The word 'Isometric' has its origins in Greek which is the case with many English words. It is derived from Greek word 'Isometros', of equal measure (the objects measured using a fixed unit). It is one of the most important outputs, if not the most important output of overall Piping effort. Isometric is a comprehensive document that holds information that is used by different people at varying stages of a project.
Purpose of Piping Isometrics:
The primary purposes of piping isometrics are enlisted below:
1. For preliminary Bulk Material Take Off (MTO)
2. For Stress Analysis
3. Issued For Construction
The level of isometric details may vary on the basis of their purpose.
Brief Overview of Practices:
Preparation of piping isometrics has seen the practices change as the tools for generating them have been and are changing. Before advent of computers from piping perspective, these sketches were prepared manually by skilled hands, which meant assiduous work on board. The arrival of computers (and Computer Aided Design), shifted most of this work from board to computers. The work was in 2-Dimensional view and still 2-D tools are used for small projects and for annotations in some cases. Now there are various 3-Dimensional design tools which are widely used to create the design. These tools have features that generate isometric, once the model is 'built'.
In today's world, the means of selection of the method for preparing the final isometrics (i.e. by hand / 2 - D / 3 - D), is driven by cost. However, every piping designer needs to know how to draw a proper manual piping isometric because the need for an isometric is often influenced by location or time. Location, sometimes a project is done in the field (at the jobsite) and is therefore all done manually. Time, for many of the "in the office" CAD based projects, the requirement for an early preliminary material take-off (Alloy, Large O.D., Lined pipe, Jacketed pipe, etc.) is before the electronic model is even started, means that piping designers must be able to draw manual isometric sketches.
The minimum inputs for isometric sketching are:
1. Plot Plan
2. Process & Instrumentation Diagram (P & ID)
3. Mechanical Data Sheets (MDS)
In some of the cases, more detailed Piping Layout will be available.
Before getting into the details of the different requirements, it is imperative to identify and define the sections of a typical isometric sheet. The isometric arrangement may differ from company to company based as per internal standards. But, it will have following details on it typically as shown in schematic below.
Now let us look into the detailed break-up of the sections mentioned above.
- North: The isometric always indicates Plant North which may or may not be collinear with Geographic North. This is one of the better examples of common sense. The Plant North is normalized with respect to Geographic North so the sketching does not require unnecessary and awkward orientation and angles.
- Isometric Drawing: The Isometric sketch is drawn here.
- Bill of Material: The Material Take Off for the sketch on the left hand side reflects here.
- Notes & Symbols: Weld symbols used on the isometric; Notes regarding Post Weld Heat Treatment (PWHT) etc. and Project related notes are covered.
- Line Data: Process data such as Line Number, Pipe Class (Specification), Design and Operating Temperature / Pressure, Insulation and Tracing etc. are covered here.
- Project Nameplate: A detail indicating the Client, the Engineering Contractor's Name and the Engineering Contractor's Logo and the project Name is placed. Alongside this, the Document number, Purpose of Issue (Information / Construction) and appropriate revision is indicated.
The above details cover most of the essential information that an isometric should possess. However, requirements to provide additional data might arise due to client or statutory requirement of the specific country or region.
Now that we have seen the contents of isometrics, let us see how the emphasis on different sections of isometric shifts as the purpose changes.
Preliminary Bulk Material Take Off:
This activity is done at the initial stages of the project. Since the data available is preliminary, the activity needs sound engineering judgment and experience. This Material Take Off is used preliminarily by Procurement and Piping departments. So it is important to keep in mind:
- The quantities for procurement shall approximate closely so that they align with the future deltas (Take off top-ups required) of materials.
- The considerations shall be reasonable from Piping Design point of view. Otherwise, it will lead to large differences in Initial Bulk MTO and IFC stage MTO.
Therefore the isometrics shall be prepared with the following considerations:
- They should cover major components and the holds as per P & ID.
- The scope of supply should be confirmed and marked clearly.
- The details of Typical Assemblies should be prepared and referenced so the sketching duration is reduced with adequate accuracy.
The stress critical lines are analyzed for flexibility. These isometrics are issued to Stress Engineer. Incomplete information may result in increased shuttling of document between the designer and Stress Engineer. For this activity, isometrics shall be prepared keeping in mind:
- The Line Design conditions i.e. Pressure, Temperature and Pipe Class (Material) shall be provided.
- Possible locations for supports, anchors and guides shall be duly marked.
- Equipment connection references for nozzle loading check shall be provided.
- Complete system sketches shall be provided i.e. all interconnected lines comprising a single stress system.
Issued For Construction:
This is the most important stage in the isometric's life cycle. The document is issued with external revision. It is used for fabrication and erection. So every detail becomes significant in addition to the requirements covered in above point's i.e.
- Shop and Field demarcation
- Each detail with respect to P& ID, MDS and Plot Plan taken care of.
a. Equipment Connection
b. Continuation Reference
c. Grid or Co-ordinates
- Reference Documents :
a. P & ID Number
b. Piping Plan Number
- Dimensions and Elevations
The IFC stage isometrics are issued to the Construction Contractors. The construction site is abuzz with all sorts of Shop and Field activities. The fabrication and erection at site sometimes requires changes be done to the IFC isometric to suit the actual site conditions.
The isometric with such 'Field Revisions' is called as 'As Built'. The 'As Built' document remains in archives to serve as reference point for future modifications.
Now that we have seen the relevant points pertaining to the preparation of isometrics, let us see the current practices.
Preparing an isometric by hand could give you a potion of patience as you have to pause for a moment to make a mental picture before you actually start sketching.
However, the availability of modern day tools means preparing and processing the isometric in its full 3D electronic form.
Nowadays, there are many 3-D Design modeling softwares that help you build a virtual 3-D Model. These softwares come with in-built capabilities to generate isometrics.
With proper administration, the interfacing with other softwares such as Reviewing (e.g. Navisworks), Stress (e.g. CAESAR-II), 2-D (e.g. AutoCAD) and Shop (e.g. SpoolGen) has become a part of modern practices that is helping in faster and more efficient designing.
In essence, the practices for preparing the isometrics will keep changing with the evolving technology and yet its core purpose and level of detail will remain intact!
About the Author
Shrivallabha Redij is a chemical engineer by qualification. He has over 6 years of experience of which last 3 are piping which includes PDMS modeling, 3D co-ordination on projects and proposals. He is currently employed by Toyo Engineering, India Limited