Correct selection of flow sensor

There are many factors that affect flow sensors. There are more than ten principles and no less than 200 types. Someone investigated more than a thousand flow sensors on site in the United States and found that 60% of them chose methods that were not suitable. Even if the selected methods were suitable, more than half of them had problems in installation and layout. It is not easy to choose the right one. In summary, the correct selection of flow sensors depends on six factors: sensor technical parameters, fluid characteristics, flow state, installation, environment, and economy. The total amount

of sensor technical parameters

and the total amount of flow (in m3 or kg) are mostly used for trade accounting, and accuracy ranks first. Flow (instantaneous quantity in m3/h, kg/h) is mostly used in process industries and is the source of information for control systems. Repeatability ranks first.

Continuous, switch The output of general flow sensors is continuous, while the switch quantity can be used for simple two-position control or equipment protection, requiring good reliability.

Accuracy Accuracy depends not only on the sensor itself, but also on the calibration system, which is an external characteristic. To explain the accuracy within what flow range, if used in a control system, it should also be considered to match the accuracy of the entire system. Note: The error indicated by the manufacturer is %Fs (upper limit) or %RD (measured value).

Repeatability Repeatability refers to the consistency of multiple measurements of a certain flow value when the environmental conditions and medium parameters remain unchanged. It is a characteristic of the sensor itself. In process industrial control systems, repeatability is often more important than accuracy. Many manufacturers mislead repeatability as accuracy. Accuracy should include repeatability and the flow uncertainty of the calibration device.

Range ratio The ratio of the maximum to the minimum flow rate within a certain accuracy range. Differential pressure flow sensors can have a larger range ratio from the sensor itself, but are restricted by the secondary table and are generally only 3:1.

Pressure loss flow sensors (except electromagnetic and ultrasonic) all have detection parts (such as orifice plates, turbines, etc.), and forced changes in flow direction (such as elbows, Coriolis) will produce irreversible pressure loss, which will increase the power of transmission to maintain normal operation. Some of the amounts are very large, and should be taken seriously today when energy conservation is advocated.

The output signal is generally a standard analog signal (0~10V, 4~20mA, etc.), which can no longer meet the requirements of system development. Communication requires digital signals. Rosemount has introduced the HART protocol and RS232/RS485 converter. RS232 is limited to within 2Km, and RS485 can reach 10Km.

Response time is the time for the output signal to react to changes in flow parameters. For control systems, the shorter the better; for pulsating flow, a slower output response is desired.

Comprehensive performance The performance indicators of the sensor are mutually constrained. For example, the upper limit of pressure in the sample is 2mpa; the temperature is 250℃, and the diameter is 1m; when the diameter is 1m, the pressure may only be 1.5mpa and the temperature can only be 200℃, which cannot be the same limit value.

Fluid characteristics

Fluid types Fluids are divided into liquids, gases, and steam. Some sensors (such as electromagnetic type) cannot measure gases; insertion thermal type cannot measure liquids.

Temperature, pressure, and density are important parameters for selecting sensors, especially parameters under working conditions. For gas flow, it is also necessary to understand whether its volume flow is in working state or standard state.

The large difference in viscosity of viscous liquids will affect the selection. For example, liquids with high viscosity should use volumetric flow sensors, and turbine, float, vortex street and other flow sensors should not be used.

Corrosion, scaling, and dirt For this type of fluid, it is not appropriate to use sensors with rotating parts and detection parts. Even for ultrasonic and electromagnetic flow sensors, errors will be caused by corrosion of the pipeline. For example, if the diameter is 50mm and the scaling is 0.5~1mm, it will cause an error of 0.5~1%. [page]

Special parameters Some fluid parameters will affect the operation of the sensor, such as the compressibility coefficient affects the differential pressure type; the specific heat and thermal conductivity coefficient affect the thermal type; the conductivity affects the electromagnetic; the speed of sound affects the ultrasonic.

