Introduction to Piping Isometrics

Piping Isometrics Introduction 

This topic of discussion gives basic information for piping isometrics, its preparation details, drafting requirements, material take off in isometrics, supports etc.

Piping isometrics drawing are a type of pictorial drawings that show the three principal dimensions of an object in one view. The principal dimensions are the limits of size for the object along the three principal directions.

Pictorial drawings consist of visible object faces and the features lying on the faces with the internal features of the object largely hidden from view. They tend to present images of objects in a form that mimics what the human eye would see naturally. They are excellent starting point in visualization and design and are often used to supplement multi view drawings.

An isometric drawing is one of three types of axonometric drawings they are created based on parallel projection technique. In isometric drawings, the three principal axes make equal angles with the image plane.

Isometrics in a single view provide a pictorial representation of the height, width and depth dimensions in a single view to provide a pictorial representation.

Piping material take off is generated from isometrics in which each individual component that makes up a pipe configuration is tabulated for purchase or procurement.

Isometrics are also used by many companies as fabrication drawings. Once drawn and properly dimensioned, isometrics are provided to fabricators who will build each piping configuration.

After configurations are fabricated, isometrics serve as an aid to the construction and erection of the facility by providing workers with the locations of tie-ins, connections, and routings

Most engineering and construction companies draw a piping isometric of every piping configuration to be installed in the facility. Piping isometrics are typically drawn in single lines irrespective of the pipe’s size, with each pipe line drawn on a separate sheet. The isometric must provide a detailed description of the pipe’s routing from beginning to end.

Isometrics are also drawn as a schematic, which means they are not drawn to scale. Isometric symbols for fittings, flanges, and valves represent all sizes of pipe. No attempt is made to represent a pipe’s actual size or pound rating graphically. This information is conveyed through the use of callouts and notes placed on the drawing.

Although piping isometrics are not drawn to scale, it should be drawn proportionally. Drawing an isometric proportionally simply means drawing a 10M run of pipe twice as long as a 5M length of pipe.

Construction Techniques for Isometric Drawing

It is quite easy creating isometric lines on isometric planes. This is done by drawing the lines parallel to isometric axes. On isometric planes, circles in principal orthographic views turn to isometric ellipses and arcs appear as partial isometric ellipses

The box technique is the most common construction technique. The box technique is also known as the coordinate technique.

Input data for preparing isometrics

Isometric drawings of pipe lines are prepared to further detail the pipe line. This detailing is required for the easiness in fabrication and erection at site.

The following data are required to prepare an isometric drawing.

1. Line list/ schedule
2. Piping GA drawings
3. Piping specification
4. PEFS/ P&ID
5. Painting Specification for the project
6. Standards for isometric symbols & Bill of Materials.
7. Standard specifications – Typical standards & connecting details of the instruments to measure flow, pressure, temperature & level.
8. General arrangement of control valve stations, utility stations etc.
9. Dimensions of valve, flanges, fittings etc.

