Chief Cui’s feed flow rate
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The Cui chemical processing unit at our plant has experienced this issue since it was put into operation this year: the flow meter shows a reading when there is no flow of material in the main pipeline. Our factory uses orifice plate measurement, with 3851 series pressure transmitters and a DCS operating system for display. However, after inspection, the following possibilities exist: Since this is our first time using the DCS operating system, there are still many aspects that are not clear to us; at first, we suspected that there was a problem with the system or the cards. But the people from the manufacturer said it wasn’t their fault. In the second step, we suspect there is a problem in the section from the safety barrier to the pressure transmitter. However, after checking, the safety barrier was fine, the signal cables were also okay, and we replaced the pressure transmitters with two units of the same model, and those worked just as well. The third possibility is a problem with the orifice plate; it may be due to an inappropriate selection or non-compliant installation. To address this, we brought in professional engineers to inspect it and reinstall it (following the installation instructions precisely), but the problem remained unresolved. Finally, suspecting signal interference, the grounding was redone and small-signal rejection was applied. It still doesn’t work. With no other option, we performed a zero-point shift for it in the DCS system. But later, when the vehicle was started, measurements were taken using another flow meter, and the results differed by several tons. :'(Sigh! To avoid embarrassment, we can only tell the workshop that there is a problem with the process. But now we are shutting down for annual maintenance, and the process has been modified according to our requirements. As production is about to start at the beginning of the year, the problem still hasn’t been resolved. :funk: I’m here asking some expert for help in analyzing what the cause is.
Is the displayed value fluctuating or stable?
A signal generator (which must be calibrated) can be used to apply a standard signal, such as 4mA, to check whether the zero reading is normal The location where it can be added is as follows: 1. At the DCS signal terminal; 2. At the front end of the safety barrier; 3. In the field transmitter junction box. If a 4mA signal is applied at the DCS signal terminal and a value is still displayed, first check whether the underlying signal is correct (Different manufacturers’ DCS systems have varying methods for viewing the underlying signals.) Then, it is necessary to check whether the algorithm is correct
The total catalytic feed flow rate, as indicated by the flow meter when it is not in operation, seems to have little to do with the DCS. Check the differential pressure transmitter on site; if there is no isolation tank, one must be installed. Also, check whether the isolation fluid inside the tank has been depleted, or if the pressure tapping pipes are partially blocked – in such cases, flushing with oil will suffice. Orifice plates introduce significant measurement errors and require frequent maintenance. Given that the total catalytic feed flow rate is important, it is recommended to use a gate flow meter or a twin-rotor flow meter, as these provide much more accurate readings with an accuracy level of 0.2%, and they also require minimal maintenance.