Keywords

Modifications; existing plant; operating procedures; pilot plants; drains and vents; plant interconnections; commissioning/decommissioning; start-up/shutdown; construction/demolition; contract operations

11.1 Modification of Existing Operations

It is important to have in place a procedure for the management of change (MOC) to ensure that all modifications are reviewed before any variation in plant, process, or operation is made. The review should recommend an appropriate method of hazard identification. Where there are significant hazards, this may be a HAZOP study. The company MOC procedure should include criteria for deciding if a HAZOP study should be done.

A very wide view should be taken as to what is a “modification.” Anything which changes a plant or a process in any way must be treated as a modification. Such changes could be to materials, catalysts, solvents, conditions, sequences, quantities, procedures, software, and so on.

When a HAZOP study is used for a modification, the basic principles of HAZOP are retained and applied. For major modifications, the study follows the steps taken for a new design. For small modifications, it is possible to proceed more quickly, using a smaller team and combining some of the roles within the team—for example, a member may act as the scribe and it may even be acceptable for the leader to have another role. It is particularly important to have the operating team represented within the HAZOP study team.

If the system was previously studied by HAZOP, then the original report may provide a useful starting point. There can be problems with defining the boundaries for the study since it is unlikely that the whole operation will be reviewed. The boundaries may have to be some distance from the point of the modification to ensure that all relevant causes and consequences are considered. The boundaries should be agreed between the project team and the HAZOP leader with the leader given the authority to extend the boundaries, if felt necessary, during the study.

11.2 Repeat Designs—HAZOP-by-difference

In some branches of the process industry, it is commonplace to install designs which are essentially the same as an earlier installation or which are made up of standard units, varying only in size from other installations. In these cases, it may be possible to do an effective hazard identification study by detailed comparison to an earlier, full HAZOP study, concentrating on any differences from the previous case. When this method is used, the team must be particularly aware of any variations in size, site, services, and interfaces with other plant.

11.3 Periodic Hazard Studies and the HAZOP of an Existing Plant

Periodic hazard studies are process hazard analyses to ensure a process plant continues to operate and be monitored to appropriate SHE standards throughout its life. While MOC reviews provide a record of incremental changes over a period of time, it may become necessary to review a system as a whole, particularly when the multiple changes may interact adversely with each other.

Such a review is particularly important if any changes to operating procedures, feeds, or products and/or modifications have been made. The requirement for such periodic studies can be legal—for example, OSHA—or company policy as best practice. There are several techniques available for such studies, including the retrospective use of Hazard Studies 1 and 2. HAZOP study should be considered as a preferred approach if the following have occurred:

HAZOP is necessarily more time-consuming than most of the alternative techniques, but has the advantage of a comprehensive outcome. Other techniques² can be valuable in identifying key issues and the need (or otherwise) for more detailed studies such as HAZOP. The choice of method will depend among other factors on the available experience base, the sophistication of the process, and regulatory requirements.

Whichever technique is used, it is important that target dates are set for completion of actions and for review of progress and subsequent periodic studies. Proper action progressing and specific periods for subsequent study may be a legal requirement where such studies are mandated.

11.4 Operating Procedures

The HAZOP methodology for an operating procedure is essentially the same as for a batch process. Such a detailed study is normally only applied to critical procedures. It requires a well-defined procedure to be available, including all significant steps and actions, an up-to-date P&ID and, ideally, structural drawings to locate valve positions. For existing plant, a tour of the process area is recommended. In preplanning, it has to be decided whether the HAZOP of the procedure comes before or after the HAZOP of the process—the normal case would be for the process HAZOP study to be done first. Also, the study must not degenerate into a procedure-writing meeting. The team composition must be correctly balanced to get the best results and must include some members who are familiar with the process, including an operator. Before the start of the study, the procedure is reviewed for clarity and the aims of the study defined. Normally, these are to identify potential hazards, operability problems, and environmental problems which may result from deviations from the procedure, especially those due to human factors.

The actual analysis follows the batch HAZOP methodology, working through the procedure stage by stage. Each stage, which may consist of a number of individual actions, is examined using the guidewords to prompt the team members to suggest meaningful deviations which are then analyzed in the usual way. In addition to the standard guidewords, “out of sequence” and “missing” can be productive. “Missing” is interpreted to mean that a step is missing from the procedure at or just before the stage examined—although such deviations could equally well be found using the guideword “no.” In the list of parameters, the phrase “complete the step” can be used to good effect, as it combines meaningfully with the guidewords “no,” “more,” “less,” “reverse,” “part of,” “as well as,” “out of sequence,” and “missing.” An unusual but occasionally useful question is “verification of success”—how is it known that a valve is closed or that a vessel is depressurized. This last point is very important in pigging and filter operations.

