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Technical Tips - Fabric Filters (Baghouses)


Industry Updates

 

Technical Tips - Electrostatic Precipitator (ESP)

 




 Improving ESP and Baghouse Performance with Flow Optimization [Save as PDF]

Optimizing flow distribution in precipitators and fabric filters (baghouses) can have a significant, positive impact on performance of the equipment by reducing outlet emissions (particulate), lowering opacity and cutting production restrictions. Flow optimization can also increase life expectancy, reduce maintenance costs (e.g. less ductwork erosion, precipitator component replacements, fabric filter bag wear). Reducing differential pressure lowers total plant operating costs.

Recognizing the importance of flow optimization, Neundorfer purchased the assets of APCO Services in 2007 to complement our strong capabilities in process consulting. More recently, we expanded our Ohio facility by purchasing a 17,000 sq ft building and converted it into a state-of-the-art model shop, further streamlining our flow modeling process which includes CFD and physical modeling. We're now able to handle more flow modeling projects, and complete them fast.

Providing results quickly is our goal, given that flow optimization modifications must to be implemented during outages. While flow optimization studies can typically take 14-20 weeks or more to complete, Neundorfer has reduced this time to less than 12 weeks.

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 Baghouse Design: Factors That Affect Fabric Filter Performance and Lifespan [Save as PDF]

Baghouse / Fabric Filter In the wake of new EPA regulations, both proposed and recently enacted, many power utilities and industrial plants are considering adding new baghouses to aid with their pollution control strategy. Before choosing a vendor and moving ahead with construction it’s important to look at some important design and operational features unique to baghouses that may not be considered by your vendor or even the A&E.

From a performance standpoint, unlike with electrostatic precipitators (ESPs), collection efficiency is not the primary consideration. Baghouses are usually 99.9% efficient, unless a bag breaks or there is some other major system error.

However, from a long term operational and maintenance perspective, several other factors are paramount: long term pressure drop limit, filter bag life, compressed air usage, instrumentation and maintenance/service accessibility during rebagging, bag length with cage design, interstitial velocities during online cleaning and, finally, air-to-cloth ratio.

Air-to-cloth ratio is a good example of a key factor that may be influenced—for good or bad—by things like industry trends, regulatory pressures, or budget cuts.

In the past several years, which one could reasonably characterize as a depressed market for baghouse vendors, an unfortunate trend has emerged: manipulating air-to-cloth ratio in baghouses by building taller and taller units to accommodate extra long bags (or forcing longer bags into existing design units).

Where once fabric filter bags generally topped out in length at around 20-26 feet, it’s becoming more and more common to see bags up to 1/3 longer than this. The problem with such designs is that they don’t take into account the mechanics of how baghouses collect particulate nor how bags are most efficiently cleaned and their core velocities.

“The devil is in the details,” says Jim Parsons, Senior Environmental Consultant at Neundorfer. “Taller bags might seem like a good idea up front to meet your specified air-to-cloth ratio, but in the long run this can create some serious cleaning and reliability issues. It pays to have someone with practical knowledge about baghouse design review your fabric filter specifications before going out for bid.”

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 Debunking Common Baghouse Misconceptions [Save as PDF]

One of the most effective ways to extend the life of your baghouse is a return to the basics. Appropriate start-up procedures and a policy of cleaning based on differential set points, for example, are essentials that should be standard at every plant using a baghouse.

To grasp why such basics are so important, though, it helps to understand the essentials of how baghouses work. This article briefly debunks the three most common baghouse misconceptions. Get these points down, and you're well on your way to maximizing the lifespan and efficiency of your baghouse.

Please note: these misconceptions do not apply if you are using PTFE or Teflon-coated bags.

Misconception #1: Bags Filter

Fabric filter bags themselves do not perform the fine filtering of particles from the gas stream. Instead, it is the control layer of dust on the bags that does the filtering. Gas flows through the filter bags, and particles collect on the control dust layer. This is why proper start-up of new bags is so critical; without the control layer, particles get embedded in the fabric itself, eventually leading to prematurely blinded bags.

