INSTALLATION AND OPERATION

Transcription

1 TROUBLESHOOTING The aim of troubleshooting your device is to trace the source of any issues/faults to a particular section of the flow meter. Having traced the fault to a particular section of the flow meter it should be possible to find a solution to the issue. General Troubleshooting The first step is determining if you have followed the manufacturer s installation instructions, and that any issues are not the result of a poor installation. Determine if there is flow present in your system oono flow, or lower than normal flow, may be attributed to a blocked strainer, malfunctioning pump, closed valves, or low liquid level in your supply tank ooif none of the above, it is most likely caused by jammed or blocked rotors within your flow meter Keep in mind that some flow will still occur with blocked or jammed rotors, unless the liquid viscosity is very high Commonly found foreign materials are weld slag, sealing tape, or debris In some cases the damage from foreign particles can be repaired by replacing the rotors, however due to the fine machining tolerances of the measuring chamber, meters with damaged meter bodies are not always repairable. ooother common causes for jammed rotors Chemical incompatibility Thermal shock (rapid expansion of the rotors without sufficient time for the meter body to expand) Use of water in an Aluminum meter Distortion due to over-pressure Drying/congealing of process liquid Incorrect re-assembly after service/ maintenance Bearing damage from over-speeding or improper start-up (excessive flow rate). If it is determined that the rotors are jammed or blocked, disassemble Oval Gear Meter for inspection and repairs. oorefer to section Removal & Installation of Oval Gear Rotors or the flow meter owner s manual for instructions on disassembly ooonce the meter has been disassembled it should be relatively straight forward to identify the damage, or the foreign material that has caused the blockage ooif there are any components showing visual damage, contact the distributor for guidance on the best course of action, and/or to purchase spare parts ooif the blockage has been caused by soft particles (e.g. PTFE tape) and there is no visual damage to the rotors, bearings, or measuring chambers, then it may be possible to simply clean and reassemble the meter. If you determine that the rotors in your flow meter are rotating correctly, but you are not receiving a signal output, then the problem lies within the electronics oopulse Output Board of the flow meter oowiring Issue ooreceiving instrument 53

2 INSTALLATION AND OPERATION Troubleshooting a Pulse Output Board The two different outputs on a Pulse Output Board are the Reed Switch and the Hall Effect. There are different methods used to troubleshoot both outputs. The most common cause of a Hall Effect failure is electrical damage caused by incorrect wiring, voltage spikes, or short circuits. oohall Effect sensors are solid-state devices, their failure tends to be sudden and permanent. oovisual Inspection of the Hall Effect sensor will usually show if a failure has occurred. Over voltage, reverse polarity, or short circuits will usually create visual damage and a smell of burning. A failed Hall Effect can only be fixed by replacement of the Pulse Output Board. The Reed Switch is a mechanical switch enclosed in a vacuum sealed glass envelope, and then encapsulated in a plastic housing. ooa Reed Switch can be easily damaged by physical impact, such as being dropped. oodamage from physical impact does not always result in immediate failure; cracks in the glass envelope will affect the performance of the sensor. Reed Switch sensors with cracked glass envelopes will eventually fail as the delicate electrical contacts degrade from contact with air. A damaged or faulty reed switch may operate well under ordinary conditions, but then fail to work at more extreme conditions. Troubleshooting Using a Display A display can be used to measure the pulses out of each sensor independently. If you are using a Reed Switch output to your receiving instrument you can check the following with a display. oono signal, oothe Reed Switch has failed ooyour wiring or receiving instrument has failed ooyou are set up incorrectly Remove the wiring from the terminals of the Pulse Output Board and connect a display directly to the Pulse Output Board with short lengths of wire. oothis isolates the Reed Switch or Hall Effect. The display should be programmed to have a K-Factor of 1 pulse/gallon or pulse/liter. The display s total display will show individual pulses as they are output from your flow meter. Be sure to check if the display requires a pull-up resistor when using accepting a Hall Effect pulse. ooif your problem is eliminated, then the problem is with the wiring or the receiving instrument. ooif your Reed switch output does not function when tested with a display, but you find that your Hall Effect output functions correctly, then you need to replace the pulse board. o o Likewise if your Hall Effect output does not function when tested, but your Reed Switch output functions correctly, then you need to replace the pulse board. In many cases if an intermittent fault exists with your Reed Switch it should simply be replaced. 54 GREAT PLAINS INDUSTRIES

