Introduction to the types and principles of seawater flow meters

Project Overview

　　Let's talk about some of the flow meters commonly used in seawater flow measurement.

　　一、Magnetic inductive flowmeter

　　Magnetic inducitve flowmeter is a flow measurement instrument that works based on the principle of electromagnetic induction. It consists of a sensor and a transmitter.

　　1.Working Principle

       Magnetic inductive flowmeter basic principle is: in general non-conductive materials made of pipes outside, installed a pair of magnetic poles N and S, to generate a magnetic field. When the conductive liquid flows through the pipe, because the fluid cuts the magnetic lines of force and produced a magnetic susceptibility to electromotive force, this induced potential by the perpendicular direction of the magnetic poles and the two electrodes lead, when the strength of the magnetic field is unchanged, the diameter of the pipeline a certain amount of this induced potential size is only related to the flow rate, the size of the induced potential is transmitted to the display instrumentation, it can be read out the flow rate.

　　2. Advantages

• 1.There are no moving parts or resisting fluids in the measuring conduit, resulting in low pressure loss and no mechanical inertia, making it highly responsive;
• 2.Wide measurable range: the range ratio is generally 10:1, up to 100:1, the flow rate range is generally 1-6m/s, which can be extended to 0.5-10m/s; the flow rate range can be from 90mL/h to more than 100,000m3/h; the pipe diameter range can be from 2mm to 2400mm or 3000mm;
• 3.It can measure the volume flow of liquids containing solid particles, suspensions or acids, alkalis, salt solutions, etc. with a certain conductivity, as well as the pulsating flow, and can carry out bi-directional measurements;
• 4. Flow signal and fluid volume flow between the linear relationship, so the instrument has a uniform scale; and the volume of fluid flow and the physical properties of the medium, flow state has nothing to do, so the magnetic inductive flowmeter only need to calibrate with water, can be used to measure the volume flow of other conductive liquids without correction;
• 5.The front straight section requirement is low compared to most other flow meters.

　　3. Disadvantages

• 1.The operating temperature and pressure should not be too high;
• 2.It has a limited range of applications and cannot be used to measure the flow of non-conductive fluids such as gases, vapors, and petroleum products, and fluids that contain a high number of larger air bubbles;
• 3.Large measurement errors will occur when the flow rate and velocity distribution do not meet the set conditions;
• 4.When the flow rate is too low, it is more difficult to amplify and measure the induced potential that is an order of magnitude in the direction of the interfering signal, and the meter is also susceptible to zero drift;
• 5.Magnetic inductive flowmeter signal is relatively weak, a slight external interference can affect the accuracy of measurement.

　　二、Ultrasonic flowmeter

       Ultrasonic flow meter is through the detection of fluid flow on the ultrasonic beam (or ultrasonic pulse) to measure the volume flow meter.

　　1.Working Principle

       Acoustic wave propagation in the fluid, the downstream direction of the acoustic wave propagation speed will increase, against the direction of the flow is reduced, the same propagation distance has a different propagation time. Propagation time method is to use the difference between the propagation speed and the measured liquid flow rate of the relationship between the flow rate, and combined with the diameter of the pipe to obtain the flow rate.

　　2.Vantage

• 1. Ultrasonic flowmeter can be used for non-contact measurement, clamp-on transducer ultrasonic flowmeter can be installed without stopping the flow of the carrier pipe, as long as the pipeline to be measured outside the installation of the transducer can be, that is, can not be disconnected or can not be punched in the existing pipeline with ultrasonic flowmeter to measure the flow;
• 2. Ultrasonic flowmeter for no flow resistance coping measurement, no additional pressure loss;
• 3.Measuring instrument coefficients can be calculated from the actual measurement of the pipe and sound channel and other geometric dimensions, you can use the dry calibration, in addition to the measurement of the pipe section with the general need for real flow calibration;
• 4.Ultrasonic flowmeter for large round and rectangular pipe, and the principle of the pipe diameter is not limited, its cost is basically unrelated to the pipe diameter;
• 5.Doppler ultrasonic flow meters can measure liquids with a high solid phase content or containing air bubbles.

　　3.Drawbacks

• 1. The ultrasonic flowmeter in the propagation time method can only be used for clean liquids and gases, and cannot measure liquids with suspended particles and bubbles exceeding a certain range; on the contrary, the Doppler method LSF can only be used to measure liquids containing a certain amount of heterogeneous phases;
• 2. External clamped transducer ultrasonic flowmeter can not be used for lining or scaling is very thick pipeline, and can not be used for lining (or rust) and inner pipe peeling (if the sandwich gas will seriously attenuate the ultrasonic signal) or corrosion is serious (to change the ultrasonic path) of the pipeline;
• 3. The Doppler method of ultrasonic flowmeter in most cases does not measure a high number of degrees;
• 4. Cannot be used for pipe diameter less than DN25mm.

　　三、Target Flow Meter

       Target flowmeter in the sixties began to be used in industrial flow measurement, mainly used to solve the high viscosity, low Reynolds number fluid flow measurement.

　　1.Working Principle

       When the medium in the measuring tube flow, because of its own kinetic energy and target plate pressure difference, and produce the target plate force, so that the target plate to produce a small amount of displacement. The size of the force is proportional to the square of the media flow rate, the target plate by the target force transferred by the target rod, so that the sensor's elastomer to produce a small amount of change, thus breaking the balance of the bridge composed of patch capacitance, resulting in the flow of the target plate on the force corresponding to the voltage signal.

　　2.Vantage

• 1. Wide measurement scale, low flow rate, especially suitable for large pipe diameter, high and low pressure gas or liquid measurement;
• 2.With integrated temperature and pressure compensation, direct output of mass or scalar flow;
• 3.With optional small signal removal, nonlinear correction, and selectable filtering time;
• 4.Simple and easy to install, good repeatability, fast measurements and extremely easy to maintain;
• 5.Resistant to dirt, high temperature, high sensitivity, anti-blocking and anti-interference, anti-impurities and other capabilities are particularly strong;
• 6.Low pressure loss.

　　3.Drawbacks

• 1. It is not suitable for working conditions where fluid switching is very frequent, and it is better to apply it in the case of continuous operation;
• 2.Accuracy is not very high, generally as a process control class measuring instruments, trade settlement caution;
• 3.Narrow range, generally meters are 10:1 range degree;
• 4.Since the target flowmeter target piece and target rod have self-weight, the zero point must be reset after installation.