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Jan 22, 2019

The orifice flowmeter is a type of differential pressure flowmeter. It has the characteristics of simple structure, convenient maintenance and stable performance. Orifice flowmeters are now widely used in coal, chemical, transportation, construction, textile, food, medicine, agriculture, environmental protection and other fields.

Ⅰ The working principle of orifice flowmeter

The pipe-filled fluid flows through the throttling device in the pipe, causing local contraction near the throttle, increasing the flow rate, and creating a static pressure difference on both the upper and lower sides.

Under the condition that the relevant parameters are known, the flow rate can be obtained by deriving the relationship between the differential pressure and the flow rate according to the principle of flow continuity and the Bernoulli equation. The basic formula is as follows:

C-outflow coefficient dimensionless

The orifice or throat diameter of the throttle under d-working conditions

Upstream pipe inner diameter under D-working conditions

Qm-mass flow Kg/s

Qv-volume flow m&sup3;/s

&szlig;-diameter ratio d/D dimensionless

Fluid density Kg/m&sup3;

Scalability coefficient dimensionless

Ⅱ The selection  of orifice flowmeter

1. Pipeline conditions:

(1) The straight pipe section before and after the throttle piece must be straight, and there must be no visible curvature. The length of the straight pipe section is generally 5D after the first 10D.

(2) The straight pipe section used for installing the throttle piece should be smooth. If it is not smooth, the flow coefficient should be multiplied by the roughness correction factor.

(3) In order to ensure the flow of the fluid to form a fully developed turbulent velocity distribution at the 1D front of the throttle, and to make this distribution a uniform axisymmetric shape, 1) the straight pipe section must be round and throttled. The front 2D range, its roundness requires it to be very strict, and has a certain roundness index. Specific measurement method: (A) On the OD, D/2, D, 2D4 vertical tube sections of the throttle, measure the inner diameter of the four pipes at least equal to each other with an equal angular distance, and take the average value D. The difference between the single measured value and the average value of any inner diameter shall not exceed ±0.3%. (B) After the throttle, 8 internal diameter single measurements are measured by the above method at the OD and 2D positions, and any single measured value is compared with D. The maximum deviation shall not exceed ± 2%. 2) A straight pipe section of sufficient length is required before and after the throttle. The length of the straight pipe section and the shape of the local resistance before the throttle are related to the diameter ratio β, see Table 1. (β=d/D, d is the orifice diameter of the orifice, and 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 upstream side of the throttle member may be in the form of the second resistance member and β = 0.7 (regardless of the actual β value), which is listed in Table 1. 1/2 of the value

(5) When the upstream side of the throttle member is an open space or a large container with a diameter ≥ 2D, the open space or the straight pipe length between the large container and the throttle member shall not be less than 30D (15D) if the throttle member and the open space or large When there are other local resistance members between the containers, the straight line between the open space and the throttle member is provided, except that the minimum straight pipe length specified in Table 1 is attached between the throttle member and the local resistance member. The total length of the pipe section shall not be less than 30D (15D).

Minimum straight pipe length on the upstream and downstream side of the throttle piece Table 1

Local thrust member on the upstream side of the throttle member and minimum straight pipe length L

Note: 1. The above table is only for standard throttling devices.

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

3. The number outside the parentheses in the above table is the value of “additional relative limit error is zero”, and the number in parentheses is the value of “additional relative limit error is ±0.5%”. That is, when there is a value in the brackets in the length of the straight pipe, the limit relative error τQ/Q of the flow measurement. The arithmetic should be added by 0.5%, which is (τQ/Q+0.5)%.

4. If the actual length of the straight pipe is greater than the value in parentheses and less than the value outside the brackets, it shall be treated according to the “additional limit relative error of 0.5%”.

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

(2) After the throttle is installed in the pipeline, the opening must be concentric with the pipeline, and the maximum allowable degree of ε allowed by the throttle shall not exceed the calculation result of the following formula: ε ≤ 0.015D (1/β-1).

(3) All gaskets should not be used with too thick materials, preferably not more than 0.5mm. The gaskets cannot protrude inside the pipe wall or it may cause a large measurement error.

(4) All valves used to adjust the flow rate shall be installed outside the minimum length of the pipe section after the throttle

(5) The installation of the throttling device on the process piping must be carried out after the pipe cleaning and purging.

(6) The pressure-receiving method of the throttling device installed in a horizontal or inclined pipe.

1) When the fluid to be tested is a liquid, in order to prevent air bubbles from entering the process pipeline, the pressure buckle should be at the position where the center line of the process pipe is ≤45°, and the positive and negative αα α1[1]

1. The standard throttle is fully used and approved by the International Standards Organization. It can be used without actual flow calibration and is unique in the flow sensor.

2, the structure is easy to copy, simple, firm, stable and reliable performance, low price;

3, a wide range of applications, including all single-phase fluid (liquid, gas, steam), part of the mixed phase flow, the diameter of the general production process, working conditions (temperature, pressure) can be measured;

4. The test piece and the differential pressure display instrument can be separated from different manufacturers to facilitate professional scale production.