Presentation of Isometrics

1. Standard A3 size isometric sheets shall be used for an isometric drawing.
2. Isometrics are drawn not to scale, but the length of pipelines shall be proportionate as far as possible.
3. Isometric can be drawn either as Area Isometric or Line Isometric.
4. It is compulsory to follow the piping symbols not only in shape but also in dimension (space).  Isometric designer should notice the difference in symbols when the end connections are changed. Also the difference in dimension for the small size & large size items, especially in the case of valves.
5. Isometrics shall be drawn with descriptive north facing towards the upper right hand corner of the isometric sheet, unless otherwise requested by Client.
6. Isometrics shall be drawn for all lines, irrespective of size. However deviations shall be taken based on Contractual Terms with Client.
7. For plants which are sub-divided into Areas having well defined Match Line (ML), separate isometrics shall be drawn for piping within each area separated by the Match Line. For other plants line isometric will be drawn from its start to end.
8. Piping shown on the isometric drawing shall be identified by the line number as stated on the engineering flow scheme and piping data sheet
9. Separate isometrics shall be drawn whenever there is a specification change.
10. The amount of piping shown on one isometric drawing shall be restricted such that the isometric remains legible
11. Long complicated lines may be shown on two or more isometric drawings. Each isometric drawing shall contain as a maximum one line number
12. Instruments mounted on an equipment can be included in one isometric sheet, provided their piping conform to the same piping specification.
13. Line thickness for representing the pipeline shall be even, irrespective of the change in pipe size.
14. When inclined lines are to be drawn, it shall be represented within boxes. Also the inclinations shall be clearly written as degrees horizontal, vertical, or both as the case may be.
15. All isometrics are to be fully dimensioned, horizontally by dimensions & vertically by elevations. The FTF (fitting to fitting) dimension shall not be used. Deviations shall be taken based on Client’s requirement.
16. Avoid duplication in dimensioning.
17. When some dimensions or tag numbers are used repeatedly the word (TYP.) typical can be used to avoid repetition. Care should be taken to avoid complication by writing (TYP) when same item of different sizes are involved
18. The dimension text shall be written parallel to the dimension line.
19. The reference location for starting & ending of each line shall be clearly identified by co-ordinates and center line elevation.
20. The position of the handwheels of valves shall be shown on the isometric drawings
21. The equipment shall be located to the center line for vertical vessels/ equipment, and to the tangent line/ reference line/ fixed support for horizontal vessels / equipment. A nozzle to be shown projected from the center line of an equipment. Indicate the location of equipment and orientation of the nozzle.
22. A pump / blower is to be located by the center line of discharge nozzle. Even though the isometric is for the suction line, indicate the location of suction nozzle by using the discharge nozzle as. reference.
23. The nozzle number to be indicated in a flag.
24. For a tapping or continuation of a line the details of the reference line shall be given. They are, co-ordinates of reference point, elevation of reference line, line number, and drawing number (COD).
25. Valves shall be located in the following manner. Face of flange for flanged valves. End of the valve (weld point) for butt weld valves. Centre line of valve for socket welded & threaded valves, except for those located by nipples, swage nipple i.e., fitting to fitting.
26. Pipe elevations shall be to the center line only.
27. Wherever reducers or swage nipples are used the following details shall be given. Concentric or eccentric, size of reducer, if the reducer is eccentric mention whether it is Flat on Top (FOT) or Flat on Bottom(FOB).
28. The following piping items shall be specifically indicated, whenever used, with the details of size & type. All threaded items, Plug, Cap, Blind flange, reducing Tee, branch weld, branch weld with pad (BRW-P), elbows with radius other than 1.5D, short radius elbow, Out of spec. items & mitred elbows.
29. Fitting which is used to take a branch is to be included in the isometric of the header.
30. The location of a specification change shall be clearly indicated.
31. When connections are to be made with lines beyond the scope of the isometric, that portion is to be drawn in dotted lines. BL for battery limits & ML for match lines of areas shall indicate the scope limit. At battery limits & match lines the following information to be given. Co-ordinates, center line elevations if not mentioned else where and the continuation drawing number,
32. When a line passes through a platform /floor the Elevation of floor/ platform details are to be mentioned.
33. Low point drain & high point vent shall be provided where required for lines even though, not mentioned in the piping key plan.
34. Flow orifice to be located to the center of orifice plate.
35. For Control valve, Rotameter, Flow orifice, Level instruments & relief valve indicate, the size, rating & end connection, face to face dimension and the tag number.
36. Rotameter to be located to the bottom face of flange.
37. Pressure gauges & temperature gauges to be located to the center line of their tapping point.
38. The orientation of flange taps in orifice flanges shall be shown in a cross section
39. The other boxes in the isometric shall also be filled in. Flow sheet reference, reference drawing number, design temperature, design pressure, test pressure, test fluid (if other than water)

Bill Of Materials on isometrics

The material take off for each line is to be entered in the space provided for it. The following things are to be taken care off before writing the MTO.

1. B.O.M, for each line to be entered in that isometric itself. Additional isometric sheets shall be used to accommodate the B.O.M. ,if required.
2. Listing of each item shall be in the ascending order of size.
3. Provide a blank space between two different types of items.
4. Entry of out of specification items to be made separately, indicating their specification on the top.
5. Tag numbers of the instruments & special items to be indicated in the tag number column.
6. Pressure rating is indicated by their schedule of pipe thickness for pipes & buttweld fittings and by pressure rating itself for flanges & socket weld fittings.
7. The description of an item such as type, facing, class, rating, end connection, and dimensional standard to be marked in the B.O.M.
8. Material description for each item can be obtained from the piping class.

Supports

Indicate the size, type, location & P.O.S. (point of support) elevation for base supports. This shall be done as per Engineering Specification for base supports.

Issue of Isometrics

After internal discipline review the isometric shall be released for inter disciplines reviews like process, instrumentation, electrical etc for their comments.

The construction issue of isometrics to site shall be only after Design Review, HAZOP and incorporation of any comments and issue of final process P&ID’s.