Primatech

PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.

The improvement of the reliability of controls and safeguards in processes often is addressed by the use of redundancy. For example, multiple sensors may be used in a voting scheme for a controller, or two relief valves may be used on a pressure vessel. However, the reduction in the failure probability of redundant systems is dependent on the avoidance of common cause failure between the redundant components.

Common cause failure is a type of dependent failure where simultaneous (or near-simultaneous) multiple failures result from a single shared cause. The cause may either be internal or external to the system. It can result from the random failure of a single (non-redundant) component, e.g. multiple flare failures owing to failure of the flare header, or systematic failures in redundant components, e.g. two pressure detectors failing owing to incorrect calibration by a mechanic.

Sources of common cause failures include:

• Utilities, e.g. electrical power, instrument air, etc.
• People, e.g. designers, operators, mechanics, etc.
• Maintenance, e.g. procedures, calibration
• External factors, e.g. lightning, flooding
• Common locations
• Environmental factors, e.g. humidity, dust
• Control systems, e.g. DCS
• Similar technologies or the same type of redundant equipment
• Process corrosion, plugging or fouling, e.g. plugging of relief valves, or sensors in a shutdown system
• Single elements supporting multiple systems, e.g. common process taps, common conduit, single energy sources, single field devices, etc.
• Susceptibility to mis-operation, e.g. activity performed under abnormal stress
  
  

Possible defenses against common cause failure include various forms of diversity to guard against their sources such as:

• Functional diversity, e.g. fire mitigation by deluge system and firewater monitors.
• Using different equipment or people to achieve the same purpose, e.g. installing a relief valve and a burst disc on a pressure vessel
• Spatial separation of equipment, e.g. using physically separate taps on a vessel for instrumentation
• Staggered testing / maintenance, e.g. servicing dual relief valves separately

Usually, it is not possible to completely eliminate common cause failures owing to commonalities of equipment manufacturing processes, raw materials, equipment components, people, utilities, etc. that cannot be avoided. However, the potential for common cause failures can be minimized by careful consideration of their sources.