JPH0778442B2 - Flow rate measuring device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a flow rate measuring device applied to, for example, a pipeline for fuel or air supplied to a heating furnace.

(Prior Art) Conventionally, the measuring method of this kind of measuring device is to use a fixed throttle mechanism such as an orifice or a venturi and a differential pressure sensor such as a differential pressure gauge or a differential pressure converter for measuring the pressure difference before and after the throttle mechanism. It is usual to use the combination alone.

(Problems to be Solved by the Invention) By the way, as is well known, the differential pressure of the fixed throttle mechanism is proportional to the square of the flow rate and sharply decreases as the flow rate decreases. Therefore, the zero drift of the differential pressure sensor (constant over the entire measurement range is constant. The flow rate measurement error due to the error that the differential pressure value of is added or reduced is shown in Fig. 2 (zero drift of the differential pressure sensor is ± ± of the maximum differential pressure).
As shown in (1%), the flow rate rapidly increases as the flow rate decreases.

On the other hand, as a method for avoiding an increase in error in the low flow rate region, there is a flow rate measurement method using a variable throttle mechanism. Since this flow rate measurement error depends on the factors listed below, each factor is A case is shown as an example in FIG.

Zero drift of the differential pressure sensor (± 1%) Zero drift related to the opening of the variable throttle mechanism (± 0.5%)
%) (Error of opening sensor and mechanical error from opening detection position to end of variable throttle mechanism, etc.) Control method of variable throttle mechanism (also used as control valve) Relationship of flow coefficient to opening of variable throttle mechanism (Characteristic of exponential function) Turn-down ratio (2
5) Ratio of other total pressure loss to the differential pressure of the variable throttle mechanism at the maximum flow rate (4) The flow rate measurement error of the flow rate measuring method using the variable throttle mechanism is
Contrary to the flow rate measurement error (FIG. 2) of the flow rate measurement method using the fixed throttle mechanism as shown in the figure, there is a problem that the error increases in the high flow rate region. In combustion control, mixing control, etc., where many types of measuring devices of this type are used, the loss due to measurement control error such as deterioration of the basic unit and deterioration of product quality is usually higher in the high flow rate region. There is a great need to improve the measurement accuracy in the flow rate region.

The present invention has been made to solve the above-mentioned conventional problems. The flow rate value obtained by the fixed throttle system is used for the high flow rate range, and the flow rate value obtained by the variable throttle system is used for the low flow rate range. Therefore, it is intended to provide a simple flow rate measuring device with high accuracy in the entire flow rate region while using a general-purpose pressure sensor.

(Means for Solving the Problem) In order to solve the above problems, the present invention relates to a relationship between a fixed throttle mechanism provided in a fluid pipeline to be measured and a flow coefficient with respect to an opening provided in the fluid pipeline. , A first differential pressure sensor and a second differential pressure sensor for detecting a differential pressure before and after each of the fixed throttle mechanism and the variable throttle mechanism. And the relationship between the flow coefficient of the fixed throttle mechanism and the flow coefficient with respect to the opening of the variable throttle mechanism are stored in advance, and this relationship, the flow coefficient of the fixed throttle mechanism, the first differential pressure sensor, and the second differential pressure sensor are stored. Based on the signals from the pressure sensor and the opening sensor, the flow rate values of the fixed throttle system and the variable throttle system are calculated moment by moment, and the flow rate value obtained by the fixed throttle system is used for the high flow rate range, while it is variable. Obtained by the diaphragm system Either output the flow rate value for the low flow rate range, or multiply the product of the flow rate value obtained by the fixed throttle system and the coefficient that increases with high flow rate, and the flow rate value obtained with the variable throttle system and the coefficient that decreases with high flow rate. And the arithmetic controller that outputs the combined flow rate.

(Embodiment) Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows a flow rate measuring device according to the present invention, in which a fixed throttle mechanism 2 is provided in a fluid pipeline 1 to be measured, a relationship of a flow rate coefficient with respect to an opening is known, and an opening sensor 3 and a variable throttle drive are provided. The variable throttle mechanism 5 to which the mechanism 4 is attached is provided, and the first and second differential pressure sensors 6 and 7 for detecting the differential pressure before and after the fixed throttle mechanism 2 and the variable throttle mechanism 5 are provided.

