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LECTURE 4*

PROCESS FLOWSHEETS

FlowsheetingFlowsheets are intended to represent and explain processes. To make them easy to understand, they are constructed with a consistent set of symbols for equipment, piping, and operating conditions. At present there is no generally accepted industrywide body of drafting standards, although every large engineering office does have its internal standards.

Process flowsheets embody the material and energy balances between and the sizing of the major equipment of the plant e.g. reactors, separators, and drums; special processing equipment, heat exchangers, pumps, and so on.Numerical data include flow quantities, compositions, pressures, temperatures, and so on.

Conventions and Format Recommended for Laying Out a Block Flow Diagram

Block Flow Diagram

Checklist of Data Normally Included on aProcess Flowsheet1.Process lines, but including only those bypasses essential to an understanding of the process2. All process equipment. Spares are indicated by letter symbols or Notes3. Major instrumentation essential to process control and to understanding of the flowsheet.4. Valves essential to an understanding of the flowsheet

5. Design basis, including stream factor6. Temperatures, pressures, flow quantity7. Weight and/or mol balance, showing compositions, amounts, and other properties of the principal streams8. Utilities requirements summary9. Data included for particular equipmenta. Compressors: number of stages; details of stages if important; SCFM (standard cubic feet per Minute) (60F. 14.7 psia)

b. Drives: type; connected HP; utilities such as kW, lb steam/hr, or Btu/hrc. Drums and tanks: ID or OD, seam to seam length, important internalsd. Exchangers: Sqft, kBtu/hr, temperatures, and flow quantities in and out; shell side and tube side indicatede. Furnaces: kBtu/hr, temperatures in and out, fuelf. Pumps: Gallons per minute, type, driveg. Towers: Number and type of plates or height and type of packing; identification of all plates at which streams enter or leave; ID or OD; seam to seam length; skirt height

Heat Exchanger

Topology of a chemical process plantWe denote every equipment as YY-ZXX YY denotes letters designated for specific equipment Z denotes to unit numberXX denotes the number of that equipment in the unit e.g.T-109 denotes 09th tower in unit 1

Processflowsheet ofthe manufactureof benzene by dealkylationof toluene

PIPING AND INSTRUMENTATION DIAGRAM (P&ID)

ACTUATORS: The function of an actuator is to adjust the position of the valve to ensure correct control of the process fluid.CONTROL VALVE: An automatic control system consists of:the fluid to be controlleda sensor for the process variablea controller which affects the actuator an actuator which modulates the valvea control valve to control the flow

Failure modeDistributed controlGeneral instrument and controller symbols

Other symbols

Letter Code for Instrument Symbols [based on ISA-5.1-1984 (R1992)]Notes:1. The letters D, G, M, N, and O are not defined and can be used for any userspecified property.2. The letter S as second or subsequent letter indicates a switch.3. The letter Y as second or subsequent letter indicates a relay or a compute function.4. The letter Z is used for the final control element when this is not a valve.

VALVE SELECTIONThe valves used for a chemical process plant can be divided into two broad classes, depending on their primary function:1. Shut-off valves (block valves or isolation valves), whose purpose is to close off the flow;2. Control valves, both manual and automatic, used to regulate flow.The main types of valves used areGatePlugBallGlobeDiaphragmButterflyNonreturn

A utility flowsheet may be provided which shows all the headers for utility inputs and outputs available along with the connections needed to the process. It provides information on the flows and characteristics of the utilities used by the plant.Vessel sketches, logic ladder diagrams, wiring diagrams, site plans, structural support diagrams, and many other drawings are routinely usedPlot plans and elevation diagrams are provided that locate the placement and elevation of all of the major pieces of equipment such as towers, vessels, pumps, heat exchangers, and so on.

Layout of the plot plan can take one of two basic configurations: the grade-level, horizontal, in-line arrangement and the structure mounted vertical arrangement. In the grade-level, horizontal arrangement, the process equipment units are aligned on either side of a pipe rack that runs through the middle of the process unit. The purpose of the pipe rack is to carry piping for utilities, product, and feed to and from the process unit. Equipment is located on either side of the pipe rack, which allows for easy access. Horizontal arrangement generally requires a larger footprint and, hence, more land than does the structure-mounted vertical arrangement.

Plant Layout: Grade-Mounted Horizontal Inline

Plant Layout: Structure-Mounted Vertical Arrangement

the location of shell and tube exchangers must allow for tube bundle removal for cleaning and repair.Locations of pumps must allow for access for maintenance and replacement. For compressors, this access may also require that a crane be able to remove and replace a damaged drive. Control valves must be located at elevations that allow operator access.

Reasons for Elevating Equipment

Piping isometrics are drawn for every piece of pipe required in the plant. These drawings are 3-D sketches of the pipe run, indicating the elevations and orientation of each section of pipe. In the past, it was also common for comprehensive plants to build a scale model so the system could be viewed in three dimensions and modified to remove any potential problems. Over the past twenty years, scale models have been replaced by 3-dimensional computer aided design (CAD) programs that are capable of representing the plant as-built in three dimensions.They provide an opportunity to view the local equipment topology from any angle at any location inside the plant. One can actually walk through the plant and preview what will be seen when the plant is built. The ability to view the plant before construction will be made even more realistic with the help of virtual reality software. With this new tool, it is possible not only to walk through the plant but also to touch the equipment, turn valves, and climb to the top of distillation columns, and so on.

THE 3-D PLANT MODEL

The PFD is divided into logical subsystems

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