Communication changes in the air
Plant-wide communications are vital; users need to know if their systems are up to par
By Rao Kalapatapu
An evolution is taking place in the design and implementation of communication and network systems for control and operation of process industries, including chemical, petrochemical, and oil and gas plants.
Even though, these systems may eventually integrate with IT networks such as corporate/enterprise LANs and WANs, special care should be taken in the separation of these systems from the normal IT networks, so it can provide maximum availability with minimum disruption for a process plant’s normal and emergency system operations. The basic building block for the plant-wide operations these days is the Distributed Control System (DCS) with its associated peripherals and interfaces. In addition, control and telecommunication subsystems form the plant communication infrastructure. These may include: Supervisory Control and Data Acquisition (SCADA) and various plant control systems; closed circuit television (CCTV) systems; public address and general alarm (PA/GA) Systems; radio systems; Private Automatic Branch Exchange (PABX) systems; access control and personnel tracking systems, and data network systems.
Depending on the type of process plant, the various local and remote control systems that interface with the DCS are:
Pipeline and remote drill site SCADA systems
Safety Instrumented Systems (SIS)
Fire and gas protection systems
Wellhead monitoring and control systems
Pump control and/or compressor control sytems
Smart transmitters, field devices valve actuator systems
Field/sensor busses and other field wiring infrastructure
Environmental equipment monitoring systems
Advanced process control systems
Asset management systems for scheduling maintenance and operation management
Interfaces to management information systems, business and IT clients to the automation networks
Closed circuit television mainly sees use for security and plant operations. The older systems are somewhat larger analog-based with VHS video taped systems, display monitors, and video switching equipment. The newer ones are digital DVD/DVR-based fiber optic networked systems and are more compact. They are easy to connect to remote monitoring systems via telecommunication LAN/WAN systems. The system specifications shall include types of CCTV (Analog/Digital, explosion proof, or weather proof), types of power supplies, types of installation (wall mount/pole mount), types of network cabling (coaxial/ fiber), and control room interfaces such as video recording and display systems.
PA/PG systems are for plant public addressing and general alarming during plant emergency and for general information dissemination. These are indoors and outdoors and interfaced with fire and gas monitoring systems. The equipment consists of indoor and outdoor speakers ranging from 6 Watts to 25 Watts, indoor and outdoor beacons, controls mentoring, and rack room racks fitted with amplifiers and control equipment. Fiber optic and copper networking is available from vendors to interconnect these speakers and beacons to central control room. The specifications include types of speakers (Indoor/outdoor, weather proof/explosion proof), types of beacons (flashing/no flashing), and wattage of speakers.
VHF and UHF radio systems with handheld and vehicle fitted radio systems work within a large plant or large geographical area for operation and maintenance personnel to contact each other during plant startup, shutdown, and during normal operation. A typical radio system consists of radio towers fitted with antennas, control room amplifier and radio network equipment, desk top control room radio units, along with handheld and vehicle mounted equipment. This is normally very expensive to install, operate, and maintain. They are also in the process of phasing out for wireless cell phone systems.
These systems do not directly interface with DCS systems, but reside on telecommunication LAN/WAN systems and can also interface with plant PA/GA and PABX systems. These could be analog/ digital based systems, and many options are available; and these radio systems are still in use in remote parts of the world. The specifications include types of radios, types of towers and antennas, quantities and required number of channels, and types of interfaces to other networks.
Plant personnel use PABX for internal and external communications. PABX systems are mostly digital-based and come in many sizes for connecting 100 to 1,000 even larger systems spread through out the plant. There could be several PABX units installed throughout the plant, but they are usually interconnected. Plant workers could install these systems, but usually telephone system suppliers or local telephone system operators commission them. The PABX system includes PC-based operator and configuration console for system configuration and maintenance, voicemail software, UPS batteries, Main Distribution systems (MDF), analog and digital telephones, and interface cards to connect other PABX systems. The capabilities can include local and international access; call forwarding and other features typical to any regular telephone system. These do not directly connect to DCS systems but are part of telecommunication infrastructure of the plant. The specifications shall include types of PABX systems and interfaces required quantities and required types and number of telephones, control room, and MDF termination details.
Watching at all times
The access control and personnel tracking system can monitor entrance and exits to key areas within the plant/port areas. It can also track personnel movement within the plant area. A number of muster and tracking points are in the plant area so personnel can gather in the event of an emergency. These systems can work as a stand-alone system, with network and workstation display facilities capable of interfacing and networking into the existing company wide access control and personnel tracking system. These do not connect to DCS systems but are part of telecommunication infrastructure of the plant. The specifications shall include types and number of cards, power supplies, and types of control room and rack room equipment.
