Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/351731
AGGREGATES MANAGER August 2014 26 problems. e fact is, if your dredge has been operational for several months, the code is probably solid, and it will not change. Most cases of a system not starting or display reading not work- ing involve a faulty field device or connection. PLC input field devices are used as system interlocks, as well as operator display annotations for alarms and status. If the PLC code is well wrien, these interlocks not only ensure safe operation of the system, but should annunciate alarm failures if the interlock is not valid. Being familiar with the PLC code or interlock schedule will be very helpful in troubleshooting control issues. When an opera- tor comments that he can't start his hydraulic pump or has a bad reading on his display for the discharge pressure, knowing which transmier and field device is the interlock is necessary. is can be found using the electrical diagrams, as Figure 1 shows, along with the PLC ladder logic printout. Go to the field device in question and look for physical damage. Next, open any junction boxes that the wiring would pass through to get from the PLC cabinet to the field device. Check for water, corrosion, or frayed wiring, which is a common issue on older dredges. If the device and all wiring appear intact, it is necessary to troubleshoot with the meter. You need to determine if the problem is the field device or an actual input module failure. If you are troubleshooting a transmier, there is a dedicated heading for this task. For the common field switch (pressure switch, push buon, flow switch, level switch, etc.), use your multi-meter set for voltage and test voltage between the input terminal at the I/O cabinet and the neutral common while the field device is exercised. If the device and wiring is good, you should see the associated voltage at the input. Now, verify if the proper PLC input light illuminates on the input module window. You may have a bad input module. If input power is not at the input terminal when testing, go to the device and open the connection box while testing for voltage there. One side of the switch should always have input power applied. If not, you probably have a bad isolated fuse, which could simply feed that field device or a bank of devices. Usually, when a complaint is that several instruments suddenly stopped working, it is narrowed down to a fuse. Now, you need to find what blew the fuse. With the fuse pulled, check which field power wire is shorted. You may have to li them from the termi- nal blocks individually to find the offensive short. It's probably a pinched wire in a junction box or water in the field box somewhere. Dredge instrumentation Typical transmiers found on dredges include pressure sensors, magnetic flow meters, encoders, density meters, ladder angle sensors (inclinometer), and RTDs. Most of these transmiers are 4-20ma signal instruments. Each of these transmiers can come in two-wire, three-wire, or four-wire configuration. It's important to know which you have for troubleshooting purposes, and they typically have the wiring termination connection and unit measure range stamped on them. When replacing a transmier, make sure to purchase the same range and wiring configuration as the original unit. is is neces- sary for scaling in the PLC. When testing 4-20ma signals, the meter is required to be in line (series) with the ma current signal. Fluke sells a 773 meter that is a nice addition to any technician's toolbox. is meter has a small retractable clamp designed to clamp around the small sig- nal wires found in a 4-20ma circuit. is avoids the require- ment to break a con- nection somewhere to read the signal. Production metering Production is usually read in tons per hour or cubic meters per hour. In some cases, an on-board com- puter calculates the production rate with signals from density meter and velocity (flow meter). Some dredges still prefer the yield meters, which have inde- pendent flow and density needles and an analog scale to infer the production via the cross needles. In either case, they rely on the accuracy Pressure transmitter Mag flow meter