JPH0321852B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring a fluid flow path using an orifice.

Generally, as a method of measuring the flow rate of fluids such as gases and liquids, an obstacle called an orifice plate is installed in a part of the flow path to partition the flow path, and the pressure change that occurs in that flow path is measured. A method of calculating the flow rate by measuring the flow rate is known. That is, if the pressure difference between both ends of the orifice is ΔP, the flow rate F is given by equation (1).

F∝√ …(1) Fluid pressure in a flow path is measured from the pressure outlet, and depending on the pressure outlet, it is divided into the Benner tap method, corner tap method, flange tap method, permanent pressure drop method, etc. .

FIG. 1 is an explanatory diagram showing the difference between these pressure measurement methods. As shown in figure a, flow path 1
An orifice 2 is installed within the flow path 1, and when fluid flows in the direction shown by the arrow in the flow path 1, changes in fluid pressure occur upstream and downstream of the orifice 2 as shown in Figure b. P ₁ , P ₁ ′, indicated by arrows in figure b
P ₁ ″, P ₂ , P ₂ ′, P ₂ ″, and P ₃ are the pressures at the pressure extraction points A to G shown in Figure a, respectively. At this time
P ₁ −P ₂ is called the Bennertup outlet pressure, and the method of calculating the flow rate from this Bennertup outlet pressure is called the Bennertup method.

Similarly, P ₁ ′−P ₂ ′ is the corner tap removal pressure.
P ₁ ″−P ₂ ″ is called the flange tap extraction pressure, and P ₁ −P ₃ is called the permanent pressure loss extraction pressure.

The relationship between the fluid pressure difference ΔP from these two pressure outlets and the flow rate F is expressed by equation (1) as described above, so when the flow rate is small, the difference in the output pressure The rate of change becomes smaller and measurement accuracy deteriorates. To solve this problem, there is a method of using two differential pressure measuring devices, for example, as shown in FIG. That is, a differential pressure measuring device 3 that can measure the full range of the flow rate in the flow path 1 and a differential pressure measuring device 4 with a small span that reaches the full range when the flow rate is 9%, for example, are provided, and the differential pressure can be measured by the three-way kettle 5. By switching the flow path flowing through the device, when the flow rate is low, the small range differential pressure measuring device 4 measures it, and when the flow rate is 30% or more, the large range differential pressure measuring device 3 measures it. , which attempts to maintain accuracy even when the flow rate is small. Note that 6 is a knob for adjusting the flow rate taken out from the differential pressure outlet.

However, since this method requires two differential pressure measuring devices, it has the disadvantage that the measuring device becomes large and expensive.

An object of the present invention is to provide a flow rate measurement method that has good accuracy even at small flow rates and can measure the flow rate with a single differential pressure measuring device.

This invention measures the fluid flow rate in the flow path by installing a plurality of pressure take-off points in the flow path, switching these pressure take-off points according to the flow rate, and connecting them to a differential pressure measuring device. It is something.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 3 is a connection system diagram of an apparatus for carrying out the present invention. An orifice 2 is provided in the flow path 1, and pressure take-off points A, D, and G are provided at positions corresponding to Benner tap take-off and permanent pressure loss take-off. 7 is a differential pressure measuring device, 8 is a three-way kettle, 9
indicate Kotsuku, respectively. Pressure take-off point A,
Fluid pressures P ₁ , P ₂ , and P ₃ are detected at D and G, respectively.

In such a measurement system, the flow rate is calculated by the Bennertapp method when the flow rate is small, and by the permanent pressure drop method when the flow rate is large. That is, by switching the three-way switch 8, when the flow rate is small, the pressure difference P ₁ - P ₂ between the Bennertap outlet A and D is applied to the differential pressure measuring device 7, and when the flow rate is large, the permanent pressure loss outlet Operate so that the pressure difference between A and G is _P1 - _P3 .

Here, assuming that the pressure difference measured by the Bennertup method is ΔP _a and the pressure difference measured by the permanent pressure loss method is ΔP _b , equations (2) and (3) hold true.