A major difference from process studies is that many of the causes of deviations are related to human factors. These may be of omission or commission. The importance of Human Factors in HAZOP studies has been considered in Section 10.2. Other possible causes include poorly written procedures, difficulties caused by poor layout, bad lighting, parameter indicators with limited or poor ranges, or too many alarms. The latter, a cause of information overload, is a topic of concern and advice is available.⁹ In assessing safeguards, a reasonable allowance can be made for the presence of the operator if close involvement with the system allows for the possibility of immediate detection and correction of the deviation. Experience of the actual conditions and the style of operation is important when making such a judgment about human factors.

Actions may suggest a change in the procedure but need not be limited to this option; instrument or equipment modifications should be recommended if they offer the best solution to a problem.

11.5 Pilot Plant and Laboratory Operations

Pilot plants and laboratories typically differ from full-scale plants and processes by their smaller scale, diversity and greater degree of human interaction. Nevertheless, there are many exceptions. Some refinery or petrochemical pilot plants can dwarf full-scale fine chemical plants. Pilot plants built to test scale-up may be single continuous stream plants and highly automated. However, the hazard study approach for all pilot plants, semi-technical plants, and experimental plants follows the same pattern. A preliminary hazard analysis—for example, Hazard Studies 1 and 2 (see Chapter 2)—should be carried out, and if the potential for significant process hazards is identified then a HAZOP study can be recommended. The same HAZOP methodology for continuous processes or batch processes, as described earlier, is used but with greater emphasis placed on the process and design uncertainty and human factors.

The design intent and limitations of the pilot plant or laboratory should be clearly defined. Constraints need to be established on the experiments, materials, and process conditions allowed in the pilot plant or laboratory. A system needs to be established which registers and evaluates new experiments. If an experiment falls outside the established constraints, then the experiment should be subjected to a preliminary hazard analysis which decides whether a HAZOP study is required. The preliminary hazard analysis should also establish whether extra studies are required—for example, reaction stability,³⁴ reactor relief requirements, emergency measures, and occupational health. Changes to the building, laboratory, plant, process, or equipment are covered by a modification procedure, as discussed in Section 11.1, page 77.

It is recommended that the HAZOP study of large continuous pilot plants initially studies the process as a continuous one (see Section 5.3.1, page 37). During the HAZOP study, extra emphasis is required on the possible inadequacies of the design. For example, a heater control may not just fail but the heater may be grossly over- or under-sized as a result of uncertainties in the process stream properties. Since starting, stopping, and aborting experimental runs is a regular feature, the process steps involved in these operations should also be subjected to a batch HAZOP study (see Section 5.3.2, page 39).

For multifunctional experimental plants (typically batch or semi-batch processes), it is recommended that a typical process is selected and subjected to a HAZOP study. The other processes are then hazard-studied in as far as they deviate from the established process in conditions (e.g., temperature, pressure, and concentration), materials (e.g., additives and solvents), and process steps. For all types of unit, the operating bands must be clearly established at the start.

For typical research laboratories, support systems should be subjected to a HAZOP study if they are critical for the security, safety, health, or environment of the laboratory. For example, the ventilation and extraction systems for a laboratory designed to handle highly toxic materials or biologically active agents should be effective and reliable—a HAZOP study of these systems should identify the potential hazards. A HAZOP study of gas supplies identifies hazards caused by failure of the equipment or operation not covered by standard designs.

For pilot plant and laboratory HAZOP studies, an appropriate team is assembled. A good team might involve the laboratory manager, operating technician or laboratory analyst, equipment specialist, maintenance manager or technician, and HAZOP leader. If the purpose is scale-up, then the respective idea developer—for example, chemist, physicist, and engineer—and a technical representative of the subsequent development stage are useful additions.

Due to the greater degree of human interaction, a greater emphasis is placed on the skill, knowledge, training, and experience of the operating technician or laboratory analyst. Involving these in the HAZOP study ensures relevance, simplifies training, and aids motivation, as well as improving the documentation of procedures.

A HAZOP study of proprietary equipment—for example, analytical test equipment—is generally not required provided the equipment has been subjected to a risk analysis by the manufacture and is installed and used only as intended by the manufacturer.

While no laboratory experiment is too small to be hazard-studied, the benefits of HAZOP study are more likely to be achieved if there is a potential for fire, explosion, significant release of a hazardous material, or other major loss. A HAZOP carried out at this stage can help to ensure that hazards are addressed at an early stage in the development of the process.

11.6 Drains, Vents, and Other Interconnections Between Plants

The vent, relief, and drains systems often link many pieces of equipment, sometimes different plants, through a common piping network. The individual pieces of equipment may operate at significantly different pressures, some equipment may be starting up while others may be shut down, and some of the fluids may be mutually incompatible. The design of these systems is often complex to reduce release to the environment and may be spread over a number of process P&IDs or split between process P&IDs and a separate set of vent and drain P&IDs.

