If the differential pressure of a orifice flow meter is greater than the compensation pressure, is the measured flow rate the accurate one?
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I would like to ask everyone. A standard orifice plate flow meter is used in the project; the oxygen production equipment is located in front of the orifice plate, and after passing through approximately 250 meters of pipeline, the flow reaches the inlet of the blast furnace fan to provide oxygen enrichment before the fan. At the start of the design, the process specified a working pressure of 25 Kpag and a temperature of 35°C; the measurement range for the orifice plate was from 4000 to 18000 cubic feet. However, in actual use, it was found that the working pressure could only reach around 4–5 Kpa, with a temperature of about 35°C. At this point, a situation arises: when the compensation pressure of the orifice plate is around 4 Kpa, the differential pressure across the orifice plate is around 4.5 Kpa. In other words, the pressure in front of the orifice plate is positive, while a local negative pressure is generated behind it. At this time, the flow rate calculated using the orifice plate is approximately 11,360 cubic meters. May I ask everyone, is the flow rate measured by the orifice plate accurate at this point? If it’s not accurate, why? As I understand it, when the flow rate through the orifice plate is high and the orifice size is small (or, as mentioned above, when the actual pressure is low), in order to allow the normal flow rate to pass through (the equipment’s capacity is limited, and the gas must flow through the orifice plate; otherwise the equipment will stop functioning), the pressure before the orifice plate rises. At this point, the compensating pressure should still be greater than the differential pressure across the orifice plate (in other words, there should be no local negative pressure behind the orifice plate). However, what is special in this case is that at the oxygen-enriched position in front of the blast furnace, the pipeline is under slight negative pressure (the blast furnace’s air inlet is connected to the atmosphere as well as to our pipelines; therefore, there is negative pressure, but it should be slight). However, considering the need to overcome the pressure loss in a pipeline of about 250 meters, the pressure at the point where the flow velocity recovers behind our orifice plate should not be negative. After several calibrations of the aforementioned pressure transmitter, the problems with it could be basically ruled out. I checked GBT2624; the standard emphasizes flow rate under full pipe conditions. Does this situation fall under flow rate under partial pipe conditions? All other installation requirements are met, such as straight pipe sections. Please help answer, thank you.
Thermodynamic compensation is carried out on the assumption that there is a reasonable deviation between the operating pressure and the design pressure; given that the deviation in your case is so large, the orifice plate needs to be recalculated
The differential pressure across an orifice plate is not equal to the pressure loss. The actual pressure loss needs to be calculated. When the pressure does not match the conditions, errors are inevitable; the values can be considered only as approximate. At such low pressures, if measurement is necessary, use a uniform-flow tube or a Venturi tube instead.
The difference between the compensation pressure and the actual operating pressure is significant; it is necessary to contact the manufacturer to recalculate and determine whether it can be used.
We contacted the manufacturer again and used the already prepared orifice plate to calculate the parameters for a design pressure of 10 KPa. The flow rate obtained using these new parameters was similar to the current value, but it differed from the production capacity of the device by about 5%. This set of equipment has been used many times before, so the output shouldn’t be that low; therefore, we considered the possibility that the measurements taken with the orifice plate were incorrect.
I wonder where the traffic deviation data comes from? It is impossible to make comparisons if all traffic volumes are not normalized
The accuracy of the orifice plate itself is not high; if there is a 5% difference from the actual value, that means an error of 0.25% in the differential pressure transmitter. I think this can be justified in situations with such low pressures as yours.
The pressure difference is too low; it seems the orifice plate is no longer functional, and other throttling devices need to be considered