Drawings and Content




The process flow diagram is the first engineering document of any consequence developed for a plant or operating facility. Only the important items of equipment along with the principal controls are shown. It is the only document which shows the plant or system design operating conditions including extreme quantities and composition on; thus, it also serves as a reference document and training tool for the client. This guide or instruction demonstrates how process flow diagrams are developed such that they will be a complete working tool.


Process flow diagrams communicate, in a simplified and brief manner, the basic flow configuration in the important operating conditions of temperature, pressure and quantity of flow for processing units and operating systems. They are also the principal source of the same information for each unit operation, or processing step, and they provide key information about the items of equipment shown on it. They are the basis for developing the more complicated and detailed mechanical flow diagrams. Process flow diagrams are also important and useful for communicating concepts when studying new process designs or ideas and for preliminary project planning. They also show the basis for economic evaluations.


For most purposes, a material balance for the flow configuration shown is a part of the PFD in table form.



Process flow diagrams are developed in advance as rapidly as possible. When they serve as the basis for the development of mechanical flow diagrams they are advanced to a rather complete state before the development of the mechanical flow diagrams is begun; then they must be kept up-to-date and reissued frequently.

Sometimes it is necessary to work on flow diagrams produced by others, in which case it is necessary to conform to existing methods to satisfy the requirements of a client. 


Quality - process flow diagrams are widely distributed within a client’s organization. They represent the quality of work your company does, and they, along with the mechanical flow diagrams, may be the only drawings influential client personnel see from which to form an opinion of the design capabilities of your engineering firm. Your goal should be to produce high quality PFD’s on every project; they must be accurate, easy to follow and complete in all appropriate detail.


Responsibility - completed process flow diagrams are the result of a joint effort by process engineering and flow diagram drafting personnel. However, process engineering is responsible for the content as well as the validity and take and integrity of the process flow diagrams on a project.


Process engineering initiates a flow diagram by providing to design drafting either a hand sketch diagram, a copy of a diagram from a similar project appropriately marked up or client furnished diagram with the necessary additions and revisions noted. Process engineering provides a configuration of the flow scheme and the important items of equipment, along with a minimum of process controls. Additional information is added to the PFD’s by process engineering as it is developed.


Types - process flow diagrams may be drawn to show the flow configuration and present operating conditions, including a material balance, for any type of operating system. They may depict a utility system as well as either a primary or secondary processing system. PFD’s are seldom used or required for miscellaneous systems.


It is not unusual to develop process flow diagrams for the major utility systems, such as steam cooling water refrigeration etc. The size and complexity of such a system are major considerations in determining the need for utility PFD. For small, uncomplicated systems, the important operating conditions may be added to the MFD instead of providing another drawing in the form of a process flow diagram


For processing systems, either primary or secondary, the flow on a PFD is shown in a feed to product type sequence as shown on MFD’s. For utility or miscellaneous system, the flow is shown both in a geographical manner and in a forward flow matter as appropriate for distribution or collection facilities.


Drawing details

The same guidelines in general practice are used in the preparation of PFD’s as for MFD’s. PFD’s are considerably more simplified in comparison to MFD’s. The simplification also extensive the symbols used to represent equipment control; not only is it less detail shown but in some cases, the symbols themselves are simplified reference as well to the equipment symbols are included in the addendum to this section for example, on PFD’s, there is only one symbol used to designate a pump regardless of type whereas on MFD’s the basic type is indicated. Other differences are noted for compressors heat exchangers etc. The symbols used to show various types of instrumentation are essentially the same for both types of flow diagrams but fewer are shown and less detail is given on PFD’s there is a notable difference however all instrumentation and control line are shown as dashed lines no attempt is made to distinguish pneumatic lines from electrical etc.



most of the same guidelines and practices used in the arrangement of MFD’s are followed for PFD’s only the differences peculiar to process flow diagrams are given in this part of the guideline.


a. P&ID


The Piping and Instrument Diagram (P&ID), based on the Process Flow Diagram

(PFD), represents the technical realization of a process by means of graphical

symbols for equipment and piping as well as graphical symbols for process

measurement and control functions. The Utility Distribution Flow Diagram

(UDFD) is a special type of a P&ID which represents the utility systems within a

process plant showing all lines and other means required for the transport,

distribution and collection of utilities. The process equipment in the UDFD can be

represented as a box with inscription (e.g., identification number) and with utility



The representation and designation of all the equipment, instrumentation and

piping should comply with the requirements of this Standard. Auxiliary systems

may be represented by rectangular boxes with reference to the separate


Dimensions of the graphical symbols for equipment and machinery (except

pumps, drivers, valves and fittings) should reflect the actual dimensions relative

to one another as to scale and elevation. The graphical symbols for process

measurement and control functions for equipment, machinery and piping, as well

as piping and valves themselves, shall be shown in the logical position with

respect to their functions.

All equipment shall be represented such that the consistency in their dimensions

is considered if not in contrast to the good representation of the equipment.



1. General rules

Drafting shall be in accordance with the guidelines as outlined in this

Standard. The drafting must ensure the ability to maintain legibility

when the drawing is reduced to an A3 size sheet.

2. Drawings sheet sizes

Diagrams should be shown on A1 size (591mm × 841 mm) Larger sizes can be used if directed by the client. Most P%ID’s and PFD’s are issued on A3 or 11x17 sheets for approval.