Misconception #2: Over-cleaning is Better Than Under-Cleaning

In most areas of our lives, the cleaner something is the better. Not so with fabric filter bags. When bags are cleaned excessively, the control layer of dust gets knocked off, and the fabric is left exposed. If a new control layer is not applied, fine dust particles get embedded in the fabric, leading to blinded bags. The decision about when to clean should always be based on differential pressure (DP) rather than time elapsed since the last cleaning. This saves bags and energy, and cuts down on emissions. (In a healthy baghouse, most emissions occur during cleaning.)

Misconception #3: Filtering Wears Out Bags

There's another reason not to clean fabric filter bags excessively: it is not filtering that wears out the fabric, but rather the cleaning process. Every time bags are cleaned, the fabric gets flexed; the indices open up, dust flows through, and some of that dust gets trapped. Excessive flexing also causes the fibers to break down. All bags eventually wear out, but they last a lot longer if cleaning is done only when necessary.

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  How Ammonia Slip Affects Fabric Filter (Baghouse) BagsHow Ammonia Slip Affects Your Baghouse [Save as PDF

Many coal-fired utility boilers now use Selective Catalyst Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR) to reduce Nitrogen Oxide (NOx) emissions. Ammonia and urea are common reagents used in SCRs and SNCRs to enhance the reaction and boost the removal of NOx. As with most additives, the right amount is okay but too much can have downstream ramifications for other equipment, such as the baghouse.

Even the most efficient SCR or SNCR system will experience a little bit of ammonia slip. But, when it becomes excessive, problems begin to occur as the dust becomes sticky. Back-end cooler compartments are typically where problems first occur. For example, when load is reduced and gas temperature drops to the dew point, dust becomes difficult to remove from filter bags.

Sticky dust results in ineffective cleaning, which snowballs into other issues: high differential pressure, increased cleaning frequency, shortened bag life. To avoid these problems, we recommend monitoring your bypass slip closely, especially at end compartments. Staying on top of this will prevent a lot of headaches.

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 Identifying Failed Pulse Jet Baghouse Diaphragm Valves [Save as PDF]

If you operate or maintain a Pulse Jet baghouse, you know how difficult it is to identify diaphragm valves that are not functioning. Once you have found the one(s) that not operating they are relatively easy to repair, yet in the meantime, they can adversely affect bag cleaning, differential pressure and consume large quantities of valuable compressed air.

A simple but effective method to find those not functioning is to gently place a small wad of tissue paper in the exhaust port of each valve. Allow all valves to cycle through a couple of times and check to see which one(s) still have the tissue left in the exhaust port.  Once identified, it’s typically just a matter of installing a new diaphragm kit or replacing the coil or solenoid and you're back to hitting on all valves.

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 Protecting Fabric Filter Bags During Startup [Save as PDF]

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.

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 Use Differential Pressure Set Points to Prevent Baghouse Over-cleaning [Save as PDF]

The typical filter media in a baghouse, with the exception of PTFE, relies on a controlled dust layer to capture fine particulate and protect bags from premature blinding restriction. (Blinding occurs when particulate becomes trapped within the fabric's indices.) If this dust layer is constantly being removed due to over-cleaning, the result is excessive emissions and greatly reduced bag life.

Preventing over-cleaning is best done by controlling the baghouse cleaning/dust layer based on differential pressure set point--High and Low. The High set point should initiate cleaning, and the Low set point should stop it.

Both settings are critical, but the Low set point is most important because it optimizes collection efficiency and bag life by ensuring the control dust layer is not being removed. This Low set point should be the minimum differential pressure you would ever want the collector to operate at under normal operating conditions.

Set points vary somewhat depending on the collector's design and the bag media being used. However, a typical range is 3.0 inches of water for Low and 6.5 inches for High. Check with your bag supplier for operating range recommendations.

Remember: over-cleaning your baghouse is worse than under-cleaning. Always clean based on differential set points to maximize collection efficiency and bag life.

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 Pulse Jet Baghouse (Fabric Filter)
            Valve SequencingUse Proper Pulse Jet Valve Sequencing to Optimize Cleaning [Save as PDF]

The sequencing and timing of pulse jet baghouse cleaning diaphragm valves is critical to optimal performance. This is particularly true if your baghouse is cleaned on-line. Never activate these valves in sequential order; if you do, a significant amount of dust will remain in the collector.