Single-phase and multi-phase refer to substances with the same physical and chemical properties in a system. Different phases have obvious interfaces. Usually, most of the phases in industry are single-phase. With the development of industry, flow measurement problems such as multi-phase flow (gas-solid, gas-liquid, liquid-solid or gas-solid-liquid) have emerged.

The state of flow

is different from many physical parameters (such as pressure, temperature, level, and composition). Flow must be based on fluid flow. There is no flow without flow.

Full pipe, part-full pipe. Generally, fluids should fill the pipe. However, when the liquid flow is small and the pipe is horizontal, part-full pipe flow may occur. At present, there are part-full pipe flow sensors.

Laminar flow and turbulent flow reflect the state of fluid flow in the pipe and affect the size of the flow coefficient. When Re < 2320, it is laminar flow. Only in a few cases, such as high fluid viscosity, small pipe, and low flow rate, will laminar flow occur. Turbulent flow is mostly seen in industry.

Closed and open channel. Most of the industries are closed pipes, and open channels are only used for liquid discharge.

Fully developed turbulent flow sensors are mostly velocity type, that is, the velocity distribution in the pipe affects the flow coefficient, so the flow sensor is required to be installed in a specific fully developed turbulent flow. It can be obtained as long as there is a length of 30 times the diameter in front of the sensor.

The flow of any parameter (flow rate, temperature, pressure, density) in the pulsating flow fluid changes with time, which is prone to errors. Additional equipment should be added to remove the pulsating flow and accurately measure it.

Installation and environment

Flow sensors often fail to work properly due to improper installation, such as: reverse installation, unsatisfactory flow velocity distribution, two phases in the pressure pipe, harsh environment, lack of necessary accessories, etc.

Pipeline layout Some sensors (such as float type) can only be installed on vertical pipes; while some can only be installed on horizontal pipes to avoid the gravity of the fluid; and if the fluid contains solid particles, the sensor should not be installed on the horizontal pipe.

Most of the flow direction flow sensors cannot be installed in reverse.

The length of the straight pipe section requires a long straight pipe section before and after the sensor, except for float, volume, and Coriolis. The insertion type sensor with throttling device and measuring point speed has the highest requirement (up to 30~50D).

The diameter of the flow sensor is narrow, which limits the selection. The reducer can be used to make up for it, but it should be noted that the diameter should still be within the normal working range of the sensor after the diameter is reduced. The flow rate should be too small and the output is too weak; the flow rate is too high and the strength is damaged.

The maintenance space should have the necessary loading and unloading and maintenance space.

Accessories For a certain sensor, you should consider installing necessary accessories, such as flow regulators, filters, gas separators, dampers, etc.

The flow sensor should be avoided in environments with high temperature, vibration, dust, corrosion, humidity, explosive and flammable, and electromagnetic interference.

Economic

initial purchase cost When you start to contact the manufacturer, you should pay attention to: treat the technical indicators advertised by the manufacturer rationally; select according to needs, and do not blindly select high indicators; pay attention to the manufacturing materials of the sensor.

The installation diameter is large, and special attention should be paid to additional issues in the installation, such as whether to install bypass pipes and necessary accessories (such as filters, flow regulators) for easy maintenance.

Maintenance and accessories Some sensor detection parts and rotating parts are prone to wear and corrosion (such as turbines, volume, orifice plates), and the maintenance volume is not small.

Calibration After some sensors have been working for a period of time, their accuracy will decrease due to corrosion and wear. If they are used for trade accounting, they should be calibrated regularly.

Operating cost Flow sensors generally have permanent pressure loss, so they will bring additional operating costs. Especially when the pipe diameter is large, its annual operating cost may be several times the purchase cost.

Loss of error If used for trade accounting, especially for more expensive energy and chemical raw materials, high-precision sensors should be selected, otherwise the economic loss caused by errors will be several times the purchase cost.

Summary

The selection of flow sensors can be based on the above six influencing factors, and the selection can be roughly based on the following procedures.

There may be more than one flow sensor that meets the same process requirements, and several sets of solutions can be made at the same time for comparison.