General
Positively Gassed determines the size related requirements of piping, fittings, and valves of a positively pressurized gas system with a single source to provide the flow and pressure needed to serve all connected appliances.
Positively Gassed is intended for relatively low pressure, constant temperature gas systems where treating the gas as ideal provides accuracy suitable for the application.
Segments
All pipe segments from the gas supply to the appliances are entered. The input for each segment includes the length, valves, and components.
Both pipe sizes and elbow quantities are automatically assigned to each segment based on the selected profiles, but those sizes and quantities but can be overridden for each segment.
Pressure
Frictional pressure loss is calculated using the Darcy‑Weisbach equation. K‑factors are used to determine the equivalent lengths of fittings and valves.
Each appliance has an independent required pressure. The required pressure at the gas supply equals the total pressure loss of the critical path segments plus the critical path appliance’s required pressure.
Sizes
Multiple sizing profiles can be created for automatic segment sizing. Each profile includes all pipe sizes from 1/2" to 18", with a flow range assigned to each size.
Profile assigned sizes can be overridden per segment, however doing so reduces the effectiveness of automatic sizing.
A built‑in solver (Excel goal seek) can generate sizing profiles automatically by calculating the flow rates corresponding to an input pressure loss rate.
Appliance Groups
Appliances can be assigned in groups, thereby allowing individual segments to serve multiple appliances and streamlining data entry.
Elbows
Profiles can be created to automatically assign elbows to segments, where each profile has a length‑to‑elbow‑quantity ratio for each size. Assigned elbow quantities can be overridden per segment.
Diversities
Diversities can be applied to individual appliances and convenience outlets to accommodate systems with modulating or cycling loads.
Features
Supports both Inch‑Pound (I‑P) and International System (SI) units.
240 segments.
1/8" to 12" pipe.
40 pipe types (roughnesses and inside diameters).
8 automatic elbow profiles.
8 appliance groups.
7 appliances per group (appliance quantity unlimited).
48 appliances.
Diversities assignable to every appliance.
4 convenience outlets.
7 convenience outlet diversity profiles.
All data is editable.
Limitations
Only “tree” type systems are supported; systems with more than one path from the supply to an appliance are not supported.
Positively Gassed uses a simplified approximation for transition regime friction factors. This approach is suitable for systems where flow is predominantly turbulent, but introduces inaccuracy proportional to the amount of piping operating in the transition regime.
Elevation differences are not included in the calculations. Therefore the accuracy decreases in proportion to the extent that elevation changes affect system pressure.
Other Considerations
Positively Gassed is not inherently a critical path calculator. However, when the critical path is clear by experience, Positively Gassed can be used for that purpose, though doing so reduces the effectiveness of automatic sizing.