The different components that make a user-defined isometric symbol include:
Symbol Origin Dynamically displays a drawing symbol area (discussed previously).
Start Point The in point of the symbol; analogous to a connect point in PDS terminology. The point where the symbol will connect to adjacent symbols.
End Point The out point of the symbol. Also, a point where the symbol will connect to an adjacent symbol.
Spindle Point The point where a valve operator attaches to a symbol. Similar to the symbol origin in the PDS world.
Tap Points Branch points on a symbol. Points where other piping components can be attached to the symbol. Tap points are not exactly
analogous to PDS taps. A redefined symbol will usually have several tap points; however, when a drawing is created normally, only one or two will be used.
Symbol Symbol graphics consist of the lines and line strings that are always drawn when a symbol is drawn in a drawing.
Lagging Lagging graphics consist of the lines and line strings that are only drawn when the lagging on the symbol is enabled.
Tracing Tracing graphics consist of the lines and line strings that are only drawn when the heat tracing on the symbol is enabled.
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The following are guidelines that should be used when assembling components into a symbol.
1. In this example, the symbol is created in a coordinate system where the x-axis is horizontal and the y-axis is vertical. The origin of the coordinate system should fall on a point of symmetry of the symbol. Thus, the y-axis will bisect the symbol. The symbol start point will usually fall on the x-axis.
2. The only valid connect point geometries that you can define are the following:
End point (for example, caps, plugs, blind flanges) Straight through (for example, valves, flanges) Offset (for example, eccentric reducer, flange)
90 degree change of direction (for example, angle valve) 3-way (for example, 3-way valve)
4-way (for example, 4-way valve)
Other geometries must be obtained through the use of fitting taps. This requirement impacts the definition of symbols in the PDS environment.
When defining a symbol in PDS, you must consider the ISOGEN symbol to which you will map when selecting the connect point geometry. If any connect point geometry does not map to an ISOGEN connect point geometry, extraction will not be performed properly.
Example:
In this example, a specialty item that resembles a lateral needs to be defined in both PDS and within ISOGEN, so that it properly extracts to a drawing.
Appendix B states that no symbols exist with connect point geometry that satisfies the requirements of the speciality item. (In fact, you can map the specialty item to a TEXX (which is a tee); however, the symbology for a TEXX is not acceptable, and the TEXX can not be redefined.)
In one solution, you can define the basic connect point skeleton of the item in both PDS and ISOGEN as a straight-through component. Although, the branch off the side can be a tap, you can not draw graphics that extend from the ISOGEN symbol for the branch. This is because you can not force ISOGEN to use a specific tap on the iso symbol. You must define several tap points and allow ISOGEN to choose the one that gives the best graphic representation of the component and connecting piping.
3. When defining graphics that are drawn between the end prep symbols, your symbol definition should not contain any end prep graphics.
4. When defining an offset fitting, the flow should go from top left to bottom right in the symbol coordinate system. For example, an eccentric reducer would be drawn with the large end on the left and the small end on the right, with the flat side down or at the bottom of the symbol. Flanged offset fittings must be created with the end prep graphics drawn integrally with the symbol.
5. If angled fittings consist of two or more legs, all legs of a redefined angled fitting must be identical. When you define the angled fitting, you draw only a single leg. The other legs are created automatically by ISOGEN. The leg that you draw must be
symmetrical about the x-axis of the symbol coordinate system. The start point of the leg must be to the left of the y-axis and the end point must be the pivot point or the origin of the symbol. All legs of an angled fitting must have a common pivot point (this requirement excludes redefinition of symbols like the angle block BA**).
Allowable and unallowable angled shapes
6. A symbol can have up to nine tap points. When distributing tap points, observe the following rules.
No more than three tap points can be placed on the x axis.
No more than three tap points can be placed on the y axis.
No more than three tap points can be placed above the x axis.
No more than three tap points can be placed below the x axis.
No more than three tap points can be placed left of the y axis.
No more than three tap points can be placed right of the y axis.
The previously-listed rules are not checked during symbol creation, but are verified when the symbol is written to the symbol library. The symbol will not be written if any of the limits are exceeded.
To use tap points that are not on the x or y axis, when the symbol is drawn, observe the following rules:
Outer tap points in the axis direction must be located at least 5 mm from the y-axis if they are to be used.
Outer tap points in the x-axis direction must be located by at least the greater of 5 mm or 10% of the symbol length down the x-axis if they are to be used.
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When a component with taps in the model is mapped to a symbol without taps, the tap point is assumed to be at the center of the symbol. None of the default ISOGEN symbols have defined tap points.
A symmetrical symbol should have symmetrically distributed tap points so that
ISOGEN always finds an appropriate tap point no matter how the symbol is oriented in a drawing.
Tap points on a flange
7. Redefined symbols can not be too large. Otherwise, problems may occur with graphic interferences within the drawings.
The amount of space reserved in a drawing for a symbol is always based on that of the old skey. Therefore, you should keep your symbol approximately the same size as the one it is based on. Using this criteria, most symbols should be between 8 and 12 mm long and 4 to 8 mm tall. Sketches of several of the original ISOGEN symbols appear in Appendix E: ISOGEN SKEY Dimensions.
8. In redefining a valve operator symbol, you must follow these rules:
The start point of the symbol must be the point where the operator attaches to the valve.
The operator must be defined on its side with the stem lying along the x-axis and the handwheel lying along the y-axis.
9. Two alternatives exist for defining pipe supports:
The pipe support symbol does not have an orientation (like the default ISOGEN pipe support symbol). In this case, the start and end points of the symbol should be located at the same point. The graphics should be symmetrical about the x-axis.
The pipe support symbol has an orientation much like a valve operator does.
In this case, the symbol definition should take place in two steps.
— Redefine the pipe support so that it consists of only a start point, an end point, and a spindle point all located at the same place. The spindle key for this pipe support should be set so that a valve operator is attached when the symbol is placed.
— Define the valve operator that will actually be the pipe support graphics.
With these guidelines, you should select commands from the tutorial and locate the various symbol components. Each tutorial command is described in the following sections.