The Reservoir junction type allows you to connect up to twenty-five pipes. At least one connecting pipe is required. The Reservoir junction is convenient for specifying a fixed pressure in your system.

A Reservoir junction applies a defined pressure and temperature (if using heat transfer) in the model at the junction location. When solving a pipe flow system, a Reservoir causes the rest of the system to distribute the flow in a manner consistent with the defined pressure.

The Reservoir Properties Window follows the second of the two basic window formats for junctions.

The Liquid Surface Elevation of the Reservoir must be defined - this value will be used along with the Pipe depth to determine the hydrostatic pressure contribution at the pipe inlet.

Surface Pressure is required, and fluid temperature is required when heat transfer is modeled.

Specifying Pipe Connectivity

If there is a single pipe connected to the junction, its elevation can be directly specified on the Reservoir Model tab. The elevation can be specified either as an absolute Elevation, or as a depth below the liquid surface. This means there are two ways to define the same pipe elevation.

If multiple pipes are connected, they can each have a unique elevation. This is displayed on the Pipe Depth and Loss Coefficients tab. In this table, all elevations must be specified as either depth or elevation, as is specified by the radio button selection below the table. Additionally, losses such as re-entrant losses can be added to the pipe connections in this table. Loss factors can optionally be specified for flow into and flow out of the pipe.

Pipes Connected Above Liquid Surface

A pipe elevation can be specified higher than that of the liquid surface. Pipes that empty into the Reservoir above the liquid surface are assumed to have liquid free fall to the liquid surface. AFT Fathom applies the proper boundary condition for above the liquid surface.

If the pipe is above the liquid surface, the only physically realistic condition is for the fluid to be flowing from the pipe into the reservoir. However, AFT Fathom will assume the fluid flowing into the pipe is the same as the reservoir fluid in order to solve the system. If this occurs a warning will be shown in the Output.

Infinite and Finite Reservoirs with the XTS Module

With the standard AFT Fathom, all reservoirs are treated as infinite. With the XTS Module, reservoirs can be infinite or finite.

  • Infinite reservoir

    • If modeling an infinite reservoir, the user can vary the surface level or surface pressure with time. Note that the liquid level is not calculated in this case, but is input by the user.

  • Finite tanks

    • Finite tank reservoirs can be open or closed. Finite reservoirs require specification of tank geometry. Typically, the liquid level and surface pressure are defined initially, and then calculated over time based on the tank geometry.

    • Open tanks do not vary the surface pressure unless the user does so through the Transient tab. Closed tanks vary the surface pressure in accordance with gas thermodynamic laws and a user specified polytropic constant.

    • For closed tanks, users can enter maximum and minimum pressures. These represent pressure control systems on the tank such as relief valves. Liquid overflow lines can be modeled with normal pipes. In such cases use of valves with event transients can be used.

  • Known parameters initially

    • In some cases, the initial liquid level or pressure may not be known, and it is desired to have this calculated based on a mass balance in the system. This can be selected using the check box options provided.

If an XTS model is run, and the pipe is no longer submerged with flow into the pipe, AFT Fathom will stop the flow in that pipe on subsequent time steps because there is no liquid to supply it.

A potential confusion exists when pipe connection data is entered as depth below the liquid surface, and then the liquid surface changes during a transient. This is not an issue in that all depth data is converted internally to absolute elevation at the beginning of the run so that liquid level changes will be handled properly.

Tip: Since transient models allow the liquid level to change, and the user may desire to run different scenarios with different initial liquid levels, it is better to specify reservoir pipe connections based on elevation and not depth.

Energy Balance

The energy balance feature is displayed only when heat transfer is being modeled. For an open system, most users desire that the temperature remains fixed, like the pressure does. However, for closed, recirculating systems, you will usually want the temperature to vary. The temperature can be allowed to vary at a junction by checking the Balance Energy option. See Open vs. Closed Systems - Balancing Energy for detailed discussion.