Flowsheets Instrumentation

Flexibility for changes, additions or upgrade of process equipment, flowsheets, instruments, etc., thus the basic canyon structure can be operated indefinitely. 

Figure 3.4. Symbols for control elements to be used on flowsheets. Instrument Society of America (ISA) publication no. S51.5 is devoted to process instrumentation terminology.

Preliminary Process Flowsheet. This will show major equipment and lines, preliminary equipment details (vessel diameter, number of trays, pump flow and driver horsepower, etc.), major instrumentation, and, it is hoped, have a material balance at the bottom of each drawing with flows keyed to a numbering system on the diagram. The process flowsheets should cover both the process and utility sides of the plant. 

From the basic process-containing flowsheet other engineering specialties develop their own details. For example, the instrument engineer often takes the requirements of the process and prepares a completely detailed flowsheet which defines every action of the instruments, control valves, switches, alarm horns, signal lights, etc. This is his detailed working tool. 

The two types of lines on a flowsheet are (1) those representing outlines and details of equipment, instruments, etc., and (2) those representing pipe carrying process or utility liquids, solids, or vapors and electrical or instrument connections. The latter must be distinguished among tliemselves as suggested by Figure 1-21. 

Figure 1-18B. Computer generated instrumentation detaii for P. and I D. flowsheet. Courtesy of Integraph Corp., Bui. DP016A0.

Figure 1-20B. Commonly used instruments for process instrumentation flowsheets. Adapted by permission, ISA Std. ANSI Y32.20—1975, ISA S5.1—1973, Instrumentation Symbols and Identification," Latest edition, 1984.

As a direct companion of the completed flowsheet, the line schedule sheet transmits the process and mechanically necessary details for proper interpretation of the piping aspects of the flowsheet (see Figures 1-24A, B, C, D). These schedules are initiated by the process engineer to further explain the requirements of the process as shown on the flowsheets. They are often and perhaps usually cooperatively completed by other engineers, particularly the piping, mechanical and instrumentation groups. 

Current technolog) allows the use of computer programs and data bases to construct an accurate and detailed flowsheet. This may be a process type diagram or a piping and mechanical/instrument diagram, depending on the input. See Figures 1-9, 1-10, 1-18A and 1-18B. 

Equipment abbreviations, 25 Instruments, 29 Flowsheets, 1-11 Block diagram, 4... 

Another widely used safety analysis method in process industry is the Hazard and Operability Analysis, better known as Hazop (Kletz, 1992). The conventional Hazop is developed to identify probable process disturbances when complete process and instrumentation diagrams are available. Therefore it is not very applicable to conceptual process design. Kletz has also mentioned a Hazop of a flowsheet, which can be used in preliminary process design, but it is not widely used. More usable method in preliminary process design is PIIS (Edwards and Lawrence, 1993), which has been developed to select safe process routes. 

Steady-state process variables are related by mass and energy conservation laws. Although, for reasons of cost, convenience, or technical feasibility, not every variable is measured, some of them can be estimated using other measurements through balance calculations. Unmeasured variable estimation depends on the structure of the process flowsheet and on the instrument placement. Typically, there is an incomplete set of instruments thus, unmeasured variables are divided into determinable or estimable and indeterminable or inestimable. An unmeasured variable is determinable, or estimable, if its value can be calculated using measurements. Measurements are classified into redundant and nonredundant. A measurement is redundant if it remains determinable when the observation is deleted. 

A process flowsheet of the dealkylation of toluene to benzene is in Figure 2.4 the material and enthalpy flows and temperature and pressures are tabulated conveniently, and basic instrumentation is represented. 

The mechanical flowsheet of the reaction section of a toluene dealkylation unit in Figure 2.5 shows all instrumentation, including indicators and transmitters. The clutter on the diagram is minimized by tabulating the design and operating conditions of the major equipment below the diagram. 

Major instrumentation essential to process control and to understanding of the flowsheet... 

Rather less freedom is allowed in the construction of mechanical flowsheets. The relative elevations and sizes of equipment are preserved as much as possible, but all pumps usually arc shown at the same level near the bottom of the drawing. Tabulations of instrumentation symbols or of control valve sizes or of relief valve sizes also often appear on P I diagrams. Engineering offices have elaborate checklists of information that should be included on the flowsheet, but such information is beyond the scope here. 

Figure 2.6. Engineering flowsheet of a gas treating plant. Note the tabulation of instrumentation flags at upper right (Fluor Engineers, by way of Rase and Barrow, Project Engineering of Process Plants, Wiley, New York, 1957).

These examples ask for the construction of flowsheets from the given process descriptions. Necessary auxiliaries such as drums and pumps are to be included even when they are not mentioned. Essential control instrumentation also is to be provided. Chapter 3 has examples. The processes are as follows ... 

Two important control instruments are to be shown on the flowsheet. These are a back pressure controller in the reactor effluent line beyond exchanger E-101 and a pH controller on the feed line of the 5% HC1 that is fed to springer R-102. The pH instrument maintains proper conditions in the springer. 

Float valves are required on all cooling tower systems. Their purpose is to supply makeup water to replace that lost by evaporation, blowdown, drift and system leakage. They are usually installed in the cold water basin and function to maintain a preestablished water level. A typical flowsheet showing the recommended arrangements for instrumentation and valves is given in Figure 7.4. 

The study leader will specify the information that must be available to the team (and which will need to have been studied before the HAZOP meeting). The most important part is likely to be the complete P ID (or flowsheet or Engineering Line Diagram, according to local terminology). It should show all vessels, pumps, process pipework, instruments, service connections and indicate floor levels or elevations. 

The Guidelines for Hazard Evaluation Procedures, Second Edition [5] also offers 45 pages of sample questions in its Appendix B. These questions cover process, (i.e., flowsheets and layout), equipment (reactors, heat exchangers, piping, and instrumentation), Operations, Maintenance, Personnel Safety, and other broad areas. 

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