JP2929159B2 - Pressure type liquid level measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure type liquid level measuring device for a closed tank.

[0002]

2. Description of the Related Art As one method of measuring the amount of liquid stored in a tank, the difference between the pressure in the liquid phase at the bottom of the tank and the pressure in the gas phase at the top of the tank is divided by the specific gravity of the liquid to determine the liquid level. The so-called pressure type liquid level measuring device to be sought is widely used in practice.

[0003] A method for mounting a sensor for measuring the pressure at the tank bottom in the pressure type liquid level measuring device is as follows: (1) A method of attaching the pressure sensor 2 to the side surface of the tank 1 as shown in FIG. 4 (2) As shown in FIG. There is a method in which the pressure sensor 2 is inserted into the tank from the top of the tank. The tanks are classified according to their structural classification as follows: (A) a tank whose gas phase is open to the atmosphere; And the pressure in the gas phase of the tank is different from the atmospheric pressure.

[0004]

The present invention relates to the above (2) and (B)
The present invention relates to an improvement in a pressure type liquid level measuring device of the type having the above-mentioned means. In this type of liquid level measuring device, it is necessary to find the difference between the pressure in the liquid phase at the bottom of the tank and the pressure in the gas phase in the closed tank. The following two methods are practically used. It has been.

(A) As shown in FIG.
The pressure of the liquid phase at the bottom of the tank is transmitted to one pressure receiving portion 3a via the diaphragm 4 and the other pressure receiving portion 3a
The pressure of the gas phase in the upper space in the tank is guided to b (back pressure part) by the pressure guiding pipe 5 to detect the pressure difference, and the meter 6 indicates the difference.

(B) As shown in FIG. 7, pressure sensors 2A and 2B for measuring the absolute pressure are provided in the liquid at the bottom of the tank and in the gas phase in the upper space in the tank, respectively, to convert the respective pressures into electric signals. An operation is performed by the electronic circuit 7 to obtain a pressure difference.

In the method (a), the measurement can be performed with one differential pressure sensor, and the differential pressure applied to the differential pressure sensor is equal to the hydraulic pressure. Therefore, the accuracy of the liquid level measurement is high, and the cost and accuracy are excellent. Since the vaporized gas of the liquid in the tank flows into the impulse pipe, droplets often occur due to dew condensation in the impulse pipe. A major drawback is that inaccuracy or failure may occur. In particular, in the apparatus having the structure shown in FIG. 6, once the liquid droplets accumulate in the pressure receiving portion, it is not easy to remove the liquid droplets, and maintenance is difficult.

On the other hand, the method (b) does not require a pressure guiding tube because the sensor is a pressure gauge for measuring the absolute pressure, and there is no problem due to the accumulation of droplets as in the method (a). Since the pressure of the liquid plus the pressure of the gas phase acts on the pressure sensor, it is necessary to use a sensor with a wide measurement range, and it is necessary to measure the liquid level with high accuracy by the method (a). Is more difficult than

In particular, when the pressure in the gas phase is high, or when the tank is shallow and the measurement range of the fluid pressure is relatively small, the ratio of the fluid pressure to the output of the sensor at the bottom of the tank is small, so that the liquid level is low. Accuracy is reduced.

The present invention provides a new type of pressure type liquid level measuring apparatus which can solve both the problems of the above methods (a) and (b).

[0011]

The means of the present invention are as follows. <Means> A pressure-type liquid level measuring device that provides a pressure sensor in the liquid in the tank and obtains the liquid amount or liquid level in the tank from the pressure applied to the pressure sensor by the liquid in the tank. The liquid pressure part is provided with one pressure receiving part for receiving the liquid pressure at the tank bottom, and the other pressure receiving part is connected to a pressure guiding tube to detect the difference between the liquid pressure and the pressure difference in the pressure guiding pipe. Pressure sensor (b) Provided in the gas phase in the upper space of the tank, one pressure receiving part receives the gas phase pressure in the tank, and the other pressure receiving part is connected to the impulse line so that the gas pressure and the pressure in the impulse line A differential pressure sensor in the gas phase that detects the pressure difference. (C) The interior is partitioned by a movable partition into two variable volume chambers, X chamber and Y chamber. The other end of the pressure guiding tube connected to the differential pressure sensor of the part, and one end were connected to the differential pressure sensor of the gas phase part The other end of each pressure guiding tube is connected to the X chamber, and the Y chamber is connected to the gas phase in the tank by a conduit. The difference from the differential pressure detected by the differential pressure sensor is reduced by an arithmetic circuit, the pressure of the liquid received by the sensor in the liquid phase portion is obtained, and the liquid amount or liquid level is measured.

