Positive displacement flow meter

Positive displacement flowmeter advantages and disadvantages 

Positive displacement flowmeter, also known as constant displacement flowmeter, referred to as PD flowmeter, is one of the most accurate flow instruments. It uses mechanical measuring elements to continuously divide the fluid into a single known volume part, and measures the total volume of the fluid according to the number of times that the measuring chamber repeatedly fills and discharges the fluid in the volume part one by one.

According to the classification of measuring elements, volumetric flowmeter can be divided into elliptical gear flowmeter, scraper flowmeter, double rotor flowmeter, rotary piston flowmeter, reciprocating piston flowmeter, disc flowmeter, liquid seal rotary drum flowmeter, wet gas meter and membrane gas meter.

Advantage:

(1) High measurement accuracy;

(2) The installation conditions of pipeline have no influence on the measurement accuracy;

(3) It can be used for the measurement of high viscosity liquid;

(4) Wide range;

(5) The direct reading instrument can directly obtain the cumulative total without external energy, which is clear and easy to operate.

Disadvantages:

(1) The results are complex and bulky;

(2) The type, diameter and working state of the measured medium have great limitations:

(3) Not suitable for high and low temperature occasions;

(4) Most instruments are only applicable to clean single-phase fluid;

(5) Generate noise and vibration.

Application Overview:

Volumetric flowmeter, differential pressure flowmeter and float flowmeter are the three types of flowmeter with the largest use, which are often used for the total amount measurement of expensive media (oil, natural gas, etc.).

In 1990, the output (excluding domestic gas meters) was 340000 units, of which oval gear type and waist wheel type accounted for 70% and 20% respectively

Electromagnetic flow meter

1. Advantages

(1) Electromagnetic flowmeter can be used to measure industrial conductive liquid or slurry.

(2) No pressure loss.

(3) The measurement range is large, and the caliber of electromagnetic flow transmitter ranges from 2.5mm to 2.6m.

(4) The electromagnetic flowmeter measures the volume flow of the measured fluid under the working state. The measurement principle does not involve the influence of fluid temperature, pressure, density and viscosity.

2. Shortcomings

(1) The application of electromagnetic flowmeter has certain limitations. It can only measure the liquid flow of conductive medium, but not the flow of non-conductive medium, such as heating water with better gas and water treatment. In addition, its lining needs to be considered under high temperature conditions.

(2) Electromagnetic flowmeter determines the volume flow under working state by measuring the speed of conductive liquid. According to the measurement requirements, for liquid media, the mass flow shall be measured, and the measurement of medium flow shall involve the density of fluid. Different fluid media have different densities and change with temperature. If the electromagnetic flowmeter converter does not consider the fluid density, it is not appropriate to only give the volume flow at room temperature.

(3) The installation and commissioning of electromagnetic flowmeter is more complex and more strict than other flowmeters. The transmitter and converter must be used together, and two different models of instruments cannot be used between them. When installing the transmitter, the selection of installation location and specific installation and commissioning must be carried out in strict accordance with the requirements of the product manual. The installation site shall be free from vibration and strong magnetic field. During installation, the transmitter and pipeline must have good contact and good grounding. The potential of the transmitter is equal to that of the measured fluid. When in use, the gas remaining in the measuring tube must be discharged, otherwise it will cause large measurement error.

(4) When the electromagnetic flowmeter is used to measure the viscous liquid with dirt, the viscosity or sediment is attached to the inner wall of the measuring pipe or the electrode, which changes the output potential of the transmitter and brings measurement error. The dirt on the electrode reaches a certain thickness, which may make the instrument unable to measure.

(5) The scaling or abrasion of water supply pipeline and the change of inner diameter will affect the original flow value and cause measurement error. If the inner diameter of a 100mm diameter instrument changes by 1mm, it will bring about 2% additional error.

(6) The measurement signal of the transmitter is a very small millivolt potential signal. In addition to the flow signal, it is also mixed with some signals irrelevant to the flow, such as phase voltage, quadrature voltage and common mode voltage. In order to measure the flow accurately, various interference signals must be eliminated and the flow signal must be amplified effectively. The performance of the flow converter should be improved. It is best to use the microprocessor converter to control the excitation voltage, and select the excitation mode and frequency according to the properties of the measured fluid, which can eliminate in-phase interference and quadrature interference. However, the improved instrument has complex structure and high cost.