3. Drawing title block


A. The following should be in the title block of all


- Revision table;

- Client Company’s name (e.g., Canuck Oil);

- Name of Engineering company (e.g., Big Time Engineering Inc);

- Name of refinery or plant;

- Engineering Company and Client company emblem;

- Drawing title;

- Client Project No.;

- Engineering Company Project Number;

- Engineering Company drawing No. (If Required);

- Client Drawing No.


b. Title block sizes and drawing dimensions:

A1 = 594 × 841 

A2 = 420 × 594 

A3 = 297 × 420 


4. Line widths


To obtain a clear representation, different line widths shall be used. Main flow

lines or main piping shall be highlighted.


An example of typical line widths is:

- 0.8 mm for main process lines;

- 0.5 mm for other secondary process lines; utility lines, and underground lines;

- 0.5 mm for graphical symbols for equipment and machinery, except valves

and fittings and piping accessories;

- 0.5 mm for rectangular boxes for illustrating Unit operations, process

equipment, etc.;

- 0.5 mm for subsidiary flow lines or subsidiary product lines and for energy

carrier lines and auxiliary system lines;

- 0.4 mm for class changes designation;

- 0.3 mm for graphical symbols for valves and fittings and piping

accessories and for symbols for process measurement and control

functions, control and data transmission lines;

- 0.3 mm for all electrical, computer and instrument signals;

- 0.3 mm for reference lines;


5. Line spacing


The standard space between parallel lines should be 10mm but can be reduced in extreme circumstances (no room) to an absolute minimum of 5mm


6. Direction of flow.


In general, the main direction of flow proceeds from left to right and from top

to bottom. Inlet and outlet arrows are used for indicating the inlet and outlet of

flows into or out of the diagram. Arrows are incorporated in the line for

indicating the direction of the flows within the flow diagram at changes of direction of the process line. If necessary for

proper understanding, arrows may be used at the inlets to equipment and

machinery (except for pumps) and upstream of pipe branches.

 If a P&ID consists of several sheets, every attempt should be made to keep the incoming and outgoing flow lines or piping on a sheet drawn in such that the lines continue at the same level when the individual sheets are horizontally aligned.


7. Connections


Connections between flow lines or pipelines shall be drawn as shown 



Below is how to show two flow lines or pipelines that are not connected:


The break is 1.5 mm on each side of the line being crossed. It is essential that the drafter is consistant in which line is to be broken and which is to remain solid. The standard is to break the vertical lines and leave the horizontal lines unbroken except in cases where the crossing lines are not of equal importance. The hierarchy of lines is generally 

A. Main Process

B. Secondary Process

C. Main Utility

D. Secondary Utility

E. Instrument

F. Miscellaneous



8. Text

A. Type of lettering


Lettering in accordance with client standard unless there is no client directive in which case the company standard will be used.


B. Height of lettering


Unless directed otherwise by client standard the height of letters should be:

- 7 mm for drawing number;

- 5 mm for drawing title and identification numbers of major equipment;

- 3 mm for other inscriptions.


C. Text


(i) Identification numbers for equipment should be located close to the

relevant graphical symbol. Further details (e.g., designation, design capacity, 

design pressure, etc.) may also be placed under the identification numbers.


ii) Line Numbers and piping


Line numbers and piping descriptions are written parallel to and above

horizontal lines and at the left of and parallel to vertical lines. If the

beginning and end of flow lines or piping are not immediately

recognizable then the line change, spec change etc. must be made easily recognizable by note and leader on the sheet.


iii) Valves and other fittings


Description of valves (Valve Numbers) and fittings are written next to the symbol and in the same direction as the direction of flow.


iv) Measurement and control functions

The symbols used should be in accordance with the requirements

stipulated in ISA-S5.1 and ISO 3511, Parts 1 and 4, latest revisions

unless otherwise specified by the client or process engineering


Equipment Location Index


P&ID sheets are usually divided into equivalent intervals either in length and width.

The divisions are designated numbers from 1 to 23 in length and alphabets from "A" to "P" 

in width. Equipment location on each diagram may be addressed by the relevant coordinates where required by client standard. This is usually addressed by a table in the upper right-hand area of the flow diagram under title of "Item Index".


All main equipment is listed by equipment number, alphabetically and

numerically and equipment location coordinates. In a separate sheet, apart from

P&IDs, an "Item Index" can then be prepared showing all equipment in the

Unit or plant with reference to P&IDs and equipment location.



2. The area above the title block on each sheet shall be completely left open for



3. The general flow scheme shall be from left to right. Unnecessary line crossing

should be avoided.


4. Process lines entering and leaving the diagram from/to other drawings in the

Unit shall be terminated at the left-hand or right-hand side of the drawing.

Lines from/to higher number drawings shall enter and leave the drawing on

the right-hand end and vice versa.


5. Each process line entering or leaving the side of the drawing should indicate

the following requirements in an identification box (see Appendix B):

a. The line service

b. The origin or destination equipment item number

c. Continuation drawing number with the relevant coordinates.

d. Process lines to/from other Units should be terminated at the bottom of the

drawing at a box indicating the following requirements (see also Appendix B):

e. The line service

f. Source or destination Unit name and number

g. The drawing number of the connecting flow diagrams with the relevant



7. All utility lines entering or leaving the diagram shall be terminated at any

convenient location at a box indicating the relevant utility service abbreviation

(e.g., CWS, CWR, IA, etc.). Standard for utility services abbreviations. “Utilities Identification Table” at the end of this section.