If you clean in sequential order, material from one row of filter bags simply gets moved to the row previously cleaned. This not only results in poor cleaning but tends to retain very fine particles on the filter bags and can lead to premature blinding.

Therefore, cleaning in a random, non-sequential order distances rows from each other and greatly reduces both re-entrainment and potential premature blinding of bags.

One way to separate rows during cleaning is to re-arrange the order of solenoid wires that lead to each valve. Most of the time, this is as simple as moving the wires on the compartment’s solenoid junction strip. In a large, multi-compartment P/J collector with PLC controllers, an even better approach to further separate the cleaning rows is sequencing each valve in separate compartments.

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 Sonic Horns Enhance
            Cleaning for R/A Fabric Filter and Shaker Baghouse Cleaning Use Sonic Horns to Enhance R/A and Shaker Cleaning [Save as PDF]

During the past 25 years, sonic horns--also known as acoustic horns--have been used successfully for supplemental cleaning energy in reverse air and shaker baghouses. The keyword here is "supplemental;" these horns are high-intensity, low-frequency generators that act on dust particles to break their adhesion/bond. Without added energy from other cleaning sources (reverse air, shaking), horns alone cannot effectively clean a baghouse over the long term.

That being said, supplemental acoustic energy can have a significantly positive effect on R/A baghouse and/or shaker cleaning. It's not uncommon to see over three inches of differential pressure improvement just by adding horns.

Not all acoustic horns are the same, and some work better in certain settings than others. The two most important factors to look for when choosing and installing horns: concentrated, low fundamental output frequency, and engineered placement within the collector so that the low tones are not canceled out.

If your baghouse already has horns installed but they don't seem to be having much of an effect, you'll want to make sure they are functioning properly. If the horns have been in place for many years, consider an upgrade. Newer models have a much more focused, low fundamental frequency (around 60 Hz compared with 125 Hz for older models). The supplemental energy from one newer horn is equivalent to about two older ones.

Acoustic horns can also be installed in pulse jet baghouse hoppers to help prevent ash buildup. If wet ash is a common problem, or hoppers aren't deep/steep enough to handle typical ash load without plugging, adding acoustic horns is often a smart move. For this application, the horns are placed into a cylinder which is mounted on the hopper sidewalls. The head of the horn sticks out the side, and the bell protrudes inside.

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 Electrostatic Precipitator Outage Startup Best Practices [Save as PDF]

Prior to an outage, be sure to record a set of benchmark T/R readings, and particularly for a major outage, a set of V/I curves as well. Then, plan to run a set of post-outage readings, once the unit is up and running normally, for comparison.

By recording T/R readings even for short outages, you will be able to see potential areas that need attention during this outage or the next one. You may also be able to address an issue that could have caused you to reduce process production if left unattended.

The V/I curves provide a benchmark of clearance issues in the box and show which T/R’s are functioning properly versus those that need to be addressed. V/I curves will also help show the effects of other performance factors such as coal and temperature changes.

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Evaluating Electrostatic Precipitator Rapper Performance [Save as PDF]

Electrostatic Precipitator RappersThe operation of all rapping systems (collecting plates, discharge electrodes and flow distribution devices) is critical to the performance of most ESPs. Failure of a single rapper or of an entire rapping system can occur. This can lead to excessive buildup on the affected precipitator components.

Collecting Plate Buildup
Excessive dust buildup on the collecting plates can lead to severe reductions in precipitator current, increase in precipitator voltage and re-entrainment caused by uncontrolled release of dust from the collecting plates.

Discharge Electrode Buildup
Buildup on discharge electrodes will inhibit corona formation. This will change the charging of particles by either increasing the “electric” diameter of the discharge electrode or wire by accumulation of low resistivity dust or blocking the flow of current altogether by accumulation of high resistivity dust.

Dust, Voltage and Sparkover
Any increase in precipitator voltage by dust buildup on plates or wires will increase the spark rate and dust re-entrainment and thus further reduce the collecting efficiency.

