Dust Collector Optimisation Improves Plant Efficiency
Fig. 1: Plants can achieve greater efficiency by integrating advanced controls in their pulse-jet baghouse dust collectors.
Every plant strives to operate more efficiently. This constant effort drives plant managers to search for opportunities to lower costs and maintenance managers to look for ways to safeguard process operations. Recent advancements made in control and sensing technologies can provide managers more precise and reliable process control as well as diagnostic data that was previously inaccessible. Therefore, upgrading key devices with the latest control and sensing technologies makes new ways to increase efficiency and save energy possible. One of these key devices that advanced controls and sensing can optimise is the pulse-jet baghouse dust collector (See Fig 1).
Many baghouses use older controls with little monitoring capability. These outdated controls can be insufficient when it comes to meeting production demands as well as today’s strict Environmental Protection Agency (EPA) regulations. Replacing them with advanced baghouse control and sensing can improve overall plant efficiency, energy saving, lower cost and help plants meet environmental standards.
The range of benefits provided by integrating advanced baghouse control and sensing is extensive and impressive. Intelligent filter cleaning lowers emissions and compressed air use by 15 per cent to 40 per cent while maximising filter life, and highly stable differential pressure (+/-0.1inWC) control maintains desired process airflow.
Advanced control and sensing can also identify leaks and failures precisely and often immediately. Users can detect and locate filter leaks by row well before emissions are visible, as well as ruptured or frozen pulse-jet diaphragms, preventing compressed air loss. Advanced control and sensing also provides continuous solenoid valve short-circuit protection and instantly detects and locates failed solenoids, preventing plugged filters.
Emerson’s ASCOTM DPT (dust and pulse technology) products (see Figs: 2 and 3) provide this level of advanced control and sensing. This monitoring, control, diagnostic and optimisation system for baghouses and cartridge-type dust collectors combines simple yet advanced control with several diagnostic features that optimise processes while lowering maintenance, operating and energy costs.
Filter leak detection
For many types of fabric filters, continuous particulate emission monitoring is now a regulatory requirement. Advanced control and sensing technology provides monitoring capabilities that help plants stay compliant.
For example, innovative charge induction technology (superior to traditional triboelectric) and a protected probe provide basic leak detection to continuous relative monitoring to calibrated mass (mg/m3) outputs. In addition, induction sensing, combined with a protective layer, enables operation with particulate buildup and process moisture, which is also beneficial for most conductive particulate applications.
There are advanced technologies that are also suitable for extreme conditions. With an unmatched level of robustness suitable for harsh industrial environments, the particulate sensor DPT P152 is free of electronics and enables optional, intrinsically safe systems for hazardous areas.
When particles flow near and around the probe, small electrical charges are continually induced into the probe by flowing particulate, creating a tiny electrical current (pico amperes). Precision high-speed digital signal processing is used to analyse and process the signal into an absolute output that is reasonably linear and proportional to the mass of particulate. A protective insulating layer flow measurement over the probe ensures reliable operation with high moisture and conductive particles without the use of an air purge (see Fig. 4).
With an intelligent pulse-jet cleaning control device, DPT P151 features an intelligent pulsing mode. This intelligent pulsing mode automatically maintains a constant pressure drop across the filter, using the least amount of compressed air. It also extends filter life through reduced pulsing and maintains low emissions as well as constant air flow by preserving the proper filter cake.
Maintaining the correct amount of dust cake on filters is essential to achieve the maximum collection potential of filter bags. When the pulse-jet cleaning system engages, it removes the excess particulate from the filter surface, leaving behind the amount needed for optimum filtration efficiency. Overcleaning leads to increased emission of particulate matter (PM), particularly fine PM. Undercleaning leads to higher differential pressure, resulting in increased resistance and fan load and a drop in upstream air flow and collection efficiency.
DPT P151, via differential pressure, uses intelligent technology for pulse-jet dust collectors to significantly improve cleaning efficiency. Pulse duration and off-time is automatically regulated.
Each time a solenoid is energised, air pressure triggers the diaphragm valve to discharge a burst of compressed air into the filters to dislodge particles that may have accumulated on the media surface.
The pulse duration produces a short, crisp pulse to create an effective shock wave in the bag. This duration is generally set to energise the pulse-jet solenoid for 0.10s. Since the frequency of pulse-jet cleaning is critical to proper dust cake retention, intelligent pulsing mode holds the pulse rate at tight differential pressure by automatically adjusting the off-time between pulses.
If pressure rises quickly, pulsing is proportionally increased automatically and vice versa (see
Fig. 5). This frequency can vary from three to more than 30s so that static differential across the collector is matched to the baghouse load to prevent overcleaning of the filter bags.
While improving collection efficiency, DPT P151 can lower system operating maintenance costs and extend filter bag life. Pulse cleaning causes temporary fatigue in the bag body, reducing its efficiency. Ensuring the minimum amount of cleaning necessary leads to reduction in filter wear, constant air flow through the baghouse, lower compressed air usage and lower operating differential pressures. DPT P151 includes a high-static alarm that will indicate a high-pressure drop condition and also when continued pulse cleaning fails to reduce the pressure drop across the filters.
Non-clogging differential pressure sensor
The correct operation of a baghouse requires the use of a pressure sensor for monitoring static differential across the filters. Many baghouse dust collectors are continuously pulsed because of problems inherent to pressure gauges. A photohelic or magnehelic gauge is commonly used on baghouses, but other options now exist.
DPT P850AD Pressure Transmitters are heavy-duty, low-pressure sensors for monitoring and controlling fabric filters. They can transmit accurate measurements below 10inWC over a broad temperature range without clogging, making them an ideal replacement for mechanical gauges and electronic sensors with small tubes that can easily clog.
For differential measurement, the flush diaphragm is mounted to the dirty side of a process, like below the tube sheet on a baghouse chamber, while the reference port is routed to the clean side. Standard 4-20mA output signal is wired directly to the DPT P151 control.
The P850AD employs a state-of-the-art capacitive sensing element with temperature compensation. Changes in process pressure or vacuum applied to the diaphragm cause deflection in the ceramic sensing element. A precise, linear 4-20mA output signal that is proportional to the pressure or vacuum is produced by the circuitry.
Compressed-air header tank pressure sensor
On pulse-jet baghouses, the pulse frequency can vary. However, the next pulse should not be programmed to fire until the compressed air pressure in the header is regained. This allows the same force of pulse for each row cleaned. DPT P151 continuously monitors a static pressure transmitter installed in the header tank servicing pulse-jet solenoid valves to ensure sufficient header pressure is available.
As new environmental regulations require higher efficiencies from pollution control equipment, baghouse maintenance and performance are more critical than ever. Plants can achieve greater efficiency, energy saving, avoid expensive fines and forced closures and lower operating expenses by integrating advanced controls in their pulse-jet baghouse dust collectors.
ABOUT THE AUTHOR:
Vijay Jawade, Sales & Marketing Director, Fluid Control and Pneumatics at Emerson India