Resistance Curve Loss Model
The resistance curve loss model is available in many different junctions. It is useful when measured flow vs pressure loss data is available for a component.
Clicking Enter Curve Data will open a new window where the input can be entered into a table. Several options for the Flow Parameter are available using the dropdown in the column header, and the appropriate units can be selected. Pressure loss can be alternatively entered as a head value.
Fill As Quadratic (Using a Single Known Data Point)
It is not uncommon to obtain only a single data point from an equipment manufacturer. For example, the data might be 3 psid pressure drop at 1 lbm/sec. In this case, the known data point can be entered into the first row of the data table. Then, selecting Fill As Quadratic will automatically insert another two data points to be used with a quadratic curve fit. The first data point will be zero pressure loss at zero flow which is physically realistic for a static component. Note that elevation differences within a junction are accounted for externally by the Solver. The last data point will be a quadratic extrapolation using the zero point and the originally provided data point. Clicking Generate Curve Fit Now will create the curve fit constants and complete the resistance curve definition.
This method is often a reasonable approach to define a static component because the pressure drop characteristics of most equipment follows the square law. As the flow rate increases, the pressure drop increases with the square of the flow rate. Defining the component with a curve fit provides a robust solution where the junction can still provide a reasonable pressure loss even if the flow rate in the model is changed. This also aids in numerical convergence when running a transient simulation with the XTS Module.
In our example, an increase in the flow rate to 2 lbm/sec would result in a pressure drop of 12 psid. Although this may not be exactly correct depending on the component, it is much better than assuming a flat 3 psid drop for all flow rates, which we know for certain is not correct.
Table 1: Example data points for pressure drop modeling using manufacturer data
| Data Point | Flow Rate (lbm/sec) |
Pressure Drop (psid) |
Comment |
|---|---|---|---|
| 1 | 0 | 0 | Always valid for passive equipment |
| 2 | 1 | 3 | Given by manufacturer |
| 3 | 2 | 12 | Extrapolated assuming square law |
Curve Fitting
Curve fitting can be performed via a polynomial or interpolated data. The Curve Fit Order can also be changed to achieve a better fit to the data, and the curve fit constants are displayed in a table.
Note: Unrealistic behavior could occur if the curve fit does not pass through the origin of the graph. This may negatively impact convergence when running transient simulations that have zero flow through the component at a point in time. See the Troubleshooting Resistance Curves topic for more information.
Reference Density
Options to correct for a reference density are available when using mass or pressure parameters. This is useful for when the pressure loss data was measured with a different working fluid than what is used in the model. See the Reference Density topic for more information.
