JPH04169819A - Bubble tube type apparatus for detecting liquid level - Google Patents

Abstract

PURPOSE:To attain prevention of crevice corrosion of a stainless steel member, and others, by a method wherein a rare gas such as nitrogen or argon which is inactive for a digestive process and does not contain oxygen except as an impurity is employed as a gas to be used for purge inside a bubble tube. CONSTITUTION:A gas of 50% N2 - 50% Ar stored 3 as a gas for purge is supplied to a constant flow rate controller 7 through a filter 4, a pressure reducing unit 5 and a pressure gage 6. While the flow rate thereof is monitored by a flowmeter 8, the gas is sent to a bubble tube 2 at a prescribed flow rate by a gas-sending pipe 9 and released as bubbles into a digestion tank 1 from the fore end of the tube. Since the gas for purge sent into the bubble tube 2 does not contain oxygen except as an impurity, it does not cause corrosion of a member formed of stainless steel in the tank 1, either form an oxygen concentration cell between the lower part of a sticking substance and the outside even when a floating matter due to sewage treatment sticks on the stainless steel member, thus enabling prevention of crevice corrosion. Since a rare gas such as nitrogen, He, Ne, Ar, etc. produces no effect on a digestive process, besides, the conditions of the sewage treatment can be maintained.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is a bubble tube type liquid level detection system for detecting the liquid level in a digestion tank for anaerobic digestion using microorganisms for the purpose of decomposing organic matter in sewage treatment. This invention relates to a detection device.

[Prior Art] Conventionally, a bubble tube type liquid level detection device has been used to detect the liquid level in a sealed tank. A bubble tube 2 is inserted into the tank 1, and the air that has passed through the filter 4 is transferred to a pressure reducer 5.
Then, the flow rate is kept constant with a constant flow regulator 7 via a pressure gauge 6, and the air is sent to the bubble tube 2 through the air supply tube 9, and is released as bubbles into the liquid from the tip of the bubble tube, and the back pressure inside the tube at that time It is designed to detect the liquid level. 8 is a flow meter, and 10 is a stop valve. That is, the back pressure P inside the pipe at this time is P=H+Pa (H・Static pressure of air corresponding to the liquid level in the tank) (Pa:
The static pressure of the air at the tip of the bubble tube) and Pa is proportional to the flow velocity.
You can get 3.

A similar conventional example is also disclosed in Japanese Patent Application Laid-Open No. 59-83017, which describes a liquid level detection device for a liquid storage tank that handles radioactive waste liquid, in which purge gas is released to the outside. It is equipped with a pump to suck the purge gas in the tank, and a flow rate regulator is provided downstream of this pump to control the flow rate, and the gas is sucked into the tank through the purge pipe. A proposal has been made to bring it back.

In such a vial type liquid level detection device, the vial is generally made of austenitic stainless steel to prevent corrosion.

Anaerobic sludge digestion tanks used in sewage treatment utilize the action of microorganisms under anaerobic conditions, so the components of the digestion gas in the tank are usually 60-65% methane and 33-33% carbon dioxide.
35%, hydrogen 0-2%, hydrogen sulfide 001-0.02%,
It is said that the nitrogen content is 0-3%, and the pH of the processing solution in the digestion tank is maintained at a neutral level of about 7.2-7.4.
Since corrosive hydrogen sulfide (H2S) is present in the digestion gas, please use bubble tubes, thermometer protection tubes,
Austenitic stainless steels such as 5US304 and 5US316 are used for parts inside the digestion tank, such as the stirring impeller and the heating steam blowing pipe.

[Problems to be solved by the invention] The corrosion resistance of stainless steel is maintained by a passive film on the surface, and the corrosion resistance of stainless steel is maintained under conditions where there is sufficient oxygen or when the liquid in contact with it is in an environment where there is no oxygen. However, in sewage treatment, the treated liquid in the digestion tank contains many suspended solids, and these suspended solids tend to adhere to the surface of stainless steel parts near the liquid surface. If air is released from the vial under such conditions, an oxygen concentration cell will be formed between the bottom of the deposit and the outside, causing severe crevice corrosion at the bottom of the deposit, and as this corrosion rate is fast, the vial will be damaged in a short period of time. The disadvantage is that a through hole is formed, and air escapes through this hole, causing an error in the detected value of the liquid level5.Also, when air is blown into the bubble tube to detect the liquid level, anaerobic conditions are maintained. There is a problem that the equilibrium of the sludge treatment conditions in the digestion tank is disrupted, and the decomposition of organic matter becomes unstable.

