Ten potential pitfalls to avoid when designing a power plant at altitude

Local cogeneration power plants are one of the cost-effective solutions that these industries typically adopt. However, designing power plants in higher altitudes presents significant challenges that require leading-edge design expertise.

1. As a main component of power plants, generating sets performance is usually tested in factory at sea level before being delivered to the site. However, in a high-altitude application, a derating factor is applied, which significantly decreases their actual net power output and can go up to 40% depending on the altitude. Additionally, and due to lower air density, the turbocharger on the engine has lower compression performance than that at sea level, which also adds to the decrease in overall engine power output. This is crucial when initially setting the power plant capacity, as it has a direct impact on needed engine quantity.

2. Cooling may also be a challenge at high altitude when not cautious: typically, radiators are designed to dissipate the engine’s heat load, calculated at sea level and at normal atmospheric conditions. Yet, at high altitude, lower air density requires more air volume for the same heat load than at sea level. This means that radiators must be oversized to properly dissipate the generating set heat.

3. Electrical motor performance is also affected at high altitude. Since air is less dense, the motor self-cooling capacity can be greatly reduced. Thus, a derating factor (between 0.82 to 0.77, depending on the outside temperature) also needs to be applied for accurate performance calculations.

4. Lower air density can also become challenging when designing gas compressors. At sea level, specifying the flow in cubic feet per minute (CFM) or in standard cubic feet per minute (SCFM) is innocuous since their values are approximately equal. However, at high altitude, the difference between actual CFM (ACFM) and SCFM becomes quickly significant. Therefore, if this aspect is not clarified with the manufacturer, it may lead to the wrong compressor sizing and selection.

5. The higher the altitude, the more significant the gap is between relative and absolute pressure, especially when pumping fluids. Thus, proper attention has to be given to the “near to” or “at saturation” level fluids, along with their thermodynamical properties at different pressures and temperatures (e.g., water flashing into steam, pump cavitation) to avoid any unwanted fluid state in the plant process.

6. Along the same lines, carrying fluids in a closed circuit requires special attention when designing the expansion tanks. For instance, manufacturers typically pre-pressurize diaphragm expansion tanks to one (1) ATM. But naturally, in a high-altitude application, this variable needs to be carefully calculated, while considering high and low network points, and validated with the manufacturer before final selection of the equipment.

7. Typical generating sets come with integrated engine-driven oil pumps for the crankshaft and other engine components lubrication. These pumps are usually designed to operate at sea level, but failing to adapt crucial design variables to high altitude (e.g., net positive suction head) can lead to oil foaming in the crankcase. Accumulation of formed air bubbles in the oil may then cause pump failure or more severe consequences. In one of our applications for instance, the pump had to be deported from the engine onto a separate skid for proper functioning.

8. When air is used as electrical insulation for electrical equipment (e.g., generators, cable terminal boxes, dry type transformers, high-voltage cables and even computer hard drives), air clearance between live parts at high altitude must be revised and the appropriate derating factor must be applied to ensure proper insulation between components. This clearance can then increase by a factor of up to 1.4.

9. The human factor also needs to be considered when at high altitude, as productivity levels decrease in conjunction with oxygen levels. This aspect can quickly involve financial challenges if not factored into the budget. Moreover, scarcity of specialized personnel becomes challenging, especially when considering site remoteness.

10. Additionally, and depending on plant remoteness, logistical complexities (e.g., schedule of equipment delivery on site, transport, installation, etc.) must be correctly assessed to avoid any schedule delays. As a result, prefabricated skids are encouraged for this type of application.

Ultimately, the items described above are only a few aspects to consider when designing a power plant in remote and high-altitude areas. Naturally, the complexity varies depending on many different factors. Thus, it is important to be rigorous when designing any equipment in these conditions, challenge the manufacturer’s technical data and use the services of professionals who have proven expertise with such power plants.

If you have questions on this subject, please contact our BBA experts.