Vibration management

There are several vibration isolators available; select the best option for the application.

By Bradley N. Rohloff, PE, LEED AP, Harley Ellis Devereaux, Southfield, Mich. July 17, 2015

Below are a few of the many vibration isolators available on the market used to isolate, reduce, or eliminate the transmission of vibration from equipment to the building housing the equipment.

Inertia bases and rail products—Inertia bases are recommended for equipment such as compressors, which can exhibit high out-of-balance forces and “top-heavy” equipment like belt-driven pumps. The concrete base reduces the amount of vertical movement and lowers the center of gravity, increasing the plan area at the base of the installation, improving stability. By adding the concrete to the base, stiffness is added, providing rigidity so the whole system can then be point-loaded on springs. The concrete surface also provides a surface that allows the pump frame to be filled with grout as required by the pump manufacturer. Without the mass of the concrete, a point-supported, base-mounted pump would quickly wear out the coupling.

Isolation padsPads may be used for small equipment to supplement air-handler internal isolation, or where there is little to no acoustical sensitivity. Isolation pads manufactured of cork and neoprene are available in many sizes. In situations where the cork and neoprene pads are not adequate, an alternative is the neoprene pad, which yields more deflection and better results than the neoprene and cork pad.

Spring hangersSpring hangers comprise 1-in. deflection for steel helical springs of various load ratings housed in steel boxes. All hangers can be fitted with a load-transfer plate to hold the equipment or piping at a fixed elevation during installation. The critical characteristic for an isolation hanger is the clearance between the hanger rod and the hole at the bottom of the hanger box.  

Rubber flexes—Rubber flexes are recommended for all piping connections to pumps, chillers, cooling towers, and air-handlers. Rubber flexes are the only connector type effective in reducing disturbances associated with the pumps pulsation frequency. Use two arch connectors wherever possible to maximize vibration and noise reduction. To ensure durability, look for connectors with DuPont Kevlar reinforcing and a steel ring embedded in the rubber flange in lieu of cable retention.

Stainless steel flexible pipe connectorsIn lieu of flexible rubber pipe connectors, braided stainless flexible connectors—though not as effective as rubber—can reduce vibration transmission from equipment to connected piping. They are necessary in lieu of rubber for temperatures higher than 250 F and when pressures are higher than those published for rubber. Because stainless steel cannot expand, neither the stainless connectors nor a series of pipe couplings can provide the same attenuation benefits as the elastomeric flex.

Rooftop unit vibration isolation—Rooftop units are a frequent source of noise and vibration complaints. All rooftop units, regardless of manufacturer, are supplied with “internal vibration spring isolators” under the fans. The rooftop unit is frequently mounted on the most flexible part of the roof, typically directly over the most noise-sensitive areas of the building. Installation of a spring-isolation curb is the recommended solution to the problem.

Bradley N. Rohloff is an associate in mechanical engineering at Harley Ellis Devereaux.