In most commercial and industrial buildings, you will find some type
of vibration isolation on the mechanical equipment. If the products have
been specified and installed correctly, they should be providing a
significant reduction in the force transmitted to the structure,
relative to the force that would be transmitted if the equipment was
“bolted down.” In our experience, there is a good chance that the
isolation is not working properly.
Most problems with vibration isolation can be easily avoided. There are three areas where problems frequently occur:
1) Improper design, selection & specification
2) Incorrect installation
3) Inadequate maintenance
Improper Design, Selection & Specification
The 2015 ASHRAE Handbook on HVAC Applications (Chapter 48 on Noise
and Vibration Control) is an excellent source of design information for
vibration isolation. We at Easyflex have developed installation
guidelines for most commonly used mechanical equipment that should allow
the engineer to specify the correct isolation for most types of
equipment. Please contact us if you need assistance for both
installation guidelines, drawings or tender specifications. Problems
arise however, either because the proper sources are not consulted or
the information on the drawings is incomplete.
One of most common errors is overloading or under-sizing of the
isolator. For springs, it should be obvious when this has occurred, but
it is surprising how often it is seen in the field. The figure below
shows two examples of fully compressed springs. A well-designed spring
will have an additional “travel to solid” of 1.5 times the rated
deflection, so the springs shown below are loaded to at least 1.5 times
their rated capacity. They should be replaced by stiffer springs
providing the specified deflection.
Overloaded spring isolators – pump base (left), spring hanger (right).
For resilient isolators such as rubber pads, overloading may not be
so obvious. Special care must be taken with this type of isolator,
especially when a concentrated load is applied directly to a pad or
sheet of material.
Housed spring isolators are commonly used as OEM equipment, but if
possible they should be avoided, due to the potential for binding,
corrosion, and debris/liquid collection. The figure below shows a housed
two-spring isolator that appears to be working properly, but closer
inspection reveals several problems related to the potential for
binding, or “short circuiting”. First, due to a slight misalignment, the
housing and the nested shell are in contact. Although a piece of
resilient material prevents metal-to-metal contact, this condition is
less than ideal (notice that the pad on one side is missing). Second,
the bolt between the two springs appears extremely close to contacting
the springs, which is also bad. Third, the potential for debris getting
into the housing and interfering with the isolation is obvious. On
rooftop installation, we have seen ice build-up and bind in the
isolator, which prevents it from moving freely.
Housed spring isolators should be avoided
For some reason, housed isolators are frequently included as part of
the original equipment on HVAC equipment such as fans and chillers. When
this occurs and it is impractical to replace them, extra care must be
taken in the installation and maintenance. Another shortcoming of some
housed isolators is that they do not include a neoprene “acoustical” cup
under the springs. This feature helps attenuate high frequency
vibration that may travel through the spring.
Incorrect Installation
By far the most prevalent installation “misstep” is to place an
anchor or other rigid metal part in parallel with the isolator. This
occurs frequently since the installer wants to hold the equipment in
place. An example is shown for a small air compressor below. In this
example, if the bolt is placed in an oversized hole, a neoprene washer
is used, and the nut is only finger tight, then this arrangement is
acceptable. There should be no metal-metal contact if vibration
isolation is to be effective. The bolt then only provides restraint
against excessive motion, not normal vibratory motion.
Another curious example is shown below. The pumps are isolated with
springs under pump bases, but the pipe stands at the end of the pumps
are supported directly on the slab below. In this case a larger base
should have been used to allow the stand to be supported by the base.
Essentially the housekeeping pad shown is too small for the pump size.
A bolt or other rigid metal part installed in parallel with the isolator can render it ineffective
Misalignment is a common problem for both spring mounts and hangers.
As shown below, misalignment can create contact resulting in “short
circuiting” of the isolation. The remedy for this lies with careful
quality control in the field. It is the responsibility of the contractor
to ensure that the springs are installed with concentric loading. For
hangers, this involves drilling overhead holes, which must be located
accurately relative the pipe being supported. Isolator alignment
requirements should be stated clearly in the project specifications.
A second type of problem that occurs with hangers is essentially the
opposite of the overloading problem discussed above. The figure below
shows a spring hanger that is obviously not supporting any load. The
clevis hanger below the spring isolator is simply too big. In this case,
either a smaller clevis hanger should have been used or the available
one modified.
Spring hanger problems – misalignment (left), oversized clevis hanger (right)
Flexible connectors are a critical element in the isolation of
equipment with connected piping. One frequently encountered problem with
braided stainless steel hose is installation in a vertical orientation,
when the vibratory motion is also in this direction. The figure below
shows an example of incorrect installation. Since the braided stainless
steel is relatively stiff along its length, it is not an effective
isolator for motion in this direction. The flexible hose should have
been installed horizontally, perpendicular to the plane of motion. It is
easy to see why this is sometimes not done, since it requires extra
space which may not be available if this arrangement was not
anticipated. If a vertical installation is required, an elastomeric
connector should be considered.
In this photo, note that the concrete inertia base has been correctly sized to support the pump stand.
Braided steel connectors should be installed perpendicular to the direction of vibratory motion (not as shown)
Inadequate Maintenance
All too often, vibration isolation is installed and forgotten.
Mechanical rooms in older buildings can be virtual museums of ancient,
neglected isolation hardware. In many cases the hardware is no longer
providing effective isolation. See the photo of a badly corroded mount
below. In spite of its bad condition, it may continue to support the
equipment for a long time, and probably does not need to be replaced
unless a vibration problem is occurring. In other cases, the isolation
should be restored by replacing or repairing the hardware.
Another case of a housed spring isolator in bad condition is shown
below. It also appears that the height-saving bracket was poorly
designed and has experienced excessive distortion.
A corroded leveling mount (left), a housed spring isolator in bad shape (right)
Oftentimes building maintenance personnel will make creative retrofits
to vibration isolation, such as in the situation shown below. Pieces of
2x4s have been used to shore up a fan on which the hangers were not
working properly, and the fan is now supported on the duct below. When
this type of situation is encountered, it should be corrected as soon as
possible. In some cases, modifications to the isolation can create a
safety hazard, especially in high seismic areas.
A creative retrofit to vibration isolation
Like the equipment itself, the vibration isolation has a finite
lifespan. It is often the case that when the isolation is in bad shape,
the equipment is also old. Hence, a major replacement or upgrade of the
isolation on existing equipment may not be a good investment. However,
any new equipment should be properly fitted with the most up-to-date
vibration isolation.
The Bottom Line
Proper design, installation, and maintenance are the keys to
providing long term, effective vibration isolation of mechanical
equipment. In building projects, vibration isolation can be a
significant part of the mechanical equipment budget. Because of this,
it is incumbent upon engineers, suppliers, and contractors to provide
the most effective isolation possible within the project budget. The
project specifications should be clear with regard to the vibration
isolation requirements. Once in place, the isolation should be well
maintained. In this way, value to the building owner will be maximized
and occupants will receive the benefits of low vibration.
