washed down by the nozzles. The powder will then accumulate over time and eventually cause plugging.
Airflow needs to be adjusted using the blower discharge damper or the balancing tee on the scrubber. The airflow should be set to pull airborne dust and vapour into the wet scrubber.
- Too little airflow and dust and vapour will not be exhausted from the mixing vessel.
- Too much airflow will result in the water spray, along with airborne powder, to be pulled into the ducting. This will cause plugging in very short period of time.
Most dry chemical storage silos utilize the guided wave radar method of level measurement. This means that a radar probe is suspended by a rope from the transmitter on the roof of the silo, to the point where the main silo section meets the cone. The radar probe does not extend all the way into the cone and usually only measures product level in the cylindrical portion of the silo. Depending on the product being stored and the size of the silo, the ‘unmeasured’ cone section of the silo may account for a significant amount of the storage capacity.
This issue can be addressed by having an appropriate offset in the calibration of the instrument.
Filter cartridges need to be changed before they become plugged.
Many operating plants schedule a change out of their dust collector filters on an annual basis. Some sites, with difficult to handle materials schedule these changes twice per year.
How often you need to change your filter cartridges will depend on the number of product deliveries that your plant receives. A site that receives six deliveries per day will have to change out their dust collector filters more often than a site that receives one delivery every two weeks.
These filters should also be changed out when the normal built in cleaning cycle, whether reverse pulse jet or shaker, does not adequately clean these filters.
Operating Tip: check to be sure that your compressed air source is clean, dry air - and that the minimum recommended operating pressure is maintained during the entire silo fill cycle.
Effective lime slaking can be accomplished with both Pebbled or Powdered Quicklime.
What’s best for you will depend on your slaking system and your ability to deal with the potential extra dust created by using only powdered quicklime. It is generally not necessary to use this more expensive powdered quicklime to create high quality / highly reactive milk of lime in a lime slaker.
High calcium pebbled quicklime is perfectly suitable for most lime slakers. It is a common misconception that powdered quicklime contains less of a grit component.
As a minimum, follow all standard on-site policies and procedures for working around operating equipment.
Lock out / Tag out procedures are critical.
In addition, slaked lime can be hot and caustic, so be sure to wear appropriate personal protective equipment (PPE).
Typical appropriate PPE will include eye protection, face shield, gloves and coveralls. In dusty conditions a dust mask, or a half mask respirator, may be required. Make sure to dilute and cool down a slaker before working on any wetted components.
Your VERTIMILL grinding media should be recharged whenever your VERTIMILL drive motor power draw drops below the minimum set point. More frequent charging, adding smaller amounts, will result in steadier power draw and steadier performance.
Modern VERTIMILLS used in lime slaking duty are designed to operate best when this power draw is between 70% and 80% of of full load amperage (FLA).
Never exceed 80% of FLA on the Vertimill drive motor.
Power draw dropping can also be an indicator of screw liner wear.
Safety First! The level of your grinding media in the Vertimill can only be checked when the Vertimill is properly Locked Out / Tagged Out!
This is called the ‘Static Ball Charge Measurement’ in the Vertimill manual. Once the mill is stopped, a calibrated rope with a non-magnetic weight securely fastened at one end can be dropped in the ball charge chute or down the main Vertimill body inspection hatch. (Often the ball charge chute is angled and convoluted so the inspection hatch is more practical.) This ball charge depth is compared to the depth measured when the Vertimill was first commissioned with a new and full ball charge.
See the attached sketch or you can call us for help determining the correct level.
One important side note - if the static ball charge rises by one foot (0.30 m) as compared to when it was first commissioned, in order to maintain the same 80% FLA maximum, your screw liners are worn and require replacement.
Some things to look for are:
- Verify that the slaker temperature sensor is reading correctly. (We suggest you measure slurry temperature with a hand held Infrared (IR) Temperature Thermometer.
- Look for product leaks from gaskets or rubber connections connecting different parts of the feed system.
