Chemical Engineering March 2011 - 60

Engineering Practice
These typically appear as different
colors on a CAD computer screen, but
print as black so that they appear to
be a single layer.
The drawing can also include a hidden
" CAD design intent layer " that
does not print. This hidden layer can
be used to explain the process intent of
the P&ID and clarify any non-obvious
design details. By way of example, I
once designed a forced-feed reboiler. A
particular butterfly valve (where the
reboiler line entered the column bottom)
was deleted during detailed engineering
to save money. The intent
was to delay vaporization until after
the valve, to prevent fouling of the reReferences
1.
Lagace, J., Project scope: The foundation of success,
Chem. Eng., February 2006, pp. 36-39.
2. Piping and Instrumentation Diagram Documentation
Criteria, PIP PIC001, Process Industry
Practices, April 2008.
3. The Engineers Tool Box - The Process Flow
Diagram (PFD), a schematic illustration of the
system; accessed at http://www.engineeringtoolbox.com/pfd-process-flow-diagram-d_465.html
4.
Informit, 1.2 Process Flow Diagram (PFD);
accessed at http://www.informit.com/articles/
article.aspx?p=1314637&seqNum=2&rll=1
5. Piping Designers.Com - Process Flow
Diagram (PFD); accessed at http://www.pipingdesigners.com/PFDS.htm
Mu?llerGmbh_Chemical
Engineering e 86x123_2011.qxd:Mu฀ll
6. Webtools - Process Diagrams; accessed at
http://webtools.delmarlearning.com/sample
chapters/1418030678_ch12.pdf.
boiler. When the design concept did not
work during startup, I was called by
the client and I explained the process
necessity of the butterfly valve. A hidden
CAD layer could have been used to
convey the importance and intended
use of the valve to the start up team.
I am a firm believer in the LPE approving
all additions to P&IDs after
the IFD issue, in order to control and
centralize all revisions. Establishing
the LPE as the central gatekeeper
for all changes will ensure that consistency
is maintained for all marks
made, and will prevent potentially
conflicting and inappropriate design
changes from being implemented by
other project participants. It also provides
an audit trail to document who
made what mark and for what reason.
A log book should also accompany
the P&IDs to capture who made a
change to the drawing, and to describe
when and why it was made. Before the
PSM stage, the log book is a simple log
book. After the Hazop stage, this document
becomes the MOC log.
n
Edited by Suzanne Shelley
Author
John C. Lagace, Jr., P.E.,
has more than 30 years of
experience (Email:
jlagace@
LeadProcessEngineer.com) in
polymers, chemicals and refining.
He holds a B.S.Ch.E. from
The Lowell Technological Institute
(now the University of
Massachusetts at Lowell) and
an M.S.Ch.E. from Pennsylvania
State University, where he
graduated Phi Kappa Phi. He
has extensive experience leading the process effort
on large E&C projects and has worked for Shell
Chemical Co. and Exxon Chemical Co. in the U.S.
and overseas, in various technical and management
positions. Lagace maintains a professional
website at www.LeadProcessEngineer.com.
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Chemical Engineering March 2011

Table of Contents for the Digital Edition of Chemical Engineering March 2011