Digital wide-range flow meter
Thread Content
This post was last edited by Wei Pengjie on 2017-4-9 at 11:40. Hello, everyone! Our company manufactures a digital orifice (nozzle) flow meter, which is primarily used for measuring steam, with a range ratio of 1:70 and an accuracy of 1%. (1) It has a wide range; the ratio of maximum to minimum flow rates can reach up to 70:1. Therefore, it is particularly suitable for measuring low-flow steam, and can help thermal power plants reduce measurement errors and associated cost losses. (2) It uses digital signal acquisition, transmission, and processing (different from 4–20mA analog signals, although such analog signals are also provided as a backup); the signals are stable and less affected by external factors. Minor resection is not required. It is currently used in the trade settlements of fifty to sixty thermal power plants, with no trade disputes to date. All marine enthusiasts are welcome to inquire!
What is the company name? It would be even better if you could send over a user manual.
How to correctly understand the range ratio of a differential pressure flow meter? The method for calculating the range ratio of such a flow meter is as follows: first, determine the differential pressure corresponding to the maximum flow rate of the flow meter; then identify the minimum differential pressure allowed by the flow meter. The ratio of these two values, taken to the power of one half, constitutes the range ratio of the flow meter. To simply take the square root of the range of the differential pressure transmitter and call that value the range ratio of the flow meter is a complete misrepresentation of the concept. Referring only to the square root of the transmitter’s range without considering the minimum differential pressure required by the throttling device for different fluids is nothing but speculation. Let’s discuss this together!
This post was last edited by shakic on 2017-4-8 at 21:54. The EJA differential pressure transmitter can measure differential pressures in the range of 0.03 Kpa to 147 Kpa. If you calculate 147/0.03, you get 4900; taking the square root of that gives 70. Thus, it is claimed that the flow measurement range is 70:1, without taking into account the effects of the orifice plate at all. Isn’t that a bit hasty?
This post was last edited by yuchenchf on 2017-4-22 08:23. It’s my first post; I’m figuring out how to upload attachments
In fact, the measurement range of the transmitter is not limited to 0.03–147. This range is the one for which our company can guarantee accuracy.
This post was last edited by Wei Pengjie on 2017-4-9 at 12:36. Bro, you have too many questions; let me answer them one by one. (1) The range ratio of the flow meter is determined by the inherent properties of the throttling element itself, right? The range ratio is determined by both the inherent properties of the throttling element itself (i.e., the discharge coefficient) and those of the differential pressure transmitter (measurement accuracy, measurement range). For example, in the case of an orifice plate, within the allowable accuracy limits, it follows the Bernoulli equation precisely; there is a maximum value and a minimum value for the flow rate. The ratio of these maximum and minimum values should represent the range ratio of the flow meter, right? When the actual flow rate is below the aforementioned minimum flow rate, the differential pressure generated across the orifice plate shows a tendency for the deviation from the White equation to increase as the flow rate decreases. This characteristic is inherent to the orifice plate itself and is independent of the differential pressure gauge, right? Theoretically, regardless of the flow rate, a pressure difference is generated whenever fluid passes through a throttling element. The flow rate at this point always conforms to Bernoulli’s equation; there is no deviation from it when the flow rate decreases. According to GB2624, the flow rate is proportional to the discharge coefficient and proportional to the square root of the pressure difference. This is based on Bernoulli’s equation and applies to any flow rate. (I can’t upload pictures, so I can only write it this way). However, the reason why people have the impression that the range of orifice plates is not high is as follows: (1) the discharge coefficient is negatively correlated with the Reynolds number, and this relationship is not linear; meanwhile, the Reynolds number is directly proportional to the flow rate, which itself needs to be measured and determined. Therefore, solving for the outflow coefficient requires repeated iterative calculations. Most manufacturers are unable to do this; therefore, a fixed value (the discharge coefficient under design conditions) must be used to simplify the formula calculations. In other words, it is simplified to flow being proportional to the square root of the pressure difference. When the flow rate decreases to less than 1/3 of the maximum value, the error in the discharge coefficient exceeds the acceptable range; in other words, the calculated flow rate cannot be used as an accurate figure. Therefore, traditional manufacturers set the range at 1:3. (2) Measurement of differential pressure. The range of differential pressure transmitters is quite wide, but most manufacturers only use a portion of this range to generate outputs corresponding to 4–20 mA analog signals; it’s meaningless to use an excessively wide range, as the discharge coefficient cannot be calculated accurately in such cases. However, when the flow rate decreases, although the analog output is 4 mA indicating a flow rate of 0, it is still possible to read the differential pressure value by using a handheld device to measure the HART signal. Our company’s calculator integrates a hand-held operator function, which allows it to read the HART signals from the transmitters (0.03 KPa to 147 KPa), with no loss during transmission. I’m wondering — when the flow rate is below the aforementioned minimum value, what mathematical principle governs the pressure difference across the orifice plate, is it Bernoulli’s equation? Trust scientists. The flow meter in the original poster’s home might have some special capabilities when it comes to handling small differential pressure signals and calculating low flow rates I’ve already explained why it’s possible to measure low flow rates. But with a flow range of 70:1, I intuitively feel that this might not be very appropriate —— I’m not very familiar with flow meters; I have a deeper understanding of pressure gauges. From a pressure measurement perspective, here are my thoughts: if the flow range is 70:1, then the differential pressure range will be 4900:1. The differential pressure gauges commonly used display values with 4 digits; those that display 5 digits are quite rare, and those that display 6 digits are even rarer. In other words, the measurement resolution of a differential pressure gauge is roughly 1/5000 of its full scale, which means its measurement capability is insufficient for handling a flow range of 70:1. It can handle ranges of no more than 10:1 at most. As for its ability to detect low flow rates, it’s seriously inadequate. I don’t quite understand what you mean by that; I’ll consult our chief engineer about it. Thinking further, for a flow range of 70:1, could it be that the flow meter in question is equipped with several differential pressure gauges with different ranges, which can switch automatically or manually? No, it’s just one EJA transmitter, only one. Our company has many cases where, without replacing any individual components, only the transmitter and secondary components were replaced, thereby directly expanding the range to 1:70. My explanation might not be perfect; I hope my colleagues can point out any mistakes!
EJA also provides different diaphragm boxes for selection, depending on the various range options. It seems that EJA didn’t mention that it’s possible to measure values ranging from 0.03 to 147 KPa using a diaphragm cell, while still maintaining an accuracy of 0.075. When did EJA claim that the ratio of the range of its differential pressure transmitters is 4900:1? Taking the M diaphragm of the EJA110A as an example, EJA specifies that the range is 1–100 KPa. My personal guess is that this implies that, while maintaining accuracy, the maximum range ratio for the M diaphragm cell is 100:1; if measuring flow rate, the range ratio is 10:1. For the M diaphragm gauge, it’s unknown whether the lower limit can be measured at 0.03 KPa as claimed by the poster But it is certain that the error is much greater than 0.075. For the M diaphragm gauge, it’s unknown whether it can measure 147 KPa as claimed by the poster But it is certain that the error is much greater than 0.075. I have something to take care of, so I’ll stop writing here for now; we can continue the conversation next time.
Yes, it is calculated using the iterative method specified in GB2624. There is a place in Beijing that does the same thing