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Fluid
Flow Design and Consulting
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INTRODUCTION
The purpose of this section is
to provide a reference point for users of the FluidFlow3 software.
There are three other important
points of reference for users of the FluidFlow3:
1. The Help files.
2. The QA files supplied with the software - see below.
3. FluidFlow3 Support
on this web site
For users new to the software
the Express
Start Guide should be worked through followed by Accutech's
QuickStart Guide, supplied with the software.
QA FILES
When FluidFlow3 is installed
a large number of example designs/files are placed in sub-folders
of the FluidFlow3 folder. Depending on the licensed modules these
folders are:
- QA 2-Phase Flow
- QA Compressible Flow
- QA Incompressible Flow
- QA Non Newtonian Flow
- QA Scripting
Reference should be made the
many designs contained in these folders for guidance on the development
of FluidFlow3 models. If
you are using a network version of FluidFlow3, these folders
will located on the server where FluidFlow3 was installed. Contact
your IT department and ask them to copy the folders to your desktop
computer.
THE BASICS
Prior to commencing a simulation
there are some basic rules that should be applied. These are:
1. Keep the flowsheet as simple as possible.
The FluidFlow3 flowsheet is a schematic. It is not intended to
be used as a pipe layout drawing; it is more like a P&ID.
- Use orthogonal layout where
possible; only use isometric when this layout provides a positive
advantage.
- Keep connector lines (pipes)
as short as possible. The more compact the model the easier to
navigate.
- Use multiple components wherever
possible - for instance a pipe containing several elbows may
be simplified by setting the elbow element to represent more
than one elbow (see Design
Example 2).
2. Build the model in sections - step-by-step:
This is probably the most important
recommendation we can give.
The FluidFlow3 copy/past
function allows pipe layouts to be copied and pasted between
flowsheets. This enables a model to be developed in parts. For
instance the system might be a ringmain supplying flow to various
facilities. Rather than developing the model in one hit, create
a separate model of each facility's pipe system and test these
models with a typical supply flow or pressure. (It may also be
possible to further break down the facility pipe system into
parts). Having developed
working sub-models these can then be assembled step-by-step into
the final model. At each assembly step, the model should again
be tested. This way, if a problem occurs, there's an audit trail
back to the last working arrangement. (See Design
Example 4).
Do not create large models
in one go and expect them to solve first time - you will make
errors in data entry and these may be difficult to find in a
large model.
3. Model convergence.
The reasons for non-convergence of a model are many. The above
two steps are crucial in avoiding this problem. However, if this
occurs the following can be investigated:
|
ISSUE |
DISCUSSION |
| Data accuracy |
Check
your data! Are node elevations
correct? Are you generating absolute zero pressures? See Example
Design 1 in Section 4. Are you generating absolute zero pressure
due to high velocities through or downstream of a piece of line
equipment. |
| Tees and crosses |
Refer
to Design Example 3 and Application Note 02 for background.
If you are confident in your data entry the reason for non-convergence
may be that a cross or tee is outside the experimental range
used in the software. Use the List Inspector to select all tees
and crosses and replace them with Connectors with No Resistance.
If this is successful, then replace the Connectors one-by-one
wth the original junction until the culprit(s) is found. Usually
its one where flow is near zero or where there is symmetrical
flow into or out of the branch. |
| Zero flow in a pipe |
Zero
flow may occur, for instance in a ringmain. This should be handled
by the Global Settings - Assumed Zero Flow value (F2 key). However,
if you identify zero or close to zero flow in a pipe, setting
the status of this pipe to OFF may assist. |
| Looped pipe systems. |
This is probably one of the trickiest
issues to handle, especially if you have loops within loops with
zero flow in these loops. Again, setting one of the pipes in
the loop to a status of OFF can help. |
| Closed pipe |
If
a closed pipe dictates the flow would be zero to outlet nodes,
say sprinklers, it sometimes helps to also set the outlet nodes
to status OFF. |
4. Tips
|
TOPIC |
DISCUSSION |
| Input
/ Output nodes or boundary locations ( Known or Assigned Pressures
and Known or Assigned Flows) |
At
any input/output (I/O) boundary to a model, you can specify a
flow or a pressure, but not both. If you specify one property,
FluidFlow3 calculates the other based on the friction loss in
the system, the head loss across fittings and static head changes. |
| Known
or Assigned Flow component as the suction supply to a pump. |
You
cannot specify a flow at an I/O node if that node is the suction
supply to a pump. The pump will generate flow dependent on the
friction loss in the pipes, head loss across equipment items
and changes in static head. |
| Control
valves. |
You
cannot have flow control valves in series. |
| Volumetric
flowrates. |
Volumetric
flowrates reflect the temperature of the fluid. To check consistency
of flow where temperature changes, use mass flowrates |
| Closed
loops in a model |
If
you have a closed loop in a model with zero flow in this loop,
it assists convergence if you close one of the pipes in the loop |
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