JP3588272B2 - Density measuring device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a density measuring device suitable for a process in which start-up, stop, and steady operation are performed relatively frequently.
[0002]
[Prior art]
One of the present applicants has previously proposed a density meter as Japanese Patent Application No. 3-159946 (Japanese Patent Publication No. Hei 8-10186). In this density meter, the differential pressure is detected from the upper and lower positions of a vertical pipe through which the fluid flows, and the density of the fluid is measured based on the detected differential pressure.
[0003]
FIG. 2 shows the principle diagram. In the figure, 101 is a vertical pipe, 102 and 103 are branch pipes branched from the vertical pipe 101 at a predetermined distance L, and the diaphragm pipes 104 and 105 of the diaphragm seal differential pressure gauge are attached to the branch pipes 102 and 103. ing. Fluid flows in the vertical pipe 101 in the direction of the arrow shown in the figure. In this case, the density γ of the fluid to be measured can be measured by detecting the pressure difference Δp (Δp = γ · L) between the detected pressure from the diaphragm 104 and the detected pressure from the diaphragm 105.
[0004]
In this case, since the pressure difference Δp detected between the upper and lower diaphragms 104 and 105 includes the pressure loss of the fluid, a restrictor 101a is provided in the inflow-side pipe to temporarily increase the flow velocity, The static pressure drop is caused temporarily. Here, if the restrictor 101a is selected so that the pressure loss and the static pressure drop become equal, the pressure difference is not included in the detected differential pressure Δp, so that it is possible to measure the density that does not vary with the flow velocity. In this densitometer, the requirement is that the aperture 101a be accurately manufactured. Otherwise, there will be a large error in the measurement accuracy of the density, making it unusable. For this reason, it is desired that the vertical tube 101 be made of a metal that allows the diaphragm 101a to be manufactured with high accuracy.
[0005]
[Problems to be solved by the invention]
However, when attempting to measure the density of a liquid having a corrosive property to a metal, such as hydrochloric acid or nitric acid, as the fluid, a material capable of withstanding the vertical tube 101 is required. For example, it is conceivable to use a resin such as a plastic or use a lined pipe. However, it is difficult to provide a high-precision aperture in such a tube. In addition, the thermal expansion coefficient of the resin is generally large, which affects the precision of the drawing. It is conceivable to use an acid-resistant metal as the material of the vertical tube 101, but it is expensive.
[0006]
Recently, as a density meter without a restrictor, a first differential pressure gauge for measuring the loss of a pipe and a second differential pressure gauge for measuring a density attached to a vertical pipe of the same diameter connected to the pipe. A density meter using a differential pressure gauge has been proposed. According to this density meter, it is possible to cancel a loss component such as a variation in flow velocity and to transmit a density signal with good followability. In addition, since there is no throttle, the fluid resistance is small, and there is little fear of adhesion of dirt and the like, and the resin can be used.
[0007]
However, although the density meter can cancel the influence of the flow velocity, the measurement accuracy cannot be expected much. For example, when controlling the concentration of hydrochloric acid used for cleaning steel sheets, it shows very good follow-up characteristics at startup and shutdown, but when the concentration control is effective to some extent and the concentration is stable (steady operation) ), The measurement accuracy becomes insufficient.
[0008]
The present invention has been made in order to solve such problems, the purpose of which is to use a resin tube, not only at the time of starting and stopping the process, but also with high accuracy during steady operation, An object of the present invention is to provide a density measuring device capable of measuring the density of a fluid having a corrosive property to a metal.
[0009]
[Means for Solving the Problems]
In order to achieve such an object, the present invention provides a method for detecting a change in the density from a first and a second differential pressure detecting means in which a pressure detecting portion is attached to a horizontal straight pipe and a vertical straight pipe having a small diameter. The density of the fluid to be measured is measured based on the detected differential pressure, and when the change in the density is small, the third differential pressure in which the pressure detecting section is attached to a vertical pipe having a diameter sufficiently larger than the horizontal straight pipe and the vertical straight pipe. The density of the fluid to be measured is measured based on the differential pressure detected by the detecting means.
