Thursday, January 1, 2015

Basic Design for Piping: Isometric Drawings

Criteria of Piping Material Application

A spool is a short piece of pipe with flanges on both ends. The word “spool” is derived from the fact that his configuration actually looks similar to the common wood spool upon which sewing thread is wound.

A spool drawing, however, is more complicated than the simple illustration above, since it involves many such “spools” on one sheet of paper. The spool drawing is an isometric drawing rather than an orthographic drawing. It is three-dimensional in that it shows both horizontal and vertical planes. The term “spool” is no longer being used as the name for these drawings. The proper term is “piping isometric”. Slang forms have shortened it to “isometric” or “isos”.

The purpose in making the piping isometric of a piping configuration is threefold:

1. It is used by the Fabricator in fabricating the line.

2. It is used by the Piping Material Group in purchasing the material for the line.

3. It is used by the field crew in erecting the line.

The iso shows routing and dimension lines of the piping only. It does not attempt to show the details of equipment and supporting structures that are found on the piping plan. The iso is usually drawn from a model or preliminary drawings. To demonstrate the use of the iso, a step-by-step sequence of events will be shown below.

Step 1

A line from the piping plan is selected to be iso’d. A typical such line is shown to the right. Note that only the horizontal plane can be seen in the orthographic drawing. The vertical portion of the line cannot be seen in full length. The iso will bring this portion of the line in clear view. Note the direction of the North Arrow.

Step 2

The first thing, which must be decided about the iso, is the direction the north arrow will be placed. Flour standard is pointing up and to the right. However, occasionally two acceptable alternatives are up and to the left or up and to the right – no other choices are commonly acceptable.

Step 3 

After orientation of the north arrow has been decided, the final spool drawing is drawn. The selection of the best configuration involves the following aspects: 

1. How the configuration will most readily adapt to the shape of the iso paper? 

2. How the configuration will result in the least amount of interference between dimension lines and extension lines? 

3. How the configuration will present the pipe most clearly? 

After drawing the configuration, the iso must be completely dimensioned. Extension lines are thin dark lines approximately 1/2” to 5/8” in length, or more if needed. These are drawn perpendicular to the section of the line being dimensioned. 

Dimension lines are thin dark lines drawn between the extension lines and parallel to the section of line being dimensioned. 

The dimension line should be drawn approximately 1/2” or more if needed from the line being dimensioned, and it should terminate with small arrowheads at the extension lines. 

In the previous example, either of the two alternatives was equally acceptable – both gave a clear view of the pipe with no crossing of dimension lines. The reason for selecting the bottom sketch was to give a north arrow orientation of “up and to the right.” This the preferred Flour Daniel standard orientation whenever possible. 

As an example of an iso where either of the two alternatives is not equally preferred, refer to the plan drawing below:

The two alternate sketches for this configuration might look as follows:

The configuration on the left has the pipe crossing itself. This is always undesirable and should be avoided whenever possible. By simply orienting the north arrow to the other alternate location, this situation is eliminated. This configuration then is preferred to the Flour standard. Another method to consider if the north arrow cannot be changed is to detach the portions of pipe that cross over. We’ll cover this later on.


Equipment Flanges

All equipment nozzles on isometric drawings are drawn double line. The call-out for these nozzles varies depending on whether the nozzle projects horizontally or vertically.

The following iso drawing indicates these two different types of nozzle call-outs:

For a horizontally projecting nozzle such as at V – 1, the following call – out information is needed:

*1. Equipment Number (V – 1)

2. Centerline Elevation (E – 110’ – 0”)

*3. Nozzle designation B1 (Obtained from Vessel Drawing)

4. Nozzle size and rating (6” – 150#) (Call – out only if nozzle is out of spec.)

*These are always shown.

A vertically projecting nozzle such as at P – 7 requires the following call – out information:

1. Equipment Number (P – 7)

2. Face of flange elevation (el. 102’ – 0”)

3. Nozzle designations or function (suction)

4. Nozzle size and rating (4” – 150# R.F.)

Out of spec equipment flanges are those which do not agree in rating with those on the connecting line. For the example below : assume the flange at P – 7 are a 300# flange. Assume that the line connecting to this pump nozzle is an “A” spec line which specifies 150# flanges. The pump flange is then said to be “out of spec”. In this case the connecting flange must be called out as shown below.


On the previous example, the mechanical flow diagram indicates a 6” line and a 4” pump nozzle. It is apparent that a reducer is necessary between the valve and the pump nozzle. This reducer must be shown on the isometric and called out as indicated in the example. The larger pipe size is always given first.

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