The Pipe Flow Expert software now contains a Compressible Isothermal Flow Calculation Engine (from version 7 and later).

In a gas system, as pressure loss occurs along a pipe, the gas density will decrease and the volume of the gas will expand. As the volume of gas increases, the velocity of the gas in the pipe will increase. Although the volume of gas and velocity in the pipe changes, the mass flow (weight of flow) in the pipe will remain constant.

Gas flow rates are therefore often referred to in terms of mass flow (weight of flow) or standard volume (which is the volume of gas at standard conditions, normally atmospheric pressure and some common temperature reference, since this standard volume also define a constant mass flow).

Pipe Flow Expert provides a choice of standard volume units for gas flow rate which include:

• SCCM (Standard Cubic Centimeters per Minute)
• SLM (Standard Liters per Minute)
• SCMH (Standard Cubic Meters per Hour)
• MMSCMH (Million Standard Cubic Meter per Hour)
• MMSCMD (Million Standard Cubic Meter per Day)
• SCFM (Standard Cubic Feet per Minute)
• SCFH (Standard Cubic Feet per Hour)
• SCFD (Standard Cubic Feet per Day)
• MMSCFH (Million Standard Cubic Feet per Hour)
• MMSCFD (Million Standard Cubic Feet per Day)

Each of the standard volume units for gas flow relate to the gas at a standard condition, however there are a number of slightly different standard reference conditions that are used worldwide depending on country and location. Pipe Flow Expert provides a choice of standard reference conditions to be used when referring to the standard volume flow rate of gas and these include:

• 0°C, 100.000 kPa.a
• 0°C, 101.325 kPa.a
• 15°C, 101.325 kPa.a
• 20°C, 101.325 kPa.a
• 25°C, 101.225 kPa.a
• 60°F, 14.696 psi.a
• 68°F, 14.696 psi.a

Mass flow can also be used to refer to an amount of gas flow and the units for mass flow include:

• Kgs/sec
• Kgs/min
• Kgs/hour
• Lb/sec
• Lb/min
• Lb/hour

Using Compressible Flow Equations

There are a number of different equations that can be used to calculate flow rate and pressure loss in a compressible gas system and the type of design and user preference often determines which equation they use to calculate the results.

Pipe Flow Expert allows for the selection of a specific compressible isothermal flow equation from a list that includes:

• General Fundamental Isothermal Flow Equation
• Complete Isothermal Flow Equation
• AGA Isothermal Flow Equation
• Panhandle A Isothermal Flow Equation
• Panhandle B Isothermal Flow Equation
• IGT Isothermal Flow Equation
• Weymouth Isothermal Flow Equation

The General Fundamental Isothermal Flow Equation (sometimes known as just the General Flow equation or the Fundamental Flow equation) provides perhaps the most universal method for calculating isothermal flow rates, however it relies on the inclusion of an accurate friction factor. The Pipe Flow Expert software provides such a friction factor by calculating this using the Colebrook-White equation.

For complex interconnected pipe systems the General Flow equation will often provide the best overall calculation result, however this approach is only made possible by the advanced software algorithms and the power of computer calculation.

The preferred method of calculation can be selected from the Calculations Tab in Configuration Options.

The calculations can incorporate the Ideal Gas Law, a custom Compressibility Factor (applied to the whole network) or the CNGA Compressibility Factor that is calculated for each pipe based on the pressures at the start and end of each pipe.

The Compressible Flow Calculation Engine will automatically take account of pressure changes within the pipe network and will automatically adjust the density properties of the gas as appropriate when performing the gas flow rate and pressure loss calculations. The equations used in the calculations currently assume isothermal flow where there is no change in temperature.

If the pipe system contains a compressor, component or valve that either significantly increase the gas pressure or significantly reduces the gas pressure then an additional fluid zone should be defined to specify the density properties of the gas at the required temperature condition.

Pipe Flow Expert will automatically account for changes in pressure within the system, however the user must define fluid properties and gas data for the operating temperature within the pipe system (or within each part of the pipe network if zone of different temperature exist).

When a user clicks to ‘Calculate’ a compressible gas system , if the calculation engine method of solution is not set to ‘Compressible Gas Flow’ then the software will prompt the user to ask if they wish to automatically switch to the Compressible Flow Calculation Engine (which is recommended).

If the user chooses not to switch to the Compressible Flow Calculation Engine then they can continue to solve their system using the standard Non-Compressible Calculation Engine, which uses the Darcy-Weisbach equation that assumes a constant density and viscosity for the gas as defined in the fluid data.

We recommend using the Compressible Flow Calculation Engine to calculate gas systems.

The compressible flow equations calculate a gas flow rate for a difference in pressure between two points. The Pipe Flow Expert software first solves the compressible network using non-compressible flow equations to find an approximate solution and then it employs the compressible flow engine to converge this to an accurate solution.

The non-compressible calculation engine operates by adjusting flows within the network to achieve a pressure balance, whilst the switch to the compressible flow calculation engine operates by adjusting node pressures to achieve a flow balance within the pipe network.

The compressible calculation engine takes account of friction loss through fittings at the start and end of a pipe, where a lower pressure but higher velocity at the end of pipe will results in a higher pressure loss through the fitting than if the same fitting occurred at the start of the pipe.

In addition the compressible flow calculation engine handles complexities that do not occur for non-compressible calculations, such as when considering a fan or blower with a flow versus head curve, where the actual operating point (flow and head value) has to balance out even when taking account of the compression and then expansion of the gas through the pipe.