Mitigating "Dimensional Drift" in Industrial Construction Work Processes

The Design Process

The goal in chemical process design is to produce a plant design that is optimal with respect to cost and performance. Plant performance involves a broad range of criteria. A good process design must not only exhibit an optimal balance between capital and operating costs, but it must also exhibit operability characteristics which will allow economic performance to be realizable in a practical operating environment. Operability considerations include flexibility, controllability, reliability, and safety.

Process Design

In chemical engineering, process design constitutes the design of chemical processes for the desired physical and chemical transformation of materials. Process design is central to chemical engineering and is considered the summit of the chemical engineering discipline, incorporating all the technical and financial considerations required for the construction and operation of a facility.

Process design applications include the design of new facilities and the modification and expansion of existing facilities. The process design exercise starts at a conceptual level and ultimately ends with the design basis for all the construction documents that will be produced in the detail design phase of the project.

Detail Design

The product of the detail design phase is the construction package that will be used to build the facility. The dimensional integrity of these construction documents varies widely depending on how the dimensional data is captured, validated, and reported. Equipment, piping, and structural steel must fit together with other new and existing components in the construction phase in order to facilitate the function of the process design. This collective installation of elements, at the surface, appears to be more intuitive than challenging, but experience teaches otherwise. The intent of this article is to define the issues and circumstances that lead to dimensional problems in detail design and the proposed technical and administrative remedies required to address these problems.

Dimensional Errors and Tolerances

There are actually two possible root causes whenever a “bust” or miss-closure is experienced during the installation phase of the project. Both are equally problematic. The two causes are:

• Fit-up problems due to dimensional errors in design, fabrication, and field erected components;
• Fit-up problems due to an excessive amount of accumulated tolerances in design, fabrication, and field erected components

A dimensional error is different from a tolerance in that dimensional errors are due to a lack of precision in the measurement process that results in incorrect dimensional data. Examples include improperly measuring and recording of as-built conditions in the field, or failing to interpret a design document properly and not building the associated component as specified.

On the other hand, a tolerance is the deviation from a standard; especially: the range of variation permitted in maintaining a specified dimension. The allowable deviation in industrial applications in many cases is set by the precision limits of the fabrication and installation work processes.

In our industry, we commonly refer to “shop tolerances” as the maximum allowable deviation from design standards acceptable in a component. Shop tolerances for heavy industrial applications are usually set at 1/8”. Given the scale of the elements, the precision of the fabrication equipment, and the measurement technology utilized, 1/8” is the best possible precision obtainable in a shop environment. It is important to remember this is the best tolerance that can be achieved and tolerances in many cases exceed 1/8” and when they do there is an increase of incidents of dimensional problems.

Both dimensional errors and accumulating tolerances can result in the failure to install elements without needing a revision of one or more of the elements.

There are three distinct project phases where dimensional errors and tolerance accumulation occur:

• Design: These are errors that occur in the measurement of existing equipment and piping. When these errors are incorporated in the design documents, fabrication will, by definition, not be compatible with existing piping and equipment in the field.
• Fabrication: Errors, such as fabricating a left hand instead of the right hand that was required, are common in every project. Owners typically do not perform extensive dimensional QA/QC in fabrication shops. Errors in interpreting and following fabrication documents frequently go undetected resulting in rework in the field.
• Installation of field fabricated elements: The layout of equipment foundations, supports and structural steel are common sources of error. Layout of cut lines on piping and equipment can also be challenging. When these errors occur, the results are usually systemic and pervasive.