Chemical Engineering December 2013 - 51

Feature Report
Engineering Practice
Calculations in Process
Engineering
Mohammad Toghraei
Engrowth Training
Design
T
he topic of carrying out process
engineering calculations in the
most effective manner can be
considered in these four ways:
1. The purpose of the calculation
2. The content of the calculation
3. The method used to carry out the
calculation
4. The performer of the calculation
The purpose of the calculation
Engineers typically carry out calculations
for a variety of purposes
including the following:
a) To conduct a technical or economical
evaluation of a plant or
specific unit (such as a heat and
material balance), to carry out
evaluations in the early stages of
the project, or for use while writing
studies or technical memos
for stakeholders
b) To size, rate or specify a piece of
equipment
c) To generate a specific operational
procedure
Sizing, rating, and specifying are
three distinctly different goals of
calculation efforts. Process sizing
means defining all the (process) aspects
of the equipment, so that the
manufacturer can produce it with
little process knowledge. Sizing efforts
typically involve calculations
that aim to identify all the unknowns
associated with one equipment
component, to enable the fabricator
to produce that component.
By contrast, when specifying an
equipment component or instrument
system, the engineer must define
a specific amount of information
for the manufacturer, and can then
expect the manufacturer to satisfy
those requirements. For example,
when an engineer plans to buy a
pressure safety valve (PSV), he or
'
she specifies the required setpoint
of the PSV, the release rate and a
few other items. This effort does
not require the engineer to design a
PSV by sizing the orifice diameter,
spring constant and so on.
It is clear that no process engineer
designs a centrifugal pump just by
calculating the impleller diameter
and vane angles. Today, process engineers
can expect that the manufacturer
will provide much detailed
information to support the process
of designing and specifying process
equipment. As a result, there is very
little " pure designing " in today's
specialized market; rather, there
is always an element of specifying
involved when carrying out sizing
calculations.
While specifying and sizing are
concepts related to equipment or
instruments that are planned to be
procured later, rating is an activity
that involves calculations associated
with existing equipment components.
During rating, the question
that the engineer is trying to
answer is whether an existing component
can be used in a specific new
application or not. Rating efforts
require evaluating current equipment
for a specific type of service
and conditions.
The content of the calculation
Calculations involve the following
three steps:
* Step 1 requires identifying suitable
input values (assumptions)
* Step 2 involves choosing and using
an appropriate methodology -
one that uses the parameters established
in Step 1 - to arrive at
an answer that has the required
accuracy in the available time slot
* Step 3 involves the evaluation
and verification of assumptions
and results
Step 1. Identifying suitable
input values (assumptions). Junior
engineers tend to struggle with
this step. Too often in the university
setting, the instructor provides the
" given values, " whereas in real-world
situations, it is up to the engineer to
determine which input values and
assumptions are relevant. This effort
requires three sub-steps:
1. Find effective parameters. Engineers
need to find the parameters
that will impact the results they are
seeking. For example, within a specific
issue, there could be three parameters
M, N and P. In this step,
these three parameters need to be
evaluated in order to recognize the
ones that are most relevant to the
problem at hand. At this stage, the
engineer should be able to evaluate
the three parameters of M, N, and
P to determine that, for example, M
and P are the only required parameters
(or the most relevant parameters)
and these will be considered
further in Step 2. Or with the level
<'
ChemiCal engineering www.Che.Com DeCember 2013 51
Line-sizing
Tank/vessel
Hydraulic calc.:
pump/comp CV
HX sizing
PSV sizing
Single flow
2-phase
flow
Liquid Gas Different
pairs
3-phase
flow
Liquid/gas
/solid
NonNewtonian
fluid
Increasing
difficulty
Knowing how specific calculations
differ can focus your efforts
Figure 1. The
most popular
types of calculations
are related
to the sizing
of lines, tanks,
vessels, pumps,
compressors heat
exchangers and
PSVs. This figure
shows these
general items and
their variations
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Chemical Engineering December 2013

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