The HAZOP therefore requires special skills:

The interface management requires at a minimum that all interfaces (lines entering and leaving a P&ID) are positively identified on all drawings and that there are no mismatches or exclusions. If there is a set of vent and drain P&IDs, each interface on the vent or drain P&ID should be labeled with a fluid description. The following data may be recorded:

If the vent and drain are included on the process P&IDs and there are many drawings, consideration should be given to preparing a special interface drawing to link all process P&IDs onto one sheet. If this is not possible, the interfaces can be labeled as above and the vent or drain treated as a system. The main parameters and guidewords are:

However, there are likely to be secondary issues such as:

A study such as the relief and blowdown review examines the dynamics within a subsection and the total system, but this does not eliminate the need to examine the total system with a HAZOP study.

11.7 Commissioning and Decommissioning

Commissioning and decommissioning occurs only once on any process and the issues are often unique. The problems of commissioning are usually dealt with during the main HAZOP study, either by inclusion as a parameter under the guideword OTHER in simple cases or by full examination in more complex cases. They usually have to be treated as a sequential operation rather than as part of a continuous process. Decommissioning is seldom considered during early project stages; it can be a complex process which merits its own HAZOP study before it is undertaken.

The main features of commissioning are:

Where the process is very critical or involves complex or high-cost machinery such as major compressors, the HAZOP procedure can be used to follow the cleaning process, for example, to:

The purging routes can be managed in a similar manner but, obviously, the rule is to ensure purging proceeds in one direction only.

Test running equipment has the potential to operate outside the normal envelope. Fluid velocities may be higher (or lower) than normal, the test fluid may have a different density, viscosity, or temperature and, in the case of gases, a different ratio of specific heats. If water is used instead of a lower density fluid, the static and dynamic heads and power draws may be excessive and the static loads on piping may be higher. If air is used for test-running compressors, there may be seal problems, horsepower limits, and high discharge temperatures.

When HAZOP is used in connection with commissioning, it is necessary to select suitable parameters in order to develop meaningful deviations. Some examples are given below:

In the case of decommissioning—leading to demolition—the sequence with which the equipment is decontaminated is followed by the HAZOP procedure. Furthermore, the procedure can follow potential issues associated with:

Ultimately, as much of the equipment as possible should be recycled and the manner in which it is decommissioned may affect the demolition. There may be some form of process waste (even residual working inventories) which has to be processed further elsewhere. This requires an abnormal operation which should be studied in detail.

11.8 Start-Up and Shutdown

In general a process plant is designed for steady-state operation. The piping configurations and instruments are designed for that objective. But there are also two dynamic modes which also must be considered—start-up and shutdown—especially important since many recorded incidents have occurred during these phases.

The main purpose of a HAZOP Study is the analysis of possible deviations outwith the design envelope during steady-state operation of a plant or process and has not handled Start-Up and Shutdown adequately. The structure of a Start-Up and Shutdown will be in the form of a procedure or set of steps which start at the introduction of inventory and end with the specification product from that step. The Start-Up and Shutdown HAZOPs are therefore best studied as a Procedural HAZOP (Appendix 5).

During these operations, pressures and temperature, and hence compositions of the process fluids, may be well outside the steady-state range or even outside the specification of the materials of construction. Also, as the illustration in Appendix 5 shows, if a step is missed or is done out of sequence there is the potential for a deviation.

This section must be treated as an overview as each unit operation must be treated separately. It should be noted that control is designed for steady-state operation and not necessarily for the dynamics of Start-Up and Shutdown. Attention must be paid to the control of a potentially unsteady state as well as specific flows only experienced during these operations.

11.9 Construction and Demolition

Construction is not a steady-state condition, and equipment may be delivered out of sequence; furthermore, there may be the need to impose unusual loads—for example, in the hydraulic testing of a steam main or gas main. The issues are likely to be the following:

Where construction is undertaken close to an operating plant, the potential for interaction may need to be considered.

Demolition is not the reverse of construction and contains its own SHE risks. Safety issues involve access and overhead/underground operations. Health hazards include toxic/flammable contamination, possible fires, and asphyxiation; some of these hazards are produced in the demolition process—for example, in the hot cutting of materials. Environmental issues could involve the disposal of lagging, spent catalysts, residues found in the equipment, and possible spills on site. The restoration of the site for other uses is influenced by the mode of demolition if it does not capture all residues. The classification of materials for recycling and the verification of cleanliness are equally important.

Specialist study methods have been developed for construction and demolition activities using checklist and/or HAZOP study approaches.

11.10 Contract Operations

It is important to define within the contract all aspects of any planned HAZOP study, including the extent to which operability problems are sought, the responsibility for cost of change, and the control of team membership. The latter may be a problem if clients request several places in the HAZOP team in addition to the contractors and representatives of licensors. The team leader should still aim to keep the team number below 10 and should consider the responsibilities of each prospective team member to ensure their presence is needed for the sections being discussed.