Plant error tolerating

An inherently safe plant1112 relies on chemistry and physics to prevent accidents rather than on control systems, interlocks, redundancy, and special operating procedures to prevent accidents. Inherently safer plants are tolerant of errors and are often the most cost effective. A process that does not require complex safety interlocks and elaborate procedures is simpler, easier to operate, and more reliable. Smaller equipment, operated at less severe temperatures and pressures, has lower capital and operating costs. 

Simplify (simplification and error tolerance) Keep piping systems neat and visually easy to follow Design control panels that are easy to comprehend Design plants for easy and safe maintenance Pick equipment that requires less maintenance Pick equipment with low failure rates Add fire- and explosion-resistant barricades Separate systems and controls into blocks that are easy to comprehend and understand Label pipes for easy walking the line Label vessels and controls to enhance understanding... 

The standard required a site review, and a head-office level review of all reported occurrences. There was good cooperation between head-office and the plant to get the system underway. Apparent inconsistencies between the definition of the class, and the examples were fairly quickly resolved, with the help of head-office. A data-base of events was established, with an extensive sort facility. The system became very well respected and well used, and much improvement of performance was achieved through experience feedback. Near-misscs were reported freely, from which much information was gained in order to facilitate performance improvement and the development of error tolerant. and error recovery systems. 

This kind of plant design is also called single error forgiving or error tolerating design. For this purpose of identifying the error propagating pathways this qualitative use and evaluation of a fault tree analysis is especially suitable. 

To determine if a process unit is at steady state, a program monitors key plant measurements (e.g., compositions, product rates, feed rates, and so on) and determines if the plant is steady enough to start the sequence. Only when all of the key measurements are within the allowable tolerances is the plant considered steady and the optimization sequence started. Tolerances for each measurement can be tuned separately. Measured data are then collec ted by the optimization computer. The optimization system runs a program to screen the measurements for unreasonable data (gross error detection). This validity checkiug automatically modifies tne model updating calculation to reflec t any bad data or when equipment is taken out of service. Data vahdation and reconciliation (on-line or off-line) is an extremely critical part of any optimization system. 

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. 

In general, the safety of a process relies on multiple layers of protection. The first layer of protection is the process design features. Subsequent layers include control systems, interlocks, safety shutdown systems, protective systems, alarms, and emergency response plans. Inherent safety is a part of all layers of protection however, it is especially directed toward process design features. The best approach to prevent accidents is to add process design features to prevent hazardous situations. An inherently safer plant is more tolerant of operator errors and abnormal conditions. 

Simplify—Design processes and facilities that eliminate unnecessary complexity and that are tolerant of human error. Example Design piping to permit gravity flow of hazardous materials in a plant, eliminating the need for pumps, which can leak. 

The variability of radiocesium concentration in undisturbed environments such as alpine pastures is very high and a high minimum of soil and plant samples would be required to estimate the radiocesium distribution median with a 95% confidence interval and a tolerable error of at least 20%. Low cost sampling with only a few samples would result in a biased estimate of the real radionuclide contamination. There is some evidence that the high variability is caused more by microscale runoff phenomenon directly after the Chernobyl accident rather than by long-term erosion processes. The high spatial variability of the Cs, the random distribution and the concentration in the first centimeter of the soil possibly causes the missing correlation of the radiocesium activity concentrations in soils and plants and therefore does not allow a soil-to-plant transfer factor to be calculated. 

Errors made by humans when interacting with technical plants do not always have to impair them seriously. This is especially true for so-called fault-tolerant or fault-forgiving systems. Thus, only errors are of interest here which have detrimental consequences. 

It is in the civil construction that relative positioning of the plant is largely determined. Cumulative tolerance errors should be avoided when addressing the plant as a whole. For large or complex plants, the person responsible for overall construction survey and setout should have the opportunity to discuss critical dimensions with the plant designer this may result in a more appropriate system of overall dimensions. 

The design of the plant should be tolerant of human error. To the extent practicable, any inappropriate human actions should be rendered ineffective. For this purpose, the priority between operator action and safety system actuation should be carefully chosen. On the one hand, the operator should not be allowed to override reactor protection system actuation as long as the initiation aiteria for actuation apply. On the other hand, there are simations where operator interventions into the protection system are necessary. Examples are manual bypasses for testing purposes or for adoption of acmation criteria for modifications to the operational state. Furthermore, the operator should have an ultimate possibility, under strict administrative control, to intervene in the protection system for the purposes of managing beyond design basis accidents in the event of major failures within the reactor protection system. 

Because of heat effects of the reactions, the calculated reactor temperature profiles from previous steps would show deviations from actual plant data. We tune the global activity factors again to ensure that the deviations of reactor temperature predictions are within tolerance. We repeat the calibration of reactor temperature profiles and mass yields ofhquid products several times until the errors of model predictions are within the acceptable tolerance. These back-and-forth procedures compose the first phase shown in Figure 6.13 which is a generalized guideline of initial calibration for the Aspen HYSYS Petroleum Refining HCR model. This follows because reactor temjjerature profiles and major liquid product yields are always crucial considerations for any hydrocracker. 

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