的安装方法在众多流体流量的测量仪表当中，最常见的一种节流式流量计要算孔板流量计了，但很多第一次使用孔板流量计的用户对它的安装方法还很陌生，下面旭东仪表厂技术人员为用户简要介绍一下孔板流量计的安装，希望对大家有所帮助： Orifice flowmeter installation method Among the many fluid flow measuring instruments, the most common type of throttling flowmeter is the orifice flowmeter, but many users who use the orifice flowmeter for the first time install it. The method is still unfamiliar. The technical staff of Xudong Instrument Factory briefly introduces the installation of orifice flowmeters to users. I hope it will be helpful to everyone:
Conditions and steps for installing the orifice plate flowmeter:
1. The straight pipe sections before and after the throttle must be straight, and there must be no visible bends. 2. The straight pipe section used to install the throttle should be smooth. If it is not smooth, the flow coefficient should be multiplied by the roughness correction coefficient. 3. In order to ensure that the fluid flow forms a fully developed turbulent velocity distribution at 1D in front of the throttle, and to make this distribution into a uniform axisymmetric shape, so:
(1) The straight pipe section must be round, and the roundness requirements for the front 2D range of the throttle are extremely strict, and there is a certain roundness index. Specific measurement method:
A. On the four vertical pipe sections of OD, D / 2, D, and 2D in front of the throttle, measure at least four individual diameters of the inner diameter of the pipe at approximately equal angular distances, and take the average value D. The difference between the single measured value of any inner diameter and the average value must not exceed ± 0.3%
B. After throttling, at the OD and 2D positions, use the above method to measure 8 internal diameter single measurement values. When comparing any single measurement value with D, the maximum deviation must not exceed ± 2%.
(2) A sufficiently long straight pipe section is required before and after the throttling member. This long enough straight pipe section is related to the form of the local resistance member in front of the throttle member and the diameter ratio β.
(3) The length of the straight pipe section between the first resistance member and the second resistance member on the upstream side of the throttle can be in the form of the second resistance member and β = 0.7 (regardless of the actual β value). 2.
4. When the upstream side of the throttle is an open space or a large container with a diameter ≥ 2D, the length of the straight pipe between the open space or the large container and the throttle shall not be less than 30D (15D). If there are other local resistance parts between the throttle and the open space or the large container, in addition to the minimum straight pipe length 1 that meets the requirements between the throttle and the local resistance parts, from the open space to the The total length of the straight pipe sections between the flow pieces must not be less than 30D (15D).
5. Connect the signal cable and power cable according to the flowmeter installation instructions.
6. Open the inlet and outlet valves, and the opening of the inlet and outlet valves must be the same.
7. Open the stainless steel three-valve balance valve, slowly open the valve at the high and low pressure end of the orifice plate, and close the stainless steel three-valve balance valve after the fluid passes through the flow meter.
计量不准确的原因有哪些？ What are the reasons for the inaccurate measurement of the orifice plate flowmeter ?
The standard orifice plate is a circular perforated sheet obtained by machining. Its throttling cylindrical cylindrical surface is perpendicular to the upstream end surface of the orifice plate, its edges are sharp, and the ratio of the orifice plate thickness to the orifice plate diameter is relatively small. The part of the orifice plate inside the measuring tube shall be round and coaxial with the axis of the measuring tube. The two end surfaces of the orifice plate shall always be flat and parallel.
1 Orifice eccentric
According to GB2624-81, the orifice plate should be aligned with the straight pipe section in the throttling device. Experiments show that the measurement error caused by the eccentricity of the orifice plate is generally within 2%. The higher the aperture ratio β value, the greater the effect of the eccentricity. Therefore, the orifice plate with a high value should not be used. 2 Orifice is bent due to improper installation or maintenance. Bend or deform the orifice plate, resulting in large flow measurement errors. Testing on the orifice plate where the flange is pressed, the maximum error caused by the orifice plate bending is about 3.5%.
3 The sharpness of the edge of the orifice plate, the wear of the entrance edge of the orifice plate is dull and not sharp or the gap is corroded, or the welding seam or the measuring flange gasket inside the orifice plate pipeline will increase the actual flow coefficient and reduce the differential pressure, resulting in The calculated air volume is too small.
Measures to improve measurement accuracy
1. Eliminate the pulsating flow in the air flow. Due to the sudden change of the gas flow rate and pressure in the pipeline, the pulsating flow can cause fluctuations in the differential pressure. The flow calculation formula of the throttling device is based on the stable flow of the blue orifice plate as Basically, when the measurement point has a pulsation phenomenon, the stability principle cannot be established, which affects the measurement accuracy and generates measurement errors. The total uncertainty of the pulsating flow is equal to the combination of the measurement error calculated according to GB / T2624-93 and the additional uncertainty of the pulsation.
Where: ET- pulsation additional uncertainty, dimensionless;-axial time-averaged velocity, m / s;-root mean square value of velocity pulsation component, m / s. (Formula application conditions ≤ 0.32)
Therefore, in order to ensure the accuracy of natural gas measurement, pulsating flow must be suppressed. Common measures are:
(1) Under the condition of satisfying the measurement capability, a measuring tube with a small inner diameter should be selected to improve the differential pressure and the aperture ratio;
(2) The use of short impulse pipelines reduces the resistance in the pipelines and makes the lengths of the upstream and downstream pipelines equal, reducing the resonance and pressure pulsation amplitude increase in the system;
(3) Eliminate the free liquid from the pipeline. The pulsation caused by the accumulation of fluid in the pipeline can be treated with an automatic pigging system or a liquid separator installed at a low place.
2. The design and installation of the measuring device should comply with the table SY / T 6143-1996
测量精度的根本原因是节流装置的几何形状和流动动态是否偏离设计标准。 The fundamental reason that affects the measurement accuracy of the orifice flowmeter is whether the geometry and flow dynamics of the throttling device deviate from the design standards. Therefore, the system must be regularly checked and maintained during use. Corrections should be made to changes in operating conditions such as pressure, temperature, and flow during actual use. A total compensation flow computer calculation scheme can be used to reduce measurement errors and ensure measurement accuracy.
3. Avoid artificial measurement errors
Strengthen measurement management, improve the technical quality of operators, and actively introduce and absorb foreign advanced natural gas measurement technology. The standard orifice plate is a circular perforated sheet obtained by machining. Its throttling cylindrical cylindrical surface is perpendicular to the upstream end face of the orifice plate, its edges are sharp, and the ratio of orifice plate thickness to orifice plate diameter is relatively small.