Equipment design, safety factors

Let s take the example of benzene, which at 12,000 ppm, is 100% LEL. The National Fire Protection Association (NFPA) states that equipment can operate, without LEL monitors or controls, if the LEL is less than 25% LEL. For benzene then, 25% LEL is equal to 3,000 ppm. This upper boundary becomes a dictating factor in the selection and design of the oxidation equipment. If the concentration is higher than 25% LEL, the NFPA requirements state that an LEL monitor is required. Using an LEL monitor, NFPA guidelines allow operation up to 50% LEL (a 2 1 safety factor). Thus, 100% LEL is explosive if the stream is at 25%, a factor safety of four exists. 

Selection and care of the hydraulic fluid for a machine will have an important effect on how it performs and on the life of the hydraulic components. During the design of equipment that requires fluid power, many factors are considered in selecting the type of system to be used-hydraulic, pneumatic, or a combination of the two. Some of the factors required are speed and accuracy of operation, surrounding atmospheric conditions, economic conditions, availability of replacement fluid, required pressure level, operating temperature range, contamination possibilities, cost of transmission lines, limitations of the equipment, lubricity, safety to the operators, and expected service life of the equipment. 

TABLE 1A Safety Factors in Equipment Design Results of a Questionnaire... 

Equipment Design Veriable Range of Safety Factor %)... 

Before any SLS equipment of substantial size is finally selected, it is essential to use the results of pilot plant tests for guidance. Although many vendors are in a position to do such work, pilot equipment should be used at the plant site where the slurry is made. Because slurries often are unstable, tests on shipments of slurry to the vendors pilot plant may give misleading results. It may be possible to condition a test slurry to have a maximum possible resistivity, but a plant design based on such data will have an unknown safety factor and may prove uneconomical. 

The conventional procedure for introducing resilience in a HEN (or general process plant) is to use empirical overdesign. That is, a nominal or conservative basis is selected for designing and optimizing the HEN. Empirical safety factors based on past experience are applied to the equipment sizes and extra units are also often introduced. However, although this empirical procedure will in general add resilience and... 

Document of material specifications and test methods. Verification of the specifications must be done to satisfy the design, and the test method must be validated where needed. Because the material is a critical safety factor, the selection of material for IOL should meet both the physicochemical and compatibility specifications described in ISO 11979-2 and 11797-3. The in-house (receiving) specifications of material should thus be documented. Where a test method is developed, the method must be validated. The equipment used for the test must be calibrated. 

Some examples of recommended safety factors for equipment design are shown in Table 6. These factors represent the amount of overdesign that would be used to account for the changes in the operating performance with time. 

Because the factor methods for calculating the depreciable capital cost are rapid methods and not based on a detailed design, many small items of equipment are knowingly omitted. Also, there are uncertainties in design and economic procedures, and bad weather, strikes, and other unforeseen events may cause delays. To correct for uncertainties and unforeseen events requires using a contingency factor or safety factor. 

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