8. Instrument, control system and software linkage signals from sheet to sheet

shall be terminated preferably at the side of sheet or in an appropriate

location at a box indicating the continuation instrument number, location, and

drawing number (


9. Equipment descriptions of towers, vessels, tanks, furnaces, exchangers,

mixers and other equipment except machinery shall be located along the top

of the flow diagram. Machinery descriptions shall be along the bottom.










Each P&ID must present all information required as listed below during

design a project in the detailed phase. Information shown on each P&ID in the

design stage will be per the client requirements and must be defined in advance of the beginning of drafting.


Vendor packages are defined with an outline of the main components 

shown in a dashed/dotted box. Packaged equipment numbers must be set to easily define what

equipment is included in a package and which package it belongs to. All equipment numbers have to be shown adjacent to each equipment and instrument of the package.


Equipment, instruments or piping that are traced or jacketed must be indicated showing the jacketing

or /and tracing.


The identification number and service presentation shall be shown for each piece

of equipment. This information shall be indicated in or adjacent to towers, drums,

heaters, tanks and heat exchangers, etc.


Equipment Indication


1. Vessels, towers, drums

a. The following are a general indication of the minimum requirements of what must be defined:

- Changes of shell diameter (if any)


- Top and bottom trays, and those trays which are necessary to locate

feed, reflux and product lines


- All draw-off trays with tray number and diagrammatic representation of

the downcommer position (e.g., side or center)


- All nozzles, manholes, instrument connections, drains, vents, pumpout

and steam-out connections, blank-off ventilations, vortex breakers,

safety/relief valve connections, sample connections and hand holes;

- Skirt or legs, top and bottom tangent lines


- Elevations above base line to bottom tangent line of column or to

bottom of horizontal drum


- The position of high high liquid level (HHLL), high liquid level (HLL),

normal liquid level (NLL), low liquid level (LLL) and low low liquid level



i) For draw-offs only "NLL" shall be shown. The other liquid positions

will be shown as required.


ii) Indication of "HHLL" and "LLLL" shall be made when they are

actuating start/stop of an equipment or machinery through a switch.

iii) "HLL", "NLL" and "LLL" shall be shown for all cases except as

specified under Note i) above.


- All flanged connections; [all connections whose purpose is not readily]

evident shall indicate the purpose (e.g., spare inlet, catalyst draw-off,



- Catalyst beds, packings, demisters, chimney trays, distributors, grids,

baffles, rotating discs, mixers, cyclones, tangential inlet and all other


- Water drop-out boots;

- Maintenance blinds for the vessel nozzles.


b. Important notes:

- All nozzles and connections indicated on the equipment data sheet

shall be shown in their correct positions.


- All indications shall be such that the consistency in the dimensions is

considered, although not necessarily to scale.


- Numbering of the trays shall be from bottom to top.


- Height of the vessel bottom tangent line shall be indicated.


- Self standing: Skirt height


- Elevated vessel: Minimum required height shall be shown as "min. ...."


- A valved drain for all columns and vessels shall be indicated.

Generally, this valve is to be located on the bottom line outside the

skirt and between the vessel and the first pipe line shut-off location

(valve or blinding flange). The drain valve shall be located on the

bottom of the vessel when:

i) No bottom line is present, or

ii) The bottom line is not flushed with the lowest point of the vessel.


- The valved vent with blind flange for all columns and vessels provided

on the top of the vessel should be indicated.


- Safety Relief valves generally located on the top outlet line

downstream of the vessel blinding location or directly connected to the

vessel should be indicated

- Utility connections on all vessel/columns shall be shown.

- One local PI shall be indicated on top of vessel/column.

- One local TI shall be indicated on the top outlet line of vessel/column.

- Nozzles identifications on vessels, reactors and towers shall be



c. The following requirements shall be described under equipment


- Vessel item number (this number will also appear adjacent to the


- Service;

- Size [inside diameter(s) and tangent to tangent length];

- Design pressure (internal/external) and design temperature;

- Indication of insulation;

- Line number of vessel trim (this applies to LG & LC connections, vents,

sample connections, etc.);

- Indication of cladding and lining (if any).

2. Tanks

a. The following requirements shall be shown:

- All nozzles, man ways, instrument connections, drains, vents, vortex

breakers, and safety/relief valve connections;

- All internals such as steam coils, air sparkers, tank heaters, vortex

breakers, water-draw off sump and etc.

b. Equipment description

- equipment item number (this number also appears adjacent to the


- Equipment Description

- equipment item number (this number also appears adjacent to the


- inside diameter and height;

- nominal capacity, in (m³);

- Indication of insulation.