Dust on Gas Distribution Systems
Buildup of dust on gas distribution devices such as perforated plates can lead to partial or almost total blockage. Blockage will cause an increase in pressure drop, or channeling of high velocity gas streams. Blockage and channeling disrupt operation and can severely reduce collecting efficiency.

Rapper Distribution
Individual rappers normally serve a precipitator section smaller than an electrical bus section. Usually, the bus section energized by one transformer/rectifier has several collecting plate and discharge electrode rappers. The failure of one rapper can substantially change the electrical power input into the affected bus section.

Rapper Failures

Electromagnetic rapper system failures can be traced to malfunctions or damage of components, such as:

  • Electrical power supplies
  • Rapper Controls
  • Wiring
  • Connecting links and shafts
  • Rapper

Pneumatic rapping system failures are most often traced to:
  • Compressed air supplies
  • Filter, regulation, lubricators
  • Selenoid valves
  • Connecting links
  • Rappers

 

Total failure of an entire rapping system is easily recognized and typically, easily corrected. Total failure of one or more individual rappers is less obvious and much harder to detect. Once detected, the rapper failure is usually easy to fix.

The partial failure or one or more rappers due to a functional deficiency that decreases the rapping efficacy to near zero is much harder to spot and is also more difficult to remedy.

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 Fuel Changes and ESP Performance [ Save as PDF]

If you have frequent changes in coal supply, or your plant has made the changeover to low sulfur and/or low sodium coal, you may also be experiencing increased opacity or even derates related to the performance of your electrostatic precipitator. SO3 conditioning may just be the solution.

Fly ash resistivity increases when using low sodium or low sulfur fuels, such as PRB coal.  SO3 (or sulfur trioxide) conditioning helps achieve optimal dust resistivity and ESP performance - either molten or granular sulfur can work, depending on the application.  Whether molten or granular sulfur is used, Neundorfer offers unique technology to ensure improved performance. Our patented remote converters make it possible to:

  • Locate the sulfur skid near the sulfur storage, reducing transport hassles
  • Place the converter near the precipitator, optimizing SO3 temperatures to avoid problem pluggage and prolong catalyst life.


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 How Boiler Performance Impacts Electrostatic Precipitators [Save as PDF]

The different areas of a power utility or industrial plant are sometimes operated as if they’re isolated from one another, when in fact every piece of equipment in the system eventually impacts every other piece of equipment. This article looks at some of the ways front-end operations (mills, boiler) impact
the performance of electrostatic precipitators on the back end.

Flue gas volume, temperature and composition, as well as particulate size and composition, is largely determined by how well combustion equipment is operated and maintained. These two sets of factors have a direct, significant impact on a plant’s ability to comply with air pollution limits set by Federal and state regulators.

Three factors in particular are especially useful when considering the effectiveness and efficiency of air pollution control systems.

1.       Unburned carbon in fly ash

2.       High temperatures at the air heater exit and/or ESP inlet

3.       Air in-leakage

The power plant is an integrated system; all parts affect one
            another.

 Unburned Carbon in Ash (Loss on Ignition, LOI)

Electrostatic precipitators (ESPs) don't do a very good job of collecting carbon; these particles re-entrain easily because their resistivity levels are low. ESP performance starts to noticeably degrade, and stack opacity levels to rise, when the ash contains more than 10 percent unburned carbon. High carbon concentration can also cause increased sparking and reduce secondary voltage levels in the ESP.

At most plants, carbon content in ash is measured regularly. If not, it is a relatively easy matter to send ash samples from the ESP hoppers to a lab (such as Neundorfer's) for analysis.

When coal is not pulverized properly, and/or there is poor combustion air distribution, the result is incomplete combustion and unburned carbon. Reducing unburned carbon means you are wringing more power out of the same amount of fuel. Optimizing your combustion system isn't just about cutting emissions; it might also reduce fuel costs.

High Air Heater Exit/ESP Inlet Temperatures

The air heater is usually the last piece of equipment before a cold-side ESP. It transfers some heat from the flue gas back into the boiler system to pre-heat combustion air. This significantly reduces the temperature of flue gas heading downstream to the ESP, from about 700F to about 300F.

