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| Simatic S7-200 controls nitrogen production system | | Requirement-based Production | Nitrogen is an important raw material in the cosmetics and food industry for manufacturing auxiliary ingredients such as collagen or lecithin. Where the gas was previously delivered and stored in tanks in production, it can now be manufactured locally as required with systems from the Carbotech Engineering GmbH. The control processes are securely "in the hands" of the powerful 224XP CPU of the Simatic S7-200 family.
The Air Liquide company supplies its customers with nitrogen. To save transport and storage, the company came up with the ingenious idea of installing nitrogen producing systems directly at the customers. The Carbotech Engineering GmbH, Essen therefore received the order to produce such a system for food production. The Gladbeck company Elektrotechnik Automatisierungstechnik Dipl.-Ing. H. Krix was contracted to provide the control technology. The special task was to unite existing standards of nitrogen systems with the end customers special specifications in one system. This meant: a redundantly structured system, flexibility in terms of volume and purity of the nitrogen and smallest possible dimensions. The electrical company, Krix, answered the control technology questions with a complete system, mainly comprising three Simatic S7-200 micro PLCs with 224XP CPU, an EM 231 RTD extension module, two TD200 micro-panels, two Sitop Power power supply units and Sirius alarm lamps. Krixs long years of positive experience with Siemens components made the choice easy. The advantages, such as compact design, easily implementable data transfer, fast programming, high performance, good operating and monitoring facilities, are in line with the company philosophy which aims at compliance of deadlines and quality solutions with a good price-performance ratio.
Continuous nitrogen production
The system by the name of Compactline Midi produces an average of 80 m3 N/h (nitrogen per hour). Up to 160 m3 N/h can be supplied if the requirement is higher. In order to guarantee a continuous nitrogen production or to be able to double the production volume easily when necessary, a redundant version was chosen, i.e. two identical systems each controlled by Simatic S7-200.
The system also based on the PSA (Pressure Swing Absorption) principle used by other nitrogen systems. Here, the property of certain materials for binding oxygen molecules at high pressure and releasing them again at low pressure is exploited specifically. To achieve a continuous nitrogen production, at least two absorption vessels must be connected so that one vessel absorbs the oxygen whilst the other regenerates. In order to load both systems evenly, both systems alternate within a pre-defined time pattern under normal conditions. After starting system A, the process air generated by a compressor is fed through an absorption vessel with CMS (carbon molecule screen). Oxygen is absorbed, the process gas, nitrogen, is produced. If the first absorber is enriched with oxygen, the second absorber is switched to and takes over the production without interruption whilst the first absorber is released into the atmosphere. This alternating operation is continued until the oxygen content has dropped from 21 percent to the process value of below two percent. Only now does the system feed the nitrogen into the product gas pipe and make it available for production. If no nitrogen is tapped, both systems switch to standby. The production restarts automatically when more nitrogen is needed. With the TD 200 Micro Panel it is possible to change limit values, cycle times or production data.
Added production safety
To avoid consequential production damage in cosmetics production due to too high an oxygen content in the nitrogen gas, the oxygen is measured permanently. In case of deviations, defects in the measuring cell or compressed air faults, system B, the partner system, is connected immediately. After both systems have been operating simultaneously for a short time to ensure a continuous production process, system A switches off via fault and system B takes over the further nitrogen production. An important feature of the system has to do with the connection of the partner system at increased nitrogen consumption or on dropping below a process pressure. To do this, the pressure is monitored by a pressure transmitter and the flow volume of the nitrogen gas by a flow meter continuously. If the value for the flow volume increases with higher consumption or the pressure drops below a defined nominal value, the partner system starts after a definable delay. The partner system stops when the nominal values are reached again. All faults are displayed on the TD 200 Micro Panel by an alarm lamp on the switch cabinet and a modem for remote monitoring. In the implementation, it was important to Elektro Krix that the redundant connection between the systems is independent of the MPI bus integrated in the controllers. The signals are therefore exchanged directly between the two Simatic S7-200 controllers so that even a total failure of a controller is detected by a falling signal and the partner system can connect automatically.
Remote monitoring available
The third Simatic S7-200 controller monitors data of the compressor and external components of the compressed air monitoring such as valves and condensation drain as well as the outside temperature as an analog value. The controller also takes over the data traffic between the two system controllers. Since the system is operated without operating personnel, Air Liquide performs the remote monitoring and operation with a modem and the monitor PLC. The data transfer takes place via the Modbus protocol. In addition to reporting faults, nominal value changes, start, stop or monitoring or changing process parameters are possible so that sending out technicians can usually be avoided.
GO! 1 / 2007 | |
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