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(1) Straight sections before and after the throttle must be straight and must not be bent by the naked eye.
(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 sparse.
(3) In order to ensure that the flow of fluid forms a well-developed turbulent velocity distribution in the 1D before the throttle, and this distribution is made into a uniform axial symmetry, so
1) The straight section must be round, and the roundness of the throttle before the 2D range is very strict, and it has a certain degree of roundness. Specific measurement methods:
(A) At the front of the OD, D/2, D, and 2D orifices of the throttling element, at least four pipe inner diameters are measured at a maximum to equal angular distance, and an average value D is taken. The difference between the single measured value and the average value of the arbitrary internal diameter must not exceed ±0. 3%
(B) After the throttling element, 8 single-diameter measurements were taken at the OD and 2D positions using the method described above. The maximum deviation of any single measured value from D was not to exceed ±2%.
2) A sufficiently long straight pipe section is required before and after the throttling piece. The long enough straight pipe piece and the partial resistance piece form before the throttling piece are related to the diameter ratio β, see Table 1 (β=d/D, d is Hole plate opening diameter, D is the inner diameter of the pipe).
(4) The length of the straight pipe section between the first resistance piece and the second resistance piece on the upstream side of the throttle may be in the form of the second resistance piece and β=0.7 (regardless of the actual β value). 1/2 of the value
(5) When the upstream side of the throttling element is an open space or a large vessel with a diameter ≥ 2D, the length of the open space between the open space or large vessel and the throttling element shall not be less than 30D (15D) if the throttling element and the open space are large When there are other local resistances between the containers, except for the minimum straight pipe length 1 stipulated in Attachment Table 1 between the throttle and the local resistance, the straightness from the open space to the throttle The total length of the pipe segment must not be less than 30D (15D).
The minimum straight pipe length on the upstream and downstream sides of the throttle
Throttle piece upstream local force piece form and minimum straight pipe length L
Note: 1. The above table only refers to the standard throttling device, and the special throttling device is available for reference.
2. The number of columns is the multiple of the inner diameter D of the tube.
3. The figure outside the parenthesis in the above table is the value of “additional relative limit error is zeroâ€, and the figure in parentheses is the value of “additional relative limit error is ±0.5%â€. That is, when there is a bracketed value in the straight pipe length, the relative error of the flow measurement is Ï„Q/Q. Should add another arithmetic 0.5% or (Ï„Q/Q+0.5)%
4. If the length of the actual straight pipe section is greater than the value in parentheses and less than the value outside the brackets, the “relative limit relative error is 0.5%â€.
(1) The DC component is installed in the pipeline. The front surface must be perpendicular to the pipeline axis. The maximum non-perpendicularity allowed shall not exceed ±1°.
(2) After the throttling element is installed in the pipeline, its opening must be concentric with the pipeline. Its maximum allowable misalignment ε must not exceed the following formula: ε ≤ 0.015D (1/β-1).
(3) All gaskets cannot be made of thick materials, preferably not more than 0.5mm. The gaskets cannot protrude into the tube wall or it may cause a large measurement error.
(4) Any valve used to regulate flow shall be installed outside the minimum length of the pipe after the throttle
(5) The installation of the throttling device on the process piping must be carried out after cleaning and purging of the piping.
(6) The pressure-recovery method of a throttling device installed on a horizontal or inclined pipeline.
1) When the fluid to be measured is a liquid, in order to prevent the air bubbles from entering the process pipeline into the tooth gap, the pressure deduction should be taken in the position where the process pipeline is offset by ≤45° from the center line.