【Daily Question 20090223】Why is it generally not advisable to use differential pressure flow meters below 30% of their full scale?

1 A differential pressure flow meter (hereinafter referred to as DPF) is a device that measures flow rate based on the differential pressure generated by a flow sensing element installed in a pipe, along with known fluid properties and the geometric dimensions of the sensing element and the pipe. The DPF consists of a primary device (sensing element) and a secondary device (differential pressure conversion and flow display instrument). DPFs are usually classified by the type of sensing element, such as orifice flow meters, venturi flow meters, and average velocity tube flow meters. Secondary devices include various mechanical, electronic, and mechatronic differential pressure gauges, differential pressure transmitters, as well as flow display and calculation instruments. They have evolved into a large category of instruments with a high degree of standardization (serialization, generalization, and standardization), featuring a wide range of types and specifications. Based on the working principle of their sensing elements, DPFs can be classified into several major categories: throttle type, dynamic head type, hydraulic resistance type, centrifugal type, dynamic pressure gain type, and jet type. Among these, the throttle type and dynamic head type are the most widely used. 2 Flow equation Where qm and qυ represent the mass flow rate (kg/s) and volume flow rate (m3/s), respectively ； 　　　　　C--discharge coefficient ； 　　　　　ε--expansibility coefficient ； 　　　　　β--diameter ratio, β=d/D ； 　　　　　d--Orifice diameter of the throttle element under operating conditions, m ； 　　　　　D--Inner diameter of the upstream pipeline under operating conditions, m ； 　　　　　ΔP--differential pressure, Pa ； 　　　　　ρ1--density of the upstream fluid, kg/m3. As can be seen from the above formula, the differential pressure is proportional to the square of the flow rate; below 30% of the flow rate, it becomes difficult to maintain accuracy. Furthermore, when the flow rate is less than 30%, the Reynolds number is often below the critical Reynolds number, resulting in unstable flow coefficients and inaccurate flow measurement. Selection Principles 　To choose the most suitable standard throttling device, the following factors should be considered during selection: 　(1) Constraints related to pipe diameter, diameter ratio, and Reynolds number range ； 　　(2) Measurement accuracy ； 　　(3) Allowable pressure loss ； 　　(4) Required minimum length of straight pipe section ； 　　(5) Sensitivity to erosion, abrasion, and contamination of the medium under test ； 　　(6) Complexity of the structure and price ； 　　(7) Ease of installation ； 　　(8) Long-term stability of use. 　　Based on the above aspects, the selection principles for standard throttling devices can be summarized as follows: (1) There are certain limitations regarding the pipe diameter, diameter ratio, and Reynolds number range for the use of various types of throttling elements in standard throttling devices; these are detailed in the standard GB/T2624-93 (or the international standard ISO5167-1). For example, orifice plates can be used in a wider range of pipe diameters compared to nozzles and Venturi nozzles, while the pipe diameter ranges for different types of conventional Venturi tubes vary significantly. 　　(2) The accuracy of various types of throttling elements in standard throttling devices, under the same differential pressure and density measurement accuracy, depends on the uncertainties of the discharge coefficient and the expansibility coefficient. The uncertainties in the discharge coefficients of various throttling elements vary considerably; in comparison, the uncertainty in the discharge coefficient of orifice plates is the smallest, while it is larger for profiled throttling elements (nozzles, venturi tubes). The reason for the larger profiled throttle elements is the poor quality of the fitted database on which the coefficient of discharge formula given in the standards is based. However, high accuracy can also be achieved by individually calibrating the profiled throttle elements. 　　(3) At the same pressure difference, the pressure loss of classical venturi tubes and venturi nozzles is approximately that of orifice plates and nozzles

The differential pressure is proportional to the square of the flow rate; below 30% of the flow rate, it becomes difficult to maintain accuracy. Furthermore, when the flow rate is less than 30%, the Reynolds number is often below the critical Reynolds number, resulting in unstable flow coefficients and inaccurate flow measurement.

In differential pressure flowmeters, the sensor generally needs to be in a laminar flow state; if the flow rate is too low, a stable laminar flow cannot be established, which affects accuracy.

The differential pressure is proportional to the square of the flow rate; below 30% of the flow rate, it becomes difficult to maintain accuracy. Furthermore, when the flow rate is less than 30%, the Reynolds number is often below the critical Reynolds number, resulting in unstable flow coefficients and inaccurate flow measurement.

Answer: In flow measurement, **the standard specifies that the range ratio applicable to throttling devices is 30%. This is because the differential pressure is proportional to the square of the flow rate, and accuracy cannot be guaranteed when the flow rate is below 30%. Furthermore, when the flow rate is less than 30%, the Reynolds number is often below the critical Reynolds number, and the flow coefficient is not constant, resulting in inaccurate flow measurement. The following approach can be taken: negotiate with the process team to reduce the maximum flow rate; if the Reynolds number is high enough, an orifice plate or differential pressure device can be used as a solution. Switch to another type of flow meter

In flow measurement, **standard regulations state that the flow ratio suitable for throttling devices is 30%. This is because the differential pressure is proportional to the square of the flow rate, and accuracy cannot be guaranteed when the flow rate is below 30%. Furthermore, when the flow rate is less than 30%, the Reynolds number is often below the critical Reynolds number, resulting in unstable flow coefficient and inaccurate flow measurement. When the flow rate is below 30%, the following actions can be taken: reduce it to the maximum extent permitted by the process, and ensure that the Reynolds number is high enough to allow the issue to be resolved through the orifice plate or differential pressure. Secondly, it can be measured using other types of flowmeters, such as turbine flowmeters.