E. Osakue PIPING DIAGRAMS AND DRAWINGS Introduction Diagrams and drawings are graphic models of objects. Diagrams are refined and neat sketches that are created with or without scale. However, good proportion is encouraged in diagramming since this enhances aesthetics. When diagrams are created to scale, they do not have complete dimensional information and specifications as necessary with drawings. Drawings are made to scale and thus bear direct relationship to the physical size and spatial orientation of the objects represented. Piping diagrams are schematic representations and are used to define functional relationships in piping systems. Spatial relationship is not evident in them, except in a general sense. They can be used to communicate with non-technical personnel since they summary the functions of complicated systems. There are at least three piping diagrams in common use. These are a) block flow diagram (BFD), b) process flow diagram (PFD), and c) piping and instrumentation diagram (P&ID). Single lines are used to represent pipes. PFD and P&ID are sometimes called flow lines, collectively. Flow lines are elaborations of schematic diagrams and are made with piping symbols that represent main equipment, instruments and fittings. Both plan and elevation views of the facility may be drawn. They provide non-technical personnel the basic information for comprehending complex systems. Flow lines are often used for preliminary estimates of the system Flow Line Conventions • Avoid flow lines crossing each other, if possible. • If flow lines of the same size cross, break the horizontal line. • ANSI standard calls for a loop or arc in the vertical line at intersections of lines. • Indicate flow direction with an arrowhead. • Label pipes: size, content, identification number. • Label equipments (name and number) or identification. • Group lines together, but must not be closer than 6 mm (¼”). • Precedence of lines: major lines, minor lines, instrument lines. • Line of lower precedence is broken if it intersects one of higher precedence. • Give adequate space between equipments to avoid overcrowding. • Show important valves, control valves, and orifice flanges. • Indicate flow direction through heat exchangers with arrows. • Show control systems. These are very important. • Label feed and product lines entering and leaving a unit.
Fig. 1: Block flow diagram (BFD)
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E. Osakue Block Flow Diagram Block flow diagram (BFD), also called a schematic diagram, is a non-scaled single line diagram. It is a simplified model of piping systems created at the initial stages of the design process. Simple shapes such as rectangles, circles, etc. are used to represent processes and equipments. BFD is a theoretical or conceptual layout of a system and its operations. It serves as a basic guide for the development of process flow diagram. Fig. 1 shows an example of a block flow diagram (BFD). • • • • • •
Conceptual sketch of a piping process Not drawn to scale Provides broad description of the system Indicates flow direction in single lines Simple shapes like rectangles or circles used to represent equipments and operations Used at the initial stages to guide creation of process flow diagrams
Process Flow Diagram (PFD) Process flow diagrams (PFDs) are a family of functional one-line diagrams showing the layout of processes and equipments. A process flow diagram is a schematic illustration of piping system where standard or conventional symbols are used to represent equipment and instruments. It shows functional relationships between the major components in the system. A PFD may tabulate process design values, different operating modes (min., normal, max.). It does not show minor components, or devices in piping systems. It may include: 1) Major equipment symbols, names and identification numbers; 2) Main piping system and flow direction; 3) major instrumentation (valves that affect operation of the system; 4) Interconnection with other systems; 5) System ratings and operational values like minimum, normal and maximum flow, temperature and pressure. Equipments are labeled with name and number assigned by the engineer or client and must be shown on the process flow diagram. Major flow lines are thick and stand out in diagrams. Each line should have a specification indication as a minimum (unit #, line #, pressure rating, NPS, insulation). Process flow diagrams are considered as preliminary drawings and are used to develop initial project estimates. Fig. 2 shows an example of a process flow diagram (PFD)
based on Fig. 1.
