More clarity during field work

Dipl.-Ing. Björn Thomessen

When parts of a system are automated, it is, for example, quite simple to replace a manually operated valve by a control valve with an attached, self-optimising, digital valve controller. Mechanically operating automatic dosing stations (B & R, Siemens, Smith) are also gradually being superseded by electronic systems, though without replacing the user by a trained technician who is capable of taking account of all transducers by programming the parameter and function codes accordingly. After the system has been “optimised” and all information successfully centralised in the central control room, the interrelationships can no longer be determined easily in the field.

Displays inside an explosion hazard area are of course not mandatory for centralised monitoring and control of processes. Not least because of the increased complexity of attaining system approvals, many manufacturers of sensors and transducers either do away with a display completely or reduce the number of displays to a minimum. The MeReX field display and batch controller (Figs. 1 and 2) covers the entire range of requirements from a field display to a complex batch controller, thereby closing the gap between simple panel-mounting pulse counters and user-friendly, self-explanatory Ex-(i) terminals. However, the disadvantage of the Ex-(i) terminals is that the intelligence is located outside the explosion hazard area. In contrast, the MeReX field display and batch controller, being part of an interlinked safety system, can easily perform monitoring and control tasks whilst keeping operation straightforward and providing full information.

The main area of application for the MeReX 5002 is currently flow measurements. The instrument is used here in its simplest operating mode for visualising counts and flow data.

The MeReX instruments are capable of processing signals from almost all flow and mass-flow measuring instruments. They are suited, among other things, for the following monitoring tasks:

l detection of broken lines,

l flow restriction,

l temperature limiting

and can also be used to prevent overfilling. In the case of the MeReX instruments, a value between 0.00001 and 9.99999 can be assigned freely to a single input pulse for each input. Basically, however, all measurement signals provided by any pulse-generating source or generator can be processed (NAMUR, passive, active), as shown in Figure 3. Moreover, the flexibility of the software, which is independent of any explosion hazard approvals, is such that user requirements can be taken into account without problems and at short notice.

The instruments in the MeReX 5002 series are equipped with a large display and a clearly laid-out keypad. All the relevant process quantities can be displayed here. The functional algorithms have been reduced to an adequate minimum. The program memory is thus able to hold a large number of display messages. For this reason, the instrument always indicates any necessary entries in plain text. A standard menu structure is used throughout, regardless of the instrument’s expansion level. Individual parts of the program can be protected by means of PIN codes if desired. Thanks to this approach, operating the instrument is very simple. For filling and dosing processes, for example, all that needs to be done is to start and stop (in the case of premature termination), reset the counter and if required change the pre-selection data (using the cursor keys). In addition, the display can be used to indicate batch sizes or target vessels that have already been stored as well as the lines which are to be used. It is thus possible – by integrating the control system in the background, of course – to allow the lorry driver or external personnel to operate the instrument in order to fill the tank.

The MeReX 5002 field display and batch controller has been developed, and is manufactured, in accordance with EN ISO 9001, and is authorised (German PTB) as a controller for pre-selecting quantities in connection with applications requiring gauging. The type-tested system consists of a gauge-able volume or mass-flow meter for liquids (except water), a MeReX 5002 and (if necessary) a gaugeable printer. Printouts can be very interesting for many applications where calibration is not mandatory. The MeReX 5002 field display and batch controllers are capable of outputting the measurement data either to various voucher printers or to an existing DIN A4 size printer in order to print delivery notes, for example.

The MeReX instruments can be operated inside explosion hazard areas. The intrinsically safe versions can be run in all zone 1 and 2 hazardous areas (EEx ib IIC T4). These instruments are identical in function and appearance to the standard models. Only the number of options which are available is slightly reduced. There is only one current output, for example, and no RS 232 interface or backlighting for the display. An isolator installed outside the explosion hazard area ensures a limited supply of power. The user can thus set up a dosing station for an explosion hazard area which is supplied with power exclusively by means of a two-wire line and a 6 bar air supply.

Within defined limits, the Pt 100 input of the MeReX 5002 permits temperature compensation for connected volume meters. If both the density and the coefficient of expansion are known for a particular medium, the value of the mass is calculated directly and displayed within a linear range. It is thus also possible to calculate the volumes under standard conditions (at 20 °C, for example).

The outputs of the MeReX 5002 field display and batch controller offer a variety of functions: the switching outputs, for example, can be used to control the solenoid valves used in filling processes. If the outputs are binary-decoded, the instrument is capable of controlling up to seven solenoid valves, so that, for instance, up to seven mixing vessels can be supplied from a single storage tank (Fig. 4). In addition, status messages can be forwarded via the contact outputs to the central control room (faults, “batch in progress”, etc.) or measurement data transferred by means of standard pulses.

The analogue outputs of the MeReX 5002 field display and batch controller can also be used to control actuators or output measurement data to a central process control system. When used as pre-set counters, the MeReX field display and batch controllers provide a means of control via the analogue output. The functionality of the instruments thus extends far beyond simple dosing or filling. The shut-down actuator, for example, or the flow control valve which is connected to the current output, and which is itself controlled by a position controller or an IP converter, can be operated during the batch process or during continuous operation. Time-consuming alignment of the start-up and shut-down stages is no longer necessary, since the set-points for the flows are entered directly.

The standard software for flow measurements and dosing comprises, among other things:

• three pre-selection functions for real time counters,

• one pre-selection function for the summation counters,

• start-up and shut-down of the batch process in a series of steps,

• post-correction,

• monitoring of the flow and pulse transducers.

The electronic assembly is fixed inside a rugged, cast-aluminium casing, which is IP 65 rated, by means of a tongue-and-groove joint, thus providing an extremely reliable seal. Large, maintenance-friendly screw terminals in the separate junction box ensure exceptionally effective electrical connections.

Bielenberg

Fax: ++49/2237/9 73 45-55

Further information cpp 260