Computer control hazop

The team should have a wide range of knowledge and experience. If a contractor designs the plant, then the HAZOP team should include people from both tlie contractor and client organizations. On a computer-controlled batch plant, the software engineer should be a member of the HAZOP team, which should include at least one other person who understands the computer logic. 

A fundamental objective of a computer system applied to automate a pharmaceutical GMP operation is to ensure the quality attributes of the drug product are upheld throughout the manufacturing process. It is therefore important that quality-critical parameters are determined and approved early in the validation life cycle. The exercise should be undertaken to a written procedure with base information from the master product/production record file examined and quality-critical parameter values and limits documented and approved for the process and its operation. In addition, the process and instrument diagrams (P IDs) should be reviewed to confirm the measurement and control components that have a direct impact on the quality-critical parameters and data. This exercise should be carried out by an assessment team made up of user representatives with detailed knowledge of both the computer system application and process, and with responsibility for product quality, system operational use, maintenance, and project implementation. This exercise may be conducted as part of an initial hazard and operability study (HAZOP) and needs to confirm the quality-related critical parameters for use in (or referenced by) the computer control system URS. 

Chung, P. and Broomfield, E. (1995), Hazard and Operability (HAZOP) Studies Applied to Computer-Controlled Process Plants, in Computer Control and Human Error (Ed. T. Kletz), Institution of... 

Andow, P. (1991), Guidance on HAZOP Procedures for Computer Controlled Plants, Her Majesty s Stationery Office, London. 

Andow P. Guidance on HAZOP procesdures for computer-controlled plants. Contract research report No. 26/1991. Health and safety executive. KBC Process Technology Ltd. 1991. 

There has been much discussion on the apphcation of HAZOPs to computer-controlled systems (computer HAZOPs or CHAZOPs) 43 particular, defence standard DEF STAN 00-56 calls for HAZOPs in relation to both systems and sub-systems, but the only guide it was able to refer to originally was that for chemical process plant. This omission has now been remedied by the formulation and pubhcation of interim defence standard DEF STAN 00-58 Hazop studies on systems containing programmable electronics. ... 

See, e.g. P.Andow, Guidance on HAZOP procedures for computer-controlled plant, HSE Contract Research Report No 26/1991 andD.J.Bums R.M.Pitblado, A modified HAZOP methodology for safety-critical system assessment mDirections in safety-critical systems,... 

With plant that is controlled by computer, the HAZOP study needs to include consideration of the effects of aberrant computer behaviour and the team carrying out the study may need to be reinforced by the software designer plus an independent software engineer able to question the philosophy of the installed software program. A technique known as CHA-ZOP has been developed for such plant which also highlights the safety critical control items. 

For computer-controlled plant, it is also recommended that at least one Chazop or computer Hazop is carried out. This addresses the hazard potential of the software (and computer hardware) itself. As with the Hazop, the Chazop may also need to be revisited as a result of changes required during commissioning. 

The conventional HAZOP documentation is supplemented to enable a review of computer control/PLC systems. The logic systems need to be as completely described as the PFD and P ED do for process systems and hardware. Several types of documentation are available ... 

However, a reasonable interpretation can be found only for part of the combinations the rest is meaningless and must be dropped. A short description of the HAZOP study can be found in [6] and [8] there are also specific standards available like [9] describing the procedure in detail. It has to be mentioned that HAZOP studies are today used for a wide variety of systems including electrical, electronic and computer controlled systems. 

The hazard study must look for potential hazards in the EUC operating with its basic process control system (BPCS). Generally, control system failures are seen as human operator errors, loop control faults or sequencing/interlock failures. However, it is also necessary for the hazard study to consider possible failure modes of any computer/PLC systems used in the BPCS or in supervisory control. This type of study falls within the scope of what are known as computer hazops or chazops . Section 3.13.3 contains some introductory notes on chazops. 

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