THE PILOT THERMAL PLANT (PTP) OF CRAN-ESSTIN


I - PLANT DESCRIPTION


II - PROCESS CONTROL


III - AUTOMATION SYSTEM


I - PLANT DESCRIPTION

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 :

1 - The boiler

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:

2 - The heat exchangers

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.

3 - The sensors

a - The temperature sensors

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

b - The flowmeters

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.

4 - The actuators

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.






II - PROCESS CONTROL

The global aims of this process control consist in

in spite of disturbances such as primary flows and temperature variations for instance.

1 - The Control loops

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).

a - The exchangers

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.

b - The boiler

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.

III - AUTOMATION SYSTEM

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).

1 - The WorldFIP fieldbus

Further information about

are available.

PCs and FBIC are connected to the medium WorldFIP through special connectors so-called TAPs.


The FBIC converters are used to interface the set of sensors and actuators to the fieldbus WorldFIP through analog and digital inputs/outputs.

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,