Page 51 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

36 Applied Process Design for Chemical and Petrochemical Plants

100
Guide:
Ope rating Pressure
50 ,......, vs
-� 'J
:'°"!',. �.f'r...... Design Pressure
Increase Over
., .. � Operating Pressure
.....
., .. � "-
... ..
"':,
u., ��
C'- ,.�
-Q. ""'>--
.. _ < '1 ......
�.�10
.. "'
., ... " I �
ct� ........ I I � -,......_
�:'. 5 """'- /
·;; Cl.>
.. >
00

Figure 1-33. Guide: Operating pressure
vs. design pressure increase over oper- 100 1,000 10,000
ating pressure. Operating Pressure, psia

If it is extremely uneconomical to design at this point, suming calculations can be avoided, or reduced to a rea-
then proper vacuum control must be installed. However, sonable minimum.
this is not the usual approach to the design. If the equip- On the other hand there are many situations which
ment can operate alternately under vacuum or positive require the detailed work before a sound decision can be
pressure, it must be designed for the worst or controlling made. In addition, it is often necessary to obtain reason-
condition. ably accurate prices for various items of equipment and
their assembly before the final decision can be made.

Time Planning and Scheduling For groups specializing in this type of design work it
well to maintain records of the time requirements, job
conditions, etc., in order to build a history upon which to
Scheduling of work in process engineering or design is base future estimating. It will be recognized that no two
a near impossibility as far as pin-point accuracy is con- projects or problems are exactly alike. However, with time
cerned. The very developmental and planning nature of certain basic similarities can be recognized, with good
the early phases, as well as the continuous follow-through judgment these records can be used to advantage. Thus,
and follow-up, make this difficult. It is seldom that one average information can have some value.
can foresee specific changes, delays, etc. Very few projects The size of a project does not always have a significant
are clear-cut and well defined ("frozen") as to scope or bearing on the schedule. Weighted judgment, taking the
design conditions except for small jobs and repeal or type of job, type of process, and type and nature of the
duplicate projects.
men with the engineering and process responsibility into
With new processes and/or products, the collection of account is necessary to align a balanced and smooth work-
physical data ( either from pilot or laboratory operations, ing team.
or from the literature), consideration and evaluation of
alternate conditions and flow schemes with the corre- Activity Analysis
sponding decisions, often become a significant portion of
the time required to complete the actual process calcula- A time study of eight graduate process engineers with a
tions and preparation of design specifications. So that this minimum of five years experience is shown in Tables 1-7
early phase of work does not unnecessarily slow down the and 1-8. The time includes process calculations, prepara-
project, it is important that close guidance and supervi- tion of specifications, discussions with vendors and han-
sion be given the individual designers and the use of dling the complete scope of small and large projects and
experience, judgment and approximations be encour- is helpful in accounting for legitimate time which was
aged. In this way many unnecessarily detailed or time con- obviously not spent in performing process calculations.