- Ensure correct flow through water lines to the slaker seals, wet scrubber and vibrating grit screen.
- Look for quicklime buildup in/around quicklime inlet.
- Slaked lime buildup at outlet or inside the vibrating grit screen.
- Sufficient exhaust suction from the wet scrubber.
- Check the HMI/DCS alarm history.
A properly adjusted and lubricated seal assembly should only require weekly adjustments and perhaps monthly packing additions.
The most common type of slaker seal in horizontal agitator shaft detention slakers is a square cross-section ‘donut’ packing. Depending on the model and size of the slaker, there might be 3, 4 or 5 packing rings in a seal assembly. The first step is always to adjust the packing by tightening the Seal Packing Retainer (also known as a follower gland). Tighten just as much as it takes to stop a leak. If the Seal Packing Retainer is bottomed out, packing donuts must be added or replaced.
Always verify that adequate packing seal flush water is flowing. Clean seal flush water is critical. Replacing the flush water with air or grease will not work.
If it seems that packing has to be replaced too often, look for worn out agitator shafts or worn out stub shafts as a possible cause. Inspect and replace these as necessary.
The PVRV is a key safety component for the silo. Whenever a PVRV activates (either under pressure or vacuum) - you must investigate!
A functioning PVRV can prevent a catastrophic structural silo failure. If it activates, the silo has either been over pressurized, or was under excessive vacuum.
Find out why:
- Over-pressurized? A common cause is plugged or blinded-off filter elements in the Bin Vent Filter (dust filter) on top of the silo. Inspect, clean and/or replace filter elements as necessary.
- Excessive Vacuum? Either your Bin Vent Filter cartridges are plugged and not allowing air flow into the silo, or you’ve experienced bridging in the silo and sudden movement of product (which is also dangerous).
- Perhaps your silo was over-filled? One possible cause is that the silo has actually been over filled. The cause might be a faulty silo level indicator or might be non-functioning high level alarms.
Under no circumstances should you ever bypass or prevent the normal operation of a PVRV!
Some bulk chemicals may degrade over time or absorb moisture from the air; some bulk chemicals may even solidify over time so it may be best to empty the storage silo in advance of a long term shut down. This will depend on your specific circumstances.
In addition, this may be an opportune time to inspect the silo’s internal structure and any instrumentation inside the silo.
A feeder calibration is a method of collecting a sample or product from a feed to correlate the RPM (speed) of the feeder to the amount (weight) of product being delivered.
The slaking control scheme requires accurate information about the quantity of quicklime being discharged into the slaker in order to properly calculate the correct flow of water to best react with the lime. A minor discrepancy in quicklime addition is compensated for in the controller. If the expected quicklime feed rate vs actual quicklime feed rate is significantly different, the slaker may overheat or not generate enough temperature resulting in a poorer quality of lime slurry.
The most common reason a slaker will be alarming with a High-High temperature alarm is if there is too much quicklime being fed into the slaker relative to the amount of water or if the incoming slaking water is bringing heat with it. Check the actual output of the feed device (see “What is ‘feeder calibration’?).
Other possible reasons include:
- Quicklime is flooding though the feed device
- Water supply is not supplying enough water (restrictions)
- Look for possible low flow alarm
- Incoming slaker feed water temperature is too high
- The temperature sensing instrument is failing
- Check the temperature of slaking water coming in before it enters the lime slaking reaction zone.
Automatic slaker controls initially adjust the water flow to match the quicklime feed rate (often to a set ratio of 4:1 water to lime by weight). Once a stable operating temperature is reached, automatic controls fine tune the water flow to maintain a final slaking temperature set point.
Automating the process ensures that the slaker is running at the optimal temperature for lime slaking - producing the most effective and most efficient lime slurry for your process. Having the most effective and efficient lime slurry will save on quicklime costs and will indirectly reduce the carbon foot print of lime use by reducing lime consumption.