piping diagrams

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PIPING DIAGRAMS

IntroductionDiagrams are neat graphic representations of objects that are not drawn to scale. They are used to define

functional relationships in piping systems and explain how the systems work. Spatial relationship is not

evident in them, except in a general sense. They can be used to communicate with non-technicalpersonnel 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 aresometimes 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 elevationviews 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

Block Flow DiagramsBlock flow diagram (BFD) is a non-scaled single line diagram. It is a simplified model of piping systemscreated 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. A block flow diagram:

is a conceptual sketch of a piping process is not drawn to scale provides broad description of the system indicates flow direction in single lines uses simple shapes like rectangles or circles used to represent equipments and operations is used at the initial stages to guide creation of process flow diagrams

Fig. 1: Block flow diagram (BFD)

Fig. 2 Block Flow Diagrams


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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 lines and flow direction;

3) Major instrumentation (valves and control valves that affect operation of the system);4) Interconnection with other systems;

5) Major bypass and recirculation lines

6) System ratings and operational values such as min., normal and max. flow, temperature and pressure

7) Composition of fluids

Process Flow Diagrams should not include:

pipe class pipe line numbers minor bypass lines isolation and shutoff valves maintenance vents and drains relief and safety valve code class information seismic class information

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. Figs. 3

and 4 show examples of process flow diagrams (PFDs) based on Figs. 1 and 2.

Fig. 3: Process flow diagram (PFD)


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Fig. 4: Process flow diagram (PFD)

Flow Line Conventions

Avoid flow lines crossing each other, if possible. Line of lower precedence is broken if it intersects one of higher precedence. Fluid or process lines

have precedence over control signal lines.

Precedence of lines: major lines, minor lines, instrument lines. If flow lines of the same size cross, break the vertical line. ANSI standard calls for a loop or arc in the vertical line at intersections of lines. Indicate flow direction with an arrowhead. Group lines together, but must not be closer than 5 mm (). Label pipes: size, content, identification number. Give adequate space between equipments to avoid overcrowding. Label equipments (name and number) or identification. 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.

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

Checklist for PFDs

a) Title block and Revision Information1. Ensure that Project/Client reference/record number show correctly.2. Check signatures (Drafter, Designer, Checker, Approver).3. Check companys name, logo, contact, etc.4. Ensure page and drawing numbers show correctly.5. Ensure that revision number and purpose show,6. Ensure that dates show correctly


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b) Drawing Information1. Use appropriate line thickness or weight,2. Lines have flow arrows on each turn.3. Lines enter page on the left and exit at the right end where possible.4. Entry and exit lines carry drawing numbers5. Flow lines have correct labels that match stream table6.

Equipment has correct label that match equipment list information7. Instruments are of the right type and logic

8. Line breaks are correct9. Check and correct errors from revision activities.10.Use highlighter to check-off items11.Check notes for spelling and correctness.12.Use zone labels for C-size and above plots.13.Battery limits are shown

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.

P&ID is a schematic illustration of functional relationship of elements and devices in a piping system.

Standard or conventional symbols are used to represent equipment and instruments. P&IDs are more

detailed versions of process flow diagrams and include all equipments, pipelines, fittings and

instrumentations. P&ID shows all piping devices and components, including the physical sequence of

branches, reducers, valves, equipment, instrumentation and control interlocks. Vents and drains are

necessary for discharging feed substances into the atmosphere or sewer. Atmospheric vents are commonon 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. It is often used for preliminary estimates of the

system. 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 toscale, 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 andoperational investigations, such as the popular Hazards and Operability (HAZOP) study. OSHA considers

P&ID as a process safety information (PSI) document that is required for process hazard analysis (PHA).Therefore, P&IDs must contain current process safety information. So they must be reviewed and updated

anytime part of a system is modified so as to reflect current systems status, design, and installation. Only

current and accurate P&IDs are used for PHAs. P&IDs are used to operate the piping systems, developorthographic layout and isometric drawings, and by construction workers. They provide information for

design and installation of devices, tie system description together by incorporating flow diagram, electric

control schematic and control logic diagram. Start-up organizations use P&IDs to prepare flushing,

testing, and blowout procedures for piping systems. Though standard symbols have been developed by

ISO/ASME/ISA, many companies in practice may not use these symbols. Legends are required in P&IDs

that use non-standard symbols.


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Fig. 6: Process and Instrumentation diagram (P&ID)

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

P&ID Checklist

In addition to the checklist information for PFDs, see that:

1. Instruments line types are correct.2. Failure mode is shown on control valves3. PSVs/PRVs have set pressures, inlet/discharge size match pipe specs.4. Branch or feed flare lines connect main/header lines from above because flare lines need to slope

downward to drain properly.

5. Double block/gate valves with drain in-between for 600 psig and above or hazardous fluidslike acids.

6. Check logical placement of valves: check valve at pump discharge, swing blind valve near gatevalve.

7. Insulation and size are correct.8. Handwheels can rotate to right angles for access as per ASME B16.51: 90o for class 150 carbon

steel of size 0.5 to 3; 45o for size 4 to 8; 30o 10 to 14.

9. Ensure that level gauges are visible to operators.10.Full vacuum rating is on required equipment.P&IDs are commonly plotted on D-size (22x34) paper, but individual companys standard may vary.

Zone labels should be included in C-size or above. Some companies require zoning labels in B-size and

above sheets. If sheets are folded, standard procedure should be followed so the companys information

shows on the upper face.


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Utility Flow LinesUtilities 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 andsometimes 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.

More Diagram Examples


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Stream TableA stream table contains a list of feeds or commodities in a process. Check and be sure all streams are

listed and have correct units. For example, ensure that gas or vapor lines do not have liquid flow ratesand verse versa. Ensure that low pressures do not flow into high pressures, that mass balance closes, that

the two sides of a heat exchanger have the same change in enthalpy. Stream table information are used to

prepare and check correctness of PFDs and P&IDs.

Equipment ListEquipment Information:

1. Vessels: capacity, size, design temperature, design pressure, full vacuum, skirt height to grade,internals/levels.

2. Tanks: capacity, size, type3. Heat exchangers: area, design temperature, design pressure, duty, power rating, elevation.4. Pumps: rated flow rate, differential head, power rating.5. Compressors: rated flow rate, discharge pressure, power rating.6. Boilers: capacity, size, design temperature, design pressure, super-heater outlet temperature7. Turbines: inlet and outlet temperatures, power rating.8. Instruments: type, mounting (field or room), control (manual or automatic)

Drains: for most equipment.

HAZOP/PHAHAZOP (Hazard and Operability) analysis or PHA (Process Hazard Analysis) examines potential causes

and consequences of fire, explosions, releases of toxic or flammable chemicals and major spills.

Substances with a flash point less than 60.5 C (141 F) or 38 C (100.0 F)depending upon the

standard being appliedare considered flammable, while substances with a flash point above those

temperatures are considered combustible. HAZOP/PHA focuses on equipment, instrumentation, utilities,

human actions, and external factors that can impact a process. The analysis helps in determining the

potential failure points/modes and hazards in a process. OSHA requires PHA to be conducted every five

(5) years after the completion of the first PHA. P&IDs must be updated and be in As-Built status for

PHA.

Can safety issues/problems arise from:

1. Any deviation in normal/operating temperature?2. Any deviation in normal/operating pressure?3. Any deviation in normal/operating flow rate?4. Operators mistake?5. Utility failures?6. Improper safety documentation?7. Maintenance activities?8. Operational activities?9. Installation activities?10.

Toxic/flammable substances?

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