Flue gas temperature is critical to ESP performance; both ash resistivity and gas volume increase with higher temperatures. The optimal operating temperature for a cold-side ESP is in the 280F-320F range. Temperatures above 350F usually have a significantly negative impact on ESP performance.

At most plants, air heater exit temperature readings are readily available in the DCS.

If air heater temperatures are higher than normal, testing the combustion system is an effective way to determine why. High exit gas temperatures can be associated with excess air—indicating poor combustion control, which wastes energy and increases flue gas volume. Poor balance across the furnace is another likely cause. Mill performance, as well as ash chemistry, can alter particle size distribution, which can significantly impact ESP performance.

Combustion system testing to find the cause of high exit temperatures may be combined with checking the air heater itself to find out if flow quantities are optimal and/or if air in-leakage is a problem.

Air In-Leakage (Tramp Air)

Anything that increases the volume of gas being treated by the ESP is bad for collection efficiency. Finding air in-leakage can be complicated, since it can get into the system at many locations. If tramp air is suspected, the hunt for a source usually starts by observing oxygen (O2) meter readings.

At most plants, O2 meters are located at the economizer exit; readings from this location are used primarily for combustion control. Under normal conditions, the boiler exit meter will read around 3% oxygen. Some plants also have an O2 meter in the stack as part of a continuous emission monitoring system (CEMS). Normally, the stack meter reads a bit higher: between 5% and 6.5%. If the stack O2 meter reading is above 8%, air in-leakage may be a significant problem.

Regular testing for air in-leakage at multiple points should be part of every plant's operating policy. Test results help direct repairs to duct-work and equipment, ensuring that whatever maintenance is done will have maximum impact.

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 Long-Term Implications of Spark-Limiting ESP Controls [Save as PDF]

Modern ESP voltage controls use sparking as a gauge for maximizing the applied voltage in the collecting fields.  This provides maximum ESP collection efficiencies.  The controls can sense a spark and extinguish (quench) it before any damage occurs.   

Optimal spark rates for specific ESPs and/or bus sections can be determined through an evaluation of voltage control data, ESP design and the process stream.

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 Monitor V/I Curves to Proactively Discover and Resolve ESP Issues [Save as PDF]

 Regular monitoring of key performance indicators is a smart way to keep your ESP running efficiently. Small problems are likely to eventually become big ones, or even de-rate crises, so it makes sense to head off issues at the pass by making best use of the operational data available.

One key indicator that's often overlooked is the V-I curve (voltage current relationship between T/R sets). Regular monitoring of V-I curves can provide useful insight into why changes in system performance occur over time. From these readings, it's possible to uncover a variety of problems, such as fluctuations in resistivity, that can be fixed more easily if caught early

While it's true taking a V-I curve reading requires ramping the T/R set down and back up, which can cause a short opacity spike (especially with outlet fields), the useful data rendered from this short blip is well worth it. Checking V-I curves should be part of your maintenance and troubleshooting routine along with voltage control calibration and resistivity testing.

Despite the potential utility of V-I curve data, almost no-one monitors this key indicator on a regular basis. So, if you haven't made a New Year's resolution yet, try this: I will regularly perform V-I curve monitoring and use this information to proactively address ESP problems.

 

Electrostatic precipitator V/I curves

Example of "normal" V-I curve values.

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Optimizing electrostatic
            precipitatorOptimizing Your Precipitator, One Step at a Time [Save as PDF]

Optimizing the performance of your power plant's precipitator might seem like a daunting task. Ever feel that there just isn't money, expertise or manpower to achieve meaningful results? This technical tip is the first in a series focused on realistic modifications that result in the biggest bang for your buck.

The most effective performance improvement projects start with evaluating what needs fixing, and then using performance prediction tools to take the guesswork out of optimization. With this phased approach, you'll find out what will make the biggest difference: internal upgrades, gas flow modifications, process improvements, or a combination of these.

Step one: collect data about your ESP’s design, internal and gas flow distribution components, fuel and ash composition, and stack test results. Together, these pieces of information provide a baseline picture of ESP performance. You can then use computer-generated and physical models to explore the cost-benefit tradeoffs of different options.

