THE PILOT THERMAL PLANT (PTP) OF CRAN-ESSTIN
This pilot plant is a thermal process controlled by several PCs which are used to monitor control LOOPS and to configure the AUTOMATION system.
The following pictures show the PTP and its components :
The boiler is a domestic one. The energy used is the gas of Groninguen (8 400 Kcal/m3). The heat-carrying fluid is water in a closed circuit (Primary circuit).
The boiler is fitted out with tree safety devices which cut the supply of gas in case of:
The heat exchangers are liquid-liquid type. The two heat exchangers allow to work with several configurations, through ON/OFF valves:
One heater is used as a load for the secondary circuit with a possibility of by-pass.
The temperature measurements
are made through PT100 platinium resistances. This kind of RDT is calibrated
in order to provide an electrical signal (4-20 mA) for a temperature variation
of 0 to 100°C. The resistance variation is a function of the temperature
in accordance with: 
Rsonde=Rc0(1+AcT+BcT^2) where
The flow measurements are made with the help of variable-pressure-drop flowmeters, associated with differential pressure sensors:
The flow Q is given by the
relationship: 
Q = K sqrt(DP/r) with
The differential drop is converted into an electrical-signal of 4 to 20 mA. Then, it is necessary to extract the square root in order to obtain a proportional signal in flow. This task is made by an external treatment.
a- The analog valves
The analog valves are
in fact pneumatic servo-motors associated with electro-pneumatic converters.
They are closed in case of lack of pneumatic energy and their aperture is facilitated by the fluid itself.
The input range is 0.2 - 1 bar.
These analog valves are non-linear components within control loops, because their static characteristics and their time reponse are not constant.
b - The electro-pneumatic
converters
They need a pneumatic
supply of 3 bars.
The input signal varies
in the range 4 to 20 mA and the output one is a pressure in the range 0,2 to
1 bar.
The global aims of this process control consist in
Several control loops are fitted out.
PID Controllers are used to realize the control function. These PID are virtual because they are implemented on PCs (see III - AUTOMATION SYSTEM).
For each exchanger, a first loop allows to adjust the Primary Flow Qp* in order to control the Secondary Temperature Tso* and a second one (internal loop) adapts this Primary Flow (Qp*).
For the Exchanger 1, this Primary Flow Control Loop is reacheable through Internet.
The output value Tc0 is maintained with a set point of gas flow Qg . A first loop allows to adjust the Gas Flow Qg in order to control the Output Temperature Tco and a second one (internal loop) adapts the Gas Flow (Qg).
c - Disturbances
Of course, disturbances
result from interactions between the sub-systems exchangers and boiler:
In order to reduce these interactions, the water flow Qc is kept constant, with the help of an other control loop applied on the by-pass valve recycling directly a part of the water flow Qrd.
This water flow Qrd is calculated from Qp1, Qp2 and Qc measurements.
The pilot plant is controlled by several PCs and two inputs/outputs converters FBIC (Field Bus Interface Converter), which are interconnected on the same fieldbus WorldFIP (Factory Information Protocol or Flux Information Process).
Further information about
and
PCs and FBIC are connected to the medium WorldFIP through special connectors so-called TAPs.
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On WorldFIP, the transmission speed is 1 Mb/s (standard) with a twisted pair as a medium. On PCs, some Automation and Fieldbus administration tasks are distributed, such as:
2 - WorldFIP / Ethernet
Data Exchange
A PC is the gateway between
both the WorldFIP fieldbus and the Ethernet network.
It is equiped with both a WorldFIP board for the connection to WorldFIP, and a 3COM509 board for the connection to Ethernet.
A SUN workstation manages the interface between the Internet Remote User and the gateway PC, as it is shown below:
The SUN workstation has also to exchange information with the PC gateway.
When an Internet Remote User tries to "play" with this Flow Control Loop,