It can be seen from the above electrode reaction formula that H + and OH - ions are produced, in which H + ions move to the cathode surface of the electrode to form H2_, and OH - ions move to the anode surface of the electrode to form O 2. The corresponding gas production of H2 is twice as much as that of O2.

A pressure control device for a hydrogen-oxygen gas generator includes a hydrogen-oxygen inlet and a hydrogen-oxygen outlet installed on the outlet pipe of the hydrogen-oxygen gas generator. The hydrogen-oxygen inlet and the hydrogen-oxygen outlet are connected with a medium cylinder. The lower part of the medium cylinder is provided with two pressure transmitter installation joints, and the two pressure transmitters are installed. Pressure sensors are installed on the fittings respectively. The delicate shape design and scientific structure sealing method of PepsiCo DPI pressure transmitters satisfy all process requirements of hydrogen and oxygen generators for pressure transmitters. In the process of the hydrogen and oxygen flowing from the hydrogen and oxygen intake to the hydrogen and oxygen outlet, the pressure of the hydrogen and oxygen is transmitted to two chambers after isolation by pressure transmission and isolation medium, and then two pressure sensors are driven by the pressure of the two chambers respectively to isolate the hydrogen and oxygen directly contacting the pressure sensor, thus eliminating hydrogen permeation, hydrogen embrittlement and return. The purpose of pressure sensor failure easily in fire.

Hydrogen and oxygen machine is the equipment for producing hydrogen fuel. It uses water electrolysis technology to decompose water into hydrogen and oxygen. Hydrogen is used as fuel and oxygen as combustion aid. It is a high-tech green energy-saving equipment. Because the hydrogen and oxygen separated gas produced by the hydrogen-oxygen machine is hydrogen and oxygen separated type, the application scope of the hydrogen-oxygen machine equipment has been greatly expanded. It is not only confined to the commonly used hot-working places, but also used to replace the traditional gas such as acetylene, propane, liquefied gas for metal cutting, metal gas welding, glass products processing, car decarbonization, vehicle-borne hydrogen and oxygen. Machinery, hydrogen fuel cell, electronic chemical industry, food processing and other fields have been widely used. Owing to the separation of hydrogen and oxygen produced, the technical bottleneck problem of easy tempering of hydrogen and oxygen mixed gases is completely eliminated in metal cutting. Therefore, the separating hydrogen and oxygen machine is used safely in the field of metal cutting.


In the aspect of control, the hydrogen-oxygen machine adopts PLC control technology, and develops a complete control program through PLC hardware configuration and program design. It realizes the start-stop, control, setting and displaying of operating parameters of the equipment, and has the functions of alarm and fault inquiry. External human-machine interface (HMI) control is implemented with industrial touch screen. The timeliness, integrity and interaction of human-machine interface control are fully considered in human-machine conversation. The interface is very friendly, easy to use, eye-catching and intuitive.

1. Control scheme design

1) Working Principle of Electrolysis

Separated water electrolysis hydrogen production unit produces H2 and O2 by direct current electrolysis of KOH (25%-30% concentration) aqueous solution. H2 and O2 entrap KOH alkali solution into hydrogen and oxygen vapor-water separators respectively for vapor-water separation (under the action of water molecular gravity). The separated alkali liquid is refluxed to the electrolyzer through the bottom end of the separator (at high level). In suppressing hydrogen, a circulating pump is needed to complete the reflux of the electrolyte.

Electrode Reaction Formula for Hydrogen Production by Water Electrolysis:

Cathode: 4E + 4H2O = 2H2_ + 4OH_

Anode: 4OH_= O 2+2H 2+4e

Total Reaction Formula: H_2 = H_2 + 1/2 O_2

It can be seen from the above electrode reaction formula that H + and OH - ions are produced, in which H + ions move to the cathode surface of the electrode to form H2_, and OH - ions move to the anode surface of the electrode to form O 2. The corresponding gas production of H2 is twice as much as that of O2.

5) System pressure

In order to ensure the safety of the hydrogen and oxygen machine in operation, the maximum internal pressure of the equipment system is set. The pressure transmitter is placed on the oxygen side, and the pressure generated by the back pressure regulating valve added on the hydrogen side is detected accordingly. When the system pressure is greater than the set pressure, the equipment is judged to be severely malfunctioning.

A/D pressure transmitter is used at the hydrogen and oxygen outlet of the equipment to input the analog data to the PLC to process the corresponding digital data. When the hydrogen outlet pressure reaches the upper limit set by the hydrogen outlet pressure, the electrolytic current will be controlled to reduce the hydrogen production, so as to reduce the hydrogen outlet pressure. Vice versa.

If the pressure at the oxygen outlet reaches the set pressure value, the oxygen vent valve opens to reduce the pressure at the oxygen outlet.

In order to facilitate user's use, the upper and lower limit pressure values of hydrogen and oxygen outlet pressure can be set on the touch screen. The principle is that the upper limit pressure should not exceed the system pressure.

6) Electrolytic Voltage Control

The amount of hydrogen produced by the hydrogen and oxygen generator is proportional to the current involved in the electrolysis, so the output of hydrogen can be controlled by controlling the output current value of the electrolytic current. In order to set the control electrolytic current conveniently, the percentage of electrolytic current can be input on the touch screen, and the 0-10V voltage signal output by D/A conversion from the PLC is added to the input control terminal of the electrolytic power supply to control the output current.

2. System Design

In order to realize the control of the whole system, a control system based on PLC is specially established. Industrial touch screen display system operation status and parameters.

S-200 is a small programmable controller in Siemens PLC products, which can control the equipment to meet the needs of automation control. It has compact structure, flexible configuration and strong instruction set, making S-200 an ideal solution for various control applications. The control system needs 6 digital inputs and 2 digital outputs (Table 1). According to the control requirements of the system, S-200 PLC is selected as the control core. In addition, an analog expansion module is needed.

In order to make the system have a better man-machine interface, set the parameters of liquid level, pressure, temperature and other field parameters, and output various alarm prompt information, the display interface of Xinjie touch screen is selected as the display setting unit. The flow chart of system software design is shown in Figure 2.
3. Concluding remarks
The control system gives full play to the advantages of PLC reliability and flexibility, as well as touch screen friendly man-machine interface function. Practice has proved that the system has strong anti-interference ability, simple operation, and is well applied in the harsher operation site.
4. Configuration List
Siemens S-200 Programmable Controller
PepsiCo DPI701 Pressure Transmitter
PepsiCo DPI10 Differential Pressure Transmitter
PepsiCo DPI Integrated Temperature Transmitter
Siemens I/O Module
Siemens 14 touch screen