Fig. 2: Process flow diagram (PFD) Creating Process Flow Diagram • Create center lines • Create equipment outlines • Insert symbols from library (primary and secondary flow lines, valves) • Add equipment and line labels • Add notes
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E. Osakue Piping and Instrumentation Diagram (P&ID) A piping and instrumentation diagram, sometimes called process and instrmentation diagram, (P&ID); is a diagram which shows the piping and installed devices in a process flow. The Institute of Instrumentation and Control defines P&ID as: A diagram which shows the interconnection of process equipment and the instrumentation used to control the process. In the process industry, a standard set of symbols is used to prepare drawings of processes. The instrument symbols used in these drawings are generally based on Instrumentation, Systems, and Automation Society (ISA) Standard S5. 1
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P&ID is a schematic illustration of functional relationship of elements and devices in a piping system. P&IDs are more detailed versions of process flow diagrams and include all equipments, pipelines, fittings and instrumentations. Standard or conventional symbols are used to represent equipment and instruments. P&ID shows all piping devices and components, including the physical sequence of branches, reducers, valves, equipment, instrumentation and control interlocks. P&IDs are the primary schematic drawings used for laying out a process control installation in a plant. It is a primary source of reference for documentation of control systems. P&ID presents functional information and provides spatial or physical relationship of devices in a general sense. It is not drawn to scale, but shows process lines, equipments, instruments, and process parameters such as flow rates, pressure and temperatures are indicated. P&IDs play a significant role in the maintenance and modification of the process that it describes. It is critical to demonstrate the physical sequence of equipment and systems, as well as how these systems connect. During the design stage, the diagram also provides the basis for the development of system control schemes, allowing for further safety and operational investigations, such as the popular Hazards and Operability (HAZOP) study. P&IDs are used to operate the piping systems. Fig. 3 shows
an example of a process and instrumentation diagram (P&ID) based on Fig. 2.
Fig. 3: Process and Instrumentation diagram (P&ID) List of P&ID items • • • • • •
All Instruments and designations All equipments with names and numbers All valves and their identifications All piping with sizes and identification Permanent start-up and flush lines Flow directions
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E. Osakue • • • • • •
Interconnection references Control inputs and outputs, interlocks Interfaces for class changes Computer control system input Identification of components and subsystems delivered by others Miscellanea - vents, drains, special fittings, sampling lines, reducers, increasers and swagers
A plan or elevation P&ID diagram may be drawn depending on the view that provides the most information. In P&ID, process line should flow from left to right. Flow lines should terminate on the right or left margin in the sheet. Each line termination should have a tag that shows line content, match point, sheet number and link to other sheets. Service devices such as pumps should be shown on the sheet where the main equipments they serve (e.g. heat exhchangers, towers, reactors, etc.) are. Main equipments are generally laid out at the middle of the sheet while service equipments are placed at the bottom. Critical instruments should be “flagged”. When vertical and horizontal lines cross, and are not connected, one should be broken.
Steps for Creating P&ID • • • • • •
Check PFD center lines Check PFD equipment outlines Add more information on equipment (Operating limits, etc) Insert more symbols (instruments, auxiliaries, etc.) from library Add line specifications Add notes
Utility Flow Lines Utilities are services that are essential for the proper functioning of a plant. These may include gas, water, sewer drains, steam, instrument and utility air, fuel oil, etc. Their piping systems must be designed too. Usually separate flow lines are developed for utilities. Vents and drains are necessary for discharging feed substances into the atmosphere or sewer. Atmospheric vents are common on tanks and vessels and sometimes on pipes. A discharge to sewer may be found on the bottom of tanks and pump cases, though it could be anywhere on a line. Piping Drawings Piping drawings are scaled drawings of plan, elevation and sections of piping systems. Others are spool drawings, piping isometrics, and 3D models. Piping isometrics and spool diagrams are probably more common than drawings. However, the use of computers and 3D piping design software readily make available scaled piping isometrics and spool drawings. Piping drawings could be complex because they show all equipment, fittings, instrumentations, dimensions and notes. Data for piping drawings are derived from process flow diagrams, piping and instrumentation diagrams, structural, mechanical, instrumentation drawings and vendors’ catalogues. Spool drawings are detail section drawings of pipes and fittings used by pipe fitters and welders during construction. Piping isometric is a 3D drawing of a single pipe run. All fittings and attachments on the pipe are shown. 3D plant representations may be a plain model or a rendered model. 3D plain models represent objects, as they would appear in reality but in vector format. They provide accurate locations, dimensions and elevations for equipment, pipes, instrumentations, etc. They thus provide a database for all components in the piping system. 3D rendered models are derived from the plain models by applying colors, texture, shading, and special effects. They give more realistic appearances to plain 3D models. As the power of the computer continues to grow, 3D model modeling will become the vogue because of their realism The drive today is 3D modeling therefore having skills in this area is an economic advantage. Drawing Index Each drawing sheet needs a reference number. Drawing index should list all drawing numbers. Describe content of each sheet. Drawing index may be inserted into plot plan if number of drawing is small.
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E. Osakue Diagram Examples
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