This time of year, it's likely that an upcoming planned outage is on your mind. How do you make best use of this time? Some urgent repairs are no doubt needed, and you can't ignore them. However, if there is time and manpower, a physical inspection of the ESP is an excellent investment of resources. The information gleaned from such an inspection often makes the difference between upgrades or modifications that hit the mark, and those that don't. After all, knowledge is power.

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 Measuring space between
            plates and wires (discharge electrodes) in an electrostatic precipitatorOptimizing Your Precipitator: Internal Components [Save as PDF]

If your precipitator isn't working as well as it should, the very first place to troubleshoot is inside. Plates, discharge electrodes (wires) and other internal components are the bones of the precipitator, and if they're not installed or maintained properly, you will have performance issues. Start by assessing clearances. The distance between wires and plates is the most basic, important aspect of an ESP's performance. If clearances are not set correctly, the result is reduced voltage, increased wire failure, more loading to downstream fields, and other problems.

It should be standard procedure before every outage to review ESP performance data to pinpoint areas that need attention. Look for lower than normal kV and mA readings; wherever these show up, mark the sections as places to focus your troubleshooting efforts. Then, during the outage, check the marked sections for clearance problems.

Re-aligning clearances might be simple, or might be complicated. In some cases, all you need to do is re-hang/replace falling or fallen wires. You might find localized plate warping caused by high hoppers, and this might necessitate replacing some of the plates. If that's not feasible during the outage, a shortcut is to simply pull out wires in sections where there are clearance issues. Sure, you'll lose some potential collecting capacity, but it will be made up for in overall improved performance. One bad apple, as they say, can spoil the whole barrel.

It can't be emphasized enough: installing and maintaining appropriate internal components--wires, plates, etc.--is vital to optimal ESP performance. Even the type of discharge electrode used makes a huge difference. For example, if there is very heavy dust loading, switching from weighted wires to another type can significantly improve performance, especially at the inlet field.

If you're planning an ESP rebuild or major set of replacements, start by assessing clearances. Consider changing the spacing between plates and switching to a more appropriate type of discharge electrode for a one-two punch. It makes a lots of sense to precisely match the discharge electrode geometry to properties of the particulate being collected, and related factors. For example, rigid discharge electrodes or barbed wires can be designed to help overcome low current levels produced by high space charge.

Also, consider the role rappers play. Older precipitators often have greater plate area per rapper than current designs. Increasing the number of rappers can help maintain power into the electrical fields and reduce the magnitude of opacity spikes.

As with a lot of other things in life, optimizing your precipitator starts from the inside out. If you avoid shortcuts anywhere, make internal components that place.

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Optimizing Your Precipitator: Power Circuits [Save as PDF]      

In a previous ESP Technical Tip about optimizing your precipitator, we explained why troubleshooting should always start inside the box with a look at plates, discharge electrodes and other internal components. Building on this, your next step is to consider electrical components and how they inter-relate to efficiently feed power into the precipitator--or not.

The two main electrical components to consider are Current Limiting Reactors (CLRs) and T/R sets. The CLR is the primary component that shapes electrical waveforms being fed into the precipitator; it does this by opposing rapid changes in current, using an electromotive force. CLRs also limit current during overload (sparking) conditions. The T/R set is a high voltage transformer and rectifier that converts A/C power to D/C and feeds it into the precipitator.

To efficiently get power into the precipitator, both CLRs and T/R sets need to be appropriate sized in relation to one another and to the ESP field. It takes two to tango, a T/R set and a CLR. If these components are mismatched, the precipitator can't run at peak efficiency.

Electrostatic precipitator power circuit

 

Ideally, CLRs should be matched with T/R sets in such a way as to provide full primary current at a target conduction angle of between 120 degrees and 150 degrees. For most precipitators, if the conduction angle falls below 100 degrees or so, some losses in collection efficiency will occur. Conversely, conduction angles above 150 degrees have no added benefit.

At many power plants, ESPs were originally built with equally sized (or rated) T/R sets on all the fields. The rating was probably chosen to best suit the outlet collecting fields, since those sections draw the most current. But, this is not an efficient way to distribute power; it creates an immediate power circuit mismatch.

