North American companies are beginning to move toward intrinsically-safe systems, already well established in Europe, that use currents and voltages too low to spark an explosion in a hazardous environment—and away from costly explosion-resistant wiring or construction. Their experiences suggest a number of tips and pitfalls to avoid when considering whether to implement intrinsic safety in an installation.
1. Evaluate by zone. Removal of one of the three sides of the fire triangle is always a good idea, but not necessarily cost-effective. Most process plants are not Zone 0 rated, which requires intrinsically safe-rated or inherently safe instruments such as pneumatic or wireless. Even wireless instruments, if they are not rated intrinsically safe, may not be suitable for maintenance in a Zone 0 plant area. For most chemical and oil and gas plants, operational areas are now classified Zone 1 and non-incendive instruments can be used. There is little to no difference in the cost of explosion-proof non-incendive and intrinsically safe instruments, but non-incendive devices do not require intrinsic safety barriers. Someday, perhaps, wireless will solve this dilemma.
2. Weigh safety alternatives. Both intrinsically-safe and explosion-proof products are going to cost a premium. However, explosion-proof tends to be so heavy-duty and huge that it causes space issues and is more likely to injure the personnel who have to install it. There is also a real possibility that the mount for an explosion-proof device will be homemade and not engineered, which could lead to other injuries. As long as the equipment is durable enough to withstand the application, intrinsically safe is recommended over explosion proof.
3. Do it right. Using the right barriers and the proper certified devices at both the field and the systems end should solve the problem. The standards clarify the requirements and it is absolutely essential that the standards be followed with zero exceptions. It also makes sense to seal conduits to eliminate the migration of hazardous gases. With all systems, proper grounding is very important. No compromise should be tolerated when it comes to safety.
4. Test everything. The most important thing when implementing an intrinsic safety system is to test it. Make sure that what has been done is correct so you can sleep well at night. Test the design and the system, make it pass all the assessments and put it as a requirement for the project. It will be first page news if something goes wrong, so make sure to supervise the installation very closely, and be sure that the design is not changed at the execution.
5. Limit risk. Don't operate equipment in hazardous areas needing intrinsic safety systems. As for operator safety, keep control signal voltages below 24Vdc. No human-operated selector or push button should have voltages over 24Vdc. It simplifies servicing and monitoring.
6. Protect controllers. If you are installing intrinsically safe systems, it’s very important to protect all automation controllers and module cards. Intrinsic barriers for fieldbus, Modbus and conventional I/O card loops is a best practice.
7. Troubleshooting issues. Intrinsic safety brings along some troubleshooting issues. Any time you decrease your current you allow the possibility that smaller voltage drops will give you bigger issues. Inputs and outputs are easily affected by loose connections on intrinsic safety circuits. Make sure all connections are tight and practice good wiring practices to minimize this issue.
8. Less maintenance. An intrinsically safe solution is recommended over explosionproof technology if only a limited number of instrument loops are involved. After implementation, this technology does not require any special maintenance attention compared to the traditional North American approach. There are even DCS I/O modules that are IS-certified and do not require a separate IS barrier. By refurbishing with low power equipment, you can reduce the surveillance rounds and maintenance checks.
9. Voltage drops. Do not neglect the voltage drop due to resistance of field wiring when designing 4-20 mA loops. During commissioning, it is common to find loops that functioned properly at low currents but stopped operating entirely as the current approached 20 mA. This is because enough voltage dropped across the field wiring to reduce voltage across transmitters to less than the compliant voltages they needed to operate.
10. Hardware intensive. Security people may think all the areas need to be explosion proof. It really depends on the experience with your equipment or plant. Look at incident statistics and the problems you see to determine where you need intrinsic safety and where not. Implementation of intrinsic safety is often very hardware intensive, so many plants try to avoid it. Many PLCs and hardware out there are Class-1 Division-2 compliant. A separate IS implementation may not be essential.
11. Alternative approaches. Early in a safety project, develop a comprehensive P&ID followed with a HAZOP review to assess risk and identify safety issues. This will allow you to investigate alternative ways to minimize risk, such as process modifications or changes in the type of process equipment. The intent is to minimize the need to implement an SIS system or to minimize SIS loops. Focus safety efforts on protecting the areas of the process most critical to the continuity of production.
12. Preliminary testing. Prior to designing a safety system, it’s essential to prepare the safety system using a preliminary testing procedure that places the process equipment out of service. Whether a safety system is operating properly needs to be verified before starting up process equipment. It should also be tested independently, without using engineering station simulation tools. This is especially important for boiler BMS. Signals from safety instrumentation may be used for process control tasks, but signals from process control loops can never be used for safety tasks. Root valves for control and safety transmitter impulse lines need to be separate.
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