3. Fired heaters, boilers, incinerators

a. The following requirements shall be shown:

- All nozzles, instrument connections, drains, vents and damper(s);

- Ducting arrangement including damper actuators where required;

- Detail of draft gages piping and arrangement;

- Waste heat recovery system (if present), such as economizer, air

preheater, forced draft fan, induced draft fan, etc.;

- Decoking connections;

- Detail of one complete set of burners for each cell and total burner

number required for each type of burner;

- Tube coils schematically in correct relative positions and all skin point


- Logic diagram of shut down system (heat off sequence);

- Number of passes and control arrangement;

- Snuffing steam nozzles and piping arrangement;

- Blow-down and steam-out connections;

- Testing facilities;

- Convection section (where applicable).


b. Equipment description

- item number (this number will also appear adjacent to the equipment);

- service;

- duty (kJ/s);

- design pressure and temperature of coils;

4. Heat exchangers, coolers, reboilers


a. The following requirements shall be shown:

- All nozzles, instrument connections, drains and vents, chemical

cleaning connections and safety/relief valves as indicated on the

equipment data sheet;

- Spectacle blinds for the isolation;

- Elevations required for process reason (e.g., reboilers, condensers,


- The connections which allow pressure and temperature survey of heat

exchanger facilities;

- The position of high liquid level (HLL), normal liquid level (NLL) and

low liquid level (LLL) for kettle type reboilers;

- Direction of flow in each side of exchanger.

b. Important notes

Due considerations should be made for proper indication in the following


- Generally, direction of flow shall be downflow for cooled media and

up flow for heated media.

- Isolation valves shall be provided on inlet and outlet lines where

maintenance can be performed on the exchanger with the Unit

operating. Provision of by-passing is required for this case.

- Shell and channel piping shall be provided with a valved vent

connection and a drain connection unless venting and draining can be

done via other equipment.

- At exchangers with circulating heat transfer media, the outlet valve

shall be of a throttling type for control of heat duty.

- An inlet and outlet, temperature indicator shall be provided on each

exchanger (on either shell or tube side) so that to facilitate checking of

heat balance around exchanger.

Type of temperature indicator shall be as follows:

- A board mounted temperature indicator (TI) shall be provided at the

inlet and outlet of all shell and tube process/process exchanger.

- For water coolers, the water side outlet shall be provided with a local TI

only. The shell side in and out shall be provided with board mo- Thermowells (TWs) shall be provided between each shell side and

tube side of the same services when the exchangers are in series.

- Local indicator type shall be provided for the requirement of local

temperature control, such as manual bypass control.

c. Equipment description

- Equipment item number (this number also appears adjacent to the


- Service;

- Duty (kJ/s);

- shell side design pressure and temperature;

- tube side design pressure and temperature;

- Indication of insulation.

d. Sequence of numbering for stacked exchangers/coolers shall be from top

to bottom.


5. Air fin coolers

a. The following requirements shall be shown:

- All nozzles and instrument connections;

- Blinds for the isolation;

- Any automatic control (fan pitch control or louver control) and any

alarm (vibration alarm, etc.);

- Configuration of inlet and outlet headers and the branches. Only one

bundle and fan shall be shown; total number of fans and bundles shall

be indicated. When multiple bundles are required, header’s

arrangement as separate detailed sketch shall be indicated;

- Steam coil and condensate recovery system (if required);

- Isolation valves (if required); isolation valves shall be provided in

corrosive and fouling services where individual bundles can be

repaired and maintained with the Unit operating;

- Valved vent and valved drain connection for each header, vent header

should be connected to closed system for volatile services;

- A board mounted TI at inlet and outlet, (the TI will monitor the process

side of each air fin service). If multiple bundles to be used for fouled

services, provide TW’s on the outlet of each bundle.

b. Equipment description

- Equipment item number (this number will also appear adjacent to the


- Service;

- Duty (kJ/s);

- Tube side design pressure (internal and external) and design



6. Rotating Machinery

a. The following requirements shall be shown:

i) Pumps

- All nozzles including instrument connections;

- Pump suction valve and strainer, and discharge valve and check

valve. Provision of wafer type check valve should be avoided

unless otherwise specified;

- Pump drains and vents piping and destination.

- The type of pump;

- Pump auxiliary system connections such as, cooling water, seal oil

and lube oil, steam, etc.;

- Detail of lube and seal oil /sealing systems, cooling water piping

arrangement, and minimum flow bypass line requirement for


- Winterization and/or heat conservation (steam or electrical) where


- Warm-up and flushing oil lines detail; a DN20 (¾ inch) bypass/drain

from the check valve to the pump discharge line shall be provided

as warm-up line for the cases specified in item 8.1.4 of this


- Pressure gage located on the discharge of each pump; the gage

shall be installed between the pump discharge nozzle and the

check valve;

- Pressure relief safety valves (if any);

- Automatic start-up of standby unit (if required);


ii) Compressors and blowers

- Type of compressor or blower;

- Start-up facilities (i.e., inert gas purge system);

- Safety/relief valves;

- Suction and discharge valves;

- Suction strainer (filter) and discharge check valve;

- Suction and discharge pulsation dampener where required;

- Valved vents and casing drains;

- Winterization (steam or electrical tracing on suction piping) where


- Lube and seal oil / sealing system and cooling water systems detail


- Interstage coolers where required;

- Surge protection (where required);

- Inlet and outlet nozzles;

- All instrument connections.


iii) Steam and gas turbine drivers

- All nozzles and connections;

- Detail of all auxiliary systems for steam turbine drivers such as

steam supply, condensate return, surface condenser and etc.;

- Detail of lube oil, cooling water, etc.;

- All instrumentations such as PI, TI, etc.;

- Safety/relief valves; relief valves shall be located between the

discharge nozzle and the outlet isolation valve; weep hole at

exhaust of the relief valve which opens to atmosphere shall be

provided to draw-off the condensate drain.

