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DCS Process Computer System

  • DCS (Distributed control system) is a microprocessor based system which gather input from various instruments and HMI, processes the inputs and sends the required output to other instrument
  • The key difference between DCS and SCADA
    • SCADA is generally used for monitoring only with limited controls.
    • The plant cannot run without DCS but can run without SCADA
    • SCADA is mainly for remote monitoring
  • DCS Network
    • DCS Networks exists due to the incapability of a single processor to compute all requirements of a plant
    • Yokogawa DCS network is called V-net, it is similar to the ethernet style and is in accordance to IEEE standards
      • Has redundancy due to it’s criticality
      • 10 Mbps speed
      • Must update data every 1 second
      • Allows fibre optic communication for long distance communication such as communication with FAR(field auxiliary room) room
        • A Far room is required because
      • A DCS network is used for communication between
        • ICS
        • FCS
        • CGW (Computer Gateway) or ACG
      • A DCS system will also have another network which is of less important. It will be also of Ethernet type. Yokogawa calls it the E-Net Network. The E-Net network communicates between
        • EWS (Engineering/Maintenance Work Station). For updating the DCS
        • HMI (Human Machine Interfac, (Yokogawa calls it ICS). This is for graphic updates
        • Historians such as Alarm PC
      • The reason for this E-Net network is to minimize traffic congestion
      • The diagram below shows a simple Yokogawa Centum CS network
      • clip_image002[4]
      • FCS Computer
        • The FCS is a microprocessor based station which performs all the control algorithms
        • The DCS contains many FCS’ connected via a network (such as Yokogawa VNet). This is:
          • To distribute the processing load
          • To ensure if one FCS goes down, the other units may can still run.
          • For centum CS, the AI+AO’s are limited to 640 and DI + DO’s are limited to 4096
          • Sub System Communication
            • Sub-system communicates with DCS through
              • ACG (Communiccation Gateway)
              • Communication (Modbus)
            • Critical Sub-systems such as IPS are connected directly to the FCS computer. The connection is by a communication card which utilizes RS485 cable. The communication protocol used is modbus. Such systems are:
              • IPS (PLC)
              • ADAS
              • MMS
              • CCC
              • HIPS
              • MOV
              • F&G System
            • Since FCS’ are scattered it is typical for a single subsystem to send multiple RS485 cables to different FCS’
            • Other subsystem
      • DCS IO’s
        • IO’s are connected through
          • Direct connection to FCS
          • Remote connection to FCS from and RIO bus which may utilizes fibre optics communication if more than 750m. Fibre optics are used due to it’s long distance reliability and able to send more data
        • The picture below shows a typical IO connection to an FCS computer. As can be seen
          • The screen cables are looped all the way and grounded at the main distribution frame cabinet (MDF) at the IE point
          • SPD’s are installed in front of IS barriers.
            • IS Barriers are only required for IS type instruments
            • SPD’s are required except for short cable i.e. if the instrument is inside the control room itself
          • The wires that goes directly to DCS IO card named AAM11 via an ELCO cable
          • Since this is a Smart HART transmitter there is a HART MUX connection from the IS barrier which goes directly to AMS system via RS485 connection cable
          • clip_image004[4]
          • IO modules for DI and DO will come before a relay board. The relays are usually manufactured by Omron
          • When an instrument sends a contact, it will hit close the circuit and causes the relay to toggle
          • DI and DO’s will have redundant IO cards if the are critical
          • Dry contact and wet contact are common terminologies to describe a particular instrument. Instrument here can be a valve, motor, MOV or relay.
            • Dry contact means that when the contact closes, no power is supplied to the receiving system. The contact is just a switch
              • One example is a relay. A relay is a dry contact if the relay itself when giving contact, does not gives any power to the receiving instrument, such as an MOV. In this case power comes from the MOV. The relay just completes the loop
              • Looking from an input expect, a level switch is a dry contact if when the contact closes, the level switch does not send any voltage to the DI relay/DCS. In this case voltage is supplied by the relay panel
            • A wet contact is vice versa, power is supplied when the contact is closed
            • As a general rule when speaking about dry or wet contact, it is to be referred to in the context of the instrument/Relay. Every connection must have a supplying voltage. If the instrument/Relay supplies the voltage, it is therefore a wet contact else it’s a dry contact. Commonly people will say, “This relay is a wet contact” means that the relay will supply power when it’s energized
            • For outputs, relays are commonly Dry since we let the valve/pump (for example) source it’s own voltage
            • For inputs, relays are commonly wet, since most DI’s are mechanical e.g. push button, indication and level switch. It is there for more feasible that the instrument let the DCS decide which voltage it prefers to use.
          • You may ask, why does a DI sensor input such as a level switch be connected to a relay board? Can’t the DI close a contact which is connected to the DI card of the DCS? Well, the reason is due to standard engineering practice where the DCS system is to be isolated from the input. This is to protect the DCS system from the instrument voltages or short circuits
          • The diagram below shows a typical redundant DI loop for a particular instrument
          •  clip_image006[4]
        • IO Status in DCS
          • CLP+ or CLP- means that the OUTPUT is clamped at a high/low limit in the system. This can be due to a lot of reasons such as the output to the control valve is full (control valve is asked to fully open) but the PV has still not reached the SV. In situation, the PID controller has failed. All other outputs from which became the inputs to this block will also be in a CLP status
          • IOP+ or IOP- means that there is a high(>20mA) or low(<4mA) current at the INPUT. A low or high current can be due to a low of reasons such as transmitter is disconnected (open loop give 0 mA), transmitter set to burst due to sensor disconnection or any other electronic failure, and also transmitter PV reading is above URV or lower than LRV
          • OOP data status occurs when there is no current to the output. This can be due to a discontinuity in the output or any other failure
          • For Control valves, regardless the control valve is fail open or fail close, 0% seen in the DCS will always be close and 100% will always be open. Even though a fail open control valve requires 4 mA to to open it, the IO card will always invert the signal to 20 mA
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