1. Characteristics of turbine flowmeter
From the previous discussion, the turbine flowmeter is a kind of flow meter with many advantages. Summed up, it has the following characteristics.
(1) High accuracy
The accuracy of the turbine flowmeter is 0. 5-0.1)% or so. In the linear flow range, the cumulative flow accuracy will not decrease even if the flow rate changes. And in a short time, the turbine flowmeter reproducibility of up to 0. 5%.
(2) Quantity Cheng Biquan
The range ratio of turbine flowmeter is up to 8-10. Under the same caliber, the maximum flow value of a turbine flowmeter is greater than that of many other flowmeters.
(3) Strong adaptability
The turbine flowmeter can be closed structure, its speed signal is non-contact measurement, so it is easy to achieve high pressure design.
If the turbine and the bearing of the flowmeter choose a material with high temperature resistance and small thermal expansion coefficient, it can be used in a wide temperature range. At this point, it should be noted that the instrument coefficient of its correction (mainly the changes in its circulation section):
K=k0[1-(βr+2βh) (t-to)] (3-23)
In the formula K, k0--when using and check the meter coefficient;
T, t0--the fluid temperature when used and calibrated;
Βr, βh are the material expansion coefficients of turbine and casing respectively.
(4) Digital signal output
The turbine flowmeter is output as a pulse digital signal proportional to the flow rate. It has the advantages of not reducing accuracy, accumulating easily, and easy to be fed into computer system during transmission.
2. Turbine flowmeter Installation and use
To give full play to the characteristics of the turbine flowmeter, the installation and use of flowmeter must also be fully noted. The following is an overview of this issue.
(1) The measured medium
The liquid measured by the turbine flowmeter is generally low viscosity (generally less than 15xl0-6m2/s) and low corrosive liquid. Although there are already a variety of dielectric measurement of the turbine flowmeter, but the high temperature, high viscosity, strong corrosive medium measurement still need to be carefully considered, take the appropriate measures. When the medium viscosity V is greater than 15xl0-6m2/s, the meter coefficient of the flowmeter must be calibrated by the solid liquid, otherwise it will produce a larger error.
The vapor-liquid two-phase flow, gas-solid two-phase flow and the concentrated-solid two-phase flow can not be measured by the turbine flowmeter.
(2) Installation of piping requirements
The measurement accuracy of flowmeter is greatly influenced by the installation of flowmeter.
The ① velocity distribution and the presence of two secondary flow in the tube are important factors that affect the accuracy of the measurement of turbine flowmeter. Therefore, the turbine flowmeter on the upstream and downstream straight pipe has-set requirements. For industrial measurement, generally requires upstream 20D, downstream 5D straight tube length. In order to eliminate two times flow, it is best to install the rectifier at the upstream end. If the upstream end can be guaranteed to have a straight section of around 20D, and add a rectifier, the measurement accuracy of the flowmeter can reach the accuracy level of the calibration.
The ② turbine flowmeter has higher requirements for the cleanliness of the fluid, and the filter must be installed before the flowmeter to keep the fluid clean. The filter can be funnel-shaped, its own cleanliness, can be measured at both ends of the differential pressure change.
③ to ensure that the liquid flow through the flowmeter is single-phase, that is, can not let air or steam into the flowmeter, when the flow meter upstream should be fitted with a cooler. For easily gasified liquids, a certain back pressure must be ensured downstream of the flowmeter. The size of the backpressure is preferable to the maximum flow rate of the flow sensor under pressure drop of twice times plus the maximum temperature of the measured liquid vapor pressure of 1. Twice times.
(3) Signal transmission line
In order to ensure that the display instrument has enough sensitivity to the pulse signal output by the turbine sensor, it is necessary to improve the signal-to-noise ratio. To this end, in the installation should prevent all kinds of electrical interference phenomenon, namely electromagnetic induction, electrostatic and capacitance coupling. Therefore, when configuring the signal transmission line, you must pay attention to the following points:
① limits the maximum length of the signal line. The maximum length of the signal line is L=DV, where V is the effective value of the output voltage of the sensing coil at the minimum flow, MV; d is the coefficient, M/MV, and its value is desirable: v<1000mv, d=1. 0. $number mv<d<5000 MV, d=1. 5. When v>5000mv, d=2. 0.
② signal transmission line should be shielded cable, in case of external induced noise. The transmission cable is required to be shielded from the display instrument side. Transmission cable can not be near the strong electromagnetic equipment, not allowed with the power line layout.
(4) Operation and maintenance
① when the piping of the turbine flowmeter needs cleaning, the bypass must be opened and the liquid cannot be cleaned through the flowmeter.
The ② piping system must be opened by bypass to prevent sudden increase in flow rate, causing the turbine speed to be too large and damaged.
③ turbine flowmeter bearings should be replaced regularly, generally according to the small flow characteristics of the change to observe the wear of the bearing.