- Warming bypass around inlet isolation valve for steam turbines; the

valve on warm-up line shall be DN25 (1 inch) globe type;

- Steam traps and condensate recovery system for the steam turbine

casing drain and upstream of isolation valve at inlet of the turbine;

- Vent line to atmosphere at turbine exhaust; the vent is required for

the start-up/test operation of the turbine.

- Detail of all firing and control systems for gas turbine drivers.


b. Equipment description

i) Pumps

- Pump item number (this number also appears below the pump);

- Service;

- Capacity, (m³/h, dm³/h for injection pumps);

- Differential pressure, (kPa

- Relative density (specific gravity) of pumped fluid at pumping


- Indication of insulation and tracing;

- Miscellaneous auxiliary piping (CW, flushing oil, seal oil, etc.).

ii) Compressors and blowers

- Equipment item number and stage (this number also appears

below the compressor);

- Service;

- Capacity, (Nm³/h);

- Suction pressure, and temperature, [kPa (g)], (°C);

- Discharge pressure, and temperature, [kPa (g)], (°C);

- Miscellaneous auxiliary piping (CW, lube oil, seal oil / sealing

system, etc.);

- Gas horse power, (kW).

c. Other requirements

- When a pump or compressor is spared, the data is listed once

commonly for both pumps and compressors at the bottom of the flow

diagram. The spare is identified by the word "Spare" below the pump

or compressor. The operating equipment and the spare have the same

number but with suffixes "A" and "B".

- Stage numbers are shown only for multistage compressors. All

compressor data for the first stage shall be indicated. For subsequent

stages only N m³/h may be omitted.

7. Miscellaneous equipment

Depending on the type of equipment (silencer, flame arrestor, filter, etc.) the

following information shall be presented:

a. All nozzles, instrument connections, vents, drains, etc.;

b. Equipment description at top of the flow diagram and including:

- Equipment item number;

- Service;

- Tracing/insulation requirements;

- Design pressure and temperature;

- Capacity


The following requirements shall be shown:

1. All instrumentation including test points;

2. Isolation valves connecting to instruments (primary connection valve);

3. Control valve sizes and air failure action (Fail Closed, Fail Open, Fail in Last Position);

4. Block and bypass valve sizes at control valve stations;

5. Level gages connection type and range, and level controllers connection type,

range and center of float (where NLL is not shown). Type, material and

tracing requirement of level gages shall be shown

6. Sequence of opening and closing for the split range control valves;

7. Solenoid shut-down devices at control valves/shut-off valves;

8. Tight shut-off valves requirements (where required);

9. Hand wheels when provided on control valves;

10.Limit switches on control valves when required;

11.Mechanical stopper and/or signal stopper on control valves when required;

12.Push buttons and switches associated with shut-down systems;

13.The instrument tag number for each instrument;

14.Analyzer loop details and special notes as required;

15.Winterization of instruments;

16.Compressor local board mounted instrumentation;

17.Software linkage and alarm and shut-down logic system. Complex shut-down

systems shall be shown as a "black box" with reference made to the logic

diagram shown on a separate sheet. All actuating and actuated devices shall

be connected to the "black box";

18.All elements of advance control and optimization systems;

19. Indication of "Readable From" for all local indicators and/or gages which shall

be readable from a designated valve.


1. General

a. All piping shall be shown on P&I Diagrams, including:

- Process lines;

- Utility/common facility branch lines (e.g., sealing and flushing lines,

cooling water lines, steam-out lines and connection, nitrogen lines,


- Flare lines, including safety/relief valves discharge lines;

- Start-up and shut-down lines;

- Pump-out lines;

- Drain and vent lines and connections;

- Purge and steam-out facilities;

- Catalyst regeneration lines;

- Catalyst reduction lines;

- Equipment and control valve bypasses;

- Detail of spool pieces, equipment internals, etc., when required;

- Steam tracing and steam jacketing.

b. All line numbers, sizes and line classification shall be shown.


c. The direction of normal flow shall be shown for all lines.


d. The points or spec. breaks at which line sizes or line specifications change

shall be clearly indicated.


e. All blinds shall be indicated on the drawings, and the symbols used shall

distinguish between tab blinds and spectacle blinds.


f. All vent and drain connections shall be identified whether screw caped or

blind flanged, if required.


g. Steam traced lines and steam jacketed lines shall be so indicated.

h. All equipment flanges, all reducers and non-standard fittings, such as

expansion bellows, flexible tubes, shall be shown.


i. All valves shall be shown by a symbol representing the type of valve. Any

special orientation or location required for process reason and/or

operability shall be shown. It is not necessary to show flanges at flanged

valves except for those cases where the flanges deviate from the piping

specification for the line in question, in which case flange and rating shall

be shown. Any isolating valve shall be shown locked, normally open or



j. Control valve sizes shall be shown.


k. All valves shown on the flow diagram shall have their size indicated by the

valve, if different from line size


l. Insulation and tracing requirements shall be covered in the line numbering

system and shown above the line. Tracing requirement shall be noted on

P&IDs by a dashed line parallel to the line to be traced.


m. Valve boxes/valve pits shall be shown by two embraced squares or

rectangular with indication of "Valve Box" or "Valve Pit".


n. Safety relief valves type, inlet and outlet size and rating and set pressure

should be shown.


o. For pressure ratings designations-nominal size and pipe component nominal

size see Appendices F & G of this Standard respectively.


2. Special requirements


a. High point vents and low point drains are shown only when they are

connected to a closed system, or are required for process reasons.


b. Utility lines originate and terminate adjacent to the equipment involved

shall be shown. Only the length of line necessary for valving,

instrumentation and line numbering is shown. Utility line origin and

terminus is indicated by reference symbol or abbreviation only.


c. Pertinent information regarding a line such as "do not pocket" or "slope",

etc., shall be noted adjacent to the line.

d. Typical air cooler manifold piping arrangement should be shown.


e. Connections on process lines which require being blanked (blind flanges used) or deblanked (blind flanges removed) for flow direction under special circumstances to be shown on P&ID.


f. Reduction and enlargement in line size are indicated by line size

designation, and reducer and expander symbols.


g. Calculated wall thicknesses and/or schedules not already prespecified in

the individual line classes shall be shown on the flow diagrams.


h. Corrosion allowances other than the nominal allowances indicated in the

individual line classes shall be shown on the diagrams.


i. All operating drains shall be noted and sized on the flow diagrams and

shall be routed to a drain funnel. Destination of the drains shall be

according to the relevant specifications. All drains carrying light

hydrocarbons (Reid vapor pressure 34.5 kPa absolute or greater) shall be

segregated from the oily sewer system, and shall be connected to the flare



j. Sample and test connections shall be shown on the diagrams where

required. Samples which require cooling and connections to the flare shall

be shown with the cooling and flare lines connections.


k. Emergency showers, eye wash fountains and utility stations shall be

shown on the Utility Distribution Flow Diagrams.


l. Any locations where slopes, straight runs, minimum mixing runs, etc., are

required for process reasons must be indicated.


m. The necessary instrumentation and piping for start-up, control and shutdown,

etc., shall be shown for any equipment on P&ID wherever applicable.


n. Break points between underground and aboveground piping with

insulating flanges (if required) shall be shown.


o. Minimum distance requirement for in line blending to be indicated.


p. Weep hole requirement to be shown.


3. Piping specialty items


a. Piping components not identified by instrument or mechanical equipment

numbers, etc., and not covered by the piping material specification, shall

be identified by assigning a Specialty Item Number or an Item Code

Number for identification symbol and shall be shown on the diagrams.


b. Symbol "M" standing for "Monel Trim" should be mentioned on the valves

on the P&IDs in services where there is a possibility of condensed water

and H2S being present except for the line classes which provide Monel

trim valves and other features. Where it is intended that the whole line

should have Monel trim valves it should also be indicated on the line list


c. ASME and non ASME Code change should be indicated for connection

where ever applicable.

4. Steam traps & winterizing system


The following requirements shall be followed:

a. Steam traps pertaining to the winterizing systems (steam tracing) are not

shown on the P&IDs except for the following cases:

- At dead ends/pockets on steam lines;

- At upstream of the Unit battery limit main block valves on steam lines;

- At all points which there is possibility of condensation;

- At upstream of the first block valve of steam line going to the steam

turbine drivers, steam coils or steam reboilers.


b. Steam trap and the relevant steam and condensate lines to be shown for

all steam reboilers, heaters, coils, etc.


c. Steam/electrical tracing requirement shall be noted on P&IDs by a dashed

line parallel to the line to be traced.


General Notes

General notes to be put on the front sheet of P&I Diagrams of each "Unit" under

title of "General Notes". Reference should be made to the front sheet drawing

No. showing "General Notes", on each P&I Diagram.


1. The following general notes shall be specified as minimum requirement:


a. All dimensions are in millimeters except as noted.


b. Elevations shown are above the highest point of paving.


c. All valves are line size unless otherwise shown.



d. This flow diagram is diagrammatic only. Design of pipe lines must be

investigated for venting of gas and vapor pockets in piping and equipment,

low points in piping, pumps and equipment for freezing and draining and

accessibility of all valves, flanges and instruments including

thermocouples etc.


e. All electronic instrumentation shall be installed away from steam lines and

high temperature heat sources.


f. For level transmitter center of float is NLL. The range shall cover the

difference between LLL & HLL.


g. Sample taping for gas samples shall be from the top of the main line. For

liquid samples tapping shall be done from the side.


h. Except for process reasons, low point drains and high point vents are not



a. All items marked (P) can be supplied as part of package Units.


b. Temperature instruments shown with "M" are provided with Monel well.







2. The following general notes may be specified as required:



i) Drains for all sizes

b. Block valves on orifice tap

• DN 15 (½ inch) single gate valve shall be provided for the all orifices of

the piping class of PN 100 (600 #) and less.

• DN 20 (¾ inch) single gate valve shall be provided for the all orifices of

the piping class of PN 150 (900#) and over.

c. Drain valve of level gages and instruments:

• Drain valves [DN 20 (¾ inch) gate valve] shall be provided.




The following design criteria shall be applied for preparation of P&I Diagrams

unless otherwise specified in the relevant piping and/or equipment specifications

of the Company. In case of any conflict, the specific piping and/or equipment

specifications will be governed.


