The rated horsepower of motor drivers should be at least 10% greater than the highest power required by the compressor (including any power transmission losses) at any of the specified operating conditions. For turbine drivers, a 20% margin is recommended to provide for deterioration in performance and variation between actual and specified steam conditions. (Moreover, the 20% margin has been necessary on smaller units to effectively handle the cyclic compressor torque.) Internal combustion engines should be rated in accordance with DEMA standards.
The DEMA continuous duty rating must be derated for site elevation, maximum ambient temperature, inlet and exhaust pressure drops, and for shaft-driven accessory loads. In addition, it is desirable to have a 10% margin between the site-derated continuous duty rating and the maximum compressor power requirement. With engines, the power margin is not as straight-forward as with other drivers owing to the different levels of conservatism of the manufacturers. A power margin is desirable because such factors as emission control devices and the cooling water temperature to the turbocharger aftercooler can affect engine capability. Engineering judgment will usually have to be applied.
Engines should also meet the DEMA requirement for the capability to carry a 10% overload for two hours out of any 24-hour period. It is also a good idea to review the engine vendor’s experience at the proposed rating with other owners of that model.