Page 13 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

1.3 The design process 7

such a process system would include one or more equipment; possibly, a heat exchanger with allied
auxiliary facilities like pumps, piping and pipe fittings that would serve the purpose.
The “expression of functional interest” is followed by documenting the “preliminary design
specifications” (PDS document). This goes beyond just the functional aspects of the proposed design
and attempts to identify and quantify the design deliverables like capacity, material and process fluid
properties, process stream conditions at inlet and outlet of the process, etc. In spite of containing
additional information, the formulation of the design problem at this stage will hardly be complete in
all aspects to embark on the design process.
Generating the complete definition of the design problem is the next step and the outcome of this
step is documented as “detailed design requirements”(DDR document). This document is prepared at
the designer’s initiative with the aim of defining all aspects of the design problem before embarking on
the design procedure. This is based on the response to a questionnaire set framed by the designer
specific to his design assignment. This questionnaire set is usually called “Basic Engineering Design
Questionnaire (BEDQ)” and requests information under various subheadings. Preparation of the
BEDQ document is the joint responsibility of the client and the designer. This serves as the designers’
starting basis and is consulted by the designer as a reference in almost every step of his activity. BEDQ
documents important issues, such as the brief background of the design case, design capacity, size/
dimensional limitations and applicable standards/codes for designs, drawings and materials. This
document even specifies the format of the design deliverables and their documentation standards.
Freezing the mutually agreed version of the “detailed requirements design document”(DRD docu-
ment) marks the start of the design activity.
Complete design of a process plant consists of the Basic Engineering, followed by Detail Engi-
neering. Basic Engineering includes process selection, equipment selection, PFD/P&ID, functional
description of the system and overall Plot Plan. The process design falls under Basic Engineering.
Equipment mechanical design; piping drawing with support structures, etc., are covered under detail
engineering. Basic Engineering for the process is usually supplied by the technology vendor in the
form of a “Basic engineering package,” and the detailed engineering is carried out by a Detailed
Engineering contractor.

1.3 The design process

Designing always starts in a concept design phase, where the designer considers potential alternatives
and compares those heuristically. Based on heuristics and experience, a finite number of alternatives
are selected. For example, in the kerosene cooling problem, the evaluation of the options considered by
heuristics will probably lead to employing a shell and tube exchanger over the other alternatives.
Selection of design solution is usually based on qualitative considerations followed by quantitative
considerations.

Qualitative considerations can be
• Soundness of the scientific concept
• Fire and Safety risk of plant equipment, manpower involved and surrounding area
• Feasibility of practical implementationdthis includes compatibility with existing components of
a plant, in case of retrofit or plant expansion, related designs as well as spatial limitations
• Economic attractiveness or advantage of one option over other(s)