With mechanical equipment, the advice for vibration isolation can be quite confusing. There are many options and just as many expert recommendations, but with a good understanding of the paths that the vibration can take, contractors can make better choices that are site-specific.
Since the mounts and hangers are where the equipment imposes the most force on the structure, this is where most kinetic energy from vibration will transfer into the building or structure, but it is also where contractors can use high deflections in springs and rubber for a high isolation efficiency. It is important to consider the location in the building in relation to potentially sensitive areas, the type, power, and RPM of the equipment, and how the mass of the equipment will deflect the supporting structure. In general, equipment with lower frequencies and more powerful motors require more deflection, but some equipment types create more powerful vibrations, which will also need to be considered. For example, a pump and a fan may have the same RPM and motor power, but the pump is moving a more dense material and impellers create more imbalance than a fan, so the pump should have isolators with more deflection. With heavy equipment, you may also need to consider the deflection of the floor due to the weight as this will negatively affect isolation.
The mounts or hangers get the most attention when it comes to vibration isolators, but the other connections often cause the most problems once the equipment is running. Pumps have piping, fans have ducts, and all equipment has an electrical conduit. In seismic areas, the restraint creates a path as well. Since the piping and ducting run throughout the building, vibrations following this path can be the most troublesome. Flexible connections should be used for both, but with the pressure in the piping system flex connectors will tighten once the system is running and can't rely on the isolation efficiency that a deflected spring or rubber mount can offer. The energy from vibration will dissipate over distance, so consider using isolation hangers for the first several support points after the flex connectors. Each elbow in the piping system will also dissipate the energy and allow the piping to flex. By allowing the piping to be more flexible and move the vibrations will release more energy before they are rigidly attached to the structure. Seismic restraints for isolated equipment should be designed so they do not create a pathway for vibration. Seismically rated spring mounts should have an air gap between the mount and the supporting spring, and for suspended equipment, only cable bracing should be used and installed to be slack. Electrical conduits should be loose and move freely after installation. Avoid hard turns that will make the conduit rigid.
The amount of attention you will need to put into isolating each path will depend on the structure and its use. Lighter-weight building materials tend to transfer energy more easily and connections that are attached farther from supporting walls or columns will generally create more noise from vibration. On the receiving end, the lighter finishes usually generate the most noise. The smooth hard surfaces or glass, drywall, or flooring act as speakers that will transmit noise at the frequency of any vibration that managed to get past your isolation defenses.
For projects with a lot of acoustic challenges contact a professional in the field of noise and vibration for assistance, but by visualizing the path before each installation you are sure to get the most from your vibration isolation system.
Tony Adamson, AScT, is a licensed technologist in BC and has decades of experience in noise and vibration isolation for mechanical, electrical, and architectural systems. He is available for seminars and training sessions. Please contact info@rainlcoudnv.com for more details.