in computer technology have provided many tools to simplify
the tasks faced by engineers, especially those responsible for
designing and managing water and wastewater systems. Selecting
the right software program for a particular application requires
such as ABZ, Inc.'s Design Flow Solutions software, allow solving
fluid flow problems quickly -- which would have been difficult
and time consuming by hand. To be truly useful, however, these
software tools must have a sound technical basis and address
those issues that are frequently important to engineers.
issues include the accuracy of the software, the data included
with the software, the type problems that the software can solve,
the flexibility to define problems, report capability, and support
from the software vendor.
and foremost, the software must calculate answers correctly.
This doesn't mean addition and subtraction (although that must
be correct too); rather, it means applying the proper equations
for each problem. Even user-friendly software is of little value
if the answers it calculates are wrong.
about how different software packages approach different problems
can generally be answered by reading the sales literature for
the product, testing the actual software, or asking the vendor
questions. It is also valuable to perform simple tests with
newly purchased software to verify the capabilities of the software,
while at the same time learning the basics of using the software.
common error in hand calculations and some software packages
is not applying Bernoulli's theorem (figure 1), or the derivative
form of this equation for compressible problems, to account
for energy conversion between static pressure, elevation, and
velocity. This conversion occurs across any change in elevation
or size (including “effective” changes in flow area
in a tee).
When solved properly, static pressure generally
increases when flow area increases due to the decrease in fluid
velocity (figure 2). When this conversion is not applied, the
calculated pressures or differential pressures are wrong. A
good software tool must automatically apply this equation, as
well as other equations, to properly account for conversion
of energy between different forms.
Other equations, such as those for calculating
hardware resistance, depend on information that may not be known
when the problem is specified. An example includes fittings
that change flow area. The ultimate flow direction of each path
must be known to correctly calculate the flow resistance; yet
for cross-connects or other similar lines, the flow direction
may not be known before the problem is solved.
Flow direction is especially difficult to determine
when analyzing casualty or off-normal conditions. To avoid requiring
that the engineer guess the flow direction, the software must
be capable of selecting the proper equation during problem solution.
There are similar concerns with selecting a friction
factor. A turbulent friction factor is generally used for valves
and fittings, while one based on flow velocity is used for pipes
and other conduits. This practice is based on actual test data
and has been commonly used for several decades.
Some tools use equations for flow resistance that
are based on flow velocity; while others choose the friction
factor based on the component type. Either of these approaches
is valid. What is not valid is to use a single friction factor
across different hardware types and sizes.
Data Included with the Software
Even proper equations need good data. A good software tool
will provide data for common hardware and fluids. This data
should include pipe and tube information (sizes and surface
conditions) and common fittings and valves.
On the fluid side, certainly water and common liquids and gases
are desirable. There should also be the capability for the user
to specify unique hardware and fluids to accurately model non-standard
items. The ability to add such user-specified items to the program's
database for future use, in addition to specifying for use in
a particular problem, also is desirable.
An engineer typically faces many different types of problems.
The information to be determined is often different from one
problem to the next. A good software tool allows problems to
be solved different ways, such as by specifying flow and solving
for pressure drop; by specifying pressure drop and solving for
flow; or by specifying pressure drop and flow and solving for
An engineer must readily convert information between many different
unit types. This requires a reference for conversion factors
and, in hand calculations, often introduces errors due to transcription
or misapplication of factors. A good software tool allows the
user to specify and view information in different units.
Many fluid flow problems faced by an engineer involve a significant
quantity of hardware and numerous flow paths. In these cases,
it is important to choose a tool that allows these larger problems
to be solved.
Software tools typically place limits on the number of flow
paths, or pipe sections with similar pipe diameters (such that
two pipes of different diameters connected together (with appropriate
size change) may count as two flow paths. The size of problems
that can be solved also can be limited based on the ability
to display a problem on the screen. A good software tool allows
the screen to be enlarged, reduced, and scrolled.
Reports – Getting Information Out
While solving problems is important, it also is important to
be able to see the results. The engineer may want several types
of information. First, a "big-picture" view of a system
showing all of the flows and pressures is useful to understand
system function and behavior.
Second, detailed information about each flow path and individual
components is useful both for design of new systems and troubleshooting
of existing systems.
It is sometimes important to consider the order of components
in a flow path, particularly in situations where the fluid pressure
approaches the vapor pressure of the fluid, where individual
component pressure drops are important, or where fluid velocity
within a flow path must be limited.
In these cases, a software tool that considers the order of
components within a flow path and allows the user to see values
for each individual component is important.
However good the software, there is always a need for technical
support, both for the computer related issues such as damaged
media or failed hardware, and for engineering issues including
questions about calculations and assumptions or issues with
a specific problem. Both the quality of the support and the
cost of the support are important.
Many vendors offer support limited by either contact methods
or by time after purchase. Technical support should not be confused
with engineering consulting, but the software vendor should
be readily available for consultation about how the program
works and the methodology applied by the program.
As always, ask before you buy.