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2Fairfax WTW
An example document

2.2

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

2.2.1

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).

2.2.2

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

2.2.3

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).

2.2.4

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).

2.2.5

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

2.2.6

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.

2.2.7

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.

†1 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.

2.2.8

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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