[0012]

1 shows the principle structure of a pressure type liquid level measuring apparatus according to the present invention. Pressure sensors A and B for measuring differential pressure are provided in the liquid phase portion at the bottom of the tank and the gas phase portion in the upper space in the tank, respectively.
Sensor A leads to liquid phase pressure P ₁ of the tank bottom on one of the pressure receiving portion, the back-pressure portion is the other of the pressure receiving portion connecting the vertical impulse line C. The sensor B guides the gas phase pressure P ₂ in the tank to one pressure receiving portion, and connects a pressure guiding tube D to the back pressure portion, which is the other pressure receiving portion.

A back pressure box 10 composed of a closed container is provided on the tank top. In the back pressure box, two diaphragms or a movable partition 12 hermetically sealed by a seal body 11 as shown in FIG. Variable volume chambers X and Y are formed. In the figure, 12a indicates a guide rod, and 12b indicates a guide portion thereof.

Thus, the pressure guiding tubes C,
D is connected to the X chamber of the back pressure box 10, and the Y chamber is connected by a conduit E to the gas phase in the upper space in the tank. Therefore, the pressure applied to the back pressure portions of the two sensors A and B is always equal to the pressure P _{3 in the} X chamber because the pressure guiding tubes C and D communicate with each other and both pressure guiding tubes are connected to the X chamber. Further, since the Y chamber is connected to the gas phase by the conduit E, the pressure is always equal to the pressure P _{2 of the} gas phase.

The hydraulic pressure P that changes with the fluctuation of the liquid level is a value obtained by subtracting the gas-phase pressure P ₂ from the liquid-phase pressure P ₁ applied to the sensor A. This is obtained by multiplying the depth H by the specific gravity ρ. Therefore, when the hydraulic pressure P is detected, the liquid depth, that is, the liquid level H can be obtained by dividing the value by the specific gravity ρ.

Here, the hydraulic pressure P is represented by P = P ₁ -P ₂ (1), and the differential pressures P _A and P _B detected by the sensors A and _B are respectively: P _A = P ₁ −P ₃ (2) P _B = P ₂ −P ₃ (3), and subtracting equation (3) from equation (2) gives P _A −P _B = P ₁ −P ₂ (4), and P = P _A −P _B (5) is obtained from the equations (1) and (4). Thus the liquid level H is determined by H = P / ρ = (P A -P B) / ρ ··· (6).

In the apparatus of the present invention, when a difference occurs between the internal pressures of the two chambers X and Y in the back pressure box, the movable partition 12 is displaced, so that the pressure P _{3 in the} X chamber increases and decreases. pressure difference between the pressure P ₂ is kept small. Therefore, the pressure P _{3 in the} X chamber is substantially equal to the pressure P _{2 in the} Y chamber, that is, the pressure of the gas phase, and the pressures of the back pressure portions of the differential pressure sensors A and B are also maintained substantially equal to the pressure of the gas phase.

The movable partition has an airtight structure, and the space formed by the X chamber and the pressure guiding tubes C and D is completely sealed.
If a gas (for example, nitrogen gas) which is dry and does not attack the structure is sealed in this space, the problem of the liquid remaining in the back pressure portion of the pressure sensor, which is a drawback of the conventional means shown in FIG. 6, is solved. Is done.

The internal pressure P _{3 of the} X chamber changes according to the temperature of the gas in the chamber, and the pressure P ₂ of the gas phase in the tank also changes depending on the operating conditions of the tank. If the amount of gas to be sealed in the X chamber and the displacement of the movable partition in the back pressure chamber are selected based on the standard, the internal pressure P ₃ in the X chamber is always kept substantially equal to the gas phase pressure P ₂ in the tank. since the differential pressure P _a applied to the differential pressure sensor a equals approximately a hydraulic P, the accuracy problems of traditional means shown in FIG. 7 is also significantly improved.

The differential pressure P _B detected by the sensor _B is
In smaller than the differential pressure P _B is detected, the measurement error included in P _B is the effect on the accuracy of the fluid pressure low, the sensor B is not necessary to use a high-precision, cost- Is also advantageous as compared to the conventional means shown in FIG.

FIG. 2 shows a specific example of the structure of the pressure type liquid level meter according to the present invention. Differential pressure P in the liquid phase detected by sensor A
A signal line 14 for sending a signal of the differential pressure P _B in the gas phase portion detected by the signal line 13 and the sensor B to send a signal of _A, connected to the arithmetic circuit 15 provided outside the tank 1, from sensor A A protection pipe 16 for accommodating the signal line 13 and the pressure guiding pipe C is vertically provided in the tank, and a sensor A is protruded from a lower portion of the protection pipe.