According to the present invention, when the density change is large, that is, when the process is started or stopped, the density of the fluid to be measured is measured with good followability based on the detected differential pressure from the horizontal straight pipe and the vertical straight pipe having a small diameter. Is done. On the other hand, when the density change is small, that is, during the steady operation of the process, the density of the fluid to be measured is accurately measured based on the differential pressure detected from the vertical pipe (tank) having a sufficiently large diameter.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments. FIG. 1A is a side view showing a main part of a process using the density measuring device according to the present invention. In the figure, 1 is a horizontal pipe (horizontal straight pipe), 2 is a vertical pipe (vertical straight pipe) connected to the horizontal pipe 1, and 3 is a tank (vertical pipe) connected to the vertical pipe 2. Hydrochloric acid flows in the horizontal pipe 1, the vertical pipe 2, and the tank 3 in the direction of arrow A in the figure. That is, hydrochloric acid, which is the fluid to be measured, flows in through the inlet 4, passes through the horizontal pipe 1, subsequently passes through the vertical pipe 2, and finally flows out of the outlet 5 through the tank 3.
[0011]
The horizontal pipe 1 and the vertical pipe 2 have the same diameter d2, and the tank 3 has a diameter d1 (d1 >> d2) that is sufficiently larger than the diameter d2 of the horizontal pipe 1 and the vertical pipe 2, for example, d1 = 4d2. Although the relationship between d1 and d2 is related to the measurement accuracy, it can be said that d1 = 4d2 is sufficient. The horizontal pipe 1, the vertical pipe 2, and the tank 3 are made of acid-resistant resin.
[0012]
Each of the horizontal pipe 1, the vertical pipe 2 and the tank 3 is provided with two branch pipes for detecting a differential pressure. The distance between the two branch pipes is separated by a predetermined distance L. That is, branch pipes 31 and 32 are provided in the horizontal pipe 1 as shown in a partial plan view of the horizontal pipe 1 from below, as shown in FIG. Is a predetermined distance L. Branch pipes 21 and 22 are provided in the vertical pipe 2, and a vertical distance between the branch pipes 21 and 22 is a predetermined distance L. The tank 3 is provided with branch pipes 11 and 12, and a vertical distance between the branch pipes 11 and 12 is a predetermined distance L.
[0013]
Then, diaphragms 61 and 62 made of a metal for hydrochloric acid resistance are assembled to the branch pipes 31 and 32 of the horizontal pipe 1, and a pressure P31 is detected by the upstream diaphragm 61, and a pressure P32 is detected by the downstream diaphragm 62. P32 is supplied to the differential pressure gauge main body 6 via the sealed liquid, and a differential pressure Δp3 (Δp3 = P31−P32) between the detection pressures P31 and P32 is detected.
[0014]
In addition, diaphragms 71 and 72 made of metal for hydrochloric acid resistance are assembled to the branch pipes 21 and 22 of the vertical pipe 1, and a pressure P21 is detected by the upstream diaphragm 71 and a pressure P22 is detected by the downstream diaphragm 72. P22 is supplied to the differential pressure gauge main body 7 through the sealed liquid to detect a differential pressure Δp2 between the detection pressures P21 and P22 (Δp2 = P21−P22).
[0015]
Further, diaphragms 81 and 82 made of a metal for hydrochloric acid resistance are assembled to the branch pipes 11 and 12 of the tank 3, and a pressure P11 is detected by the upstream diaphragm 81 and a pressure P12 is detected by the downstream diaphragm 82, and the detected pressures P11 and P12 Is supplied to the differential pressure gauge main body 8 via the sealed liquid, and the differential pressure Δp1 between the detection pressures P11 and P12 (Δp1 = P11−P12) is detected.
