Hazards and Operability Studies

A hazard and operability study is a procedure for the systematic, critical, examination of the operability of a process. When applied to a process design or an operating plant, it indicates potential hazards that may arise from deviations from the intended design conditions. 

The technique was developed by the Petrochemicals Division of Imperial Chemical Industries, see Lawley (1974), and is now in general use in the chemical and process industries. 

The term operability study should more properly be used for this type of study, though it is usually referred to as a hazard and operability study, or HAZOP study. This can cause confusion with the term hazard analysis , which is a technique for the quantitative assessment of a hazard, after it has been identified by an operability study, or similar technique. Numerous books have been written illustrating the use of HAZOP. Those by Hyatt (2003), AIChemE (2000), Taylor (2000) and Kletz (1999a) give comprehensive descriptions of the technique, with examples. 

A brief outline of the technique is given in this section to illustrate its use in process design. It can be used to make a preliminary examination of the design at the flow-sheet stage and for a detailed study at a later stage, when a full process description, final flow-sheets, P and I diagrams, and equipment details are available. 

Corrosion-resistant materials of construction would be specified, but external corrosion is possible due to nitric oxide fumes allow minimum factor = 0.1. 

Welded joints would be used on ammonia service and mechanical seals on pumps. Use minimum factor as full equipment details are not known at the flowsheet stage, factor = 0.1. 

The index works out at 21 classified as Light. Ammonia would not normally be considered a dangerously flammable material the danger of an internal explosion in the reactor is the main process hazard. The toxicity of ammonia and the corrosiveness of nitric acid would also need to be considered in a full hazard evaluation. The process unit risk analysis would be completed when the site for the plant had been determined. 

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Here we shall restrict consideration to safety and health considerations that can be built in while the design is developing rather than the detailed hazard and operability studies that take place in the later stages of design. The three major hazards in process plants are fire, explosion, and toxic release. ... 

As the design progresses, it is necessary to carry out hazard and operability studies. These are generally only meaningful when the design has been progressed as far as the preparation of detailed flowsheets and are outside the scope of this text. 

Design procedures are developed with the intention of improving the safety of equipment. Tools used in this step are hazard and operability studies and quantitative risk analysis (ORA). The following scheme may be used ... 

Control of Plant and Process Modifications Many accidents have occurred because plant or process modifications had unforeseen and unsafe side effects (Sanders, Management of Change in Chemical Plants Learning from Ca.se Histories, Butterworth-Heinemann, 1993). No such modifications shoiild therefore be made until they have been authorized by a professionally quahfied person who has made a systematic attempt to identify and assess the consequences of the proposal, by hazard and operability study or a similar technique. When the modification is complete, the person who authorized it... 

HAZOP (Knowlton, 1989 Lees, 1980 CPQRA, 1989, pp. 419-422). HAZOP stands for hazard and operability studies. This is a set of formal hazard identification and elimination procedures designed to identify hazards to people, process plants, and the environment. The techniques aim to stimulate in a systematic way the imagination of designers and people who operate plants or eqmpment so they can identify potenti hazards. In effect, HAZOP studies make the assumption that a hazard or operating problem can arise when there is a deviation from the design or operating intention. Corrective actions can then be made before a real accident occurs. 

Hazard and Operability Study (HAZOP) A systematic qualitative technique to identify process hazards and potential operating problems using a series of guide words to study process deviations. 

R. E. Knowlton, Hazard and Operability Studies, The Guide Word Approach, Chemetics International Company, Vancouver, BC, 1981. 

A HAZOP (Hazard and Operability Study) involves a fonual review of proeess and instrumentation diagrams by a speeialist team using a stmetured teehnique, based upon key words. These eomprise property words and guide words , e.g. as in Table 12.4. 

Hazard and operability study prior to implementation (repeat when in use) Updating of all operating instructions, notices, procedures Removal/isolation of obsolete plant/lines Control of changes in personnel (see training)... 

Refresher course on fundamentals of safety, hazard recognition, procedures Participation in hazard and operability studies on existing operations and procedures... 

