A recent study showed that 44 percent of accidents were due to poor instrument specification, 6 percent of accidents were caused by errors during install and commissioning, and 15 percent occurred due to errors in design and implementation. This means that 65 percent of industrial accidents were ready to happen before operations even began.
“We often prefer to think that accidents are the cause of operator error, but the reality is they are often the result of an instrumentation issue,” said Mark Murphy, P.E., Technical Director of Electrical and Control Systems Engineering, Fluor, during a safety instrumented systems (SIS) session at Emerson Process Management's annual user's group meeting.
According to Murphy, there are three keys to instrument safety:
• Know that SIF (Safety Instrumented Functions) is important for every component in your system, so give equal amounts of attention to each device.
• Understand SIL (Safety Integrity Level) ratings. Part of this understanding means knowing that using a SIL 2 transmitter with a SIL 2 PLC and a SIL 2 valve does not equal a SIL 2 SIF.
• Focus on the lifecycle of the device, not just the implementation of it.
To help attendees better understand what differentiates SIF from SIS from SIL, Murphy said that SIF covers individual devices such as sensors, controllers, and final elements such as valves and actuators; SIS (Safety Instrumented Systems) is collection of SIFs; and SIL is a device’s specified integrity level related to SIF requirements.
Paying close attention to the safety aspects of instrumentation is critical because 92 percent of system failures can be traced to the final element (50 percent of failure causes) and sensors (42 percent of failure causes).
“Everyone wants to be involved in designing the logic solver component of a system, but this only accounts for 8 percent of failures,” Murphy added. “So [instrumentation] is where the efforts really need to be focused.”
Beyond sensors, field devices, and final element considerations, Murphy also pointed to two other system components that can have a direct impact on system operation and, therefore, system safety. These two components are solenoid valves and instrument air supplies.
Solenoid valves tend to be forgotten, but they are the cause of 50 percent of valve failures. Murphy cautions engineers and technicians to be aware of the build-up of particles in the solenoid valve. He adds that the air supply to the solenoid valves should be filtered, monitored, and protected from outside elements.
Instrument air supplies are an external piece of an SIS, and so they tend to be overlooked during plant retrofits when capacity is often increased. As a result, instrument air supply is often not increased along with the plant capacity, which can lead to pressure drops. "As part of your safety preparations, you need to consider what will happen if this air supply system fails,” Murphy said.
