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Section 8 INSTRUMENTATION, OPERATION, AND MAINTENANCE A pneumatic dust-collection system is made up of many interrelated components. The efficiency of the entire system suffers if a single component is not working properly. To assure maximum efficiency, operating personnel must monitor the system continually and inspect and maintain it on a regular schedule. This section covers basic aspects of the instrumentation, operation, and maintenance of dust-collection systems. The section is not intended to be comprehensive. Day-to-day operation and maintenance of a system requires much more information than can be given here. Operators should have at hand the original design data and drawings for their dust-collection systems. When a system is modified, the data and drawings should be modified accordingly. The responsible people should study these materials until they are thoroughly familiar with the layout and intended performance of the system. They should also study the instruction manuals provided with the equipment and keep the recommended tools and spare parts readily available. Ins trumentation Various instruments and associated hardware are available for monitoring and controlling dust-collection systems. Besides contributing to efficient operation, this equipment warns of potentially hazardous breakdowns. Commonly used equipment and its functions are described briefly below. Types of Equipment A magnehelic gauge is a pressure-measuring device with a direct-reading dial. Normally it is used to measure the pressure drop across a fabric filter. A photohelic gauge is also a pressure-measuring device. It has a direct-reading dial like a magnehelic gauge, but also has electrical contacts that can be wired to an alarm, such as a light or horn, on a control panel. The contacts are actuated by pressure. Usually they are set at the upper and lower pressure limits. A manometer is a handy device for measuring static pressure at any point in a dust-collection system. It is a glass or plastic tube shaped like a long U and filled with oil or antifreeze. The tube is marked to indicate pressure in inches of water. 79
80 A pitot tube is a device that can be used with a manometer to measure velocity pressureâthe pressure exerted on a flat plate by a stream of air flowing perpendicular to the plate. Velocity pressure can be used to determine volume flow in a duct. A level indicator is used to detect plugging of a filter hopper with dust. One type of level indicator is a rotating paddle wheel driven by a torque motor through a friction clutch. The clutch slips when dust builds up in the hopper and covers the paddle wheel. A second type of level indicator is a pressure-sensing diaphragm that detects a buildup of dust. A third type is a probe having a dielectric field that changes when the probe is surrounded by dust. Each type of level indicator is tied electrically to an alarm that signals when the hopper is plugging. A broken-bag detector usually is a photoelectric cell in the air outlet of the filter. It detects abnormal discharge of dust from the filter and actuates an alarm. A zero-motion switch signals failure of rotating equipment, such as the rotary discharge valve on a filter. A pressure-differential switch can be preset to close and open at specified pressures. It is used to control "demand cleaning" of filter bags. The cleaning system starts when the pressure drop across the filter reaches the high setting of the switch and stops when it returns to the low setting. A recording pen can be used with a pressure sensor to record a permanent, continuous log of pressure at a desired point. Basic Monitoring Equipment It is recommended that a dust-collection system be equipped with basic monitoring devices at least sufficient to warn of potentially hazarous malfunctions. An adequate installation would include the following devices; Filter 1. A magnehelic gauge to give the pressure drop across the unit. A manometer could be used instead, but may not work properly in extreme heat or cold. If the operation of the elevator is monitored from a central control panel, it may be desirable to replace the magnehelic gauge with a photohelic gauge wired to an alarm at the control panel. 2. A pressure-differential switch to control "demand cleaning" of the bags.
81 3. A broken-bag detector. 4. A level indicator in the filter hopper. 5. A pressure gauge with recording pen to log pressure drop across the filter. 6. A microswitch at the filter inlet to close down the system when cleaning. 7. A zero-motion switch to signal stoppage of the rotary discharge valve on the filter hopper. Exhaust Fan 1. A magnehelic gauge to give static pressure between inlet and outlet of fan. 2. A pitot tube calibrated to give direct reading of airflow in system. Ductwork 1. A manometer installed at each hood to indicate suction at hood. Electrical Interlock Besides being equipped with basic monitoring and control equipment, the moving components of a dust-collection system should be electrically interlocked. The interlock should be arranged so that shutdown of any component will shut down the system. It is also recommended that a time-delay sequence be built into the electrical system. The order of start-up would be; 1. Pneumatic system or screw conveyor taking dust away from filter. 2. Rotary discharge valve on filter. 3. Bag-cleaning motors on filter. 4. Exhaust fan on the system. To shut down the system, the foregoing sequence would be reversed. The grain elevator handling should be interlocked so that they cannot operate without the dust system in operation. Using the Instruments Proper instrumentation can tell you basically how a pneumatic dust-collection system is performing. Figure 8-1 shows the relationship of the components of a typical system and the points where instruments may be installed. The uses of the instruments are described briefly below. Again, however, keep in mind that the description is not intended to be comprehensive.
B2 Blast Gates Exhaust Air Static Pressure Check Points Hoods Ducts (Pickup Areas) Reverse Air Blower Magnehelic Gauge Level Indicator Blower and Blow-thru Airlock Feeder if Dust is to be Pneumatically Conveyed to Bin Rotary Airlock Discharge FIGURE 8-1 Typical dust control system. The figure shows an exhaust system because the fan draws air through the collector. If the fan is ahead (i.e., pushing air through) of the collector, it is a blow system. Most cyclone collectors are attached to blowing systems, and most filter collectors are attached to exhausting systems. Filter Pressure A typical installation of a magnehelic (or photohelic) gauge on a fabric filter is shown in Figure 8-2. It is useful to mark (red-line) the face of the gauge to show the lower and upper pressure limits for acceptable operation.
