Q: A petrochemical plant has a PLC-based fire and gas (F&G) system with monitoring in the control room. We need to add monitoring at the fire station. Cabling between three stations (two control rooms and one fire station) and using mimic panels is not possible because of many barriers. I’m thinking about sending signals from the PLCs to the computer through network cards and antennas. Distances are about 400 meters and 1 kilometer. The amount of data transmitted ranges from 1 to 30 MB.
Again, cabling or using fiber-optic is not possible, so we have to think of another choice. I can get help from the IT department, but I would like the benefit of your advice based on actual experience concerning probable difficulties. I think industrial antennas are better than general ones. I don't know much about radio modems, I need to search and see the advantages. The IT department has used MikroTik router board antennas to Internet from server to other buildings with similar distances, so maybe I could use their sth or sxt models.
I want to find best (technical and economical) solution, beginning with your comments on whether this system could work or not, and how to select the right devices.
Rahim Rahim / [email protected]
A: Use a Moxa industrial wireless Ethernet modem and dual antenna system. Moxa has modems that cover 2.4 GHz and 5 GHz ranges (depending on the region). I've used wireless Ethernet systems for line-of-sight distances up to 3 km. As a back-up, you can have a telephone alarm sound at the fire station (from your master fire alarm panel). Telephone alarms work at 48 VDC.
Raj Binney / [email protected]
A: A number of makers supply industrial data relay radios. I'd rely on their guidance for the associated installation and antennas.
As a long-time ham radio operator, I'd also suggest careful review at installation and on follow-up. Interference can prevent reliable signals and the radio path might not be exactly as it appears. The people may have to be educated about the antennas to protect against carelessly “adjusting” them. There is a great deal of information available on antenna design and fabrication. Depending on your situation, there may also be the possibility of an Internet connection.
If control is involved, you must design with the thought that loss of data transfer is likely, and the field part of the system must consider and address how loss of connection can be detected, then define how the system is to react. A full safety review may be needed.
Cullen Langford / [email protected]
A: There are some very specific questions that you'll need to answer to determine the proper antenna type (probably a Yagi directional), frequency band, and technology (probably WiMax) because of the distance and data rates, but it may be possible to use WiFi with a directional antenna and direct line of sight between antennas. Consider:
• In which country and/or region will these radios be operated?
• Does the end user have a licensed frequency they want to use, or is the license-free ISM band OK?
• Is this a new wireless installation/network, or will you be adding more radios to an existing network?
• How many radios will be added at this time? Will there be more later that must be accommodated?
• What other radio networks are in the area which might cause interference?
The chances are that the IT department will not have any of the technical knowledge to make this work. Don’t use a standard wireless network card. Get a radio modem with an Ethernet connection, and connect your PC and PLC to the radio modems. The radio modems have much stronger signals, better noise rejection, and a wider choice of frequencies and antenna types. This is needed in a refinery where there will be a lot of reflections and occasional disturbances. If your refinery is in a cold location, then occasional steam clouds from the process units may block signals unless a high antenna is used with low-loss cables.
Dennis Brandl / [email protected]
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A: For most PLCs, the “network interface card” you describe is an Ethernet interface that's normally used with application protocols such as EtherNet/IP, Modbus/TCP or Profinet. These are intended to be wire/cable connected to a network. You'll need to buy an industrial-grade Wi-Fi router with an external antenna connection. If you use a high-gain antenna such as a parabolic dish or Yagi designed for the 2.4 GHz band, and correctly aim these antennas at the control room systems with similar directional antennas, covering distances to 1 km should be no problem. Antenna alignment is critical. Wireless links such as this have been in use in IT for many years when public roads must be crossed or between distributed buildings on a campus. The equipment is widely available, economical, reliable, well proven and not too expensive.
A: I understand that you plan to send data via wireless. This being critical information (going to fire brigade), I suggest using a fiber-optic connection from the PLC.
H.S.Gambhi / [email protected]
A: I would not recommend microwave communications with dedicated antennas, since the fidelity and integrity of the signals could be compromised.
Since the distances are quite large, and due to the fact that F&G systems have stricter codes, I'd recommend using a fiber-optic cable, probably a four- or six-conductor, 62.5-micron size, connecting the F&G system to a signal/fiber-optic transmitter, and have a dedicated fiber/signal decoder at the other end. This cable would be armored for direct burial, and that way damage issues are taken care of.
General antennas will not work in microwave frequencies, which is what you'd be using. Also, remember that you will end up contacting the pertinent local/national radio licensing authority for the pertinent licenses—400-800 MHz ranges or higher are regulated by all governments, and for speed of transmission and accuracy you will need these.
The problem with using the PLC or SCADA network access is that you need to make sure that the network extends to all areas of the plant and that it's in a secured network, so no cyberattack can affect it.
In general, if no network cables are laid between the locations you're back to the starting point where the main issues are:
1. How to transmit the signal from the source—F&G panel to the end users. You either use cables (fiber-optic or copper) or you use radio.
2. If you use cables, is there an existing cabling system capable of transmitting the signals for the required distance with the required accuracy and repeatability?
3. If you use copper cables, the distances may be too large.
4. If you use network cables, you'll need signal boosters and multiplexers.
5. If you use fiber-optic, you'll need multiplexers.
6. If you use radio, you'll need to get the proper government approvals and licenses, then buy an RTU with multiplexers.
Another way is to get an RTU connected to a microwave antenna, and have another RTU in the different locations to output the signals required. For this you'd need the following:
1. A study to determine the optimal placement of the antennas, both sending and receiving.
2. A radio license for the company for the frequencies that you'll use.
3. Three radio transmitters with I/O cards, so the different areas can communicate with each other.
4. The transmission towers, which in the case of 1-km distance, would need to be the right height to get line-of-sight to the receiving units.
5. Hardware cabinets with RTU (I/O) cards and radio system (probably dual systems to guarantee a modicum of redundance in case of failure).
6. Configuration of the RTU, radios and F&G systems.
7. A monthly maintenance plan to verify that the radio antennas aren't covered (this diminishes the signals), maintain battery racks feeding the system, and operate the radios and I/O cards.
What you would have to weigh is the cost and maintenance of an antenna-based system (cheaper, more maintenance) versus a fiber optic system (more expensive, less maintenance). If your company prefers wireless, I'd recommend you get the proper radio licenses before you start, but it’s not the optimal system.
Alex (Alejandro) Varga / [email protected]
This column is moderated by Béla Lipták, automation and safety consultant and editor of the Instrument and Automation Engineers’ Handbook (IAEH). If you have an automation-related question for this column, write to [email protected].