Further, an arithmetic controller 8 for inputting the signals from the first and second differential pressure sensors 6 and 7 and the signal from the opening sensor 3 is provided. This arithmetic controller 8 is formed by using a microprocessor or the like, and is stored in advance in the form of a mathematical expression of the flow coefficient of the fixed throttle mechanism 2 and the flow coefficient with respect to the opening of the variable throttle mechanism 5, or in the form of a table. Both the flow rate values of the fixed throttle system and the variable throttle system are calculated moment by moment based on the above-described relationship and the above sensor signals.

Further, the arithmetic controller 8 switches the flow rate value obtained by the fixed throttle mechanism 2 and the flow rate value obtained by the variable throttle mechanism 5 or outputs the weighted averaged flow rate value signal 6 depending on the flow rate region. In addition, it has a control function to set the flow rate or the differential pressure or the opening of the variable throttle mechanism to a predetermined value.
An opening control signal is sent to the variable aperture drive mechanism 4 to operate the opening of the variable aperture mechanism 5.

Therefore, first, as a first embodiment of the usage of the flow rate value by the fixed throttle system and the flow rate value by the variable throttle system in the arithmetic and control unit 8, a switching method when the flow rate measurement value is mainly used for an instruction (monitoring etc.) will be described. .

In this method, the flow rate value of the fixed throttle system is a predetermined value, for example,
It is switched at the time when it reaches 50%, and if it is more than that value, the fixed throttle system flow rate value is used, and if it is less than that value, the variable throttle system flow rate value is used.

Needless to say, the same effect can be obtained by using the variable throttle system flow rate value or the opening of the variable throttle mechanism instead of the fixed throttle system flow rate value as the signal for determining the switching time point.

Although this method is simple, there is a high probability that a difference will occur between the two flow rate values at the switching point, and there is a high probability that control disturbance will occur due to a sudden change in the flow rate value at the time of switching. .

Next, as a second embodiment of selectively using the flow rate value of the fixed throttle system and the flow rate value of the variable throttle system in the arithmetic and control unit 8, since the flow rate measurement value is mainly used for control, the flow rate value by the above switching is changed. A weighted averaging method that eliminates sudden changes will be described.

As shown in the following equation, the flow rate value Qs of the fixed throttle system and its maximum flow rate value
Let the power of the ratio with Qsm be the weighting function value G, and let that power be W.

G = (Qs / Qsm) ^W (1) Then, the flow rate value of the fixed throttle system is directly multiplied by this weighting function value, and the flow rate value Qd of the variable throttle system is multiplied by 1's complement of this weighting function value. , And the addition of both as in the following equation is the weighted averaged flow rate value Qe.

Qe = G · Qs + (1-G) · Qd (2) Fig. 4 shows the error of each sensor of the fixed throttle system and the variable throttle system, that is, the error of both flow rate values is the same as in Fig. 2 and 3, In addition, the error of this weighted averaged flow rate value when the power W of the equation (1) is 3 is shown. In the high flow rate region, it is close to the fixed throttle system of FIG. The error characteristic is close to that of the variable aperture system shown in FIG.

Needless to say, the same effect can be obtained by using the variable throttle system flow rate value or the variable throttle mechanism opening degree as the variable signal of the weighting function instead of the fixed throttle system flow rate value.

Also, the power of equation (1) is not limited to 3,
If this value is made small, it approaches the flow rate value of the fixed throttle system, that is, the error characteristic, and if it is made large, it approaches the flow rate value of the variable throttle system, that is, the error characteristic.

Furthermore, the weighting function is not limited to the equation (1), and may be, for example, an application of a trigonometric function or an exponential function. It is also possible to make the flow rate value, that is, the error characteristic, closer to the flow rate value, that is, the error characteristic of the variable throttle system, in the low flow rate region.

