Four outdoor diesel generators were designed for a hospital, and sound levels were better than expected.
By Tom Divine, PE, LEED AP, Smith Seckman Reid, Houston
The project was a greenfield community medical center project, including a 500,000-sq-ft hospital, two 130,000-sq-ft medical office buildings (MOBs), a central utility plant (CUP), and a parking garage. The proposed expansion called for up to an additional 600,000 sq ft of hospital space, and two more MOBs.
The standby generation system was selected as a parallel system of four outdoor 2-MW, 5-kV diesel generators installed at the CUP location, with two units installed initially and two to be installed with future expansion. The generator sets were surrounded on three sides by a brick surround with roughly 30% free area, with the exterior wall of the CUP on the fourth side.
The space available for the generator yard was not sufficient to allow adequate outflow space for the radiators.
The nearest point in the hospital was roughly 800 ft from the generators, corresponding to an inverse-square sound attenuation of a bit more than 30 dB relative to the 23-ft standard. The MOBs were not intended to operate continuously in a prolonged power outage. The nearest point of the property line was 125 ft from the generators, with light commercial properties adjacent. The municipality allowed 68 dBa at the property line for commercial adjacencies.
The distance from the generators to the property line corresponds to an inverse-square attenuation of 14 dB. The permissible total sound-pressure level of the four-generator installation was therefore 82 dB. Modeling the installation as a point source with four times the sound power of a single generator gave an allowable sound-pressure level of 70 dB at 23 ft.
Enclosures of 70 dBa were installed. Enclosures also were equipped with a diverter assembly to redirect the radiator air upward, extending the length of the enclosures but eliminating the necessity for clearances at the radiator outlet.
The system provided better sound attenuation than expected. The improvement may be due to the effect of the surrounding walls, which were not included in the calculations, or to the fact that the radiator outlet was directed upward, away from the property line. The sound-pressure level at the property line was below 60 dB during generator testing, with two generators installed.
Tom Divine is a senior electrical engineer and project manager at Smith Seckman Reid Inc. He is a member of the Consulting-Specifying Engineer editorial advisory board.