Equipment, selection

Equipment used in a safety instrumented function must be carefully chosen. The instrumentation must be fuUy capable of performing the functional requirement. All equipment must be justified so that the end user is totally confident that the instrumentation wiU properly perform in 

Materials used in the instruments must be compatible with process materials if the instrumentation sees process wetted service. Process environmental conditions must not exceed the instrumentation ratings. The functional safety of the instrument must be assessed. AU justification decisions must be documented as part of project records. 

Special attention must be paid to the support systems e.g. power supplies, air suppUes, wiring methods. Eor energize to trip designs, these systems become safety critical since they have a direct impact on safety. 

ANSI/ISA-84.00.01-2004 (lEC 61511 Mod) requires that equipment used in safety instrumented systems be chosen based on either lEC 61508 certification to the appropriate SIL level or justification based on prior use criteria (ANSI/ISA-84.00.01-2004 (lEC 61511Mod), Part 1, Section 11.5.3). However the ANSI/ISA-84.00.01-2004 (lEC 61511 Mod) standard does not give specific details as to what the criteria for prior use means. Most agree however that if a user company has many years of documented successful experience (no dangerous failures) with a 

Most agree also that prior use requires that a system be in place to record all field failures and failure modes at each end user site. Version records of the instrument hardware and software must be kept as significant design changes may void prior use experience. Operating conditions must be recorded and must be similar to the proposed safety application. 

More than one equipment design may be suitable for a particular application. Often the initial approach is to replace it in kind. However, it is wise to evaluate the features of the present unit s operation in light of the process requirements and priorities. For example, is it labor intensive Are copious volumes of wash required  

Ever-increasing environmental concerns may make it necessary to evaluate the existing process to reduce emissions, operator exposure, limit waste disposal of filter aid, or reduce wash quantities requiring solvent recovery or wash treatment. Breakdown of an old piece of equipment often provides the opportunity and justification to improve plant conditions. New grass roots designs may have the tendency to revert to industry standards. This is also the opportunity to improve conditions or substantiate the current equipment of choice. 

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Walas, S. M., Chemical Process Equipment Selection and Design, Butterworth, Reading, Mass., 1988. 

L. Theodore and A. J. Bwomzox., Air Pollution Control Equipment—Selection, Design, Operation and Maintenance, Prentice-Hall, Englewood CHffs, N.J., 1982. 

Special safety constraints apply to equipment selection, design, and operation in nuclear reprocessing (269). Equipment should be reHable and capable of remote control and operation for long periods with minimal maintenance. Pulsed columns and remotely operated mixer—settlers are commonly used (270). The control of criticaHty and extensive monitoring of contamination levels must be included in the process design. 

Operating conditions vary radically depending on the type of equipment selected but typically temperatures used are in the range of 50 to 180 °C and pressures of 446 to 3550 kPa (50 to 500 psig) are sufficient (11). 

The efficiency of the Rankine cycle itself can be increased by higher motive steam pressures and superheat temperatures, and lower surface condenser pressures in addition to rotating equipment selection. These parameters are generally optimized on the basis of materials of constmction as well as equipment sizes. Typical high pressure steam system conditions are in excess of 10,350 kPa (1500 psi) and 510 °C. 

SOURCE Adapted from Walas, Chemical Fiocess Equipment Selection and... 

Equipment Selection Ideally, selection of equipment to produce a gas-in-hquid dispersion should be made on the basis of a complete economic analysis. The design engineer and especially the pilot-plant engineer seldom have sufficient information or time to do... 

G-7. Stern, Air Follution, 3d ed., vols. 3-5, Academic, Orlando, FL, 1976-77. G-8. Strauss, Industtial Gas Cleaning, 2d ed., Pergamon, New York, 1975. G-9. Theodore and Biionicore, Air Follution Control Equipment Selection, Design, Operation and Maintenance, Prentice Hall, Englewood Cliffs, NJ, 1982. 

Equipment The equipment selected for CCD circuits may consist of multiple-compartment washing-tray thickeners or a train of... 

Implicit in cake filtration is the removal and handling of solids, since the cake is usually relatively dry and compacted. C es can be sticky and difficult to handle therefore, the ability of a filter to discharge the cake cleanly is an important equipment-selection criterion. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

Many successful types of reactors are illustrated throughout this section. Additional sketches may be found in other books on this topic, particularly in Walas Chemical Process Equipment Selection and Design, Butterworths, 1990) and Ullmann Encyclopedia of Chemical Technology (in German), vol. 3, Verlag Chemie, 1973, pp. 321-518). 

Factors in Control-Equipment Selection In order to solve an air-poUrrtion problem, the problem mrrst be defined in detail. A nrrm-ber of factors mrrst be considered prior to selecting a particrrlar piece of air-polhrtion-control eqrripment. In general, these factors can be grorrped into three categories environmental, engineering, and economic. 

Equipment Selection Criteria and Guidelines A number of factors should be considered in order to determine when to select a blowdown drum, cyclone separator, or quench tank to handle a multiphase stream from a relief device. Among these are the plot plan space available, the operating limitations of each type, and the physicochemical properties of the stream. 

Increasing the TOV level, i.e. choosing a higher protective level (V,es) of the arrester, may increase the life of the arrester but will reduce the margin of protection for the protected equipment. Selection therefore should be done judiciously bearing all these aspects in mind. 

Theodore, L., and Buonicore, A. J, "Air Pollution Control Equipment Selection, Design, Operation and Maintenance." Prentice-Hall, Englewood Cliffs, NJ, 1982,... 

Each chapter contains a section on recommended resources, which includes Web sites. Some of these sites contain additional tools such as software that you can download for calculation purposes or for equipment selection information or financial calculations. Developing a strong library of references and Web sites is important to acquiring skills and staying abreast with the technology advances in air pollution abatement. 

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