0. Introduction GDS (Toxic and Hazardous Gas Detection System), including combustible gas alarms and hydrogen sulfide alarms, is a crucial safety device for ensuring personal safety in factories. Currently, secondary meters/alarms are commonly used for alarm display. A certain tank farm has 10 combustible gas alarms and 10 hydrogen sulfide (H2S) alarms. Due to the long distance of the wires and the use of 24-core cables, capacitive charge buildup occurs between the wires, causing signal interference. This interference causes the secondary meters of the alarms to frequently generate false alarms, rendering them unusable. Because the tank farm covers a large area, the signal wires are numerous and complex, and they are laid in the same junction box as the 220V instrument power supply lines. The secondary meters of dozens of alarms located at greater distances are thus affected by interference, resulting in two main forms: first, frequent false alarms, where the primary meter indicates no exceedance, but the secondary meter shows a high alarm; second, synergistic alarms, where one primary meter detects a high alarm signal, but multiple secondary meters display high alarms, disrupting normal production. The root cause of the problem is that the signal wire of the combustible gas alarm is not shielded and there is no dedicated cable tray for it. Since the source of interference cannot be accurately determined on-site, replacing all the wires would be a large-scale and costly undertaking. See the diagram below: [img=546,380]http://cms.cn50hz.com/files/RemoteFiles/20090204/428848001.jpg[/img] Figure 1: Schematic diagram of the alarm interference system. However, replacing all the wires is wasteful, involves a large amount of work, and poses a high risk of open flame in the tank area. Therefore, a less labor-intensive repair solution needs to be explored to solve the problem and protect the initial investment. 1. Selection of Modification Solution Solution 1: Replace all the interfered signal wires. Re-lay cable trays and replace multi-wire cables with three-core cables, with a separate cable for each primary meter. Option 2: There is a fire pump room 150 meters from the site. The signal lines from 20 meters will be run to the pump room, and a PAC (Power Acquisition Controller) will be installed inside. The signals will be converted to digital signals and centrally transmitted to the main control room. Only one fiber optic cable is needed between the pump room and the main control room, forming a field digital communication system. The estimated cost is shown in the table below: [img=324,406]http://cms.cn50hz.com/files/RemoteFiles/20090204/428848002.jpg[/img] Assessment: Because fiber optic cables are not affected by electromagnetic interference, the problem can be solved without interference between lines. However, the pump room environment imposes strict installation conditions on the PAC: for safety reasons, the data acquisition controller and cards must be placed in an explosion-proof enclosure. However, the sealed explosion-proof enclosure has poor heat dissipation performance, and the high temperatures in summer may exceed the PAC's operating temperature range. Option 3: Both of the above modification options require cable replacement. All signals must be converted by secondary meters and displayed as contact signals, making it difficult to observe specific alarm values ​​and very unintuitive. Therefore, replacing the primary meters with analog meters capable of outputting 4-20mA current signals cost 140,000 yuan for 20 units. Simultaneously, the secondary meter display is eliminated; the analog signal directly enters the main control room through the original 24-core cable and connects to the PAC, with the industrial control computer acting as the host computer to display the data. Since current signals have stronger anti-interference capabilities than analog signals, coupled with the filtering and anti-interference characteristics of the data acquisition controller, the signal interference problem should be solved. Evaluation: This solution avoids the huge workload of replacing cables and qualitatively upgrades the system, greatly improving measurement reliability and automation. However, the investment is substantial. Solution Four: Replace the primary meters with three-wire analog meters capable of outputting 4-20mA current signals, still using the original 24-core cable. Since each primary meter is changed from four-wire to three-wire, the spare cable can be used as a shielding wire for grounding, preventing excess charge from causing interference. Additionally, a 20A DC regulated power supply is installed in the main control room to power the primary meters, increasing the power supply and further improving stability. The estimated cost is shown in the table below: [img=252,223]http://cms.cn50hz.com/files/RemoteFiles/20090204/428848003.jpg[/img] Assessment: This implementation plan does not use a secondary meter as a display device, saving 20 x 0.4 million = 80,000 yuan. 2. Solution Verification Comprehensive analysis suggests that Solution 4 is the best. This solution not only fully utilizes existing cables but also comprehensively improves signal quality. More importantly, this solution saves money (80,000 - 4.2 million = 38,000 yuan). However, its effectiveness needs to be proven experimentally. If Solution 4 is not ideal, then Solution 3 is also unsuitable. If Solution 4 fails, consider verifying Solution 2, but the explosion-proof problem of the PAC needs to be solved, and using an explosion-proof box will bring heat dissipation problems. If the other three solutions cannot solve the problem, then only Solution 1 can be implemented for modification. 2.1. System Configuration for Option Four: The hardware for the PAC (Physical Accounting Controller) is the Advantech ADAM-5550KW. This controller handles a large volume of analog signal acquisition, and multiple systems based on it are already in use at our plant, making it our first choice due to its stable operation. Its strong processing power also ensures sufficient system capacity for future upgrades to all instruments in the tank area (including oil level, temperature, flow rate, etc.). For monitoring, the Human-Machine Interface (HMI) software is Kaiwu 2000, a domestically developed software widely used in the petrochemical industry, with multiple systems already in use at our plant. It has a dedicated driver program for the ADAM-5550KW controller. Since the Advantech ADAM-5550KW controller runs on the WinCE operating system, we will temporarily use an Advantech industrial PC to run the Kaiwu 2000 configuration software during this system upgrade. Later, when time permits, the configuration software will be ported to the controller for direct connection to a monitor. This will save the cost of an industrial PC. 1. Introduction to PAC PAC (Programmable Automation Controller) was proposed by Craig Resnick, a senior researcher at ARC Advisory Group, and is defined as follows: [*] It has multi-domain functionality, supporting at least two functions such as logic, motion, drive, and process control within a single platform. [*] It integrates multi-process software functions such as HMI and soft logic on a single development platform, using common tags and a single database to access all parameters and functions. [*] The processing flow designed by the software tools can span multiple machines and process control processing units, realizing processing programs that include motion control and process control. [*] It has an open, modular architecture that can cover the needs of industrial applications from factory machinery and equipment to process control operating units. [*] It adopts recognized network interface standards and languages, allowing devices from different vendors to exchange data on the network. [＊] Advantages of using a soft logic PAC controller: 1. Powerful PC-BASED functionality: [＊] Large storage capacity [＊] Numerous convenient and flexible communication ports [＊] Powerful floating-point and mathematical operation capabilities [＊] Complex control algorithms (fuzzy control) [＊] Powerful software functions, facilitating the addition of function libraries [＊] Embedded low-cost Ethernet 2. Programming software:

[list][＊] Multi-tasking architecture [＊] International standard programming standard IEC61131-3, [＊] Supports five programming languages ​​simultaneously: ladder diagram, function block, ST, IL, SFC, etc., [＊] Quick to learn programming, seamless switching.

PAC Soft Logic Controller—ADAM-5550KW [img=442,159]http://cms.cn50hz.com/files/RemoteFiles/20090204/428848004.jpg[/img]

[＊] CPU: AMD Geode GX533 (GX2) [＊] I/O Capabilities: 8 slots [＊] LED Indicators: Power, CPU, and Communication [＊] Memory: 128 MB DDR SDRAM with 512KB Battery Backup [＊] 1x CompactFlash Card (Internal) [＊] Operating System: Windows CE 5.0 [＊] Real-time Clock: Yes [＊] Watchdog Timer: Yes [＊] Communication Protocols: Modbus/RTU and Modbus/TCP [＊] Maximum Nodes: 32 [＊] Media: 2x Ethernet Interfaces with RJ-45 Interface [＊] Transmission Rate: 10M/100M bps (10/100 Base-T)

[img=580,173]http://cms.cn50hz.com/files/RemoteFiles/20090204/428848005.jpg[/img] The field signal is directly connected to the terminal of the AI ​​card ADAM-5017. The signal value is collected and processed by the ADAM-5550KW, and the data is put into the MODBUS storage area. The MODBUS TCP protocol is used to transmit the data to the industrial control computer, and then the Kaiwu 2000 software completes the graphic display. The ADAM-5550KW was programmed using the KW-Multiprog development tool. The hardware configuration was simple and straightforward, as shown below: [img=557,391]http://cms.cn50hz.com/files/RemoteFiles/20090204/428848006.jpg[/img][img=549,546]http://cms.cn50hz.com/files/RemoteFiles/20090204/428848007.jpg[/img] Figure 2 Schematic diagram of the modified alarm system structure 2.2 Application effect of scheme four This scheme uses Advantech PAC to convert the primary field meter into digital signals and transmit them centrally to the main control room, forming a digital anti-interference advantage to replace the secondary meter to complete the alarm function. After the scheme was put into use, the original strong noise signal was effectively suppressed, the interference amplitude did not exceed 0.3%, the false alarm phenomenon was completely eliminated, and the reliability of the system was enhanced. Moreover, the advantages of computer systems over secondary meters are that the data display is intuitive and accurate, and it has historical record functions, which is widely welcomed by process operators. As a display device, the industrial control computer can receive more data, providing a sufficient hardware platform for future instrument expansion. The digital data of the industrial control computer can be uploaded to the real-time database via the network to display the values ​​of combustible gas and hydrogen sulfide content. Therefore, we believe that Scheme 4 fully meets our requirements and is the best solution. So we will not verify other schemes. Thus, we have successfully solved the problem. [*] 3. Summary By running the latest high-tech computer and control equipment, we have revitalized the original 300,000 yuan equipment with only a small investment and fundamentally improved the application effect. Moreover, we have minimized the amount of engineering work required for the transformation. The successful implementation of this project has yielded the following experience: 1. A new solution has been provided for handling the inter-line interference faults that are often encountered in the instrumentation profession, namely, using PAC to convert signals and eliminate interference. The noise filtering function of PAC (including the Advantech ADAM-5550KW used in this project) is quite effective in the signal acquisition process, providing a corrective means to solve on-site interference without waste. 2. The replacement of secondary instrument displays with computer display technology is an inevitable trend in technological progress. Computers used in industrial control are extremely reliable, possessing not only powerful processing capabilities and flexible, intuitive display formats, but also providing real-time data interfaces. This represents a qualitative leap in control capabilities. 3. The PAC hardware and software system built in this project is a large-capacity platform capable of meeting the future needs of upgrading all instruments in the tank farm (including oil level, temperature, flow rate, and other metering values). This prepares the way for improving the automation level of the tank farm.　[＊] References: 1. ADAM-5550KW Series User's Manual Ed-1.0.pdf [/list]