In addition, the plant data network systems can connect the plant personal computers. It includes 10/100/1,000 Mbit LAN switch racks, personal computers, printers, PC, Internet, and Intranet software typical for any PC system installed in the offices of plant engineering and operating personnel. These do not directly connect to DCS systems but are part of telecommunication infrastructure.
Linking voice, data networks
The backbone communication system is a combination of private and public switched telephone networks. Private networks can be copper, coaxial, fiber-optic, or wireless radio networks. Public switched networks may be wired or wireless type networks leased from local telephone/cable or satellite companies that will allow the linking of all real time voice and data networks spread around large areas.
All network architectures loosely start with the International Standards Organization (ISO) standard seven layer’s Open Systems Interconnection (OSI) model. The object of the OSI model is to establish a framework that will allow any system or network to connect and exchange signals, messages packets, and addresses. The model makes it possible for communications to become independent of the plant operating systems and shields the user from the need to understand the complexity of the backbone network.
ISO Layer 1 covers physical media such as cables and connectors. The network technologies also called LAN technologies or network specifications are in Layer 2. These specifications dictate how the physical media are shared and how control access goes to the shared medium. For that reason, Layer 2 (the data link layer) is also the media access control layer (MAC). The most widely used network technology is Ethernet, but there are several others such as Token Ring, Asynchronous Transfer Mode (ATM), and Fiber Distribution Data Interface (FDDI).
A typical network consists of nodes such as computers and other subsystems, a connecting medium (wired or wireless), and network equipment such as routers, switches, or hubs. In the case of the Internet or Intranet, all these pieces working together allow each node to send information to other nodes that could be on the other side of the world.
In addition, typical networks today use two different addressing mechanisms on top of each other; and addressing is a necessity for data exchange between any two-networked machines. The lowest level addressing is the hardware based MAC access (Ethernet addresses hardware addresses). The MAC addresses go within a single LAN. MAC addresses go in to the hardware (typically network adapters). The telecommunication system design philosophy includes specification of a backbone telecommunication transmission system whose components include:
Physical media (OSI Layer 1) such as copper and fiber-optic cable networks, other wired or wireless networks
Physical network devices (OSI Layer 2 and Layer 3) for linking the above physical media include routers, switches, firewall equipment, and terminal servers—this equipment provides interfaces and interconnection of various communication sub systems.
Switch is on
In general, telecommunications facilities switch, combine, amplify, and transmit information over chosen media. The hardware consists of transmitters to convert the voice/data signals to suit the transmission media, wiring, switches, routers, bridged, and multiplexers to amplify and carry the signals to a receiving station. The software consists of multiplexing controls and communication protocols.
There are two types of multiplexing, circuit switching and packet switching for allocating bandwidth on the communication channels. Circuit switching most often is synonymous with voice traffic. In circuit switching, it will find an available path, seize it, and dedicate it to exclusive use of participants. Packet switching is a bandwidth allocation technique that utilizes the bandwidth only when there is data to transmit. Public switched networks use circuit switching where for private local networks packet switching is more common. The LAN networks include RS-232, FDDI, Token Ring, and Ethernet. The WAN networks include X-25, Frame Relay, ATM, and Synchronous Optical Network.
Fiber optic/copper cables and connectors, fiber optic and electronic transmitters and receivers, multiplexers, routers, and switches play an important part in the formation of telecommunication networks.
The implementation phase covers detailed engineering, procurement, installation, and commissioning of telecommunication and transmission systems and networks. So in this phase, specifications and bill of materials must be ready for each subsystem, and tenders should float out to the selected vendors. Based on the bid evaluation of the quotes received for each subsystem, select the vendors for supply of equipment for each subsystem. The subsystem vendor drawings provide additional wiring and installation details for all the equipment supplied by these vendors. The implementation phase thus may include the following:
Detailed engineering, which includes vendor drawing reviews, factory acceptance testing, physical cabling/wiring layouts for fiber optic/copper/coax cabling systems and interconnections to:
Plant automation DCS/SCADA/ other control systems
SIS, Fire and GAS systems
CCTV, PA/GA, PABX and remote card access systems
Corporate IT network interface firewall routers and switches
Arrangements for telecommunication infrastructure implementation phase may also include:
Selection of public terrestrial telecommunication system vendors for across the country connections with Frame Relay, ATM, and dedicated leased telephone lines etc
Selection of public wireless networks such as Satellite/UHF/VHF radio systems based LAN/WAN for remote facilities such as drilling well head control systems and various production and transmission pipeline monitoring and control systems
Commissioning phase includes site acceptance testing of each subsystem after installation, interconnections testing, and putting these in to operation.
ABOUT THE AUTHOR
Rao Kalapatapu P.E. (firstname.lastname@example.org) is a supervising control systems engineer with Washing Group International Co., Houston.
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