83 Suction Connection I Collector Pressure Connection Upper Chamber Filter Sleeve Lower Chamber FIGURE 8-2 Typical magnehelic gauge installation. The best method of determining whether a collector is operating effectively is to check the magnehelic gauge. If filter resistance is above 3-4 inches of static pressure (SP difference between the upper and lower chamber), check the filters for excessive dust accumulation. Filters are designed to operate at a pressure of 3 to 5 inches of water. (Pressure at start-up may be less than 1 inch until a mat of dust begins to build up on the fabric.) Once the filter reaches equilibrium pressure, the magnehelic (or photohelic) gauge becomes an indicator of the operation of the entire system. So long as the gauge reads between 3 and 5 inches of water, the system will be delivering design volume flow. When pressure exceeds the upper limit, the filter should be checked for malfunction. If none is found, the excessive pressure means that the bags should be cleaned or replaced. Normally, the bags should not require attention until after many months of operation. When filter pressure falls below the lower limit, the system, again, should be checked for malfunction. The most likely problems are broken filter bags, a partially plugged main duct, closed blast gates in branch ducts, or slipping belts on the system's exhaust fan. Filter Hopper Level indicators should be included with all new filters, and retrofit is recommended for existing filters. An indication of dust buildup in the filter hopper points to one of several problems; failure of the rotary discharge valve on the hopper; plugging downstream of the filter; or bridging of dust in the hopper.
84 Manometer-Pitot Tube The manometer is used to balance a dust-collection system initially. It is also used periodically to recheck static pressures at hoods or other points. The use of the instrument is shown in Figure 8-3. Note that atmospheric pressure at the open end of the manometer depresses the column of water against the lower pressure inside the duct. The instrument is marked so that the vertical distance between the ends of the column is pressure in inches of water. Manometer -Inches of Water Which Varies from 1" to 2!4" FIGURE 8-3 Typical hood and blast gate. Figure 8-4 shows the use of the pitot tube and manometer to measure velocity pressure. The pitot tube is a combination of two tubes, as shown. The leading tube measures total pressure in the duct, and the trailing tube measures static pressure. The differenceâthe velocity pressureâis shown by the manometer.
85 Velocity pressure can be used to obtain airflow in a duct. The velocity of the air in the duct in feet per minute is 4,000 times the square root of the velocity pressure in inches of water as shown in Figure 8-4. The velocity (V) times the cross sectional area (A) of the duct gives airflow (Q) in cubic feet per minute (i.e., Q = AV). Tables are available that give airflows for a range of velocity pressures and duct sizes. Vel. = 4000 V Vel. Pressure Vol. = Vel. x Area of Pipe â¢Based on Std. Temp, and Pressure. FIGURE 8-4 Pi tot tube. The pitot tube, an instrument for measuring velocity pressure, consists of two tubes joined together as shown. Special pitot tubes are required for measuring air flow in a pipe containing dust. The standard ASME tube is unsatisfactory because the holes in the tube plug with dust quickly. The pitot tube is inserted in the pipe at right angles to the air flow and a traverse is made to get the average velocity pressure reading. Maintenance Maintenance should be kept constantly in mind when designing and installing dust-collection systems. Likely problem points such as elbows, long horizontal runs, should be made accessible for inspection and routine maintenance or repair. Inspection ports should be provided as necessary. Remember that a poor design cannot be well maintained.
86 It is recommended that one individual be assigned to monitor the operation of the dust-collection system. The individual assigned should have all manufacturers' maintenance manuals readily available. He should be thoroughly familiar with the manuals so.as to be able to pinpoint trouble should it occur. The individual responsible for the system should follow a regular schedule of inspection and maintenance. The exact schedule will depend on the particular system and the number of hours it operates per day or week. However, a sample maintenance schedule is shown below. Table 8-1 shows a sample trouble shooting guide. Sample Maintenance Schedule Daily 1. Check magnehelic-gauge readings on all filters. 2. Check for dust in clean-air outlet from filter. 3. Check filter hoppers for continuous discharge of dust. Weekly 1. Check and record magnehelic-gauge readings on all filters. 2. Check fan and motor bearings for excessive heat or vibration. 3. If high-pressure pneumatic conveying equipment is used to dispose of dust, check the positive-displacement pump for vibration, overheating, and proper lubrication. Also, compare reading on pressure gauge with previous readings. Clean air-inlet filter or replace as necessary. It is important to follow the manufacturer's recommendation on this piece of equipment. Monthly (or at manufacturer's recommended intervals) 1. Check oil in all gearmotors. Do not overfill. Six Months 1. Check belt tension on all V-belt drives.
87 TABLE 8-1 Sample Troubleshooting Guide Problem Possible Source Abrupt rise in magnehelic-gauge reading Tubes plugged; tube-cleaning mechanism faulty No magnehelic-gauge reading Plugged tubing to gauge; gauge defective; fan off Dust in (normally) clean air outlet Filter tube torn or missing Filter hopper plugged Debris over outlet, or rotary valve not operating No airflow Fan not running; main airflow damper closed Excessive buildup of dust in ductwork Low airflow; high filter back pressure or obstruction in hoods; closed blast gates Main exhaust-fan motor overloads (straight-bladed fan) Main airflow damper open too far Rotary-valve gearmotor overloads Obstruction in valve