In view of the special circumstances in sewage treatment, the present invention has developed a bubble tube type liquid level detection device that does not cause crevice corrosion on the stainless steel members in the digestion tank (and does not affect the treatment conditions). The purpose is to provide.

[Means for solving the problem] For this reason, as a gas used for purging inside the bubble tube,
For the digestion action in the digestion tank, a gas is used which is inert and does not contain oxygen except as an impurity and is composed of nitrogen and at least one rare gas such as He, Ne, Ar, etc.

Further, as a purge gas, the gas generated by the digestion action in the digestion tank can be taken out and pressurized, and the flow rate can be adjusted and used. In this case, it is desirable to extract the gas from a partition that covers the liquid surface around the bubble tube, or to remove the corrosive gas by passing the gas in the digestion tank through a desulfurization device.

[Effects] Therefore, the purge gas sent into the bubble tube does not substantially contain oxygen, which prevents corrosion of the stainless steel components in the tank, and prevents suspended matter from sewage treatment from forming on the stainless steel components. Even if it adheres, it does not form an oxygen concentration cell between the lower part of the deposit and the outside, and crevice corrosion does not occur.

Furthermore, since neither nitrogen nor the above-mentioned rare gases have any effect on the digestion action in the digestion tank, they do not impair the sludge treatment conditions even if they are blown into the digestion tank.

In addition, when using the gas in the digestion tank, sulfide gas is removed via a desulfurization device to prevent corrosion of constant flow regulators, pressure converters, etc.

[Example] The embodiment shown in the figures will be described below.

In the embodiment shown in FIG. 1, 1 is a digestion tank, 2 is a bubble tube, and 500 ml of gas is stored in a storage tank 3 as a purge gas.
%N250%Ar gas is supplied to a constant flow regulator 7 via a filter 4, a pressure reducer 5, and a pressure gauge 6, and the flow rate is monitored by a flow meter 8 while being controlled at a constant rate by an air supply pipe 9 via a stop valve 10. It is sent to the bubble tube 2 at a flow rate of , and released as bubbles into the tank from the tip of the bubble tube.

The back pressure in the bubble tube at this time is measured by a pressure transducer 12 through a throttle valve 11 and converted into a liquid level signal 13. The gas in the pressure transducer after detection is released into the atmosphere by the valve 14. In addition, 2] is the sludge in the tank, 22 is the breakwater tube surrounding the bubble tube, 23 is the stirring impeller, 24 is the stirring motor, 25 is the heating steam blowing pipe, 26 is the thermometer, and the breakwater tube and other All members are made of stainless steel SUS3Q4.

As a result of one year of operation using the detection device of this example configured in this way, no corrosion was observed on the stainless steel components, and the deposits on the surface were black, the same color as the floating matter in the digestion tank. It was early.

On the other hand, for comparison, in a digestion tank with the same configuration using air as the purge gas, after one year of operation, suspended matter adhered to the water surface of the bubble tube and breakwater pipe, and this A through hole had occurred at the bottom of the part, causing an error in the detected liquid level. In addition, hydrogen sulfide was oxidized by the oxygen contained in the purge gas, producing sulfur, which turned the surface of the deposits yellow, indicating a change in the anaerobic treatment conditions.

In addition, in the above-mentioned example, 50% N2 was used as the purge gas.
-50% Ar gas is used, but it is not limited to this, and any rare gas that does not substantially or qualitatively contain oxygen other than as an impurity may be used, such as He, Ne, Ar, Kr,
A similar effect can be obtained by using a gas consisting of at least one of Xe, Rn, and nitrogen.

FIG. 2 shows another embodiment, in which the same parts as in the embodiment of FIG. 1 are given the same reference numerals. 15 is a compressor, 16 is an intake pipe, and 17 is a return pipe from the pressure converter 12.

Gas generated by the digestion action in the upper part of the digestion tank 1 is taken out through the intake pipe 16, compressed and pressurized by the compressor 15, pressure-regulated through the filter 4 and pressure reducer 5, and supplied to the constant flow regulator 7. The gas is sent to the bubble tube 2 through the air supply pipe 9 at a constant flow rate while monitoring the flow rate with the flow meter 8, and is discharged into the tank from the tip. The gas pressure applied to the pressure transducer 12 at this time is measured and converted into a hydraulic pressure signal 13. The gas after detection is returned into the tank through a return pipe 17.

