Latest Liquid Turbine Flowmeter Features and Installation Requirements
1. Turbine flow meter features
As can be seen from the previous discussion, the turbine flow meter is a flow meter with many advantages. To sum it up, it has the following features.
(1) High accuracy
The accuracy of turbine flowmeters is around (0.5-0. 1)%. In the linear flow range, the accumulated flow accuracy does not decrease even if the flow changes. And in a short time, the reproducibility of the turbine flowmeter can reach 0.05%.
(2) Wide range ratio
Turbine flow meter range up to 8-10. With the same caliber, the maximum flow rate of the turbine flowmeter is greater than many other flowmeters.
(3) strong adaptability
Turbine flowmeter can be made into a closed structure, its speed signal is non-contact measurement, so it is easy to achieve high pressure design.
If the flowmeter's turbine and bearing select materials that are resistant to high temperatures and low thermal expansion coefficients, they can be used over a wide temperature range. At this time, attention should be paid to the correction of its meter factor (mainly the change of its flow section):
K=K0[1-( R+2 H)(t-to)] (3-23)
Where K, K0 - meter factor when used and calibrated;
t, t0 - fluid temperature during use and verification;
R, H are the material expansion coefficients of the turbine and the casing, respectively.
(4) Digital signal output
Turbine flowmeter output is a pulsed digital signal proportional to flow. It has the advantages of no decrease in accuracy during transmission, easy accumulation, and ease of feeding into computer systems.
2. Installation and use of turbine flowmeters
In order to give full play to the characteristics of turbine flowmeters, full attention must be paid to the installation and use of flowmeters. Here is an overview of the issues in this area.
(1) Measured medium
Turbine flowmeter measured liquid, generally low viscosity (usually less than 15 × l0-6m2/s), low corrosive liquid. Although there are already turbine flowmeters for various media measurements, the measurement of high-temperature, high-viscosity, strongly corrosive media still requires careful consideration and appropriate measures. When the viscosity of the medium v ​​is greater than 15×10-6m2/s, the flowmeter meter coefficient must be calibrated with solid solution, otherwise it will produce a large error.
The vapor-liquid two-phase flow, gas-solid two-phase flow, and concentrated-solid two-phase flow cannot be measured by turbine flowmeter.
(2) Installation piping requirements
The installation of the flow meter has a great influence on the measurement accuracy of the flow meter.
1 The uneven distribution of flow velocity and the presence of secondary flow in the tube are important factors affecting the measurement accuracy of the turbine flowmeter. Therefore, turbine flowmeters have certain requirements for upstream and downstream straight pipe sections. For industrial measurements, generally require 20D upstream and 5D downstream straight pipe length. To eliminate the secondary flow, it is best to install a rectifier on the upstream side. If the upstream end can guarantee a straight pipe section of about 20D, and install a rectifier, the measurement accuracy of the flowmeter can reach the accuracy level of the calibration.
Turbine flowmeters have high requirements for the cleanliness of fluids. Filters must be installed before the flowmeter to ensure fluid cleanliness. Filter can be used funnel type, its own cleanliness, can be measured at both ends of the differential pressure change.
3 To ensure that the liquid passing through the flowmeter is single-phase, that is, air or steam cannot be allowed to enter the flowmeter, an air eliminator should be installed upstream of the flowmeter when necessary. For easy gasification liquids, a certain back pressure must be guaranteed downstream of the flowmeter. The back pressure can be twice the pressure drop of the flow sensor at the maximum flow rate plus 1.2 times the measured liquid vapor pressure at the highest temperature.
(3) Signal transmission line
In order to ensure that the display instrument has sufficient sensitivity to the pulse signal output by the turbo sensor, the signal to noise ratio must be improved. To this end, various electrical interference phenomena, namely electromagnetic induction, electrostatic and capacitive coupling, should be prevented during installation. Therefore, when configuring the signal transmission line, the following points must be noted:
1 Limit the maximum length of the signal line. The maximum length of the signal line is L=dV; where V is the output voltage effective value of the sensing coil at the minimum flow rate, mV; d is the coefficient, m/mV, and its value is desirable: when V<1000mV, d=1 .0; when 1000 mV<d<5000 mV, d=1.5; when V>5000 mV, d=2.0.
2 The signal transmission line should be shielded cable to prevent inductive noise from the outside. Require that the transmission cable be grounded on the display meter side shield. The transmission cable must not be close to strong electromagnetic equipment, and it must not be arranged with the power line.
(4) Operation and maintenance
1 When the pipeline of the turbine flowmeter needs to be cleaned, the bypass must be opened and the cleaning liquid cannot pass through the flowmeter.
2 When the pipeline system is started, it must be bypassed first to prevent the sudden increase of the flow rate, causing excessive turbine speed and damage.
3 Turbine flowmeter bearings should be replaced on a regular basis, generally according to the changes in the characteristics of small flow rate to observe the wear of the bearing
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