Safety Lifecycle

Lets take a look at each stage of the safety lifecycle.


Risk Analysis


Once a concept has been developed and documented, the equipment under control (EUC) has been defined a hazard analysis needs to be done to determine the risk level of every safety function. Then, determine the safety integrity level (SIL) for each function.


The standard provides quantitative and qualitative methods to determine risk levels. Once risk level is determined they are compared to the tolerable risk criteria that have been decided by person responsible. If the risks calculated are higher than the criteria measures have to be taken to lower the risk factor. The measures taken can either be process changes or safety procedures.


The standard allows for other means of risk reduction-other technology safety-related systems and external risk reduction facilities. But, does not consider them within the scope.


Performance Criteria


Here it is important to figure out the type of safety system required. The standards provide performance criteria in the form of safety integrity levels (SIL). These values are derived from the probability of failure on demand (PFD). The safety system must satisfy the integrity requirements. A performance verification is needed in order to know if the system meets the requirements.


Performance Verification


Manufacturers need third party verification. There are third party organizations like TUV that provide certification on safety systems. End users and engineering companies will generally not accept a system unless it has been third party certified.


Planning


While evaluating the risk factor, a major planning effort is required for:

• Installation & commissioning
• Validation
• Operation and maintenance

This is when the importance of documentation really stand out. If the planning is not properly documented potential hazards could be implemented into the safety system. If the detailed specifications, procedures and tests are not performed by competent people there is very high probability that system failure will be prominent during the system life.

Installation & Commissioning/Verification Testing

Both should be carried out in strict accordance with the plans. Documented evidence that all of safety related sytems, other technology and external risk reduction facilities are ready for installation.

Operation & Maintenance

Should be carried out in strict accordance with the plan. Need records of all modification made that have an impact on the hazards recorded in the assessment. All modifications must still meet functional safety requirements. The modifications will need justification documentation an impact analysis study and full reports covering everything.

Decommissioning

During decommissioning all the functional safety requirements must still be meet. An impact analysis must be carried out and include adjacent EUC's and the impact on there study.

IEC 61508 Overall Lifecycle

Introduction/Contents of Standrds

Introduction

New safety standards are constantly emerging and most standards implement specific instrumentation. This is generally a costly modification to a plant, so companies may try and avoid installing them. In order to convince companies to implement the the new standards, they need to know that installing the new standard would bring economic benefits to them.


Economic Justification

Economic benefits rather than a safety benefits will get company's to accept the new standards. If implementing the new standard will make there plant run and shut down more efficiently, installing the new standard will pay for itself. So, not installing the the new standard will be more costly than installing it.

All control systems fail. These fallers can either be safe or dangerous. Safe failures initiate a process shutdown without a process demand and dangerous failures initiate a shutdown (SD) with a process demand. A safe SD affects product availability and a dangerous SD affects process safety. Purpose designed safety systems can dramatically reduce system fallers, which in turn increases the safety and production. What better economic justification.

When you look at the benefits of standard implementation, it shows that safety pays you.

Lifecycle Concept

The lifecycle of the plant is considered when justifying the economic benefits of standard implementation. The standard support this through the lifecycle concept for safety systems. The lifecycle concept is just the basis of the standards, they contain more than that.

Contents of Standards

Ther are 2 standards that deal with Critical Control Systems (Safety Instrumented Systems, SIS).

IEC 61508 published in 1999

• Generic standard. Specific industry standards will follow. The standard specifically for process industries IEC 61511 will be based on ISA S84.01
  

ISA S84.01 published in 1996

• Specific to the process industries.
  

The IEC standard will be an international standard. We will be most concerned with this standard. This standard addresses a large number of issues which fall into six categories.

Lifecycle: This standard covers the SIS from concept to the time the system is taken out of service.

• Concept and planning
• Risk/Hazard Analysis
• Risk criteria
• Risk allocation
• Implementation
• Installation and commissioning
• Operation and maintenance
• Modifications
• Decommissioning

Documentation: Documentation must be present for every phase in the life of the SIS. The types of documents required are Policies, Procedures, Methods, Justifications and Verifications. Each of them must include the following.

• Objectives
• Requirements
• Scope
• Inputs
• Deliverables
  

Risk Assessment: The qualitative and quantitative methodologies to determine the risk levels in the process.

• Inherent risk
• Acceptable risk
• risk reduction
• Quantitative and qualitative risk assessment
  

Performance Criteria: The standards provide the criteria in the form of Safety Integrity Levels (SIL).

• Provide in SIL form
• SIL 1-3 apply to process industries
• SIL is defined by the Probability of Failure on Demand (PFD)
• Risk Production Factor (RPF) is the inverse of (PFD)

Management of Change (MoC): This means that someone has to be assigned responsibility for each phase of the safety lifecycle. Documentation and revision tracking procedures have to be put in place.

• Every phase has someone assigned to take responsibility
• Every phase must be documented and planned
• Adequate documentation procedures must be in place
• Adequate revision tracking procedures must be in place

Competence: The standards insist that responsibility for safety lies with all levels of people involved with the system. The management 's role is to determine the risk level acceptable and the documentation for every stage of the decision making through the life of the system.

• Someone will be held responsible
• Adequate qualifications
• Adequate experience

Reasons for Control System Failure: A survey was conducted by the Health and Safety Executive (HSE) in the UK. They found, as shown in the diagram below, the causes for control system failure. Notice that Specification, Design & Implementation and Changes after commissioning account for almost 80% of the failures. This is why competency i a major issue in the standard.