Functional Safety 2014

Functional Safety 2014

– 5

November 2014

Page 13 of 14

Typical examples of some of the human errors during installation and testing are:

Mis-calibration of the instrument such as a level/pressure transmitter;

ii.

Forgetting to re-open and lock the block valves under a relief valve after maintenance and

before the relief valve is returned to normal service;

iii.

Leaving the transmitter/sensor root valve closed causing an unsafe failure;

iv.

Leaving the entire safety instrumented function in by-pass mode after maintenance or after

some other human intervention (such as an unintended error or as a necessity during start-

up)

Statistics from the Process Improvement Institute shows that Probability of Human Error (PHE) varies

from 0.1 to 0.001 depending on the industry and the control of human factors. For example:

PHE = 0.01 to 0.04 for a relief valve being returned to normal operation after maintenance

due to leaving the block valves in the by-pass position;

ii.

PHE = 0.2 while working 30 days of 12 hour shifts during a refinery shut down (In some

countries, there are now restrictions for maximum of 12 days shifts)

Below are some of the recommendations for minimising systematic human failures:

For installation and testing:

Include a test override switch as part of the safety instrumented function;

ii.

Include position switch/indicators on the by-pass block valves;

iii.

Consider the use of an alarm to indicate an active by-pass;

iv.

Apply the two man rule for routine tasks and safety critical activities;

Enforce the use of Installation or Testing procedures;

vi.

Where modification takes place, the system should be re-tested to ensure consistency with

the requirements;

For Testing Procedures:

Be accurate and complete;

ii.

Be clear and concise with an appropriate level of detail (Too detailed and it may be hard to

follow; too little information and it may be difficult to carry out the task correctly);

iii.

Identify any hazards;

iv.

State necessary precautions for hazards;

Reflect how tasks are actually carried out;

vi.

Ensure Procedures are accessible;

vii.

Use consistent terminology;

viii.

Use an appropriate format;

ix.

Use familiar language;

Promote ownership by users;

xi.

Be current and up to date;

xii.

Be supported by training;

Conclusion

There are various reasons for systematic failures; some are inevitable but some may be avoidable.

Outcomes from various studies concluded that a large percentage of accidents were contributed to