Assembly piping of pumps


1. Valve size selection basis for pumps

Generally, the size is likely different between pump suction line and pump

suction nozzle, or pump discharge line and pump discharge nozzle. In case

that, pump nozzle is one or more sizes smaller than the line size, the size of

block valve shall be in accordance with the following:



Nozzle Block Valve at Pump Suction

- One size smaller than line

- Two or more sizes smaller than line

o Same as suction line size

o Select one size smaller than line at Pump Discharge

o Smaller than discharge line select one size smaller than line


2. Pump strainer

The suction strainer of pumps shall be selected in accordance with

DN 80 (3 inch) and larger T

DN 50 (2 inch) and smaller Y

Strainers DN150 (6 inch) and larger shall have DN 25 (one inch) drain valve.


3. Pump vents and drains


Pump casing drains and vents shall be routed to closed system such as flare

for the following services:

• Fluids containing toxic material;

• Fluids with a Reid vapor pressure greater than 34.5 kPa (abs) at pump

operating temperature.

• In addition to the above, the vent of casing for the vacuum service should be

routed back to the suction vessel to make out the pressure balance prior to

the pump operation. Drain of hydrocarbon pumps shall also have disposal to

oily water sewer in all cases in addition to the above requirements unless

otherwise specified.


4. Warming-Up line

The provisions for warming-up of pump is required for the pump operated at

170°C and higher or when the process fluid solidifies at ambient conditions or

the fluids are corrosive or toxic.


5. Auxiliary piping of pump

Details of auxiliary piping such as, cooling water, plant water, steam and

condensate, mechanical seal flush fluid, etc., which are required as per pump

data sheet shall be shown on a separate drawing. Reference to the auxiliary

piping drawing shall be noted under the pump description.


Steam-Out, Drain and Vent for Vessels

 The vent valve shall be directly mounted on the vent nozzle with blind flange.

 A blanked off ventilation nozzle shall also be provided on the top of the all

horizontal vessels near the end opposite the manway.


 Vent connections must be located on top of the vertical and horizontal



 The drain valve will be provided as follows:

• For low pressure services, up to design pressure of 3800 kPa, provide

single block valve with blind plate.

• For high pressure services over design pressure of 3800 kPa, or where

the nature of liquid requires it, provide double block valves with blind plate.


Bypass for Safety/Relief Valve

The bypass shall be provided for venting the hydrocarbon gas or toxic gas to

flare system while plant shut-down or start-up. Provision of bypass shall be as

per following criteria:

1. Vessels:


Bypass shall be provided unless otherwise specified in the relevant

Company’s specifications.

2. Piping/equipment:

a. Gas service

• If there is other purge line to flare on same stream line, bypass is not

required for safety/relief valve.


• In case of no purge line to flare for toxic or flammable hydrocarbon,

bypass valve shall be provided. The size of bypass valve and line shall

be same as the vent size of piping/equipment.

o Liquid service

Bypass valves are generally not provided for liquid service unless

otherwise specified.


Block and Bypass Valves for Control Valve


Block valves shall be line size while the bypass valve is to be same size as control valve and full port


Line Numbering


1. Line numbers shall be assigned to all lines with the following origins and


o From individual equipment item to individual equipment item;


o From line to individual equipment item and vice versa. Another number is

required for the line located at the downstream of equipment;


o From line to line (exceptions: control valve bypass, block valve warm-up

and equalizing bypasses, and safety/relief valve bypass);


o From unique equipment to the same unique equipment item (except level



o From line or equipment to atmosphere, funnel, or closed drainage system

(exception: continuous process vent stacks and process drains).


2. Pipe line numbers shall be prefixed, from source to Unit battery limit with the

Unit number of the Unit of origin.


3. A new line number is required when the pipe design condition can vary (e.g.,

downstream of the control valve assembly) or when a new piping class is to

be specified.


4. Line number shall be held up to the point where the line ends to the header or

Unit battery limit block valve. All branches to and from header shall have an

individual line number.


5. All utility headers (systems) including all steam, water and sewer lines shall

be numbered with their respective Units. All branches serving a specific Unit

will be numbered with that Unit.


6. Line numbers shall be selected so that consecutive line numbers are grouped

first by common service. Spare line numbers may be left between the



7. All process lines routed from Unit to Unit shall be assigned on interconnecting

line number. Within the process Unit(s), Unit line numbers are to be assigned.

The interconnecting Unit P&I Diagram is to show every interconnecting

process line and indicate the line numbers inside the process Units at the

Units battery limits.


Philosophy of Instrumentation Installation


Piping design is responsible up to and including the first valve off of the process or utility line. Process engineering/instrument engineering then marks up what they need for the installation of the instrument and piping design does drafting only.


Utility Connections

Utility connections to process line and equipment for steam and nitrogen shall be

as follows:

Connections to process line and/or equipment  



Notes on utility tie-in:


• The isolation valve may be omitted if the process line is open to



• Provide a drain at downstream of check valve to check for leakage.


• Provide spectacle blind and block valve for N2 service.


• Main block valve for steam service shall be at the branch point from steam



• This configuration shall be used for low pressure steam (all sizes). For

medium and high-pressure steam double block valves with bleeder

between the valves is required.