Because of high particulate loading, the inlet fields typically draw 80% less current than the outlet fields. T/R set sizes/ratings should progress from front to back of the ESP, getting increasingly larger.

Matching the size of T/R sets and CLRs to each field's power demands is key to optimizing the power circuit. Result: improved particulate charging, boosted field strength and enhanced overall ESP collection efficiency.

 

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 Optimizing Your Precipitator: Performance Testing [Save as PDF]

In previous installments of this series on optimizing electrostatic precipitators, we examined the value of collecting data about your ESP's design, the importance of properly maintained internals, and the relationship between different power circuit componentsNext we move on to performance testing, another tool for determining just how well the ESP is working and what modifications will result in the biggest bang for your buck.

The performance testing discussed here is sometimes referred to as "EPA Method 17." It involves inserting probes that contain filter into test ports while the ESP is running to measure gas dust loading. At the same time, flow, temperature and gas composition are determined. Testing is done at the inlet and at the outlet, and results are compared to determine overall collection efficiency.

The more representative data you can collect about how the ESP is operating today, the better off you'll be when it comes to correlating test results and determining what actions should be taken. Since the goal is to understand what's actually happening with the ESP, Method 17 typically involves 10-12 test runs (up to three runs per test condition) under varying conditions. For example, one run might be done at full load with normal load, another at normal load with no rapping, and another at 75% boiler load with no soot blowing and reduced ESP power input.

Method 17 testing provides a baseline picture of how well the ESP is running. For a more full picture of efficiency, it helps to simultaneously test fuel and ash samples, and collect other information such as primary/secondary current and voltage readings, CEMS data (Oxygen, NOx, opacity, etc.), and boiler O2%. All of this information can then be fed into a computer performance model to see the potential effect different modifications will have on collection efficiency.

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Troubleshooting ESP Discharge Electrode Suspension [Save as PDF]

This article discusses suspension considerations encountered by users of weighted wire discharge electrodes.

There are two discharge electrode frames to consider in weighted wire applications:

  1. Upper wire frame from which wires are hung.
  2. Lower wire frame which maintains horizontal stability of wires by fixing the position of the wire and its attached weight


Isolating the discharge electrode system from grounded sections of the precipitator is essential for proper performance. Electrical clearances should be maintained at a distance near the plate-to-plate distance. With 9” plate spacing, a distance of 8 inches is usually acceptable, meaning that the discharge system structure should be no closer than 8 inches from the nearest ground source.

The most critical consideration in the design of discharge electrode systems is the alignment of components to set and maintain electrical clearances that maintain high operating voltages.  

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Troubleshooting ESP Secondary Voltage Dividers [Save as PDF]

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.

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 
              Secondary Current

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
       .5A

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.  

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Troubleshooting ESP Wire Failures [Save as PDF]

Common wire failures in discharge electrode systems can be a very serious problem with the potential to force the shutdown of a precipitator’s complete electrical field. Common causes of wire failures include:

  • Warped plates that change the wire-to-plate distance
  • Overvoltage
  • Metal fatigue
  • Chemical attack
  • Mechanical failure
  • Eroded wire frames
  • Full hoppers
  • Improper weight sizes
  • Oscillation
  • Swinging lower wire frames
  • Corrosion
  • Improper rapping


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Troubleshooting T/R Sets [Save as PDF]

A common question when troubleshooting precipitator problems, especially when the focus is on the high voltage control system, is whether or not the transformer-rectifier set is operating properly.

What is a transformer rectifier set? Commonly called a “T/R set,” the transformer rectifier is made up of a transformer (to step up normal service voltage (V) to the required kilovolt (kV) range) and a rectifier (to convert the high AC voltage to a direct current (DC) voltage supply). The T/R set is part of the precipitator electrical circuit, which consists of the service power supply, a modern voltage controller, a current limiting reactor, the T/R set and finally, the precipitator electrodes.

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 Understanding Current Limiting Reactors (CLRs) [Save as PDF]

Purpose of the CLR

1. Provide wave shape smoothing and absorb transients caused by phase fired SCRs.
2. Provide current limiting during transient overload (sparking) conditions.