[0016]
(Density measurement principle)
The pressure difference Δp between two points generated while the fluid flows through the pipe having the inner diameter d can be calculated by the following equation (1).
Δp = λγν ² L / (2d) + hγ (1)
Here, λ is the loss factor, γ is the density of the fluid, ν is the flow velocity, L is the horizontal distance between the two points, and h is the vertical distance between the two points.
[0017]
When the volumetric flow rate Q flows from the inside diameter da tube (first tube) into the inside diameter db tube (second tube), the flow velocity ν ₁ in the second tube is equal to the inside diameter da of the first tube and the second inside diameter da. If the flow velocity in the first pipe is ν ₂ , it can be calculated by the following equation (2).
ν ₁ = (da / db) ² ν ₂ (2)
Further, the time t required for the fluid to pass through the distance L can be calculated as the following equation (3).
t = L / ν (3)
[0018]
[Density measurement from differential pressure Δp1 generated in tank 3]
Here, the pressure difference Δp1 generated in the tank 3 will be examined.
The differential pressure Δp1 generated in the tank 3 is represented by the following equation (4) by the above equation (1).
Δp1 = λγν ₁ ² L / (2d1) + hγ (4)
Further, the flow velocity ν ₁ in the tank 3 is represented by the following equation (5) by the above equation (2).
ν ₁ = (d2 / d1) ² ν ₂ (5)
[0019]
When equation (5) is substituted into equation (4), it is expressed as the following equation (6).
Δp1 = λγL [(d2 / d1) ² ν ₂ ] ² / (2d1) + hγ (6)
In the expression (6), when d1 >> d2, the expression (6) can be approximately regarded as the following expression (7).
Δp1 ≒ hγ (7)
Since the vertical distance h in the equation (7) is a constant value (h = L), when the density γ of hydrochloric acid changes, Δp1 changes. Therefore, the density γ of hydrochloric acid can be known from the change in Δp1.
[0020]
The time t1 during which the hydrochloric acid passes between the branch pipes 81 and 82 in the tank 3 can be calculated as t1 = L / ν ₁ by the above equation (3). In this case, by substituting the equation (5) as flow rate [nu ₁ in the vessel 3, expressed as the following equation (8).
t1 = L / [(d2 / d1) ² ν ₂ ] = L (d1 / d2) ² / ν ₂ (8)
[0021]
In this case, since d1 ≧ d2, the passing time t1 has a large value. In the method of observing the change in the differential pressure Δp1, when the density does not change and is almost stable and changes gradually, as can be understood from the equation (7), there is no uncertain factor, so that the density is measured with high accuracy. It has the advantage that it can be. However, when the change in the density of hydrochloric acid occurs in a short time, it is difficult to quickly measure the density following the change, which is a disadvantage.
[0022]
[Density measurement from differential pressure Δp2 generated in vertical pipe 2 and differential pressure Δp3 generated in horizontal pipe 1]
Next, the differential pressure Δp2 generated in the vertical pipe 2 will be examined.
The differential pressure Δp2 generated in the vertical pipe 2 is expressed as the following equation (9) from the above equation (1).
^{_{Δp2 = λγν 2 2 L / (}} 2d2) + hγ ···· (9)
In this case, since d1 >> d2, the above equation (9) cannot be expressed like the above equation (7). Therefore, it is not possible to measure the differential pressure Δp2 and then measure the density γ.
[0023]
However, if the differential pressure Δp3 generated in the lateral pipe 1 is used, it can be changed as in the above equation (7). That is, since the differential pressure Δp3 is h = 0, it is expressed as the following equation (10).
^{_{Δp3 = λγν 2 2 L / (}} 2d2) ···· (10)
Here, when the difference between Δp2 and Δp3 is obtained,
^{_{Δp2-Δp3 = λγν 2 2 L}} / (2d2) + hγ-λγν 2 2 L / (2d2) = hγ ···· (11)
Which is similar to equation (7). Since the vertical distance h in this equation (11) is a constant value (h = L), when the density γ of hydrochloric acid changes, Δp2−Δp3 changes. Accordingly, the density γ of hydrochloric acid can be known from the change in the difference Δp2−Δp3.
