How to choose the wiring of the gas detector
For fixed gas detectors, on-site gas detection and controller centralized display interlocking system configuration is essential. How do you choose between the two different wiring methods that are common on the market, bus system, and split system? This time, AIYI Technologies analyzed the advantages and disadvantages of the two for customers' reference when choosing.
First, how to distinguish between the bus system and the split-line system:
As the name implies, the bus system means that all products are connected in series with one line. For bus wiring, as long as one pipeline is equipped, all devices are connected across this pipeline. The controller knows the location and situation of each device through the address code of the detector.
The split system is sometimes called a multi-wire system, that is, each detector is equipped with a pipeline to the controller, and the controller can see the location of each device at a glance.
In addition to the different wiring methods, the bus and split-lines are different in many other places. For example, the bus system communication is RS485 signal, and the split system is 4-20mA signal; the bus system is a 4-wire system, and the split system is more For a 3-wire system (rarely for the 2-wire system) and so on. In fact, the easiest way to distinguish between the bus and the split-line is to look at the controller terminals. If the connection is 4-core wire and the number of cables is far less than the number of gas detectors, it must be a bus system. If the access is a 3-core cable and the number of cables is equal to the number of gas detectors, it is split-line wiring.
Second, the advantages of the bus system:
1. Reduce the troublesome overhaul caused by an inconsistent equipment signal system.
The signals used by many of our existing DCS systems are not a unified standard. Analog signals include different current and voltage signals of 4-20mA and 0-5V, which bring great inconvenience to maintenance work. The use of a bus control system has no such inconvenient factors, and the data is transmitted in the same form on the data line, which enhances the reliability of the data.
2. Reduce installation workload and improve work efficiency.
The obvious advantage of the bus system is that the wiring is less, the wiring is simple, and the cost is less. Four bus system, two signal lines, two power lines, wiring is simple and convenient.
3. Improve the safety of the unit to a certain extent.
In the current DCS system, there are many cases where signal grounding burns the IO card of the control system or short-circuits the power supply of the control system, resulting in the system running in a fault state or unplanned outage. It is very difficult to troubleshoot after such an accident, which is both laborious and time-consuming. The generation of Fieldbus can easily solve this problem, and at the same time, it is convenient to find the faulty equipment.
Third, the disadvantages of the bus system:
1. Information lag
Data refresh is too slow, and the more anxious it is, the more difficult it is to get up. This is where many operators criticize the bus control system. If there are many points of distribution, the amount of single-point transmission data is large, and the transmission signal delay time is longer. It cannot respond well to the scene in a timely and effective manner. Especially for safety control instruments, it may cause major accidents.
2. Dangerous concentration
With the rapid development of the petrochemical industry, the scale of the equipment is becoming larger and larger and continuous. In addition, the chemical process has the characteristics of flammable, explosive, high temperature, high pressure, toxic, harmful, and corrosive, and the production control system will slightly fail. It will cause catastrophic accidents and cause irreparable losses in production, equipment, personnel, and environment. Therefore, for the control system of large-scale petrochemical production equipment, its safety and reliability are overwhelmingly important and necessary. Even if the control system fails or is damaged (any control system cannot fail or fail), it must be guaranteed. The affected control points should be as few as possible, and the affected production surface should be as narrow as possible. In this way, after the bad spots are removed, the production personnel can maintain safe production according to the relevant process parameters; the maintenance personnel can deal with the individual dead spots loosely and easily, and of course, it is safe.
How to set up the field bus system instrument? Each mainline segment (four buses) leading to the site drags multiple bus-type instruments on site. The four-bus mainline is both the power supply bus for multiple measurement detectors and the bus for transmitting signals up and down. Undoubtedly, the four-bus mainline drives multiple gas detection instruments in a production area or production unit. Any failure of this field bus trunk will affect all nodes on the four bus trunks. As for the continuous production of petrochemical production equipment, multiple measurement points and multiple gas detectors in a production area or production unit are suddenly out of control at the same time. What a terrifying scene will be faced with rumbling production equipment. This is a typical manifestation of the concentration of danger.
Although users are reminded in the application guide that the twisted pair cannot be grounded, short-circuited, or open-circuited, the user must "absolutely" follow the guidelines, but how can the ever-changing production site be "absolutely" done? Even if it can be done, the relevant cards, isolation barriers, components, terminals, etc. of the bus system can never go wrong? The above failure examples are typical examples of "dangerous concentration".
3. Power supply problem
The power supply problem is also a problem that has always plagued the bus system. For occasions with explosion-proof requirements on-site, the DC24V power supply should be used. If all instruments are powered by a centralized bus, the voltage drop on the power supply bus may cause the terminal instruments to not work properly. Therefore, bus system products often have special instructions on the power supply. Generally, a machine that can be driven by a DC power supply requires no more than 30 channels, reducing the power supply current and reducing the voltage drop on the line; the power supply should be close to the site, which requires the power supply to be installed in the on-site explosion-proof box This is very detrimental to the heat dissipation of the power supply, will greatly reduce the life of the power supply, and even dangerous accidents may occur.
4. Cost issues
Many people think that the bus system cost is low. If it is only from the perspective of the line and construction, it saves costs. However, if the bus system cannot be compatible with the same type of products on the market (non-Modbus protocol) from the perspective of the entire project, you can only find the original There are manufacturers to replace, maintenance and debugging require corresponding customized software and some professional peripheral equipment, the cost and maintenance cost is much higher than the split-line system.
Fourth, the advantages of the split-line system:
1. Synchronous data transmission and timely response
Compared with the bus system, each gas detector of the split-line system communicates with the controller separately, which can timely transmit the on-site situation to the control part, so that the monitoring personnel can make timely and effective judgments, and the peripheral control equipment should be timely Effectively make corresponding control actions to avoid dangerous accidents.
2. Dangerous dispersion
There are multiple detectors installed in the same production section or production unit. Once a detector line fails, it will not affect the normal operation of other detectors, so that it can still protect the leakage in the section.
3. There is no limit to the power supply.
For decentralized detectors, the power supply is not as strict as the bus system, which is relatively simple.
4. Simple replacement
For the split-line gas detector, different brands can be replaced at will later (limited to toxic gas, flammable gas is included in the CCC certification, product matching requires certification, but can not be replaced is only a policy requirement, non-technical issues).
Fifth, the disadvantages of the split-line system:
1. Complex wiring
There is a lot of wiring, heavy workload, complicated wiring, complicated installation and construction, and high cost of materials.
2. There are many types of transmission signals and large interference
The transmitted signals are 0-5V, 4-20mA, pulse amount signal, level type or contact type switch value signal, power supply resistive current signal (greater than 50mA), etc. The interference between the signals is inevitable and maybe to a certain extent. Malfunctions such as false alarms and zero drift will occur.
In general, the introduction of the advantages and disadvantages of the bus system and the split system is only for each other. Both are very mature solutions in the market. Which one is more suitable for the specific choice. It is recommended that you can contact AIYI Technologies to help you choose.