This article covers the four commands used to calculate pressure differences between two nodes, compute a Design Factor that scales the model’s total system load to a target value, and calculate pipe efficiency along a flow path to calibrate the model against field data. All four commands require a model that has already been solved and balanced before you run them.
GDI Commands
Calculate Delta P (Linear)
Calculates the pressure difference between a pair of selected nodes, using the absolute pressure values directly.
- With a solvable model open, the GDI Window displayed, and the model already solved and balanced, select Calculate Delta P (Linear) from the GDI Command List.
- At the Select First Node prompt, left-click a node. It is highlighted.
- At the Select Second Node prompt, left-click a second node. It is highlighted.
- A message reports the calculated difference in the absolute pressure values of the two selected nodes. Click OK to clear it.
Also runs by typing CALCULATEDELTAP or CALCDP on the GDI Command Line.
Notes
- The two nodes don’t need to be adjacent or connected — any two nodes in the model can be selected.
- This linear pressure difference is the form used in pipe flow equations such as the Darcy-Weisbach equation. See the GASWorkS User’s Manual for full descriptions of the Pipe Flow Equations GASWorkS supports.
\Delta P = P_1 - P_2
\Delta P = P_1 – P_2
where P1 and P2 are the absolute pressures at the two selected nodes.
Calculate Delta P (Squared)
Calculates the difference of the squared absolute pressures between a pair of selected nodes.
- With a solvable model open, the GDI Window displayed, and the model already solved and balanced, select Calculate Delta P (Squared) from the GDI Command List.
- At the Select First Node prompt, left-click a node. It is highlighted.
- At the Select Second Node prompt, left-click a second node. It is highlighted.
- A message reports the calculated difference in the squared absolute pressure values of the two selected nodes. Click OK to clear it.
Also runs by typing CALCULATEDELTAP2 or CALCDP2 on the GDI Command Line.
Notes
- The two nodes don’t need to be adjacent or connected.
- This squared pressure difference is the form used in pipe flow equations such as the Institute of Gas Technology (IGT)-Improved equation. See the GASWorkS User’s Manual for full descriptions of the Pipe Flow Equations GASWorkS supports.
\Delta P^2 = P_1^2 - P_2^2
\Delta P^2 = P_1^2 – P_2^2
- where P1 and P2 are the absolute pressures at the two selected nodes, each expressed in units such as psia2.
Calculate The Design Factor
Calculates an adjustment factor that scales the model’s total system load to a specified target flow value.
- With a solvable model open, the GDI Window displayed, and the model already solved and balanced, select Calculate Design Factor from the GDI Command List.
- On the Design Factor Calculation screen, type a value for the Desired Total System Flow and select a dimensional unit, then click Calculate.
- The Current Total System Flow and the resulting Calculated Design Factor are displayed. Click Apply to use the calculated value, or Close to keep the current Design Factor and return to the GDI Window.
- If Apply is clicked, respond Yes to the confirmation message (showing the Current and Calculated Design Factor values) to apply it to the entire model, or No to end the command and preserve the model as-is.
- If Yes, a message may remind you to solve the model to calculate the new pressure and load values. Click OK to clear it.
Also runs by typing CALCULATEDESIGNFACTOR or CALCDF on the GDI Command Line.
Notes
- The Design Factor is a dimensionless decimal applied during solving to selected node loads in the Calculated Values section of Node Data — for example, a Design Factor of 0.5 reduces the affected loads by 50%. It does not change any underlying node or customer load data, and it can be viewed afterward on the Solution Data screen (accessed via Solve The Model).
- For a node, Base Load is multiplied by the Design Factor only if Adjust Base Load is “Yes.” External Load (the sum of assigned customer loads for customers with Adjust Load set to “Yes”) is multiplied by the Design Factor only if Adjust External Load is “Yes.” External Load (Fixed) — the sum for customers with Adjust Load set to “No” — is always excluded from the Design Factor calculation. Total Load sums all the other node load values.
- Solve and balance the model again after applying a new Design Factor:
L_{adjusted} = DF \times L_{base}L_{adjusted} = DF \times L_{base}
- where DF is the dimensionless Design Factor and Lbase is the Base Load or External Load value being adjusted (only for loads whose Adjust option is set to “Yes”).
Calculate The Flow Efficiency
Calculates the hydraulic efficiency along the flow path between a pair of selected nodes, to calibrate the model’s pressures against known field values.
- With a solvable model open, the GDI Window displayed, and the model already solved and balanced, select Calculate Efficiency from the GDI Command List.
- At the Pipe And Boundary Selection Style prompt, choose manual selection along a User-defined flow path, or automatic selection along the primary upstream or downstream flow path, from the GDI Prompt List, or right-click to accept the default method.