[0024]
The time t2 during which the hydrochloric acid passes between the branch pipes 71 and 72 in the vertical pipe 2 can be calculated as t2 = L / ν ₂ by the above equation (3). The time t3 during which the hydrochloric acid passes between the branch pipes 61 and 62 in the horizontal pipe 3 can be calculated as t3 = L / ν ₂ by the above equation (3). Therefore, the time t12 when the gas passes through both branch pipes of the horizontal pipe 1 and the vertical pipe ₂ is expressed as t12 = 2L / ν ₂ .
[0025]
In this case, since ν ₂ >> ν ₁ , the passing time t12 is short. The method of observing the change in the difference between the differential pressures Δp2 and Δp3 has an advantage that if the density of hydrochloric acid changes in a short time, the density can be quickly measured following the change. However, since the differential pressures Δp2 and Δp3 are measured and further subtracted to obtain a density signal, the number of uncertain factors is increased by that much, and the density signal is compared with the method of measuring the density from the change in the differential pressure Δp1 generated in the tank 3. The accuracy is poor, which is a disadvantage.
[0026]
When controlling the density of hydrochloric acid to a predetermined density, the fluctuation of the control width frequently occurs at the start of operation, so that the density must be tracked quickly. Thereafter, when the control system is stabilized and the control for diluting hydrochloric acid is stabilized, a quick response of the density measurement is not required, but rather a high-precision measurement of the density is required. Further, when the operation is stopped, the control becomes unstable, so that the density must be tracked quickly.
[0027]
Therefore, in the present embodiment, when the density change is large, that is, when the operation is started or stopped (when the process is started or stopped), the differential pressure Δp2 generated in the small diameter vertical pipe 2 and the horizontal pipe 1 The density of hydrochloric acid is measured with good followability by adopting a method of observing a change in a difference from the differential pressure Δp3 generated in the above. On the other hand, when the change in density is small, that is, when the control for diluting hydrochloric acid is stabilized (during the steady operation of the process), a method is employed in which the change in the differential pressure Δp1 generated in the tank 3 having a sufficiently large diameter is adopted. Thus, the density of hydrochloric acid can be accurately measured. Thus, the density of hydrochloric acid can be accurately measured not only at the time of start-up and stop, but also at the time of steady operation.
[0028]
In this embodiment, the case where the density of hydrochloric acid is measured as a fluid having a corrosive property to a metal has been described, but the density measurement of another fluid including a fluid having no corrosive property to a metal has been described. It is also possible to do. In this embodiment, since a high-precision throttle is not required, a resin material can be used for the horizontal pipe 1, the vertical pipe 2, and the tank 3, and a fluid having a corrosive property to a metal such as hydrochloric acid or nitric acid can be used. When the density measurement is performed, it is excellent in that an inexpensive acid-resistant resin can be used without using an expensive acid-resistant metal.
[0029]
When a resin is used, it is desired that there is no concern about the structural strength in terms of material strength. In particular, a differential pressure type densitometer requires a branch pipe for taking out a differential pressure up and down, and the structure becomes tall when considering the upstream straight pipe distance necessary for stabilizing the flow. This is unstable in structural strength. Further, since the fluid is hydrochloric acid, fluid leakage due to damage to the device is absolutely not allowed. In consideration of this point, the device is devised as follows.
[0030]
As can be seen from equation (5), the flow velocity of Δp1 is very low because the inner diameter of the tube is d1 >> d2. Therefore, even if there is no upstream straight pipe distance of the tank 3, the density measurement error due to the distance is small and can be ignored. As can be seen from FIG. 1 (a), it can be summarized almost only by the height of the tank 3.