In addition, as shown in FIG. 3, a gas collection chamber 18 is provided to cover the liquid surface around the bubble tube together with the breakwater in the digestion tank.
If the digestion gas from the sludge resulting from the digestion action obtained in the gas collection chamber is taken out from the intake pipe 16,
The composition of the gas sent to the compressor and the gas released from the bubble tube are the same, and there is no effect on the anaerobic treatment conditions in the digestion tank. 19 is a pressure regulating valve in the gas collection chamber.

Figure 4 shows a system that prevents corrosion of the equipment due to the corrosive sulfide gas contained in the digestion gas when using the digestion gas in the digestion tank. A device 20 is provided. therefore,
Corrosive sulfide gases such as hydrogen sulfide and mercaptans contained in the digestion gas taken out from the digestion tank 1 and sent to the compressor 15 are collected and removed to prevent the equipment from corrosion. Note that the desulfurization device 20 may be attached to the digestion tank 1, and is not limited to the one for the NaOH solution shown in the embodiment.

[Effects of the Invention] As described above, according to the present invention, the purge gas in the bubble tube is made of at least one of nitrogen and rare gas, which is inert to the digestive action and does not contain oxygen except as an impurity. Since gas is used, local corrosion of the stainless steel parts in the digestion tank can be reliably prevented, and there is no error in the detected liquid level over a long period of time (and a highly reliable bubble tube type liquid level needle can be provided). At the same time, it is possible to perform stable sludge treatment without changing the anaerobic treatment conditions.

In addition, by using the gas in the digestion tank as the purge gas, there is no need to prepare a separate purge gas and there is no need for a storage tank. By collecting the corrosive gas contained in the gas, there is no need to use materials with excellent corrosion resistance such as austenitic stainless steel for piping and equipment for liquid level detection, and the process is easy. Copper alloy or the like is used, and there is an advantage that even if gas leaks from the liquid level detection circuit, there is no risk of corrosion of the electric circuit or the like.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a block diagram showing another embodiment, Fig. 3 is a partial sectional view of an embodiment improved from Fig. 2, and Fig. 4 is a separate diagram. Fig. 5 is a block diagram showing a conventional example. 1 is a digestion tank, 2 is a bubble tube, 3 is a storage tank, 4 is a filter, 5 is a pressure reducer, 6 is a pressure gauge, 7 is a constant flow regulator, 8 is a flow meter, 12 is a pressure transducer, 13 is a liquid position signal, 1
8 is a gas collection chamber, 20 is a desulfurization device, 21 is sludge, 22 is a breakwater pipe, 23 is a stirring impeller, 24 is a stirring motor, 25
2 is a heating steam blowing pipe, and 26 is a thermometer. Patent applicant Yasuyo Electric Manufacturing Co., Ltd. 1 Figure 3 and υ Figure 4

Claims (1)

[Scope of Claims] 1. A constant flow of gas is sent to a bubble tube installed in a sealed digestion tank for sewage treatment that processes sludge by anaerobic digestion of microorganisms, and the gas is released as bubbles from the tip of the bubble tube, and the gas inside the tube is released. In a bubble tube type liquid level detection device that detects the liquid level in a tank by measuring the back pressure of A bubble tube liquid level detection device characterized in that it uses a gas consisting of at least one of nitrogen and rare gases. 2. A constant flow of gas is sent to a bubble tube installed in a closed digestion tank for sewage treatment, where sludge is treated by anaerobic digestion of microorganisms, and the gas is released as bubbles from the tip of the bubble tube, and the back pressure inside the tube is measured. 1. A bubble tube type liquid level detection device for detecting a liquid level in a tank by using a bubble tube type liquid level detection device, characterized in that gas in the tank generated by a digestive action is used as a purge gas in the bubble tube. 3. The vial type liquid level detection device according to claim 2, wherein the purge gas is gas collected in a partition provided around the vial and covering the liquid level. 4. The bubble tube type liquid level detection device according to claim 2 or 3, wherein the purge gas is gas generated by a digestion action in a digestion tank and is extracted through a desulfurization device.