Installation requirements of orifice flowmeter

1. Overview of Orifice Flow Meter

The orifice flowmeter is a high range ratio differential pressure flow device that combines a standard orifice plate with a multi-parameter differential pressure transmitter (or differential pressure transmitter, temperature transmitter and pressure transmitter). It can measure the flow of gas, steam, liquid and natural gas, and is widely used in process control and measurement in the fields of petroleum, chemical industry, metallurgy, electric power, heating, water supply, etc.

2. Orifice flowmeter installation requirements

1. Pipeline conditions:

(1) The straight pipe sections before and after the orifice flowmeter must be straight and must not have any visible bends.

(2) The straight pipe section used to install the throttling device should be lubricated. If not lubricated, the flow coefficient should be multiplied by the roughness correction.

(3) To ensure that the flow of the fluid forms a fully developed turbulent velocity distribution 1D before the throttling device, and to make this distribution into a uniform axisymmetric shape,

1) The straight pipe section must be round, and the roundness requirement for the 2D range before the throttling device is very strict, and there is a certain roundness target. Detailed weighing method:

(A) Measure at least 4 single measurements of the inner diameter of the pipe at OD, D/2, D, and 2D before the throttling device at approximately equal angular intervals, and take the average value D. The difference between any single measurement of the inner diameter and the average value shall not exceed ±0.3%

(B) After the throttling device, 8 single values ​​of the inner diameter are measured at the OD and 2D positions using the above method. The maximum deviation of any single value compared with D shall not exceed ±2%

2) A sufficiently long straight pipe section is required before and after the throttling device. This sufficiently long straight pipe section is related to the partial resistance member before the throttling device and the diameter ratio β, see Table 1 (β=d/D, d is the orifice opening diameter, D is the inner diameter of the pipe).

(4) The length of the straight pipe section between the first resistance member and the second resistance member on the downstream side of the throttling member can be taken as 1/2 of the value listed in Table 1 according to the method of the second resistance member and β = 0.7 (regardless of the actual β value).

(5) When the downstream side of the throttling device is a closed space or a large container with a diameter ≥ 2D, the straight pipe length between the closed space or large container and the throttling device shall not be less than 30D (15D). If there are other partial resistance members between the throttling device and the closed space or large container, in addition to the minimum straight pipe section length 1 in accordance with the rules in Table 1 between the throttling device and the partial resistance member, the total straight pipe section length from the closed space to the throttling device shall not be less than 30D (15D).

2. The number of rows is a multiple of the inner diameter D of the tube.

3. The numbers outside the brackets in the above table are the values ​​of "additional absolute limit error is zero", and the numbers inside the brackets are the values ​​of "additional absolute limit error is ±0.5%". That is, when there is a value in the brackets in the length of the straight pipe section, the limit absolute error of the flow measurement τQ/Q should be arithmetically added by 0.5%, that is, (τQ/Q+0.5)%

4. If the actual straight pipe length is greater than the value in the brackets but less than the value outside the brackets, it must be handled according to the "additional limit absolute error of 0.5%".

(1) When the DC component is installed in the pipeline, its front end face must be perpendicular to the pipeline axis, and the maximum allowable non-perpendicularity must not exceed ±1°.

(2) After the throttling device is installed in the pipeline, its opening must be concentric with the pipeline, and its maximum allowable eccentricity ε shall not exceed the result calculated by the following formula: ε≤0.015D (1/β-1).

(3) All gaskets cannot be made of too thick materials, preferably not more than 0.5 mm, and the gaskets cannot protrude into the pipe wall, otherwise it may cause large measurement errors.

(4) Any valve used to adjust the flow should be installed beyond the minimum pipe length after the throttling device.

(5) The throttling device installed on the process pipeline must be stopped after the pipeline is cleaned and purged.

(6) A method of taking pressure in a throttling installation on a horizontal or inclined pipeline.