AFT
Impulse provides a practical tool for the piping systems engineer to address the
complex subject of liquid piping systems waterhammer
and surge transient events.
AFT Impulse incorporates an integral steady-state solver to calculate the
initial conditions and seamlessly transfer these to the transient solver. Based
on the same, proven solver in AFT Fathom, steady- state solutions are determined
using a modified Newton-Raphson matrix interation. AFT Impulse’s transient flow
solution engine is based on the Method of Characteristics, which utilizes time
steps and pipe section lengths that are an integral fraction of the wave speed.
This permits a direct solution of each time step, making AFT Impulse highly
efficient.
The following outline will provide you an overview of AFT Impulse's interface
and capabilities.
The Workspace
AFT Impulse models are built in a graphical environment, displaying the model in
a familiar schematic format. AFT Impulse models contain two constructs, pipes
and junctions. Junction types included are:
- Reservoir (up to ten pipes may be connected to each)
- Branch (up to ten pipes may be connected to each)
- Pump (as pump vs. head curve with transient speed)
- Assigned flow
- Assigned pressure
- Stop valve
- One way valve
- Relief valve
- Control valve (pressure or control)
- Gas accumulator
- Liquid accumulator
- Surge tank
- Spray discharge
- Vacuum breaker valve
- Dead end
- Volume Balance
- General component (user defined component)
An AFT Impulse model is 'assembled' by dragging the junctions to be included
onto the Workspace and connecting them with pipes (this can be done in any order
and pipes and junctions can be added and deleted at any time). The resulting
system schematic clearly displays the connective relationship between system
components and pipe runs in the same way traditional schematics and diagrams do.
Model Data
The Model Data window provides a tabular listing of the input data. This is a
great aid for some types of input review. For example, knowing that your pipe
lengths are the range of a few feet to several tens or hundreds of feet, a scan
down the pipe length column will quickly reveal an input value with an extra
digit or two or a length unit mistakenly specified as miles instead of feet.
Double-clicking on an item within the Model Data window opens its Specifications
window for editing.
Pipe Sectioning
As the Method of Characteristics divides pipes into section lengths that are an
integral fraction of wave speed, and modeled pipe length is segment
length
times the number of segments, there is inherently an approximation in the
modeled pipe lengths. The smaller the section lengths and time steps used, the
closer this Fundamental to the Method of Characteristics used for modeling is
the division of pipes into an integral number of segments with a time step
duration related to this segment length by the wave speed (speed of sound
through the fluid). Since, in most cases, there are a number of different pipes
lengths in a system and, usually, no one segment length will integrally divide
into all pipes, modeled pipe lengths will approximate the actual length of some
of the pipes. Smaller segment lengths result in a more accurate approximation
but result in a longer analysis run time. A traditionally laborious aspect of
the Method of Characteristics is analyzing the trade-off between smaller pipe
segments and the required run time.
AFT Impulse's Pipe Sectioning greatly simplifies this task. The user specifies a
range of mininimum and maximum number of pipe segments to be considered and
maximum length approximation error, and AFT Impulse will calculate and display
the available pipe sectioning schemes that meet these requirements.
Scenario Manager
A powerful aspect of modeling software is the ability to change the system and
study the effects - 'what if'. AFT Fathom's Scenario Manager raises this 'what
if' capability to new levels by, first, allowing the user to define variations
of a model and save them within one model file and two, automatically keeping
common attributes between scenarios synchronized.
Scenarios may vary by almost modeling parameters; pipe sizes, fluids, pump
curves, valve settings, indeed, scenarios may differ by the elements present.
For example, a variante of a model may have an additional branch or loop than
its 'parent'.
Changes in higher order scenarios are automatically passed downward to their
'children' for the parameters that are linked. Consider, for example, having a
base scenario with several variants each with different pump curves. To change,
say, the size of the pump discharge line on all variants, the Base can is
selected, the change made and automatically passed to the 'children', both
saving time and ensuring the variants remain synchronized in their common
attributes.
Output Window
A tabular display of calculated results, the Output window is organized into
General, Pipes and Junctions sections, which can be displayed all at once
or
individually. In addition to general information regarding the model and
analysis, the General Section provides tabs with specifically formatted output
for valves, pumps and other specialized system components as well as a summary
of when various events occurred. The Pipes section displays initial steady-state
data, transient max/min by pipe and transient output data for each pipe segment
at each time step. The Junctions section provides summary output for all
junctions along with tabs for each type of junction present in the model and
transient junction data for each time step. Design alerts may be specified that
will automatically highlight output values outside of the specified range for
flow, pressure or velocity.
Output format, units and which items are displayed in the output are readily
selected through Output Control, and may be changed without requiring re-
running the analysis. Output Window results may be exported to a file for
incorporation into post processing calculations, such as forces exerted on pipe
supports.
Graph Results
Graphs of a wide variety of output data may be readily generated using Graph
Results. Flows, pressures, velocities, and more, may be plotted vs. time. Both
instaneous flow and integrated flow plots are supported. Graph formatting may be
customized by the user.
Fluid Properties
Fluids may be chosen from the standard AFT Fathom database of 9 fluids, user
specified or from the optional Chempak Property Database add-in. With the
Chempak add-in almost 700 fluids are available as well as the ability to
assemble non-reacting mixtures. Non-Newtonian fluid modeling is supported using
Bingham Plastic, Power Law and paper stock viscosity models.
Customization
AFT Impulse provides extensive customization features including:
Workspace appearance (colors, line widths, fonts, junction sizes and appearance,
location of pipe numbers and names, notation and more are all user selectable.
Fluid, Component, Material and Insulation Databases - AFT Impulse comes standard
with a database of 9 fluids and engineering data for eight commonly used piping
and tubing materials ranging from steel to cast iron, copper and PVC pipe.
Additional fluids and pipe materials may be readily added to these standard
databases along with and type of component modeled as a junction. Specified a
pump that you will use again? Click on 'Add Component to Database' and it's
readily available from a drop down list. In fact, you can easily compile
catalogs of pumps, valves, heat exchangers...virtually any kind of piping
component to be quickly and easily selected within AFT Fathom. Better yet,
database files can be shared either by copying between stand alone computers or
across a network. Selecting which databases are used in a particular model,
creating new databases and modifying existing ones is easily handled through the
Database Manager.
Additional Losses
In addition to modeling fittings and valves as junctions, loss values for these
components may be directly included in the pipe through Additional Losses. Loss
values may be directly specified or you may select from the comprehensive
database including data from a variety of standard references for over 500
commonly used fittings and valves.
AFT Impulse : System Applications
Following is a partial listing of the many systems on which AFT Impulse has
been used to model transient waterhammer and surge events:
- Refined product tank farm distribution system
- Cape Canaveral launch pad LC17 overpressure suppression system
- Coal processing plant therminol cooling system
- Surge suppression alternatives for 50 mile irrigation pipeline
- Nanoscale bubble jet print mechanism
- Offshore oil platform blowout preventer hydraulic system
AFT Impulse : Industry Applications
AFT Impulse has been applied with great success in many industries,
including:
- Overland oil transport
- Refined petroleum products and distribution
- Chemical plants
- Municipal water supply systems
- Offshore drilling
- Space launch vehicle support systems
- Coal processing
- Bubble jet printers
If you have any further queries concerning this
software, please drop us a note
and we will get back to you as soon as possible.
