Data acquisition and control in petrochemical plants

Omniflex expertise in last mile networking solves Petrochem Plant data acquisition challenges

The roles of telemetry and data acquisition are very often closely related and indeed use the same information from the plant. Remote control applications generally require the use of some form of telemetry, to get the signals from one location to another over some distance. This is generally a point-to-point system, however, modern telemetry systems are no longer confined just to the one-to-one applications with plants requiring more complex architectures for telemetry data acquisition and control.

Analysers are generally located within the plant close to the processes they are analysing, often in shelters as the area may be classified as hazardous. In many cases the system needs to use either existing plant cables/plant infrastructure to transfer the signals. This is where the Conet LAN comes into its own, running up to 10 km on screened twisted pair the connections to the analysers can be made by simply using spare instrument cable pairs. Omniflex's T2 Maxiflex CPU is designed specifically for data acquisition and telemetry applications. They require no user application programs when used for data acquisition or telemetry application making them a suitable platform for this type of system. Simply plug them in; connect field wiring and power up.

The analysers are connected via NIMs (network interface modules) using an RS232 or RS485 port on the analyser. The NIM has downloaded to it the Analyser Serial Protocol enabling the Maxiflex to interrogate and retrieve data from the analyser. This enables the supervisory system to gain access to all the features of the analyser from a remote location. Analyser information and its status are assembled into data interchange table (DIT). Using a technique called 'network subscriptions' (same idea as subscribing to a magazine from the newsagent) the data required by either party can be shared between themselves. Maxiflex is configured using a Windows-based configuration management package which edits the Maxiflex DIT table and invokes the network services necessary to implement the data transfer between either end of the system. These configuration files can be saved and stored on the PC/laptop. Configuration can be done at the unit itself or via the Network, which is especially useful when access to the other end of the system is difficult. These tools also facilitate browsing of the system for engineering staff. At the supervisory end of the system, Maxiflex can assemble the collected data into a single contiguous block, which allows more efficient transfer of data to the supervisory system.

Figure 1. Typical system overview

Being modular and flexible, Maxiflex can accommodate systems of varying size and those with primitive front end instruments right up to sophisticated measurement devices with on board communications ports, RS232/RS485, Modbus, Hart, Ethernet, etc.

Network media

* Copper cable.

* Fibre-optic.

* Ethernet backbone.

* Combinations of all of the above.

Maxiflex networks can be implemented on copper cable (screened twisted pair - instrument cables) up to 10 km between the furthest points. Other network media such as fibre-optic cable or Ethernet plant backbone etc can also be used. A Maxiflex data acquisition system can also comprise combined elements of the above media and network topologies:

* Point-to-point.

* Multidrop - bus.

* Complex - star, ring, combinations.

* Redundant link.

Configuration is easily made using a free Windows-based software package called 'OmniSet' (Figure 2) and a serial cable available from Omniflex. Conet Explorer is a complete suite of software for configuration management, programming and terminal access which has full features to cater for the needs of the end user's system. Simple selection of pull down options allows the user to configure the CPU completely. This configuration includes the ability to create multidrop systems using the Omniflex CONET local area network up to 10 km on twisted pair cable.

Figure 2. Sample configuration screen

Maxiflex provides a cost-effective solution to data acquisition applications and simplifies the field I/O systems by eliminating the need to write time-consuming application programs for them.

Should some control or interlocking be required this can be integrated within the system as an application program. An IEC61131 programmed CPU can also be used for applications where the user may want to write control applications in ladder or function block. Maxiflex I/O systems cover analogue inputs, voltage, current, thermocouple, RTD and digital inputs, potential free and voltages up to 200 V d.c. series contact chains, pulse inputs up to 50 kHz, Modbus devices, custom RS232 or RS485 protocols, analysers like Vista, Panametrix, Siemens, etc.

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