3 Testing with a Digital Multimeter A digital multimeter (DMM) can be used to check the condition of your wiring, the presence of power at the Pulse Output Board terminals, or can be used to check the operation or accuracy of an analogue loop (4-20mA signal) The DMM cannot be used to check the presence of a pulse output. When troubleshooting a Hall Effect output the DMM can be used to measure the following ooinput Voltage to your Pulse Output Board Connect the DMM across the +VDC and -0V (COM) terminals of the Pulse Output Board oomeasured voltage should be between 5-24VDC for correct function of the Hall Effect sensor. ooif voltage is not present or is below 5VDC, then the Hall Effect sensor will not function Check the DC voltage supply and the associated wiring oovoltages above 28VDC will have caused over-voltage failure, and over 24VDC may have caused failure if voltage spikes have occurred. In cases of over-voltage failure, replacement of the Pulse Output Board will be required. oothe other factors that can be checked with a DMM is the presence of a pull-up resistor Check resistance across the +VDC terminal and Signal terminal stuck Reed switch contacts Check continuity across Reed switch terminals with the Pulse Output Board removed from meter All of these tasks are carried out using the resistance or continuity setting on the DMM When troubleshooting an Oval Gear flow meter fitted with a 4 20 ma module or RT12 Electronic Instrument, the analogue output can be checked using a DMM in series oothe DMM acts as the receiving instrument, and will display the circuit current during programming or operation. It is important that the series resistance shown be used (see figures 5-32 & 5-33) oothis method isolates the instrument from the PLC or receiving instrument, and will allow the user to attribute any faults to either the 4 20 ma module (or RT12), or to the receiving instrument. If the current measurement on the DMM is correct, then the PLC or receiving instrument should be inspected for defects If the current measurement on the DMM is absent, or inconsistent, then the 4 20 (or RT12) should be inspected for defects The problem could be with the signal output from the flow meter to the 4 20 module (or RT12) Check the condition of the wiring Check the presence of welded or 55

4 INSTALLATION AND OPERATION Figure 5-32 Figure 5-33 Testing with an Oscilloscope The Oscilloscope can be used for more advanced pulse output board troubleshooting. An oscilloscope is most useful for detecting the presence of an output signal, and will allow diagnosis of some output problems The oscilloscope should be connected to voltage between the signal terminal () and the -0V (COM) terminal; and will chart the voltage measurement against time allowing you to view the signal in real time ooit measures features of your signal such as voltage, frequency, and duty cycle. Shown Below there are various output signals that can be seen when inspecting signal outputs on an oscilloscope oothe first signal trace (see figure 5-34) shows a normal output from a Hall Effect sensor; there is some cyclic variation in pulse width in the signal, which is a function of magnetism tolerances in the magnet manufacture. Pulse widths are usually even within 10%, but the amount of variation shown below is normal and not a cause for concern. Signal traces 2 and 3 are examples of normal signal output from a Reed Switch; o o Trace 2 (see figure 5-35) is showing low frequency operation (approximately 5Hz) 56 GREAT PLAINS INDUSTRIES

5 In trace number 2 the square wave output can be seen with a nice square ON transition and more of a gradual transition to the OFF state (note that the voltage measurement across the Reed Switch will measure zero volts when the switch is ON and will measure the supply voltage when OFF ) The gradual OFF transition is due to the mechanical nature of the Reed Switch, and is caused by the time taken for the contacts to open after the magnetic field is removed. ootrace 3 (see figure 5-36) is showing high frequency operation (approximately 60Hz). the switch will reach a point where it does not have sufficient time to open, before closing again. At this point operation becomes very inconsistent and unreliable. The error shown here in Trace 4 (see figure 5-37) has 1 missing pulse in every 4 pulses, which is a symptom of a bad magnet Similarly, if you are missing 2 pulses in every 4 pulses, this is a symptom of a failed Hall Effect sensor (1 meters and larger). This type of diagnosis should always be confirmed by testing the Hall Effect K-factor to avoid a false diagnosis. Trace number 3 shows the same switch operating at full speed; it can be seen that the gradual OFF transition is more significant here. If the switch is operated at a frequency that is too high, Figure 5-34 Figure 5-35 Signal Trace 1 - Hall Effect Normal Operation Signal Trace 2 - Reed Switch Low Frequency Figure 5-36 Figure 5-37 Signal Trace 3 - Reed Switch High Frequency Signal Trace 4 - Hall Effects Fault Diagnosis 57