Pipe Flow Expert User Guide
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- 200 Pipe Flow Expert User Guide
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For systems that contain compressible fluids the following should be noted. The mass flow rates entering the system and the mass flow rates leaving the system must be balanced. Normally In-Flow or Out-Flow values are entered using units of mass flow or units of gas flow at standard volume (volume at standard temperature and pressure). Users should avoid specifying gas flow rates in regular volumetric units such as m³/hour since these refer to actual volume of gas at whatever density has been defined in the fluid data (i.e. the density at some 200 Pipe Flow Expert User Guide pressure condition). Volume in standard units such as SCMH (Standard Cubic Meters per Hour) should be used instead. Where volumetric In-Flow rates entering the system are to be used, these values should generally be specified in gas flow standard volume units, such as SCFM, or in mass flow units, such as lb/min. If the flow rates are entered in regular volume, such as ft³/min, then this defines the actual flow rate of the fluid at the density specified in the fluid data for the current fluid zone (and not the uncompressed volumetric flow rate of the gas at standard conditions). Where volumetric Out-Flow rates leaving the system are to be used then these values should generally be specified in gas flow standard volume units, such as SCFM, or in mass flow units, such as lb/min (the same as for In-Flow values as described above). With the Non-Compressible Flow Calculation Engine, Pipe Flow Expert uses a constant value for the fluid density throughout each individual fluid zone in the pipeline system. Where regular volumetric flow rates are used to specify the In-Flows and Out-Flows to the system, the individual density for each fluid zone is used to convert from volumetric flow rate units to the mass flow rate units used internally by Pipe Flow Expert. As described above, we would recommend not specifying flow rates in volume units related to the density of the fluid zone, but rather use mass flow unit or gas flow standard volume units (volume at standard conditions). The calculations are performed using mass flow rates to achieve mass flow rate continuity at each node and an overall pressure balance within the pipeline system. When using the Non-Compressible Flow Calculation Engine, the effects of pressure changes on the fluid density are not modeled. Note 1: The Fluid density at the compressed fluid condition can be calculated using the normal density of the compressible fluid and the fluid pressure. Compressed fluid density = Normal fluid density x (Fluid pressure + Atmospheric pressure) /Atmospheric pressure Example: If a volume of 10 m³ of air at normal temperature and pressure is compressed to 6 bar g The Fluid density would be: 1.2047 x (6.000 + 1.01325) / 1.01325 = 8.3384 kg/m³ Note 2: The Actual volumetric flow rate of the fluid at the compressed fluid condition can be calculated using the uncompressed volume of the fluid at standard condition and the compressed fluid pressure. Actual flow rate = Uncompressed fluid volume x (Atmospheric pressure / (Fluid pressure + Atmospheric pressure)) Example: If a flow of 10 m³/s of air at normal temperature and pressure is compressed to 6 bar g The Actual flow rate would be: 10 x (1.01325 / (6.000 +1.01325)) = 1.445 m³/s |