ΔP _a =P ₁ −P ₂ (2) ΔP _b =P ₁ −P ₃ =(1−m)(P ₁ −P ₂ )… (3) Here, m is the aperture ratio of the orifice. The full range of flow rate F _T can be determined by the permanent pressure drop method, and by determining the range of the differential pressure measuring device, the flow rate F at large flow rates can be measured using equation (4).

F=K√ _b … (4) Here, K is a constant. Then, when the flow rate becomes small, switch to the Bennertup method and measure in the range for large flow rates determined above. Since the relationship shown in equation (5) holds, √ _b = √1−√ _a … ( 5) When the flow path is less than the maximum flow rate √1-, the Bennertup method can measure the flow rate from 0 to F _T ×√1- using the full range of the same differential pressure measuring device. Therefore, it is possible to prevent a decrease in accuracy when the flow rate is small, which occurs when the flow rate is measured using only a single pressure difference.

In this example, the method of measuring by switching between the Benertap outlet and the permanent pressure loss outlet has been explained, but the switching of these pressure outlets is not limited to the above, and measurement accuracy must be taken into account. Various other combinations can be selected. Furthermore, the measurement of the pressure difference is not limited to two pressure outlet ports, and the pressure difference between any number of ports may be measured.

It is also possible to provide hysteresis to the switching of the pressure outlet so as to prevent the flow rate near the switching point from being switched unnecessarily.

FIG. 4 is a diagram showing the relationship between the pressure difference and the flow rate when switching the pressure outlet has hysteresis in the embodiment of FIG. 3. That is, from the maximum flow rate F _T to F _T ×√1−×α (α<1), P ₁ −
Measure using the permanent pressure drop method using P ₃ , and when the flow rate becomes lower than that, switch to the Bennertup method and measure using P ₁ − _{P 2.} After that, the flow rate increases and F _T ×√
When it reaches 1-m, switch to the permanent pressure drop method again.

By providing hysteresis in the switching operation in this way, stable measurements can be performed.

Furthermore, since the measurement signal is unstable and inaccurate during the switching of the three-way switch, the reliability of measurement accuracy can be increased by adding a function to hold the value before switching until the switching is completed.

As explained above, according to the present invention, the flow rate is measured by switching the pressure outlet in accordance with the change in the flow rate, so a single differential pressure measuring device can be used to measure from small flow rates to large flow rates. It has the excellent effect of being able to perform measurements without deteriorating accuracy.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing the difference in pressure measurement methods, Fig. 2 is an illustration showing an example of a conventional flow measurement method using two differential pressure measuring devices, and Fig. 3 is an explanatory diagram showing the difference in pressure measurement methods. The connection system diagram of the device, FIG. 4, is a diagram showing the relationship between the pressure difference and the flow rate when switching the pressure outlet has hysteresis. 1...Flow path, 2...Orifice, 7...Differential pressure measuring device, 8...Three-way socket, A, D, G...Pressure extraction point.

Claims (1)

[Claims] 1 An orifice is installed in a part of the flow path, and a common pressure take-off point is established on the upstream side of the flow path through this orifice, and a pressure take-off point for the Bennertup method and a pressure take-off point for the permanent pressure drop method are established on the downstream side of the flow path. and a switching means for switching between the pressure take-off point for the Bennertapp method and the pressure take-off point for the permanent pressure drop method, and the differential pressure between the pressure take-off point switched by the switching means and the common pressure take-off point. When a differential pressure measuring device is installed and the switching means selects the Bennertup method pressure extraction point, the fluid flow rate is measured by the Bennertup method based on the differential pressure of the differential pressure measuring device, When the switching means selects the pressure take-off point for the permanent pressure drop method, the fluid flow rate is measured by the permanent pressure drop method based on the differential pressure of the differential pressure measuring device, and the fluid flow rate measured by the Bennertup method is applied to the orifice. A method for measuring a flow rate, characterized in that when the flow rate exceeds a predetermined flow rate range based on the aperture ratio of the flow rate, the switching means is switched to the pressure take-off point side for the permanent pressure drop method.