This is the seventh article in our series about Power Design Pro™ software and it discusses the software’s mechanical design capabilities for sizing pipe.
When installing a generator, mechanical design issues need to be adequately considered. For natural gas generators adequate gas piping is essential to correct unit operation. If the piping is too small, the generator will have issues picking up and carrying load. For indoor generators, correct exhaust piping is also critical to unit operation. If the piping is too small, the engine will experience too much back pressure and likewise have issues picking up and carrying load.
To address these critical needs, Power Design Pro™ performs mechanical design of incoming gas line piping and outgoing engine exhaust piping. These features are accessed on the mechanical design tab on the left tool bar, after you have made your generator selection. (The generator selection is made on the project tab, load summary page. The generator can be selected automatically by the program’s sizing algorithm or manually by the user. The fuel type is also selected on the project setup page.)
The gas piping module is a tool to help determine the required fuel pipe size for gaseous generators. The gas piping tool supports single unit and Generac’s Modular Power System (MPS) configurations.
The program uses supply gas pressure as a starting point and then sizes the pipe to ensure adequate gas pressure at the generator under full fuel flow. When entering the supply gas pressure, the internal pressure drops within the supplying service and system pressure regulators need to be included. It is common that the available pressure through these devices will drop down under high gas flow. The supply gas pressure entered should match the available pressure under this full gas flow condition.
The gas piping module is designed to support sizing a single generator or a combination of generators on one gas line. When the single module check box is selected, the program pulls the gas flow requirement for a single generator module of the size listed. The program then utilizes the length of the piping run and the number of elbows and tees to calculate the available pressure at the generator.
The program provides four methods of sizing the gas pipe – three automatic methods and one manual method. Each of the automatic methods allows the user to select a different sizing requirement and provides the proper pipe size. With the manual method, the user selects the pipe size.
The first automatic sizing option is limiting the piping to 0.5” of water column pressure drop. This option will match most gas piping handbook calculations. This method allows for very little pressure drop and requires the largest size pipe. Though the most conservative, this method provides the most robustness if the input gas pressure is less than expected.
The next two options are based on utilizing a percentage of the potential allowable pressure drop to accommodate normal system fluctuations. For example, if the input is 15” of water column and the generator requires 5”, there is the potential for the piping to drop 10” of water column and still meet the generator’s requirements. Though possible, this is not desired because the piping is not sized to allow for any variation in the input gas pressure which may occur. But 33% of allowable pressure drop does seem reasonable (assuming the gas source pressure is reliable). The program defaults to 33% of allowable pressure drop with an option for 50% of allowable pressure drop.
The final option is a manual pipe size selection. This allows the user to input various pipe sizes to see the effects on available generator gas pressure.
The exhaust piping module determines required pipe size for exhaust. Fundamentally, the exhaust piping tool works very similar to the gas piping module. The program does a pressure drop calculation and selects the correct pipe size to meet the maximum back pressure constraint of the engine. The user can enter the length of the piping run and number of elbows. For engines with dual exhaust piping, the program can size the piping for a combined exhaust or separate piping.
Mechanical design considerations are essential for a successful generator installation and the capabilities provided by Power Design Pro™ make sizing gas and exhaust piping easy. Simply enter a few basic elements and the program provides you with a pipe size recommendation.