XTS - Extended Time Simulation

What is the XTS module?

An optional add-on module to AFT Fathom which allows users to simulate system transient behavior.

While steady-state modeling answers many design questions, some questions cannot be adequately answered without considering how systems behave over time. Questions such as how long it will take to fill a tank require a dynamic system model. The AFT Fathom XTS module answers such questions.

The XTS module allows you to model transient system behavior. Users can specify the time duration of the simulation, time step size, control system parameters, and how components such as pumps and valves operate over time. Operations such as valve position changes can occur during a specified time schedule, or can occur in response to events in the system thereby simulating control system actions.

Users can also specify tank volume on “finite” tanks, so that tank draining and filling can be simulated. Tanks can be open or pressurized, with the gas pressure automatically calculated as the liquid level changes.

The XTS module is a powerful tool which extends AFT Fathom’s powerful modeling capabilities into the dimension of time.

Note: Transient modeling with XTS does not support heat transfer.

How Does the XTS Module Work?

The XTS module can be described as using a lumped or quasi-steady approximation to time simulation. This means it represents the transient system behavior as a sequence of pseudo-steady-state solutions. In between each pseudo-steady-state solution it adjusts transient parameters, performs mass balances on tanks, and changes component operations as specified by the user.

As a thought experiment, consider a system which you want to simulate for ten minutes in one minute increments. This would require eleven time step solutions (time zero and each minute up to ten). You could manually do this with standard AFT Fathom by running a steady-state model eleven times, and in between each run adjusting the input parameters for the next run based on the results of the previous run. The XTS module automates this manual process.

Besides the automation benefits, the XTS module offers additional benefits such as transient output data management, consolidation and display, and graphing tools to review the transient results.

Using the XTS Module

The user has the option of activating or not activating the XTS module when AFT Fathom first loads. After AFT Fathom is loaded, the XTS module can be activated or deactivated for use from the Modules panel in the Analysis Setup window or the Tools menu. Whether or not XTS is activated impacts the Analysis menu, Analysis Setup window, Status Bar and multiple other AFT Fathom functions as is discussed in the Modules panel topic.

If the XTS module is active, the user can still run models in Steady Only mode by selecting this under Time Simulation on the Analysis menu. Hence there are three possibilities for XTS:

XTS is not active
XTS is active and operated in Steady Only mode
XTS is active and operated in Transient mode

Types of Transients Modeled

Reservoirs -

Reservoirs can be modeled as finite tanks with user specified tank geometry.
Finite reservoirs with known geometry allows AFT Fathom to model liquid level changes over time to simulate tank filling and draining and its impact on the flow in the system.
Finite reservoirs can be open or enclosed. If enclosed, then surface gas pressure changes are automatically updated.

Pump transients -

Pumps can be started or stopped based on time or event transients
Pump and Pump as Turbine speed can be changed with time
Pump flow or head can be changes with time
Controlled pump setpoint can be changed with time
Pump as Turbine Load Rejection

Valve transients -

Valves can be closed or opened based on time or event transients

Control Valve transients -

Control valve setpoints can be varied based on time or event transients

Other junction transients -

Spray Discharge junctions can be closed or opened based on time or event transients
Assigned Flow junctions can have flow varied based on time or event transients
Assigned Pressure junctions can have pressure varied based on time or event transients
Reservoir junctions can have liquid level or surface pressure varied based on time or event transients
Three-way valve junctions can have valve position varied based on time or event transients