Many accidents occur because process materials flow in the wrong direction. Eor example, ethylene oxide and ammonia were reacted to make ethanolamine. Some ammonia flowed from the reactor in the opposite direction, along the ethylene oxide transfer line into the ethylene oxide tank, past several non-return valves and a positive displacement pump. It got past the pump through the relief valve, which discharged into the pump suction line. The ammonia reacted with 30m of ethylene oxide in the tank, which ruptured violently. The released ethylene oxide vapor exploded causing damage and destruction over a wide area [5]. A hazard and operability study might have disclosed the fact that reverse flow could occur. 

We put a lot of effort into improving safety by adding protective equipment onto our plants, new and old gas detectors, emergency isolation valves, interlocks, steam curtains, fire insulation, catchment pits for LPG storage tanks, and so on. We also introduced new procedures, such as hazard and operability studies and modification control, or persuaded people to follow old ones, such as permits-to-work and audits. 

This incident shows the importance, during hazard and operability studies (see Chapter 18), of considering abnormal conditions, such as failure of utilities, as well as normal operation. 

Should the design team have foreseen that the condensate might freeze A hazard and operability study (see Chapter 18) would probably have drawn attention to the hazard. 

The managers and engineers who authorize modifications cannot be expected to stare at a drawing and hope that the consequences will show up. They must be provided with an aid, such as a list of questions to be answ ered. Such an aid is shown in References 1 and 2. Large or complex modifications should be subjected to a hazard and operability study (see Chapter 18). 

This chapter is not concerned with accidents on the road. Rather, it describes some of the many incidents that have occurred while tank trucks and cars (known in Europe as road and rail tankers) were being filled or emptied. Section 18.8 shows how hazard and operability studies have been used to spot potential hazards in filling systems, and Section 22.3 describes some runaway reactions in tank trucks and cars.. 

A hazard and operability study w as carried out during design, but flow of steam from the treatment vessel to the reactor was never considered as a possible deviation, perhaps because the team... 

In this case the design was not at fault. The operators did not understand the design philosophy. Would this have been foreseen in a hazard and operability study (Section 18.7), and would special attention have been paid to the point in operator training ... 

The incidents listed earlier in this chapter and many others could have been foreseen if the design had been subjected to a hazard and operability study (hazop). This technique allows people to let their imaginations go free and think of all possible ways in which hazards or operating problems might arise. But to reduce the chance that something is missed. 

Hazard and Operability Studies, Chemical Industries Association, London,1977. 

The MIC storage tank was contaminated by substantial quantities of water and chloroform—up to a ton of water an VA tons of chloroform— and this led to a complex series of runaway reaction [4]. The precise route by which water entered the tank is unknown several theories have been put forward, and sabotage seems the most likely [2], though whoever deliberately added the water may not have realized how serious the consequences would be. Hazard and operability studies (Section 18.7) are a powerful tool for identifying ways in which contamination and other... 

Well-conducted hazard and operability studies (Chapter 18) could have prevented about half the incidents (but only 40% in the 1980s and 1990s). The incidents they could not have prevented include, for example, mechanical failures and installation of the wTong material of construction. 

The incidents described could occur in many different types of plants and are therefore of widespread interest. Some of them illustrate the hazards involved in activities such as preparing equipment for maintenance and modifying plants. Others illustrate the hazards associated with widely used equipment, such as storage tanks and hoses, and with that universal component of all plants and processes people. Other incidents illustrate the need for techniques, such as hazard and operability studies, and protective devices, such as emergency isolation valves. 

An opportimity for error recovery would have been to implement a checking stage by a supervisor or independent worker, since this was a critical maintenance operation. However, this had not been done. Another aspect of the unforgiving environment was the vulnerability of the system to a single human error. The fact that the critical water jacket flow was dependent upon a single pump was a poor design that would have been detected if a hazard identification technique such as a hazard and operability study (HAZOP) had been used to assess the design. 

All of these factors determine the stress experienced by the workers and the extent to which operational errors will be recovered before disastrous consequences have ensued. In this context, hazard identification techniques, such as hazard and operability studies (HAZOP), failure modes and effects and criticality analysis (FMECA), fault trees, and others are useful in making the process environment more forgiving. 

A hazard and operability study (HAZOP) was used to identify potential hazards, the most serious of which is aii unrecoverable release from the storage tank. 

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