【Weekly Question 20090608】On-site indication and central control indication of flow meters

1. It has happened due to line grounding or a mismatch between the DCS and the gauge’s range. 2. Units and range, as well as the output method (linear or square root). 3. The flow meter was fluctuating; upon inspection, foreign objects were found on its components, and after cleaning them, the fault was resolved

In production, the following issues were encountered: 1. When testing the system with pure water, the electromagnetic flow meter did not display any readings; however, after adding some industrial salt to the pure water tank, the readings became visible again. 2. During the evaporation of the alkaline solution, it was observed that the reading on the liquid level gauge did not match the value indicated remotely; after removing the water accumulated at the connector terminals of the instrument, the reading became consistent.

1. During the work process, there were indeed 2 instances where the values shown on the field indicators did not match those displayed on the ICS in the control room. In one case, the value shown on the ICS was only half of the value indicated by the field instrument. Upon comparing the DCS configuration parameters with those of the field transmitters, it was found that the measurement range specified in the DCS configuration parameters was half of that of the field transmitters. After changing the measurement range in the DCS configuration parameters from 50 N3/h to 100 N3/h in accordance with the process requirements, normal operation was restored； Secondly, the on-site reading of a pressure transmitter was about 20% lower than the reading on the ICS; when a standard signal generator was used to generate a 4–20mA current signal and supply it to the DCS, everything worked normally. Analysis indicated a fault in the transmitter output module; therefore, the module was replaced and sent for testing, where it passed. After reinstallation, the ICS displayed values that matched those on-site. Through experiments such as simulations, it has been concluded that DCS measurements are more accurate. 2. During configuration, the measurement units must be consistent, and the measurement ranges must be equal ； The output signal of the field transmitter is consistent with the signal received by the DCS ； The device tag numbers are consistent ； The device locations on the flowchart match those in the actual site ； On the process diagram screen, the standard drawings of the equipment match the actual devices on site, etc. 3. Experience: A newly installed Wiltech electromagnetic flowmeter was in use for about a month when the user reported that the cumulative value displayed on the field transmitter was much lower than that shown on the DCS. After comparing the configuration parameters, it was found that those of the field transmitter had been changed. It turned out that after the parameters were entered, they were not uploaded and saved to memory; the power outage the previous night caused the data on the transmitter to be “lost.” The transmitter was reconfigured as required, and the parameters were uploaded and saved. Two months have passed since then, and the measurement results are quite normal.

This post was last edited by meetkey on 2009-6-9 at 11:15: 1. The table headers do not match the square root of the remote transmission signals, nor do they match the measurement ranges； 2. The DCS has small-signal rejection, but the meter head does not have such rejection ； A small amount is displayed in the table header, while no display is shown on DCS. 3. The remote transmission module circuit board is damaged (capacitors/resistors are faulty, etc.) ； It is displayed in the table header, but not in DCS. That’s about all I can think of; if there’s anything else, please add it.

1. For instruments that require an external power supply, such as electromagnetic and mass flow meters, the current signal is transmitted from the transmitter, and there is a possibility of issues with the wiring; therefore, measurements are generally more accurate on-site. For two-wire instruments such as differential pressure and vortex flow meters, as long as they are set correctly, the signals at the field site and in the control room are similar. 2. I have encountered situations where the readings from the electromagnetic flowmeter did not match those displayed on the DCS; it turned out that the range set in the DCS configuration was different from the range set on the field transmitter.

1. Have you encountered inconsistencies in the instructions from these two parties during your work? What causes it? Which one is more accurate? Of course, there are cases of inconsistency. For example, when measuring the flow rate of a gas, the value shown by the main controller is usually the compensated flow rate, while on-site the display is based on differential pressure or a percentage; therefore, the main controller should provide a more accurate reading. 2. What parameters need to be ensured to be consistent between the field and the central control system during configuration? During configuration, the liquid level, pressure, differential pressure, etc. should be set to match the actual on-site height ； 3. Based on your work experience, give an example of a problem related to traffic that you handled personally. A newly installed split-type electromagnetic flowmeter was giving inaccurate readings, showing values about 40% higher than the actual value. Various adjustments were made to check it, but the problem persisted. Eventually, it was discovered that the sensor part had been mistakenly installed with another unit (which hadn’t been used yet), as the diameters of the two units were different. For one DN80 and one DN100 unit, during installation only attention was paid to the tag number of the converter; not much attention was given to the sensors (in fact, the tag numbers of the sensors were very poorly marked). After swapping the sensors, the fault was immediately resolved!

The readings from the on-site device and the remote transmitter may not match. Possible reasons for this discrepancy include: 1. Differences in the configuration parameters of both devices (range, units, limits, output method, etc.)； 2. Check the control room to see if compensation is applied ； 3. Is there duplicate square rooting? ； 4. Are the working locations identical (especially for externally powered instruments)? ； 5. Cable laying methods, wiring, etc.

The problem encountered during testing was that the flow rate displayed on the central control system did not match the flow rate shown on-site, which led to misguidance of the operators and resulted in liquid overflowing from the tower. 1# wwxxss_1

1. Have you encountered inconsistencies in the instructions from these two parties during your work? What causes it? Which one is more accurate? To measure the flow rate of gases and vapors, the flow rate displayed by the main control unit is the compensated value, while on-site it is usually either the differential pressure or the flow rate under actual operating conditions; therefore, the main control unit should provide a more accurate measurement ； 2. What parameters need to be ensured to be consistent between the field and the central control system during configuration? During configuration, the range should match that of the field instrument ； 3. Based on your work experience, give an example of a problem related to traffic that you handled personally. The on-site reading of the newly installed electromagnetic flowmeter differed from the reading displayed on the central DCS system. The process engineer assumed, based on experience, that the on-site reading was accurate; however, upon inspection it was found that the measurement range set in the DCS configuration did not match that of the electromagnetic flowmeter on site. After making the necessary adjustment, the issue was resolved.

I have encountered situations where the on-site indication of a flow meter did not match the indication shown in the central control system. The reason for this was that the analog input of the DCS card used standard current signals, and the on-site flow output also utilized standard current signals. However, since the card received its power from an external 24V DC supply, this resulted in differences between the internal and external signals, with significant fluctuations. It is likely that although both sources provided 24V DC voltage, issues arose due to differences in ripple levels and voltage stabilization circuits. The problem was resolved by replacing the card with one that did not require an external power supply.