The use of advanced electronic control system in process plants is growing because it increases productivity. But instruments and devices used in an electronic control system are vulnerable to corrosion. If the system is not properly protected, it may lead to breakdown and distortion of control information. An electronic system should therefore be designed to be inherently robust to perform well in a corrosion-prone environment.
A corrosive environment may be the result of gaseous contaminants like ammonia, mercaptan, hydrogen sulphide, hydrochloric and sulphuric acids, moisture, and high relative humidity. Tobacco, smoke and smog also cause corrosion. Since electronic control systems have to perform in highly corrosive environments of industry, they must be well-shielded to protect them from damage through dust, moisture, and gases. However, it is practically impossible to seal a system in an airtight enclosure. A detachable cover or an access door is required for time-to-time servicing, repairs, testing, and system reconfiguration. This is especially true for local control panels of rotating equipment.
Even if a system is housed in a sealed cabinet or an enclosure, the environment within becomes corrosive the moment the system is opened. Corrosion can be minimised by maintaining the proper temperature of the room where the electronic control system is installed. This can be done by using air-conditioners and keeping the room as airtight as possible.
Control systems comprise a number of high-density components’ cards that generate heat which is dissipated through in-built cooling fans. Since these fans continuously throw out hot air, the proper functioning of the air-conditioner must be ensured at all times to avert the malfunctioning of the cards and hence the system.
Connectors, circuit boards, and wire wrap connections are most susceptible to degradation by environmental conditions. Separable contacts, such as two-piece card connectors, switches, and sockets for ics, can cause failure of the process control system. Common contaminants like active sulphides and reactive oxides on combining with oxidants such as nitrogen oxide react actively with base metal which can lead to high contact resistance, giving rise to intermittent process failures. The use of quality connectors helps overcome this problem.
It must be ensured that printed circuit boards used in the system are of high quality and the copper on the pcb remains insulated by a thin protective layer. This is important as the pcbs are exposed to etchants, flux, and residues of process chemicals. For system durability, protective coating of exposed pcb surface is essential. In pure sulphide environments, exposed copper may get etched. A properly selected and applied coating on a pcb provides a banter to both dust and corrosive gases. The longevity of an electronic module in a corrosive environment depends upon the quality of each component. Most resistors and capacitors used today have a polymer coated outer jacket which protects them from damage by environmental conditions. However, all polymeric materials are permeable. If moisture or contaminant penetrates this protective jacket, the component will fail over a period of time. To overcome this, hermetically-sealed components are used in military-grade applications.
All electronic designs should minimise the use of potentiometers. It is believed that pots cannot survive in harsh conditions. However, certain specially-manufactured pots do withstand a wide temperature range and humidity. Thus the selection of components of proper grade minimises the failure rate of control cards.
Digital electronic control systems do provide long-term reliable service in harsh process plant applications. But the capacity to withstand environmental conditions must be an inherent quality of the components, the interconnection sockets, and the cabinet itself. Their protection holds the key to successful performance of electronic systems for long. Process plants require protection against events that might cause damage to equipment, personnel, and production. This can be achieved through the following strategies:
Wherever possible, control components should be insulated from sources of contamination and corrosion at the time of original plant design.
Specifying corrosion-resistant materials that are effective in a particular environment, especially in the case of circuits that cannot be isolated from corrosive atmosphere.
Tuesday, June 12, 2007
Electronic Systems in Corrosive Environments
Electronic systems installed in corrosive industrial environments face premature destruction if protective features are not in-built in them. For critical systems to withstand and perform flawlessly in such environments, it is important to shield them effectively to minimise the damaging impact of corrosive elements
