Equipment failures

These causes are treated in tlie sections tliat follow. 

The equipment of a processing system is designed according to process conditions witli a view to containing tlie chemical(s) and maintaining the control parameters required to produce tlie desired product. Equipment failure can generally be attributed to one or more of tlie following hazards  

Most equipment failures occur under abnonnal conditions, especially elevated pressures and temperatures. The design of equipment presents internal and external constraints. External limits may arise from physical laws, while internal limits may depend on tlie process and materials. In any case, if these limits are exceeded, tlie chance of an accident is greatly increased. 

It is impossible to quantify the possible hazards associated with each piece of equipment. To supplement tlie brief introduction to process equipment presented in Cliapter 5, possible failures and hazards associated with typical equipment are presented below. 

Pressure vessels and tanks may be used for reactions, storage of raw materials, conlaimnenl of chemical inlennediales or products, and pliase separations. Tlie 

Fatal accidents happen every year from sudden asphyxia. If a major spillage occurs, evacuate the area immediately, and keep away from the cloud of cold vapour. 

Certain materials are not suitable for use in cryogenic systems because they suffer a ductile to brittle transition as flie temperature is reduced below ambient, which can lead to brittle failure. Such materials include, for example, carbon steels and plastics and they must not be used. 

Generally, metal fatigue limits are higher at low temperatures so that fatigue failure is less likely. However, brittle failure of pipes and tanks can lead to spillages and serious consequences. 

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Crude oil and gas from offshore platforms are evacuated by pipeline or alternatively, in the case of oil, by tanker. Pipeline transport is the most common means of evacuating hydrocarbons, particularly where large volumes are concerned. Although a pipeline may seem a fairly basic piece of equipment, failure to design a line for the appropriate capacity, or to withstand operating conditions over the field life time, can prove very costly in terms of deferred oil production. 

These systems have been operated in extremely low quality (and radioactivity contaminated) industrial environments for the past several years without any major equipment or component failures. Utilizing specialized operating/warm-up procedures, they have operated in low grade, out-of-doors, dust ridden, rain-soaked, industrial environments at temperature ranges which greatly exceed the original equipment manufacturers (OEM) specified limits. The systems have been successfully operated at ambient temperatures of minus 10 to plus 103 degrees Fahrenheit without any pre-mature or un-anticipated equipment failures. 

There are other distributions available to represent equipment failures (10), but these require more detailed information on the device and a more detailed analysis. Eor most situations the exponential distribution suffices. 

A PPM program is needed to avoid equipment failures, utiHty outages, and production intermptions. From a cost savings angle it is extremely important to do preventive maintenance in order to avoid breakdowns. Periodic inspections and a good lubrication program uncover conditions that could lead to breakdowns. When problems are found eady, they can be taken care of without work intermption and costly repairs. Sometimes faciHty managers are so afraid of downtime that preventive maintenance is done too often. In other cases production does not allow adequate time to provide proper maintenance. 

The formation of anodic and cathodic sites, necessary to produce corrosion, can occur for any of a number of reasons impurities in the metal, localized stresses, metal grain size or composition differences, discontinuities on the surface, and differences in the local environment (eg, temperature, oxygen, or salt concentration). When these local differences are not large and the anodic and cathodic sites can shift from place to place on the metal surface, corrosion is uniform. With uniform corrosion, fouling is usually a more serious problem than equipment failure. 

Fig. 9. Pitting corrosion is damaging because it can lead rapidly to equipment failure.

Eault tree analysis (ETA) is a widely used computer-aided tool for plant and process safety analysis (69). One of the primary strengths of the method is the systematic, logical development of the many contributing factors that might result ia an accident. This type of analysis requires that the analyst have a complete understanding of the system and plant operations and the various equipment failure modes. 

ETA breaks down an accident iato its contributing equipment failures and human errors (70). The method therefore is a reverse-thinking technique, ie, the analyst begias with an accident or undesirable event that is to be avoided and identifies the immediate cause of that event. Each of the immediate causes is examined ia turn until the analyst has identified the basic causes of each event. The fault tree is a diagram that displays the logical iaterrelationships between these basic causes and the accident. 

The result of the ETA is a Hst of combiaations of equipment and human failures that ate sufficient to result ia the accident (71). These combiaations of failures are known as minimal cut sets. Each minimal cut set is the smallest set of equipment and human failures that are sufficient to cause the accident if all the failures ia that minimal set exist simultaneously. Thus a minimal cut set is logically equivalent to the undesired accident stated ia terms of equipment failures and human errors. 

The foUowiag symbols are used ia fault tree constmction to display the iaterrelationships between equipment failures and a specific accident ... 

Fault Tree Solution. Solving the fault tree means obtaining the minimal cut sets. The minimal cut sets are all the combinations of equipment failures that can result in the fault tree TOP event. Computer programs are requked to determine the minimal cut sets for large fault trees (72). The solution method has four steps ... 

Minimal cut sets are then ranked. Two factors are considered in the ranking procedure. The first factor considers stmcture, ie, a one-event minimal cut set is more important than a two-event minimal cut set. The implication is that one event is more likely to occur than two events, two events are more likely than three events, and so on. The second factor considers ranking within equal-size minimal cut sets. The general ranking rules consider the probabihty of human error, active equipment failure, and passive equipment failure (73). 

This ranking implies that human errors are more likely to occur than active equipment failures (functioning equipment, such as a mnning pump) and that active equipment failures are more likely to occur than passive equipment failures (static, nonfunctioning equipment, such as a storage tank). 

Quahtative simulation is a specific KBS model of physical processes that are not understood well enough to develop a physics-based numeric model. Corrosion, folding, mechanical wear, equipment failure, and fatigue are not easily modeled, but decisions about them can be based on qualitative reasoning. See Refs. 178 and 292. 