[0031]
After taking out the pressure P21 from the upstream diaphragm 71 for Δp2, there is a bent pipe 9 and the pressure P22 from the downstream diaphragm 72 is measured. Although a swirling flow is generated in the bent pipe 9, the flow is stabilized while flowing through the distance L, and no error occurs in the differential pressure Δp2. Since only the height from the branch pipe 21 from which the pressure P21 is taken out to the branch pipe 22 from which the next pressure P22 is taken out is required, the apparatus can be made compact. For Δp3, when the pipe is horizontal, it is easier to make a straight pipe distance than when it is vertical. For this reason, the upstream outlet is taken out of the horizontal pipe.
[0032]
In this embodiment, the tank 3 is connected after the vertical pipe 2, but may be connected before the horizontal pipe 1.
Further, in this embodiment, the diaphragm 71 is provided for the differential pressure gauge main body 7 and the diaphragm 62 is provided for the differential pressure gauge main body 6, but the diaphragm 71 is omitted, and the detected pressure P32 from the diaphragm 62 is reduced. It may be provided to the differential pressure gauge main body 7 as the detection pressure P21 by 71. Conversely, the diaphragm 62 may be omitted, and the detected pressure P22 from the diaphragm 71 may be applied to the differential pressure gauge main body 6 as the detected pressure P32 from the diaphragm 62.
[0033]
【The invention's effect】
As is apparent from the above description, according to the present invention, when the density change is large, that is, when the process is started or stopped, the follow-up is performed based on the detected differential pressure from the horizontal straight pipe and the vertical straight pipe having a small diameter. The density of the fluid to be measured is measured with good accuracy, and when the density change is small, that is, during the steady operation of the process, the density of the fluid to be measured is accurately measured based on the detected differential pressure from the vertical pipe (tank) having a sufficiently large diameter. Using a resin tube, it is possible to accurately measure the density of a liquid that is corrosive to metals, not only when starting or stopping the process, but also during steady operation. .
[Brief description of the drawings]
FIG. 1 is a side view showing a main part of a process using a density measuring device according to the present invention, and a partial plan view of a horizontal pipe portion in the density measuring device as viewed from below.
FIG. 2 is a principle diagram of a conventional density meter disclosed in Japanese Patent Application No. 3-159946 (Japanese Patent Application No. 8-10186).
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Horizontal piping (horizontal straight pipe), 2 ... Vertical piping (vertical straight pipe), 3 ... Tank (vertical pipe), 11, 12, 21, 22, 31, 32 ... Branch pipe, 4 ... Inlet, 5 ... Outflow port, 6, 7, 8 ... differential pressure gauge main body, 61, 62, 71, 72, 81, 82 ... diaphragm, 9 ... bent pipe.

Claims (2)

First differential pressure detecting means for detecting a differential pressure between a detected fluid pressure from a first pressure detecting unit and a detected fluid pressure from a second pressure detecting unit attached to a horizontal straight pipe at a predetermined distance; When,
A detection fluid pressure from a third pressure detection unit and a fourth pressure detection unit attached to the vertical straight pipe connected to the horizontal straight pipe and having the same height difference as the predetermined distance to the vertical straight pipe having the same diameter as the horizontal straight pipe; Second differential pressure detecting means for detecting a differential pressure from the detected fluid pressure from
Fifth pressure attached to a vertical pipe connected to one of the horizontal straight pipe and the vertical straight pipe and having a diameter sufficiently larger than the diameter of the horizontal straight pipe and the vertical straight pipe with an arbitrary height difference A third differential pressure detecting means for detecting a differential pressure between the detected fluid pressure from the detecting unit and the detected fluid pressure from the sixth pressure detecting unit;
When the density change is large, the density of the fluid to be measured is measured based on the differential pressure detected from the first and second differential pressure detecting means. When the density change is small, the density from the third differential pressure detecting means is measured. A density measuring apparatus characterized in that the density of a fluid to be measured is measured based on a detected differential pressure. 2. The density measuring apparatus according to claim 1, wherein the second pressure detecting section and the third pressure detecting section are also used.

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