(7) Higher price

Ultrasonic flow meter

1. Advantages

(1) Ultrasonic flowmeter is a non-contact measuring instrument, which can be used to measure the fluid flow and large pipe diameter flow that are not easy to contact and observe. It will not change the flow state of the fluid, will not produce pressure loss, and is easy to install.

(2) It can measure the flow of strongly corrosive medium and non-conductive medium.

(3) The measurement range of ultrasonic flowmeter is large, and the pipe diameter ranges from 20mm to 5M

(4) Ultrasonic flowmeter can measure the flow of various liquids and sewage.

(5) The volume flow measured by ultrasonic flowmeter is not affected by the thermophysical parameters such as temperature, pressure, viscosity and density of the measured fluid. It can be made into fixed and portable forms.

2. Shortcomings

(1) The temperature measurement range of ultrasonic flowmeter is not high. Generally, it can only measure the fluid whose temperature is lower than 200 ℃.

(2) Poor anti-interference ability. It is easy to be disturbed by bubbles, scaling, ultrasonic noise mixed by pumps and other sound sources and affect the measurement accuracy.

(3) The requirements for straight pipe section are strict, which is the first 20d and the last 5D. Otherwise, the discreteness is poor and the measurement accuracy is low.

(4) The uncertainty of installation will bring large errors to flow measurement.

(5) The scaling of the measuring pipeline will seriously affect the measurement accuracy, bring significant measurement error, and even the instrument has no flow display when it is serious.

(6) The reliability and accuracy level are not high (generally about 1.5 ~ 2.5), and the repeatability is poor.

(7) Short service life (the general accuracy can only be guaranteed for one year).

(8) The ultrasonic flowmeter determines the volume flow by measuring the fluid speed. Its mass flow should be measured for the liquid. The mass flow measured by the instrument is obtained by multiplying the volume flow by the artificially set density. When the fluid temperature changes, the fluid density changes. The artificially set density value cannot guarantee the accuracy of the mass flow. Only when the fluid velocity is measured and the fluid density is measured, can the real mass flow value be obtained through operation.

(9) The price is high.

Vortex flow meter

1. Advantages

(1) The vortex flowmeter has no movable parts, and the measuring element has the advantages of simple structure, reliable performance and long service life.

(2) Vortex flowmeter has a wide measuring range. The range ratio can generally reach 1:10.

(3) The volume flow of vortex flowmeter is not affected by the thermal parameters such as temperature, pressure, density or viscosity of the measured fluid. Generally, separate calibration is not required. It can measure the flow of liquid, gas or steam.

(4) It causes little pressure loss.

(5) The accuracy is high, the repeatability is 0.5%, and the amount of maintenance is small.

2. Shortcomings

(1) The volume flow of vortex flowmeter under working condition is not affected by the thermal parameters such as temperature, pressure and density of the measured fluid, but the final measurement result of liquid or steam shall be mass flow, and for gas, the final measurement result shall be standard volume flow. Mass flow or standard volume flow must be converted by fluid density, and the change of fluid density caused by the change of fluid working condition must be considered.

(2) The main factors causing the flow measurement error are: the measurement error caused by the uneven flow velocity of the pipeline; The medium density when the fluid working condition changes cannot be determined accurately; The wet saturated steam is assumed to be dry saturated steam for measurement. If these errors are not limited or eliminated, the total measurement error of vortex flowmeter will be very large.

(3) Poor vibration resistance. External vibration will cause measurement error of vortex flowmeter, and even can not work normally. The high velocity impact of channel fluid will cause additional vibration of the cantilever of vortex street generator and reduce the measurement accuracy. The influence of large pipe diameter is more obvious.

(4) Poor adaptability to measuring dirty media. The generator of vortex flowmeter is very easy to be polluted by medium or entangled by dirt, which changes the geometry size and has a great impact on the measurement accuracy.

(5) High requirements for straight pipe section. Experts pointed out that the straight pipe section of vortex flowmeter must ensure the first 40d and the second 20d in order to meet the measurement requirements.

(6) Poor temperature resistance. Generally, vortex flowmeter can only measure the fluid flow of media below 300 ℃.