Failure logic. This logic provides for responding to abnormal conditions, such as equipment failures. 

Inherently Safer Design Rather than add on equipment to control hazards or to protect people from their consequences, it is better to design user-friendly plants which can withstand human error and equipment failure without serious effects on safety, the environment, output, and efficiency. This part is concerned with this matter. 

For many years the usual procedure in plant design was to identify the hazards, by one of the systematic techniques described later or by waiting until an accident occurred, and then add on protec tive equipment to control future accidents or protect people from their consequences. This protective equipment is often complex and expensive and requires regular testing and maintenance. It often interferes with the smooth operation of the plant and is sometimes bypassed. Gradually the industry came to resize that, whenever possible, one should design user-friendly plants which can withstand human error and equipment failure without serious effects on safety (and output and emciency). When we handle flammable, explosive, toxic, or corrosive materials we can tolerate only very low failure rates, of people and equipment—rates which it may be impossible or impracticable to achieve consistently for long periods of time. 

Once the fault tree is constructed, quantitative failure rate and probability data must be obtained for all basic causes. A number of equipment failure rate databases are available for general use. However, specific equipment failure rate data is generally lacking and. 

Usually reactions are carried out without mishaps, but sometimes chemical reactions get out of control because of problems such as using the wrong raw material, using raw materials containing trace impurities, changed operating conditions, unanticipated time delays, equipment failure, or wrong materials of construction. 

Frequently a piece of equipment is used in different processes during its lifecycle. This could result in process conditions that exceed the safe operating limits of the equipment. Equipment inspection may provide a poor prediction of the equipment s useful life and reliability, due to the change of material handled or change in process chemistry over the life of equipment. Batch operations are also characterized by frequent start-up and shut-down of equipment. This can lead to accelerated equipment aging and may lead to equipment failure. This chapter presents issues and concerns related to the safe design, operation, and maintenance of various pieces of equipment in batch reaction systems, and provides potential solutions. 

Select equipment to minimize inadvertent contact as a result of equipment failure... 

Vibration during Check plow and linkage for loose compo-plowing out—can nents/wear lead to premature. sharpen plow or use serrated blade for hardened equipment failure heels and a potential ignition source— Manually remove heel more frequently see above. Plow at lower bowl speed Advance plow more slowly Make sure plow system is well damped Avoid air actuated plows Avoid use of full depth plows with hard cakes Use nitrogen knife to scrape centrifuge ... 

Table 1 shows prevalent examples of misconceptions about QRA. Many are actually generalizations that are too broadly applied. Two of the most common misconceptions concern (1) the lack of adequate equipment failure data and (2) the cost of performing QRA. 

There are a variety of ways to express absolute QRA results. Absolute frequency results are estimates of the statistical likelihood of an accident occurring. Table 3 contains examples of typical statements of absolute frequency estimates. These estimates for complex system failures are usually synthesized using basic equipment failure and operator error data. Depending upon the availability, specificity, and quality of failure data, the estimates may have considerable statistical uncertainty (e.g., factors of 10 or more because of uncertainties in the input data alone). When reporting single-point estimates or best estimates of the expected frequency of rare events (i.e., events not expected to occur within the operating life of a plant), analysts sometimes provide a measure of the sensitivity of the results arising from data uncertainties. 

Frequency Phase 1 Perform Qualitative Study, Typically Using HAZOP, FMEA, or What-if Analysis. To perform a qualitative study you should first (1) define the consequences of interest, (2) identify the initiating events and accident scenarios that could lead to the consequences of interest, and (3) identify the equipment failure modes and human errors that could contribute to the accident... 

Frequency Phase 3 Use Branch Point Estimates to Develop a Ere-quency Estimate for the Accident Scenarios. The analysis team may choose to assign frequency values for initiating events and probability values for the branch points of the event trees without drawing fault tree models. These estimates are based on discussions with operating personnel, review of industrial equipment failure databases, and review of human reliability studies. This allows the team to provide initial estimates of scenario frequency and avoids the effort of the detailed analysis (Frequency Phase 4). In many cases, characterizing a few dominant accident scenarios in a layer of protection analysis will provide adequate frequency information. 

Metal particles from wear and rust particles from reservoir and oil piping corrosion can lead to premature equipment failure and oil deterioration. It is important to provide suitable filtering equipment to remove these particles from the system. 

It is generally agreed that a shelter with a roof having ridge ventilation and with curtain walls not extending lower than 8 feet above the operating platform would be freely ventilated. Because a gas compressor would not be a source of hazard, except under abnormal conditions such as an equipment failure, this type of compressor shelter is usually classified as a Division 2 area. 

Any gas stored or transfeiTed under pressure represents an energy souree. If piping or equipment failure oeeurs, energy is given out as the gas rapidly expands down to virtually atmospherie pressure. Elastie strain energy in the walls is also given out, but, by eomparison, this is relatively small. 

Accidental release, spillage Transport incidents Overfilling of containers Equipment failure Unexpected reactions Runaway reactions... 

The use of appropriate instruments to monitor equipment operation and relevant process variables will detect, and provide warning of, undesirable excursions. Otherwise tliese can result in equipment failure or escape of chemicals, e.g. due to atmospheric venting, leakage or spillage. Instruments may facilitate automatic control, emergency action such as coolant or pressure relief or emergency shutdown, or the operation of water deluge systems. 

Older cook styles called for addition of phenol, formaldehyde, and water followed by alkali. Once the alkali was added, strict temperature control was the only barrier to a runaway reaction. A power or equipment failure at this point was likely to lead to disaster. Every batch made involved a struggle between the skill of the operator and capability of the equipment to control the exotherm versus the exothermic nature of the reactants. Most of the disasters that have occurred were due to utilization of this cooking method. 

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