Baghouse Technical Tip:
Protecting Bags During Startup
Startup is the most critical time in a bag’s life
The first 24-48 hours of a filter bag’s operation can be the most critical in determining its overall life expectancy. The new bag’s unprotected fabric is vulnerable to high velocity particulate becoming embedded into the indices of the media. When this happens, trapped dust accelerates the blinding process, increases the average drag/restriction coefficient, and significantly shorten the bag’s life.
Flow rate permeability is an excellent way to measure the life stages and life expectancy of a bag. Permeability for new bags is usually in the range of 25-60 CFM/ft-2. For seasoned bags, the range is usually 5-10 CFM/ft-2. A blinded bag’s permeability may be less than 2 CFM/ft-2. Considering this large disparity in flow rates, it’s easy to see how new bags/compartments are able to receive a lot more gas/dust at a higher velocity than those that are old and blinded.
The key to preventing premature blinding is to follow three simple rules during startup.
1. Apply a compatible protective precoat material to the filter bags before starting the process gas flow.
2. Limit the gas flow to the new bags/compartment; a flow level at or near the design filtering velocity/air to cloth ratio is ideal.
3. Reduce or stop the cleaning energy until the dust cake builds up to a level that requires cleaning.
Starting up a new bag is a critical operation that should not be taken lightly. Following the rules outlined above greatly reduces the chances of premature bag damage. Putting in this effort and attention now will pay dividends later in the form of a longer bag life, and energy/cost savings from reduced operating differential pressure.
Visit our website to learn more about recommended precoat materials and startup procedures for your process. Or, call us anytime for advice: (440) 942-8990.
Precipitator Technical Tip:
Troubleshooting Secondary Voltage Dividers
Ensuring accurate operating signals to voltage controls is the key to optimizing precipitator performance and protecting your equipment. Proper maintenance yields significant long-term benefits.
Maintaining peak performance for precipitator voltage controls requires accurate transformer/rectifier (T/R) feedback signals. These signals are also important tools for troubleshooting sections with poor performance. One of the most important feedback signals is secondary voltage or kV. Although this is one of the most commonly inaccurate signals found in many installations, correcting these errors is relatively simple. Click here to see schematic showing where a voltage divider fits in the circuit.
1. One clue that kV feedback is inaccurate: secondary voltage is operating at unusually low levels, yet the primary voltage indication is normal for the amount of current displayed. If this happens, you can check for inaccuracies by estimating the secondary voltage and comparing the estimate to the actual reading.
Use the following formula to estimate secondary voltage.
Primary Current x Primary Voltage x .7
Here’s an example: a T/R set is running 73 Amps, 440 Volts and 500 Ma (.5A). In this case, you’d fill in the formula as shown below.
73A x 440V x .7 = 44,968 Volts or 45 kV
This method of calculating secondary voltage is a useful troubleshooting tool when you experience a missing or suspicious signal. The formula only works, though, if the conduction angle is above 100 and the other three signals are calibrated.
If you discover that the signal is not, in fact, reading correctly, the next step is to recalibrate and verify the signal using the control manufacturer’s guidelines. Be very cautious when calibrating feedback signals: you’re working on a live control with high voltage!
Signals from voltage dividers allow the voltage control to read and monitor secondary voltage. This protects the T/R set from over-voltage, which can damage diode bridges and the secondary transformer. How can you tell if a voltage divider has failed or is failing? You can reasonably suspect this in the following scenario: a voltage control shows no reading in the secondary voltage readout display, but it is still operating and the other readout values are normal.
When a voltage divider fails, replace it. Many voltage dividers are located in the T/R set tank, which makes them difficult to access. Retrofit dividers are available that can be installed in the high voltage switch enclosure or bus duct—a much more accessible location. Visit our website for examples of retrofit voltage dividers.
Neundorfer Introduces Heavy Duty Voltage Dividers
The previous article discusses the importance of secondary voltage feedback and how to troubleshoot inaccurate voltage divider signals. If you are considering replacing voltage dividers because they have failed or need to be more reliable, check out Neundorfer’s latest innovation. Neundorfer engineer's designed the Heavy Duty Voltage Divider for applications where traditional dividers do not last long because of tough environmental conditions. Click here and scroll down to learn more about this unique design!
2010 Seminar and Workshop Schedule
Increase your knowledge, and earn professional development credits, by attending these learning events.
Fabric Filter Seminar at the Energy Generation Conference. January 26-28, 2010. Bismarck, North Dakota.
Precipitator Workshop. February 1-2, 2010. Neundorfer Training Facility, Willoughby, Ohio.
Fabric Filter Workshop. February 3-4, 2010. Neundorfer Training Facility, Willoughby, Ohio.
Precipitator Users' Group. May 25-27, 2010. Neundorfer Training Facility, Willoughby, Ohio.
New! Precipitator Users' Group Labs. May 25-26, 2010. Neundorfer Training Facility, Willoughby, Ohio.
Autumn in Ohio
It’s autumn in Northeast Ohio, and the foliage colors are beautiful! This photo, taken by Mae in LeRoy Township (about 30 minutes East of Neundorfer’s headquarters in Willoughby), shows a field of mature soybeans, a stray corn plant, and trees adorned in their fall finery.