I have a question regarding the selection of flow meters

The difficulty you face in making a choice stems mainly from ignoring the characteristics of differential pressure measurement in flow meters. The relationship between flow rate (Q) and differential pressure (ΔP) for orifice plate flow meters is such that differential pressure is proportional to the square of the flow rate. With this clarified, let’s take a look at your requirements again: 1. Data related to the process: – Normal flow rate: 1 t/h – Maximum flow rate: 3 t/h 2. Relationship between flow rate percentage and differential pressure percentage (meaning of the square root scale): The specifications state that “the normal value should be within 60%-85% of the range of the differential pressure transmitter, with the maximum value not exceeding 95%”. These percentages refer to the proportion of the differential pressure value relative to its range, not the flow rate percentage. In other words, for example, if the full-scale differential pressure of your gauge is set at 100 kPa, then: when the flow rate is at its maximum value (3 t/h), the differential pressure is around 95%, so it will be approximately 95 kPa ； - The flow rate is at the normal value (1 t/h), and the differential pressure should be between 60% and 85%, that is, around 60–85 kPa. 3. Let’s perform a simple calculation to verify this based on the square relationship between differential pressure and flow rate: Assume that when the maximum flow rate \(Q_{max} = 3\) tons/hour, the differential pressure reaches 95% of the full scale (i.e., 95 kPa). For a normal flow rate of \(Q_{nor} = 1\) ton/hour, the corresponding normal differential pressure is: At this point, the normal differential pressure represents only about 11% of the full scale, which is far below the requirement specified in the standards (60%–85%). Clearly, this does not meet the requirements. 4. How to understand “range ratio of 1:10”: The range ratio is the ratio of the maximum measurable value to the minimum value that can be measured accurately. It should be noted, however, that the range ratio of flow meters is usually expressed in terms of differential pressure, rather than directly in terms of flow rate. Since \( Q \propto \sqrt{\Delta P} \), a differential pressure ratio of 10:1 corresponds to a flow rate ratio of \(\sqrt{10} \approx 3.16:1\). In other words, when the range ratio of your instrument’s differential pressure is 10:1, the actual measurable flow rate range is only from about 3% to 100%. 5. If the process does not specify a minimum flow rate, how can it be determined? The process does not specify a minimum value; generally, it can be determined as a certain percentage of the normal flow rate (such as 30% or even less). Since the maximum flow rate is already known, the calculation can be done using both the normal and maximum design values. If there is a significant difference between normal operating conditions and maximum flow rate (for example, in your case the maximum flow rate is three times the normal value), the differential pressure orifice plate may need to be resized, or a different type of flow meter should be used instead. 6. Practical solution: – Adjust the inner diameter of the orifice plate to increase the differential pressure, so that at normal flow rates ΔP is above 60%, and the maximum flow rate does not exceed 95%; the specific inner diameter can be determined using specialized calculation software. - Once the orifice plate size is determined, the \"measurement range\" is simply set as the upper and lower limits of measurement for the corresponding differential pressure transmitter. For example, under normal operating conditions, the flow rate reaches 60% of the full scale. If the corresponding differential pressure is 60 kPa, then the differential pressure corresponding to three times this maximum flow rate would be nine times greater (following the square relationship), i.e., 540 kPa. Clearly, this value cannot be achieved in practice (as it exceeds the specified limit by 100%). This indicates that you need to readjust the orifice diameter to meet the requirements. 7. Practical advice: Under standard conditions, the data you provided (with a significant difference between normal and maximum loads) is actually not suitable for measurement using just one standard orifice plate with a single transmitter. In general engineering practice, when encountering such a situation, one of the following solutions can be considered: – a combination of two orifice plates or a differential pressure transmitter with multiple ranges ； - Use vortex flowmeters, turbine flowmeters, and electromagnetic flowmeters with a wider range ； - If only a single standard throttling device can be used, the inner diameter of the orifice plate should be adjusted appropriately so that the differential pressure under normal operation meets the specifications as much as possible (e.g., over 60%), and the maximum flow rate should have its limits relaxed somewhat, but still remain within 95% (based on the limits permitted by the specifications). Here are some brief selection suggestions for reference: – When the difference between the normal flow rate and the maximum flow rate is large (a ratio of 3), it is difficult for a standard orifice plate to meet the requirements of having a differential pressure within the range of 60%-85% at the normal flow rate, with this value not exceeding 95% at the maximum flow rate; it is recommended to recalculate the inner diameter of the orifice plate. - If standard orifice plates must be used, accuracy at one end must be sacrificed (which is generally not recommended), or a dual-sensor/dual-range approach or other types of flow meters should be employed. The above content is intended to help you understand the principles behind selecting a differential pressure flowmeter. It is recommended to use specialized instrument selection software to calculate the orifice diameter and subsequently determine the actual measurement range. .