2. Connections to vessel for steam-out

a. Permanent steam-out connection:





b. Temporary steam-out connection  


Unit Battery Limit Installation

1. Process lines:





Notes on unit battery limit installation requirements:

• On hydrogen, nitrogen, toxic gases and all high-pressure fluids

(Pressure greater than 4000 kPa), provide double block valves, spectacle blind and drain as shown.

• Provide for each process line (not included in item 1 above) an isolation

valve, spectacle blind and drain as shown.

• Provide a flow indicator and recorder shown on board for each process

stream entering and leaving each Unit. Do not duplicate measuring

elements in the same stream within one block area.

• Provide a board mounted TI on each process stream entering and leaving

the Unit where a flow integrator is provided. Do not duplicate with TI’s

required for other purposes. Generally, the TI to be located at downstream

of the flow element.

• Provide a sample station for all products leaving and/or entering the Unit.

Product streams leaving Units shall be piped at the Unit limits to the

relevant slops header (light or heavy slops) as well as for the start-up (offspec.)


• Provide a local PI on each process stream entering and/or leaving the

Unit. Do not duplicate with PI’s required on the same streams. PI may be

board mounted as required.

• Special attention should be made to the possibility of avoiding duplication

of some or all the above-mentioned hardware on the adjacent Units.














2. Utility lines (see figs. Below):









Notes on unit battery limit installation requirements:

a. Provide valves, drains and instrumentation as shown in Fig. 11 for the

following cases:

- low pressure steam (all sizes);

- Medium pressure and high-pressure steam [sizes smaller than DN200

(8 inch)];

- Boiler feed water.

b. Provide valves, drains and instrumentation as shown for

medium and high-pressure steams for DN200 (8 inch) and larger sizes.

c. A line size boot at upstream of the first isolation valve shall be provided

and shall discharge condensate to the condensate recovery system

through steam trap. The boot and steam trap requirement is not needed

for boiler feed water streams.


Sample Connections

For sample connection symbols, reference to be made to Appendix X and

Appendix XX.(Appendix to come)


Steam Trap Assembly

For individual steam trap symbols reference to be made to Appendix X of this


1. Steam trap assembly with internal strainer for different services:


a. Winterizing (see below):

The following symbol can be used to demonstrate the steam trap

assembly configuration as shown below in and UFDs in

order to avoid duplication.


b. Heat conservation

The following symbol can be used in place of the steam trap assembly

shown below on P&IDs and UFDs.




2. Stream trap assembly with external strainer 





1. The Utility Flow Diagram(s) (UFDs) shall be prepared as separate drawing

titled as "Utilities Distribution Flow Diagram". The distribution of utilities for

plant operation shall be shown on the drawing. The utilities for plant operation

are generally classified as follows where applicable:

- Several grades of steam;

- Several grades of condensate;

- Boiler feed water;

- Cooling water and sea water;

- Raw (fresh) water;

- Plant and potable water;

- Fuel oil and fuel gas;

- Instrument and plant air;

- Nitrogen;

- Inert gas;

- Seal oil/flushing oil;

- Closed circuit hot oil system;

- Flare and blow-down;

- Chemical system such as caustic and ammonia.

The above utilities are classified into several groups and shown on diagram(s)

in accordance with the next articles. A dedicated drawing shall be prepared

for "Flare and Blow-down".


2. Utility Flow Diagrams shall be presented in accordance with the requirements

stipulated in this Standard for P&IDs where applicable.


3. Utility Flow Diagrams shall show main distribution/collection headers and

finger headers with their isolating facilities and instrumentation. The branch

line and subheader arrangement shall be shown as practical as possible.


4. Indication criteria of connection between P&IDs and UFDs is according to the

following general philosophy:

a. The indication of isolation valve shall not be duplicated on P&ID and UFD.

b. Valve and instrument which will be used for the normal operation shall be

indicated on P&ID, such as:

- Block valves for water cooler inlet and outlet;

- Block valves for snuffing steam of fired heater;

- Globe valve for steam injection control;

- Control valves for fuel control.

c. Valves which will be used only for start-up and shut-down shall be

indicated on the UFD such as:

- Header isolation valve for steam purge connection;

- Isolation valve for fuel gas or fuel oil.


5. Utility/common facility branch line header valves at the process Unit battery

limit shall be shown. The Utility Flow Diagram shall also indicate any valve in

utility/common facility individual branch lines required for process and

maintenance operations even if these valves may be physically located in the

pipe rack or the sequence of branches may allow in the future for a single

valve to serve several branch lines.


6. Isolation facilities shall be indicated for:

- finger areas;

- process Unit block areas;

- at position of change from pipe rack to pipe rack.


7. The finger area is defined as being the area that serves a process

area which may consist of one or more process Units. In addition to the

equipment that is located alongside the finger pipe rack, the finger area also

includes the equipment located alongside the main pipe rack.


8. Utility Flow Diagram shall be arranged to cover the whole refinery/plant area

and these are divided into separate sheets each with corresponding match

lines. Depending on the complexity and extent of the particular utility/common

facility, sheets may be combined, extended or omitted as required.


9. All equipment that is supplying a particular utility common facility either from

the system (e.g., steam boilers) or from a process Unit (e.g., waste heat

boilers) shall be shown in a "box" in geographical location. This "box" shall

give relevant equipment number(s), Unit number and sheet number of the

drawing in which the equipment is detailed.