When the variable throttle mechanism 5 is not also used as a control valve, the differential pressure of the variable throttle mechanism 5 can be arbitrarily selected. For example, the relationship between the flow rate coefficient and the opening degree of the variable throttle mechanism 5 is an exponential function characteristic, and when the differential pressure is controlled to be constant, the flow rate measurement error is almost constant in the entire flow rate region. Since an error related to the opening degree of
In the high flow rate region, the error becomes larger than the flow rate measurement error by the fixed throttle mechanism 2. Therefore, even in such a case, it is effective to properly use both depending on the flow rate region.

(Effects of the Invention) As is apparent from the above description, according to the present invention, the relationship between the fixed throttle mechanism provided in the fluid pipeline to be measured and the flow coefficient with respect to the opening provided in the fluid pipeline is shown. A known variable throttle mechanism, an opening sensor attached to the variable throttle mechanism, a first differential pressure sensor and a second differential pressure sensor for detecting a differential pressure before and after each of the fixed throttle mechanism and the variable throttle mechanism. The relationship between the flow coefficient of the fixed throttle mechanism and the flow coefficient with respect to the opening of the variable throttle mechanism is stored in advance, and this relationship, the flow coefficient of the fixed throttle mechanism, the first differential pressure sensor, and the second differential pressure are stored. The flow rate values of the fixed throttle system and the variable throttle system are calculated from time to time based on the signals from the sensor and the opening sensor, and the flow rate values obtained by the fixed throttle system are used for the high flow rate region, while the variable throttle system is used. Flow rate obtained by the system The value is output for the low flow rate region, or the product of the flow rate value obtained by the fixed throttle system and the coefficient increasing at high flow rate and the flow rate value obtained by the variable throttle system and the coefficient decreasing at high flow rate And the arithmetic controller that outputs the combined flow rate.

In this way, the flow rate value obtained by the fixed throttle system is used for the high flow rate range, and the flow rate value obtained by the variable throttle system is used for the low flow rate range, thereby reducing the flow rate measurement error due to the error of the differential pressure sensor. Therefore, it is possible to measure flow rate with high accuracy using a general-purpose differential pressure sensor.
It is possible to easily obtain a device that measures the flow rate to be measured with required accuracy.

[Brief description of drawings]

FIG. 1 is a system diagram of a flow rate measuring device according to the present invention, FIG.
3 and 4 are diagrams showing the relationship between the flow rate and the flow rate measurement error. 1 ... Fluid line, 2 ... Fixed throttle mechanism, 3 ... Openness sensor, 4 ... Variable throttle drive mechanism, 5 ... Variable throttle mechanism, 6,
7 ... Differential pressure sensor, 8 ... Arithmetic controller.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Akira Nakayasu 2-4-7 Kyomachibori, Nishi-ku, Osaka City, Osaka Prefecture Chugai Prox Co., Ltd. (72) Hideaki Narahara 2-4-4 Kyomachibori, Nishi-ku, Osaka City, Osaka Prefecture No. 7 inside Chugai Prox Co., Ltd.

Claims (1)

[Claims] 1. A fixed throttle mechanism provided in a fluid pipeline to be measured, a variable throttle mechanism provided in the fluid pipeline with a known relationship of a flow coefficient with respect to an opening degree, and a variable throttle mechanism attached to the variable throttle mechanism. An opening sensor, a first differential pressure sensor and a second differential pressure sensor that detect a differential pressure before and after each of the fixed throttle mechanism and the variable throttle mechanism, a flow coefficient of the fixed throttle mechanism in advance, and the variable throttle mechanism. The relationship of the flow coefficient with respect to the opening of the fixed throttle system is stored, and based on this relationship, the flow coefficient of the fixed throttle mechanism, and the signals from the first differential pressure sensor, the second differential pressure sensor, and the opening sensor. The flow rate values obtained by the fixed throttle system are output for the high flow rate region, while the flow rate values obtained by the fixed throttle system are output for the low flow rate region. Or by fixed aperture system An arithmetic controller that outputs a flow rate that is the sum of the product of the obtained flow rate value and the coefficient that increases at high flow rates and the product of the flow rate value obtained by the variable throttle system and the coefficient that decreases at high flow rates. A flow rate measuring device comprising: