2Fairfax WTW
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An overview of the control system

The water treatment works will be controlled by a Siemens Simatic Process Control System (PCS 7 system). This system has four basic components, the Automation Systems (AS), the Operator Station Server (OSS), the Operator Station clients (OSC) and the Engineering Station (ES).

The Automation Systems interface directly with the plant and control the plant devices (motors, valves &c.) and instrumentation (flow, level, pressure sensors etc.).

The Operator Station Server (hereafter just the Server or OSS) interfaces with the Automation Systems and collates the data from those systems (instrument reading, device status, events etc.).

The Server will also identify alarm conditions within the process itself, within devices and within instrumentation. These conditions along with critical plant data will be logged and stored for offline analysis.

The Operator Station Clients provide the operator interface to the plant — allowing the operator to start and stop automated processes, change set-points, take manual control of plant devices and generally operate the plant. The OSCs also provides a visual representation of the plant, allowing the operator to monitor the state of the plant devices (stopped, running, opened, closed, in fault etc.) and react to anomalous conditions within the plant. Each OSC communicates with the Server, reading the data to be displayed and writing operator instructions to the system.

The Engineering Station is used to configure (programme) the system.

PCS 7 has its own standard library of objects (referred to as the Advanced Process Library or APL) and this library will be used as the basis for all devices, instrumentation and graphical representations.

The APL is compliant with the current engineering standards for alarm management and process control graphics (these are the EEMUA 192 and 201 standards respectively).

The control system is constructed around a three network architecture: terminal bus (linking the OSCs to the Server), plant bus (linking the Server to the Automation Systems) and field bus (linking the Automation Systems to the plant equipment). A schematic of this arrangement is shown in Figure 2.2

Of the three networks, the terminal bus and plant bus are Ethernet networks and the field bus is a ProfiBus DP (distributed peripheral) network.

The electrical signals from the plant devices and instruments are wired to the remote IO racks; these are situated in various panels distributed throughout the plant; these remote IO racks transmit the various status signals from the device to their associated AS. Similarly, commands from the AS (open/close, start/stop etc.) are passed to the devices by the remote IO racks.

All signals to and from the remote IO racks are transmitted across the ProfiBus DP (field bus) networks.

The configuration used to control the water treatment works consists of:

  • 1 × Server

  • 2 × Operator Station Clients (control room and filter house)

  • 2 × Automation Systems

  • 6 × remote IO racks distributed around the plant

  • 1 × Engineering Station (located in the control room)

Figure 2.2 - Control system architecture schematic

Figure 2.2   Control system architecture schematic


The Automation Systems (AS)

The plant is controlled by two AS units one using the 417‑4 CPU, controlling the main plant (inlet, filtration and outlet) and a smaller unit using the 414‑3 CPU controlling the dosing systems.

The plant is controlled by the AS units; these are programmed by a Siemens proprietary programming package (Simatic Manager) and support both continuous control and sequential control. The continuous functions (those functions that are always executed) are written using the continuous flow chart (CFC) programming language; sequential operations are written using the sequential flow chart (SFC) programming language.

Special user defined functions are also supported by the AS units; these are written in a derivative of Pascal called structured control language (SCL).

All three of these languages: CFC, SFC and SCL are compliant with the IEC 61161-3†1 standard.

†1 IEC 61131-3 (2013) is the International Electro-technical Commission standard specifying the syntax and semantics for a unified series of programming languages for programmable logic controllers (PLCs).


Remote input and output (IO) racks

All the device signals are hardwired to remote input and output (IO) racks, each IO rack is located in one of the electrical panels positioned around the plant. The following is a list of the electrical panel, their location and the remote IO racks they contain (Table 2.2).

The locations of the panels around the site are shown on the site general arrangement drawing Figure 2.3.

Panel Location IO Rack Description
E001 Auxiliary Room None Main server panel
E002 Auxiliary Room AS01-Rack 01 Inlet works & services control
E003 Cavendish valve house AS01-Rack 02 Cavendish feed flow control
E004 Coverley valve house AS01-Rack 03 Coverley feed flow control
E101 Filter house AS01-Rack 04 Filter & backwash control
E401 Outlet works AS01-Rack 05 Outlet works control
E601 Dosing room AS02-Rack 01 Dosing control
Table 2.2   Electrical panels & remote IO racks

Each IO rack is connected to its associated AS via a ProfiBus network. There are two ProfiBus networks, one for AS01 and one for AS02 (see § 2.2.8)

Figure 2.3 - Fairfax WTW site general arrangement

Figure 2.2   Control system architecture schematic


The Operator Station Server (OSS)

The Fairfax WTW control system has a multiple station operator system consisting of two Operator Station Clients and one Operator Station Server. The Server connects to the two AS units and reads the process data (process values, alarms messages etc.) from the AS units and writes any operator commands to the AS units. The server also manages the archiving of process data for trending and historical records; and manages the users of the system.

Whilst not an operator station in itself (it does not, for example, display plant mimics etc.) it does make all the system data available to the system operators via the OSCs.

The OSS is a Dell PowerEdge R630 rack mounted server running the Windows Server 2008 R2 (service pack 1) operating system.

The OSS has two network interfaces (both Ethernet based): firstly the Plant Bus network used to connect the Server to the AS units (see § 2.2.7); and secondly the Terminal Bus, used to connect the Server to the two OSCs (see § 2.2.6).


The Operator Stations Clients (OSC)

The two Operator Station Clients form the interface between the operator and the plant; the operator can, from either OSC, see the state of the plant from the process mimics, has access to all alarm information and can control the plant by issuing commands.

Both OSCs have the same user interface and the operator can control any part of the system from either OSC.

Each operator has an individual logon for the system (administered by the OSS) and can logon to either OSC. Different users have different privileges within the system and depending on the level of privilege can perform different actions within the system.

The two OSC are linked to the Server via the Terminal Bus network (see § 2.2.6).

Both OSCs are Dell T5810 Precision Workstations running Windows 7 Ultimate (service pack 1).


The Engineering Station (ES)

The Engineering Station (ES) contains the engineering tools needed to configure the system; it is in essence the development platform for the system. It contains the following:

  • Hardware editors for managing the system components

  • Network configuration tools

  • Software development systems for the AS and OS systems

  • Deployment software for distributing the developed configurations to the various system components


The Terminal Bus network

The Terminal Bus is a 1 Gb/s Ethernet (1000 Base T) network using the TCP/IP protocol; it connects the Server and the Engineering Station with the two OS Clients.

The Terminal Bus uses Siemens Scalance X-300 (copper wire type) industrial Ethernet switches to manage the network.


The Plant Bus network

The Plant Bus is a 100 Mb/s Ethernet (100 Base T) network using the ISO†2 protocol. It connect the Server and Engineering Station with the two AS units, it also allows communications to take place between the two AS units themselves.

The Plant Bus uses Siemens Scalance X-200 (copper wire type) industrial Ethernet switches to manage the network.

The Plant Bus network is configured as a redundant ring.

†2 The ISO Industrial Ethernet protocol is a communication protocol based on the media access control (MAC) address of a device; it is both faster and more secure than TCP/IP and is exclusively used when communicating with the AS units.


The ProfiBus network

The ProfiBus network is used to connect the two AS units to their remote IO racks; there are two ProfiBus networks: the first links AS01 to its five remote IO racks (see Table 2.2), the second links AS02 to its single remote IO rack; the two networks are independent and are not connected together in any way.

Both ProfiBus networks are of the distributed peripheral (ProfiBus DP) type and are configured to carry remote IO signals to the AS units. Both networks communicate at 1.5 Mb/s.

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