Help with instrument calculation problems
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Orifice plates are used in combination with differential pressure transmitters to measure fluid flow; the measurement range of the transmitter is 0~80 Kpa, corresponding to a flow rate of 0~100 t/h. After the instrument was put into use, it was found that the actual flow rate in the pipeline was very low, with the transmitter’s output being only around 5.5 mA. If it is desired for the transmitter to output around 8mA, how should the measurement range of the differential pressure transmitter be adjusted? What will be the maximum flow rate measured after this adjustment?
The orifice plate comes with a predetermined pressure measurement range, which cannot be changed arbitrarily. Trying to solve the problem by changing the transmitter or the range of the DCS won’t work. If the normal flow rate is already low, it simply means that the orifice plate chosen is too large; was this not taken into account during the initial design?
The flow meter was not selected appropriately; it is necessary to provide the manufacturer with proper parameters once again, so that they can recalculate and manufacture the orifice plate. It is not possible to measure flow by simply changing the parameters.
This is just a calculation problem from a book; two cases need to be considered: with square roots and without them. Find a collection of questions for instrument technicians; there are likely to be the original questions in it.
This one. . The range should not be changeable. At the time of production, the size of the throttle device is determined; let’s use a smaller one
A differential pressure of 80 KPA indicates that the flow rate in the design condition is quite high (higher flow velocity = higher differential pressure), and this is commonly seen in medium/high-pressure steam applications. For general liquid materials, the pipe size is selected based on a flow rate of 2 meters per second; for gases, it is usually 10 meters per second. Based on this experience, a differential pressure of 50 KPA is usually sufficient; you can figure out how many orifice plates are needed by doing some calculations yourself. Changing the transmitter to full-scale differential pressure is one solution, but if the flow rate is low, the Reynolds number may not even meet the accuracy requirements; this is a problem related to the flow rate itself. @jiaguoyun @WindHowling
It is well known that the output current of a differential pressure transmitter is 4–20 mA. The corresponding relationship is as follows: (20–4) mA corresponds to a differential pressure of (80 KPa), while this same current level corresponds to a flow rate of (100 t/h). In practice, (5.5–4) mA corresponds to a differential pressure of (7.5 KPa), resulting in a flow rate of (30.618 t/h). The target values are: (8–4) mA corresponds to a differential pressure of (20 KPa) and a flow rate of (50 t/h). If it is desired for the transmitter to output 8 mA, and the ReD requirement is met, then the measured flow rate should be around 50 t/h. @HEJIYUER
The differential pressure range needs to be recalculated; it can’t be changed just as one wishes
I completely agree with you, provided that the transmitter does not output a square root value.