How to choose bus and branch line of gas detector? |How to choose bus and branch line of gas detector?


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How to choose bus and branch line of gas detector?

Article source: AI Yi Technology popularity: - Time of publication: 15:39:00, July 12, 2017

For the fixed gas detector, the system configuration of field gas detection + centralized display interlock of controller is essential. For the common bus system and branch line system in the market, how to choose? This time, AI Yi technology will analyze the advantages and disadvantages of the two for customers' reference when choosing.

1. How to distinguish bus system from branch line system

Bus system as the name implies, that is, a line in series with all products. For the bus system wiring, as long as one pipeline is provided, all the equipment is connected on the pipeline. The controller can know the location and situation of each equipment through the address code of the detector.

The split line system is sometimes called multi line system, that is, each detector is equipped with one pipeline to the controller, and the controller can clearly understand the position of each equipment at a glance.


 Comparison of AI bus and branch line

In addition to the difference in connection mode between bus and branch line, there are differences in many other places, such as RS485 signal for bus system communication, 4-20mA signal for branch line system, 4-wire system for bus system, and 3-wire system for branch line system (few for 2-wire system). In fact, the simplest way to distinguish bus and branch line is to look at the controller terminal. If the access is 4-core wire and the number of cables is far less than the number of gas detector, it must be bus system. If the access is 3-core wire and the number of cables is equal to the number of gas detectors, then the wiring is divided.

2 Advantages of bus system:


1. Reduce the troublesome maintenance problems caused by inconsistent signal system of equipment.

The signals used in many DCS systems are not unified standards. Analog signals include 4-20mA and 0-5V current and voltage signals, which bring great inconvenience to the maintenance work. The bus control system has no such inconvenient factors at all. 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 advantages of bus system are less wiring, simple wiring and less cost. 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 case of power failure of DCS, or the control system is out of control due to power failure. It is very difficult to investigate such accidents, which is time-consuming and time-consuming. The production of Fieldbus can solve this problem easily and provide convenience for finding out the fault equipment.


3、 Disadvantages of bus system:

1. Information lag

Data refresh is too slow, and the more urgent, the more difficult it is, which is a lot of operators criticized the bus control system. If there are many distribution points and a large amount of single point transmission data, the transmission signal delay time will be longer, which can not effectively and timely respond to the site situation, especially for safety control instruments, it may cause major accidents.

2. Concentration of risk

With the rapid development of petrochemical industry, the scale of the plant is becoming larger and larger. In addition, the chemical process has the characteristics of flammable, explosive, high temperature, high pressure, toxic, harmful and corrosion. If the production control system is slightly lost, it will lead to catastrophic accidents and cause irreparable heavy losses in production, equipment, personnel and environment. Therefore, for the control system of large-scale petrochemical production plant, its safety and reliability are the most important and necessary. Even if the control system fails or is damaged (any control system cannot fail or damage), the affected control points should be as few as possible, and the affected production area should be as narrow as possible. In this way, after the bad points are removed, the production personnel can maintain safe production according to the relevant process parameters; the maintenance personnel can deal with individual bad points more easily and safely.  

How to set up fieldbus instrument? Each segment leading to the site (four bus) drives several field bus type instruments. The four bus main line is not only the power supply bus of multiple measuring detectors, but also the bus for transmitting signals up and down. Undoubtedly, the four bus trunk line drives multiple gas detection instruments in a production area or production unit. Any slip of this fieldbus trunk will affect all nodes on the four bus trunk. For the continuous production of petrochemical production units, a production area or production unit of multiple measuring points, multiple gas detectors suddenly out of control, facing the rumbling production device will be a terrible scene. This is the typical concentration of danger.  

Although the user is reminded in the application guide that the twisted pair cannot be grounded, short circuited or open circuited is the criterion that the user must "absolutely" abide by, but how can the ever-changing production site "absolutely" do it? Even if we do, the relevant cards, isolation barriers, components and terminals of the bus system will never have problems? The above fault examples are typical examples of "hazard concentration".

3. Power supply problem

Power supply is also a problem perplexing the bus system all the time. For the occasions with explosion-proof requirements on site, DC24 V power supply should be used. If all instruments adopt centralized bus power supply, the voltage drop generated on the power supply bus may lead to the failure of terminal instruments. Therefore, the bus system products often have special instructions for power supply. Generally, a DC power supply can drive a machine with no more than 30 channels, so as to reduce the power supply current and voltage drop on the line. If the power supply is close to the site, it requires that the power supply be installed in the explosion-proof box on site, which is very harmful to the heat dissipation of the power supply, which will greatly reduce the service life of the power supply and even cause dangerous accidents.

4. Cost issues

Many people think that the cost of bus system is low. If only from the point of view of line and construction, the cost will be saved. However, from the perspective of the whole project, the bus system can not be compatible with the same type of products on the market (non Modbus Protocol), so we can only find the original manufacturer to replace it. The maintenance and debugging need corresponding customized software and some professional peripheral equipment, The cost and maintenance cost are much higher than that of the distribution system.

4、 Advantages of split line system:


1. Synchronous transmission of data, timely response

Compared with the bus system, each gas detector communicates with the controller separately, which can timely transmit the field situation to the control part, so that the monitoring personnel can make timely and effective judgment, and the corresponding peripheral control equipment can make corresponding control actions timely and effectively, so as to avoid the occurrence of dangerous accidents.

2. Risk dispersion

Multiple detectors are installed in the same production section or production unit. Once one detector line fails, it will not affect the normal operation of other detectors, so that the leakage in the section can still play a protective role.

3. There is no limit to power supply.

For the distributed detector, the power supply is not as strict as the bus system, which is relatively simple.

4. Easy to replace

For the split line gas detector, different brands can be replaced at will in the later stage (only for toxic gas, combustible gas is included in CCC certification, product matching needs certification, but it is only a policy requirement, not a technical problem).

5 Disadvantages of dividing line:

1. Wiring is complex

There are a lot of wiring, heavy workload, complicated wiring, complicated installation and construction, and high cost of installation and materials.

2. There are many kinds of transmission signals and large interference

The transmitted signals include 0-5V, 4-20mA, pulse signal, level type or contact type switching value signal, power source resistive current signal (greater than 50mA), etc. the mutual interference between signals is inevitable, and to some extent, it may cause false alarm, zero drift and other fault phenomena.

Generally speaking, bus system and branch line system The introduction of advantages and disadvantages is only for each other They are very mature solutions on the market. Which one is more suitable for you? We suggest that you can directly dial the 400-025-9821 hotline of Aiyi technology, and the professional after-sales engineer can help you choose.

Nanjing Aiyi Technology absorbed Integrated solutions and Gas detection analyzer , Dust detector , Gas detection alarm , Oxygen analyzer , Combustible gas detector Etc. production, sales, R & D, after-sales one-stop technology service providers Hotline: 400-025-9821

Keywords: Gas detector, bus system, split line system
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