- At the Select Start (Target – Incorrect Pressure) Node prompt, left-click the node whose calculated pressure doesn’t match its known field-recorded value.
- At the Target Pressure For Node X prompt, type the known field pressure value and press Enter.
- At the Pressure Tolerance prompt, type the acceptable tolerance value and press Enter; the calculation is considered complete once the resulting pressure falls within this tolerance of the target.
- If the Select End Node (Base – Correct Pressure Node) prompt appears, left-click a node with a known-correct pressure, typically a regulator outlet or a supply point into the system.
- At the Reset Current Efficiency Values prompt, choose Yes to set the selected pipes’ efficiencies to 1.00 before the first iteration, or No to start from their current efficiencies.
- If a pipe selection prompt appears, select the pipes along the flow path to include using the chosen selection method.
- A message reports when the calculation is complete. Click OK to clear it.
Also runs by typing CALCULATEEFFICIENCY or CALCEFF on the GDI Command Line.
Notes
- This command is part of the calibration process — adjusting pipe efficiencies tunes model pressures to match field data at given flow conditions. Low efficiencies (below 0.80) may indicate a model error or a field issue such as a flow blockage; high efficiencies (above 1.20) usually indicate an error in the model or recorded data, or an unsuitable pipe flow equation for the modeled conditions.
- Hydraulic Efficiency is found in the Hydraulic Data Items section of Pipe Data, in both the Data Panel and the Pipe Data Report, and can be viewed or set manually from either location.
- Recommended practice: set efficiency to 1.00 for every pipe to be included before starting; when selecting pipes manually, generally include only pipes along the primary flow path between the Base and Target nodes; and only include pipe-type Hydraulic Type features along that path.
- Each iteration assumes a consistent flow rate across the flow path, uses only the pressure drop across the selected pipes to compute an efficiency correction factor, and applies that factor to those pipes. If more than one flow path exists between the Base and Target nodes, applying the factor may shift the flow distribution — the “primary flow path” selection methods will detect such a change and include pipes along the new path automatically.
- Pipe efficiency can also be changed directly: in the Data Panel’s Pipe Data, via the “Pipe Efficiency” Set item on the Multiple Edit Specifications screen, via the “Efficiency” Set item on the Mass Update Specifications screen (with Pipe Model Data as the Item Type), in the Pipe Data Report, or via the “Pipe Efficiency” Set item on the Trace & Update Data Specifications screen.
Notes & Considerations
- All four commands assume the model has already been solved and balanced; run Solve The Model first if it hasn’t been.
- If Allow Undo Of Data/Graphic Changes is enabled (Utilities > Options & Settings > Environment), click the Undo icon to reverse the last data or graphic change.
See Also
- Solving & Tracing the Model
- Editing Customer, Node & Pipe Data
- Example: Selecting, Moving & Viewing Features
- Example: Fittings, Panels & Palettes
- GDI Utilities: Undo, Redo, Cancel & Housekeeping
FAQ
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What kinds of tasks live under Analysis & Model Utilities?This subcategory covers running and interpreting the hydraulic analysis itself — solving and tracing the model — along with the calculations built on top of it (pressure drop, design factor, flow efficiency), routine load and supply-main housekeeping, and general-purpose model utilities like undo, redo, and node renumbering.
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When should I use Solve versus Trace?Solve calculates the full hydraulic results for the model — pressures, flows, and related values at every feature. Trace instead follows the gas flow path through the system from a starting point without running a full solution, which is useful for quickly confirming connectivity or flow direction before committing to a full solve.
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Where do I go if I made a change I want to undo, or need to renumber nodes after editing?Both live in GDI Utilities: Undo, Redo, Cancel & Housekeeping. Undo and Redo step backward and forward through your data and graphic changes, while Renumber The Nodes and the other housekeeping commands there handle cleanup tasks like reducing redundant nodes or resetting the numbering sequence after a lot of editing.
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Can I calculate pressure drop or design factor without re-running a full solve?Yes — Calculating Pressures, Design Factor & Efficiency covers these as standalone calculations: Calculate Delta P (Linear), Calculate Delta P (Squared), Calculate The Design Factor, and Calculate The Flow Efficiency can each be run on their own rather than requiring a full model solve.
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How do I bulk-update customer loads or move a batch of customers to a different supply main?Customer Loads, Supply Mains & Maintenance covers both. Multiply Customer Loads and Multiply Node Loads scale load values across a group at once, while Reassign A Customer’s Supply Main and Reassign The Customer’s Supply Main Numbers handle moving customers between supply mains.