Maintenance: Automation Systems: Surviving in the Mill Environment
April 1, 2002 By Pulp & Paper Canada
Automation Systems encompass a wide variety of equipment such as PLCs, DCSs, PC-based controls, dedicated OEM systems, as well as field sensors, actuators and control and power cabling.Typical automat…
Automation Systems encompass a wide variety of equipment such as PLCs, DCSs, PC-based controls, dedicated OEM systems, as well as field sensors, actuators and control and power cabling.
Typical automation systems are implemented with a distributed control strategy; operating stations may be in control rooms or on the factory floor. Processors and I/O racks are in electrical rooms, dedicated DCS rooms or out on the floor. Final control elements (sensors and actuators) are distributed throughout the process.
Each of these areas can present a different environment to automation equipment. This environment can be divided into two categories: the physical environment and the electrical environment.
The physical environment includes factors such as temperature, dust, corrosive gases, vibrations, liquid splashes or infiltration and impacts.
The electrical environment can stress equipment due to transient voltage surges, voltage rises and sags, harmonic wave form distortion, electro-magnetic interference (EMI) and radio frequency interference (RFI)
Process control and automation equipment typically used in pulp and paper mills is very robust and will take a lot of abuse. However, installing equipment in a harsh environment without sufficient protection can lead to:
1. Reduced life expectancy.
2. Increased maintenance and repair activities.
3. Reduced quality of maintenance activities.
Survivability of automation equipment is largely determined by the environment in which it is installed.
An analysis of all environmental conditions for a given application, as well as incorporation of the appropriate protective measures in the installation and in the selection of the equipment location will go a long way to enhancing reliability and survivability.
Once the environmental conditions as well as the design and installation details are known, it is a simple but important task to determine what preventive maintenance will be appropriate for the equipment.
Automation equipment which is not industrial application hardened demands special care in installation and maintenance planning.
Conditions affecting automation systems include:
Operator stations have a tendency to collect dust and debris and should be vacuumed on a regular basis with a static-free vacuum cleaner.
Care must be taken to secure power cords and data cabling that may be dislodged by personnel.
Segregating power supply to control equipment on separate, regulated power circuits is recommended. You don’t want your DCS processors on the same power source as the microwave in the lunch room.
Electrical Rooms and DCS Rooms:
Thermometers or temperature switches on processor and I/O cabinets can provide early warning of overheating damage.
Dust accumulation can be a problem and requires regular inspection, cleaning and replacement of air filters.
Single-point grounding methods should be applied for computer-based systems. Care must be taken to ensure that the original grounding scheme is respected when additions or modifications are made.
NOTE: The automation equipment manufacturer’s recommended grounding schemes do not necessarily meet Code requirements and should be examined.
Interconnection of control systems such as PLC and DCS equipment through data communication links can be a problem if the two systems are powered from different sources. Common mode voltage levels can be high enough to garble data and cause erroneous operations. The same phenomenon can exist when connecting systems which are in separate buildings because of slightly different ground levels. Optical fibre links or optical couplers will solve this problem.
Computer processor-based automation equipment often contains switch-mode power supplies. These devices may fail due to overheating in a high harmonics environment. DCS, PLC and computer-based controllers are frequently powered from UPS systems or static power conditioners which effectively isolate the equipment from most electrical power problems including harmonics.
PLC racks are often powered from ferro-resonant transformer type voltage regulators. Typically these devices also eliminate harmonic distortion and can be purchased with transient voltage surge suppressors (TVSS).
A correctly-sized TVSS represents a good insurance policy against damage to electronic equipment which is not powered from a UPS or power conditioner. Furthermore, all data cables (except fibre optic) running to control equipment outdoors should be protected by TVSSs.
Care must be taken to avoid running power cables and data cables in close proximity. This often occurs when additions or modifications are made to existing systems. Of course, fibre optic data cables are immune to EMI problems.
More and more fibre optic cables and ethernet devices are making their way into process control systems. OEM system suppliers seem to be moving quickly in this direction. It is essential to recognize that the equipment and installations being provided do not necessarily meet industrial duty requirements. Commercial hubs piled on desks with 9-Volt power adaptors are a recipe for loss of control. Industrial-hardened rack-mount ethernet equipment is available and will increase reliability.
Plants producing chemical pulps can generate gas emissions which are highly corrosive to electronic equipment. Depending on wind direction, these gases can be sucked into any electrical room in the plant and will eventually cause corrosion and automation equipment failures. The time frame is dependant on the gas concentration. The appropriate air scrubbers must be provided to treat electrical room air ventilation.
Major physical problems encountered on the factory floor are temperature, vibrations, contamination and impact damage.
Operating stations, processors, etc., on the floor require temperature controlled cabinets which require regular inspection and maintenance to insure reliable operations.
Factory floor cabinets require the same attention to power quality and grounding as mentioned above. Grounding cables should be inspected regularly as over time they tend to get damaged, severed or misappropriated.
Intelligent field instruments are becoming the norm. Many contain microprocessors and are connected on communication networks instead of the classic 4-20 ma current loop. These devices (although hardened) can be more sensitive to temperature, contamination, vibration and electrical power quality than their ancestors. Survivability will depend on adequate consideration of the physical and electrical environmental conditions when specifying the installation standards.
Three instruments that every mill should have:
1. Power transient recorder.
2. Harmonic distortion metre.
3. Ethernet cable and connector tester.
Roger McCuaig has an Electrical Engineering degree and a Masters degree in Project Management. He has over 25 years experience in the pulp and paper industry as Project Engineer, E & I Superintendent and as a Consultant. He is presently working as a Senior Project Manager for Wood, Banani and Associates, Electrical Engineering Consultants with offices in Ottawa, ON, and Gatineau, QC.
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