Maloperations

Many processes require equipment designed to rigid specifications together with automatic control and safety devices. Consideration should be given to the control, and limitation of the effects, of equipment malfunction or maloperation including ... 

Tolerant of maloperation or poor Continuous plant Batch plant... 

Irregular Plant maloperation Process plant Dust and fume extraction plant... 

Irregular Plant maloperation (e.g. unauthorized venting ) Plant failure - spillages, pipe joint failures Start-up/shutdown Dismantling/demolition Unauthorized waste incineration (rubbish burning) Fires ... 

Additionally, consequences of possible process maloperations, such as incorrect charging sequence, contamination of reactants, agitation failure and poor temperature control, adding reactants too quickly. 

Number of days since last accidental release of hazardous material. This measure distinguishes between routine emissions (such as from storage tank vents, or low pressure steam discharges) and accidental emissions resulting from maloperation or breakdown. Events that might count would be safety valve releases, accidental releases into inappropriate drainage systems and unconfmed spills during maintenance. 

The Maloperation Data Base, covering events of less than accidents severity... 

These tests demonstrated that the Lurgi Rectisol process provides an extremely pure synthesis gas which can be charged directly to the metha-nation plant without problems of sulfur poisoning of the nickel catalyst. However, in order to cope with a sudden sulfur breakthrough from Rectisol as a result of maloperation, a commercial methanation plant should be operated with a ZnO emergency catchpot on line. 

Irregular Plant maloperation Process plant Dust and fume extraction plant Flare stack Emergency/occasional flaring Plant failure Process plant — emergency venting Extraction/collection plant (cyclones, precipitators, filters, scrubbers)... 

In a plant for the continuous nitration of chlorobenzene, maloperation during startup caused the addition of substantial amounts of reactants into the reactor before effective agitation and mixing had been established. The normal reaction temperature of 60°C was rapidly exceeded by at least 60° and an explosion occurred. Subsequent investigation showed that at 80° C an explosive atmosphere was formed above the reaction mixture, and that the adiabatic vapour-phase nitration would attain a temperature of 700° C and ignite the explosive atmosphere in the reactor. See l,3-Bis(trifluoromethyl)benzene, above... 

Equipment should tolerate maloperation, poor installation or maintenance without failure. E.g. expansion loops in pipework are more tolerant to poor installation than bellows. The construction materials should be resistant to corrosion and physical conditions. For most applications metal is safer than glass or plastic. 

From Table 6 it can be seen how the selected parameters have a connection to the basic principles of inherent safety. For instance the subindices of equipment safety and safe process structure contain several characteristics of inherent safety such as limitation of effects or tolerance to maloperation. It is practical to include several characteristics into few parameters, since the inherent safety principles are both very broad and overlapping. The philosophy behind them cannot be described just by one process parameter. The selected parameters are discussed in more detail on the following pages. 

Tolerance - resistant to maloperation corrosiveness equipment safety safe process structure... 

B. Due to maloperation of equipment in service e.g., spurious relief valve operation... 

Unlike relief system sizing for non-reacting systems, a considerable amount of experimental information is normally required for the design of chemical reactor relief systems. It is necessary to assess all the credible maloperations and system failures that may occur on the process/ plant to determine the reaction runaway that requires the largest relief system. The Workbook also summarises the main steps necessary to do this. 

The procedure to determine both the basis of safety for the reactor (see Annex 1) and the worst case scenario for that basis of safety is iterative. The same screening tests which help determine the worst case for pressure relief sizing may lead to the conclusion that pressure relief is not the best basis of safety. The results of screening tests may also indicate that it is worthwhile to seek a more inherently safe solution by designing out the possibility of certain maloperations or system failures (for example, if the screening indicates that a very large relief system would be required). 

Is there scope for obtaining a smaller relief system size by designing out maloperations or system failures by means of inherent safety or otherwise (see Annex 1)... 

The full range of process maloperations, including system failures that might lead to process runaway will first have to be considered by a systematic evaluation of the plant and process concerned141. These may, for examplel be due to human error, hardware failure, or due to failure of a computerised sequence controller. To assess the likely/ credible maloperations accurately, it is recommended that personnel who will be operating the plant are involved in the hazard assessment. 

A list of typical maloperations is given, in Figure 3.2. However, it should be noted that these will be specific to the process and plant concerned and this list should not be regarded as comprehensive. ... 

It may not be credible that multiple simultaneous failures and maloperations occur. In considering what is credible, the following points may be useful ... 

It will then be necessary to check whether or not the credible maloperations can lead to exothermic runaway. Information on this is given in reference 1. Where it is shown that runaway can occur, and it is decided that emergency pressure relief may be used as part of the basis of safety, then it will be necessary to carry out further work to identify the worst case for relief system sizing. ... 

The worst case will normally be the maloperation that results in the highest rate of temperature and/or pressure rise over the relief range (dependent on whether the system is tempered or untempered). However, this assumes that other parameters used in the relief sizing calculations remain constant, e.g. reactor fill ratio, mass of reacting species, physical properties. Where any of the other parameters used in the calculations are significantly affected by the maloperation under consideration, then this will also have to be taken into account in selecting the worst case. In such cases, it may be necessary to roughly calculate the relief size. 

At this stage, a number of credible maloperations will have been defined that can lead to vessel over-pressurisation. In order to cope with all the credible runaway scenarios, the relief system will need to be sized for the "worst case runaway" reaction that can occur, and this is normally the maloperation that will give rise to the highest rate of temperature and/or pressure rise over the relief range. 

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