Current Limiting ReactorThe typical T/R set power supply uses phase fired Silicon Controlled Rectifiers (SCRs) to regulate current flow to the T/R set during each half cycle of the power line. The precipitator load is capacitive under normal operating conditions and low resistance or shorted load during a spark. The precipitator equivalent load is reflected to the T/R set primary side. Without inductance of the CLR, there would be a significant primary current surge (high di/dt) each half cycle of the line at SCR turn on, as the T/R primary circuit voltage makes a step change to the line voltage.

The CLR provides an inductive load regardless of what is happening on the T/R set secondary, providing sinusoidal current waveforms on the primary and secondary sides of the T/R set.

During sparking, the secondary of the T/R set is effectively shorted. The T/R set typically has an impedance of 6%, meaning that during a spark condition, the primary current could rise to 1/0.06 or more than 16 times the T/R set rated primary current. The SCRs and other primary circuit elements would be severely stressed and would fail from the repeated surges. The CLR has a typical impedance of 30-60%, which limits surge currents to 1.66: 3.33 times the rated primary current. This is within the repetitive surge current ratings of well designed SCR assemblies, ensuring long service life.

CLR Sizing

CLRs should be designed to complement T/R set ratings to provide full primary current at a target conduction angle of 120-140 degrees. It has been documented that, for most precipitators, as conduction angles decrease below about 100 degrees, some losses in collection efficiency can be expected, while conduction angles above 140-150 degrees have negligible benefit, and in some cases can cause operational problems.

Spark over voltage in a precipitator limits the peak voltage that can be applied to the bus section, while average voltage determines the collection efficiency. In a normally operating precipitator, any mechanism that can raise the average voltage without increasing the peak voltage will improve collection. A T/R set energized with full conduction sinusoidal current produces the maximum ratio of average secondary voltage to peak secondary voltage. By targeting 120 degrees, an operating margin allows increasing power if needed, while only slightly reducing the average to peak secondary voltage and current ratios.

Practical Application

We attempt to select CLR and T/R sizes and taps to create a system that reaches primary and secondary current limit at 120-140 degrees conduction angle. Unfortunately, many of the installed high voltage systems have components that are improperly sized. As long as the T/R set is not undersized, the problem can be fixed by proper selection of CLR parameters.

The most common errors are CLRs that have insufficient inductance and/or T/R sets that have a substantially higher milliamp rating than needed. If the T/R set current rating is oversized (a common problem in inlet fields) or the CLR has insufficient inductance, the T/R will run at low conduction angles, reducing collection efficiency. In either case, increase the CLR inductance until a 120-140 degrees conduction is achieved at the highest bus section operating current. If full conduction angle is achieved without sparking or reaching current limit, try a lower inductance CLR tap.

If the T/R set has multiple primary taps, selecting a different tap may improve performance. If full conduction angle is achieved without sparking, try a lower voltage/higher current T/R set primary tap. If sparking typically occurs at conduction angles below 120 degrees, select a higher voltage/lower current primary tap. If primary current limit is reached substantially before secondary current limit, increase the CLR inductance and/or select a different T/R set primary tap.

Sizing Formula

CLR inductance (mHy) = [10 * (CLR target percent impedance) * (Line Voltage)] / [377 * (T R primary current rating)]. If the T R set impedance is significant, deduct it from the CLR target impedance.

Typically, CLRs are sized to give 50% impedance and are rated for the T/R nameplate primary current rating. Installing a multi-tap CLR with taps providing 40%, 50% and 60% impedance provides flexibility in tuning the system and is recommended.

As an example, for 50% impedance with a 480 Volt line supply and a 146 Amp T R set, the required inductance would be:

L (mHy) = (10 * 50 * 480) / (377 * 146) = 4.3 mHy.

If the T/R set is oversized for the electrical bus section, sizing the CLR inductance to the current rating of a properly sized T/R set can improve performance. If this is done, use a multi-tap CLR. Select the lowest inductance tap to give 40% impedance for the T/R nameplate current rating and select the mid or high impedance tap to match 50% impedance for the ideal size T/R set. The CLR current rating should be sized to the highest anticipated operating current, and the AVC primary current limit must be set no higher than the lower of CLR or T/R current rating.

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