When the signals of the differential pressures P _A and P _B are input from the signal lines 13 and 14, the arithmetic circuit 15 subtracts the differential pressure P _B from the differential pressure P _A based on the above equation (6). The liquid pressure P is obtained, and the liquid pressure P is divided by the specific gravity ρ of the liquid to output a liquid level H. A stainless steel welding bellows 17 is used for the movable partition in the back pressure box, and sufficient durability, high flexibility and sufficient displacement are obtained.

In a conventional tank or the like which is large and has a large measuring range of liquid level, a sensor for receiving the liquid phase pressure is provided at a position other than the bottom of the tank, and the measuring range of each sensor is divided and switched to improve the measuring accuracy. Although the means has been put to practical use, the apparatus of the present invention thus has a plurality of sensors A ₁ , A
_2, even when armed with the A ₃ can be applied by communicating the X chamber of back-pressure portion and the back圧箱of each sensor in the impulse line F.

[0024]

Since the pressure type liquid level measuring apparatus according to the present invention has the above-described configuration, the following effects not provided by the conventional apparatus can be obtained. (1) Since the movable partition has an airtight structure and the space formed by the X chamber and the pressure guiding tube is completely sealed, the liquid does not stay in the back pressure portion of the pressure sensor. (2) The differential pressure P _B detected by the sensor _B is smaller than the differential pressure P _B detected by the sensor A, and the measurement error contained in P _B has a small effect on the accuracy of the hydraulic pressure. A highly reliable measurement result can be obtained without using a high-precision device. (3) If the operation is normal, the differential pressure detected by the differential pressure sensor B is a pressure difference between the X chamber and the Y chamber caused by the resistance force when the movable partition is displaced. When the gas filled in the chamber has an abnormality such as leakage, the sensor B
Output value exceeds the normal range, and the self-diagnosis of abnormality can be performed by the output value of the sensor B. (4) By connecting a suction pump and a negative pressure measuring device to the X chamber of the back pressure box and lowering the pressure in the X chamber, a differential pressure can be artificially applied to the differential pressure sensor. Operation check and re-calibration can be performed without removing the, and maintenance is easy.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing the principle structure of an apparatus according to the present invention.

FIG. 2 is a longitudinal sectional view showing an embodiment of the present invention.

FIG. 3 is a longitudinal sectional view showing another embodiment of the present invention.

FIG. 4 is a longitudinal sectional view showing a conventional example.

FIG. 5 is a longitudinal sectional view showing another conventional example.

FIG. 6 is a longitudinal sectional view showing still another conventional example.

FIG. 7 is a longitudinal sectional view showing still another conventional example.

[Explanation of symbols]

 Reference Signs List 1 tank 10 back pressure box 11 seal body 12 movable partition 13, 14 signal line 15 arithmetic circuit 16 protection pipe 17 bellows A, B differential pressure sensor C, D impulse pipe E conduit F impulse pipe

Claims (1)

(57) [Claims] A pressure type liquid level measuring device for providing a pressure sensor in a liquid in a tank and obtaining a liquid amount or a liquid level in the tank from a pressure applied to the pressure sensor by the liquid in the tank. A liquid-phase part provided at the bottom liquid-phase part, receives liquid-phase pressure at the tank bottom at one pressure-receiving part, and is connected to a pressure-guiding tube at the other pressure-receiving part to detect a liquid-phase pressure and a pressure difference within the pressure-guiding pipe. (B) Provided in the gas phase in the upper space of the tank, one pressure receiving part receives the gas phase pressure in the tank, and the other pressure receiving part is connected to a pressure guiding tube to A differential pressure sensor in the gas phase for detecting the pressure difference in the pipe. (C) The inside is partitioned by a movable partition into two variable volume chambers of an X chamber and a Y chamber. The other end and one end of the pressure guiding tube connected to the differential pressure sensor in the liquid phase are connected to the differential pressure sensor in the gas phase. The other end of each of the pressure guiding tubes is connected to the X chamber together, and the Y chamber is connected to the gas phase in the tank by a conduit. The difference between the differential pressure detected by the differential pressure sensor of the phase part is reduced by the arithmetic circuit, the pressure by the liquid received by the sensor of the liquid phase part is determined, and the liquid amount or the liquid level is measured. Type liquid level measuring device.