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IEC 61131-3 improves Fuels ManagementAnton Käsbeck, Sakura Endress Co., Ltd., Japan
Fuels management is an area of the process industry where instruments and software might stay over a decade or longer in place. The use of a standardized language like IEC 61131-3 and new technology like a SoftPLC offer a substantial benefit in the operation of a tank farm or refinery.
Market SituationSakura Endress Co., Ltd., Japan together with the Whessoe Group have become the major supplier of tank gauging systems for the oil & gas industry worldwide. Until recently, customers simply asked for highly accurate level gauges, but now the vast majority demand turnkey solutions for tank gauging systems which include:
What are the needs of tank gauging customers?
Flexibility:Only few countries build new tank farms, most systems are installed as add-on and must be compatible with a variety with existing hardware.
User friendliness:User friendly Operator screens: Tank operators have very limited experience with computers or software.
Tank gauging systems are installations, which operate over a very
long period of time (i.e. 10 to 20 years). Service people face an extremely
wide range of products out in the field.
How does a typical tank farm or refinery look like?A tank farm can be divided into three components:
The SCADA system , where data is stored and visualized. Located in the control room. In former times simple local display with a connection to a DCS system. New systems include impressive graphic displays, interfaces to other computers, LAN connectivity, etc.Additionally device setup, local display, etc. are available. "Figure 2: Instrumentation on a tank" shows a tank with typical instrumentation:
Figure 2: Instrumentation on a tankTank Gauging is mostly a refitting or add-on business: Except few countries in South East Asia there are hardly any new tank farms built. Requirements include:
Tank farms cover large areas, cable lengths up to 3 km
Upgrades often have to use existing cabling and connect existing instruments.
Computer and software knowledge is usually limited at customer site
Moving oil in and out from tanks is an essential functionality of a complete tank gauging system. Control programming via IEC 61131-3 is beneficial.
Configuration model for IEC 61131-3The full model for the possibilities described above in IEC 61131-3 consists of three independent resources connected via a fieldbus. Each of the resources is located on a control unit with a powerful processor and a multitasking operating system:
Resources exchange runtime information via SEND and RECEIVE function blocks, which allow application programming, independent of the fieldbus used. This is especially important for integration of existing applications.
Systems with reduced functionality will be available and can be custom tailored. The location of controls programs and their interfacing are dependent on system requirements and are part of a system design.
Special requirements for control programming in a tank gauging systemA crude oil tank with a diameter of 30m and a height of 20m may take more than a day to fill up. It seems no problem to interrupt a process of filling for a couple of seconds, but it needs to be guaranteed, that the so-called transfer movement is continuously controlled and monitored. Movements will be used as an example for control in tank gauging systems.
What is a movement? A movement involves one (or several) sources and one (or several) destinations
What are the requirements to control a movement?
Control of the valves and matching measurements is the task to be programmed with IEC 61131-3:
- Control is done by the operator from the SCADA station, independent of where the actual control is located. A operator command is transmitted to the SoftPLC, where it is defined as an input signal VAR_INPUT (%IX). The scanning of this input is done in a task (PUMP_TASK), which is event driven by %IX.
- The input signal (e.g. to start emptying a tank) is then processed in the programmed control logic in a PROGRAM PUMP, where
Implementation of the runtime engineThe components of a tank gauging system as described above allow several options to integrate the runtime engine. Other factors in a tank gauging system and implementation cost give advantages and disadvantages for each of them:
A SoftPLC could be integrated into the control unit of a sensor , where electronic boards are already designed for modular communication and control of the sensor
The RTU interface allows the integration of a specifically designed board for a SoftPLC or a SoftPLC as an additional task on the main board.
software used for tank gauging is a modular system running on Windows NT.
A SoftPLC is integrated as an additional task under Windows NT. An interface
to the database of the SCADA software allows read/write access.
Implementation of a Fieldbus interfaceEach of the components above running a control unit connects via a fieldbus. Sakura Endress Proservo NMS53 is a servo gauge sensor with a modular design. Communication interfaces are separate boards in the sensor, which have a standard interface to the communication board.
The Whessoe Coggins RTU/8130 interface is based on a main board with up to 4 fieldbus cards. Special interfaces for tank gauges as well as standard field busses are available.
The SCADA software FuelsManager will interface to a fieldbus via built in interfaces or via OPC (OLE for Process Control).
A control unit can be integrated into each of these components. With this concept customer requirements at each level can be satisfied.
Market needs define which approach is cost effective and where benefits are highest.
Application PROGRAMMINGTank gauging systems are delivered as turnkey solutions to the companies running a tank farm or refinery. The end customer himself usually does little or no change. Systems are configured and programmed at the manufacturer of the tank gauging system.
How can the usage of a standard programming language like IEC 61131-3 improve the performance of a system integration center (SIC)?
ConclusionThe language standard IEC 61131-3 and commercially available SoftPLC give manufacturers of industrial controls equipment a better chance of offering complete tank gauging systems.
Manufacturers benefit from:
Customers benefit from: