
Fluid Flow Software Tools Help
Avoid Engineering Errors Advances in computer technology are
providing better tools to simplify the tasks faced by engineers, especially those
responsible for designing and managing process lines that handle fluids and gases. Data Collection The first step in solving a fluid flow
problem is to find data for the hardware and the fluid being used. Hardware data includes size and flow resistance and can be provided in
different forms (K factor, equivalent length, and Cv are common). This data is found in various textbooks and from
hardware manufacturers, and keeping track of multiple sources can be difficult. In comparison, software tools typically include
data for common hardware. Software tools also
may permit the user to add specialty items to customize the data available in the program. This data must then be used with the
proper equations. Equations used can include,
for example, the Colebrook equation for friction factor, the real gas equation for fluid
density, the Darcy equation for frictional head loss, and Bernoulli's theorem for
conservation of energy (Figure 1). Knowledge
of which equations are needed and should be used is left to the engineer. Equation Selections The proper equations must be used in
the right sequence to correctly solve a problem. A
common error in hand calculations is not applying Bernoulli's theorem to account for
energy conversion between static pressure and velocity.
This conversion occurs across any change in elevation or size (or change in
flow area such as across a tee). When solved
properly, static pressure generally increases when flow area increases due to the decrease
in velocity. Figure 2 shows energy profiles
for a horizontal pipe and enlarger. Static
pressure for the pipe decreases because of frictional loss.
Static pressure for the enlarger increases due to the decrease in velocity. A similar effect is seen across tees in a flow
network where the flow stream divides. A good
software tool will automatically apply the proper equations. Other equations have subtle
requirements or assumptions that are commonly overlooked.
For example, hardware resistance information such as K factors are provided
or calculated for a specific diameter. K
factors for different items in a problem can only be added together if they are based on
the same diameter. Otherwise, K factors must
be corrected to a common diameter before they are added.
A good software tool will correct automatically for size differences. A similar issue occurs with hardware
resistance in determining the friction factors to be used.
The correct friction factor for any given item depends on several things,
including the size and type of the item. Fittings,
such as valves, almost always use a turbulent friction factor while pipes use a value
based on actual fluid velocity. A good
software tool will account for these situations automatically and consistently, thus
preventing errors. Complex Calculations One additional issue with using equations results from using desired data together with known data in an equation. Often engineers confuse what is truly known about a problem and what is desired, and attempt to solve an equation with more information than is mathematically required. This often leads to solutions that don't make sense. A good software tool doesn't allow this error to occur. This can be accomplished in several ways, including warnings by the software. The engineer's job if further complicated because values must often be converted into different units. This requires another reference for conversion factors, and often introduces more errors as the factors are transcribed incorrectly or not applied consistently and correctly. Finally,
one of the biggest issues with hand calculations is performing these complex calculations
correctly. Ensuring that mathematical errors
are not made in a calculation is extremely difficult.
Even repeated reviews of calculations often do not identify such errors. Tools such as spreadsheets can be used to reduce errors in
calculations. Spreadsheets, however, are generally tailored for very specific
problems. The effort to make them useable for a wide variety of problems would
be essentially the same as creating a new software package. By: Nicholas J. Capik, Vice President, ABZ, Inc., Guest Author

