Seeing as I suggested this topic, I should chime in I suppose. I'm not sure I can add anything that hasn't already been said, but I'll try.
I'll start by saying that it is a fact that an understanding of what constitutes "good" dimensioning practice on piping arrangements has by and large been forgotten or is ignored. Defining good and bad is not easy in just a few words and without examples; however I’m sure everyone reading this can appreciate that dimensions must have meaning and relevance for the end user of the drawing, the constructors. All drawings must clearly communicate the designs to construction, and dimensioning plays a large role in this. Here are a few general rules for dos and don’ts:
• All equipment is located by two coordinates (commonly a Northing and an Easting). Equipment coordinates vary by equipment type: centerline of motor shaft and centerline of discharge nozzle for horizontal pumps; centerlines of motor for vertical pumps; horizontal centerline and centerline of fixed saddle for shell and tube exchangers and horizontal vessels; centerlines for vertical vessels; centerlines for tanks.
• All equipment requires an elevation. Equipment elevations vary by equipment type: horizontal centerline of suction or motor shaft for pumps; horizontal centerline for shell and tube exchangers and horizontal vessels; bottom tangent elevation for vertical vessels; underside of baseplate for tanks.
• All supports are located by two centerline of steel coordinates.
• All modules require a datum point and two centerline of steel coordinates.
• Dimension from a relevant point, such as from the centerline of a column.
• All dimensions are to centerline or to face of flange.
• Use centerline elevations when connecting to equipment.
• Use BOP elevations for grouped lines, such as in piperacks.
• Choose the most appropriate form of dimensioning, i.e. chain or tail dimensioning. For instance, chain dimension from centerline to centerline of branch connections; tail dimension from the datum point to the centerlines of pipes laid in a piperack module.
• Do not “double dimensionâ€. Especially, do not dimension between two points already located by coordinates.
• Do not use horizontal dimensioning on sections.
• Do not dimension from a fictitious reference point, such as a matchline.
• Do not chain dimension between pipes at different elevations.
It’s possible to blame poor dimensioning on any number of factors, such as: the belief that we have to live with the set-up of the automatic drawing generation; the belief that drafting training is no longer necessary; a claim that there is no time left during the checking and backdrafting stage to make corrections. No doubt each of us could generate such a list, but I believe poor dimensioning can be summed up in one word: Ownership, or more precisely the lack of it.
The designer who produced manual or 2D CAD piping arrangements took great pride in the content and presentation of these drawings. For them this deliverable was a mark of their professionalism, and it spoke volumes about their understanding of piping design and the needs of construction. The designer working in 3D does not usually start or develop their designs with this end product in mind, and quite often they are totally disassociated from this deliverable. The more junior personnel may well be delegated the task of generating and editing the piping arrangements. Ownership in the finished product is lacking.
Thankfully, I also believe that this can be corrected with a few simple recognitions and a change of emphasis in drawing creation:
• Recognition that CAD proficiency does not replace experience; training in drafting and construction execution, amongst many other things, is still very necessary in order to create good designs and good drawing presentation that will communicate these designs.
• Recognition that there is no such thing as an acceptable fully automated piping arrangement drawing generation. Some manual editing is always required.
• Recognition that poorly dimensioned drawings are vastly diminished in their usefulness, may lead to costly fabrication and construction errors, and, while they may no longer be considered the calling card of the designer, they are still the calling card of the company. Given that the reputation of a company largely rests on the quality of these documents, it’s worth putting a little time and effort into them and using your best people.
• Create examples of how piping arrangements are to look. Make these a company/project standard.
• Have the designers create the piping arrangements for their designs in order to introduce and maintain an element of responsibility and ownership.
Best wishes to all.
...Normally these people are really just CAD operators or computer geeks who are in charge because the "leadership" thinks that because AutoCAD is a computer program that it takes a computer orientated personnel to manage it! ...And normally end up doing so badly.
