12
The Practical Side of
Vibration Testing and Analysis
Kevin P. Gaghan
|
NEBB’s Sound and Vibration Committee
(
In collaboration with Stuart McGregor and Jim Huber)
Vibration testing and analysis. This has been referred
to in a number of different ways, most of which are not
meant to be complimentary. Voodoo box, black magic,
rocket scientist stuff, etc. It is an unknown for most
people, and if something is unknown, people either run
from it or attack it.
Customers come in many forms and with many types
of needs. The job of a S&V Certified Professional (CP) is
to provide a service to a customer which solves their
vibration problem. Keep in mind that when vibration
becomes an issue, it is a bad thing. Vibration has always
existed in mechanical equipment, it just becomes bad
when it impacts people, money, and equipment life,
otherwise it goes unnoticed.
The most common reason that I have found in the
mechanical contracting and service industry for
needing vibration correction is basic fan balancing.
A lot of specifications in new equipment establish a
maximum allowable vibration, usually at fan speed, to
determine whether correction needs to be taken before
allowing the equipment to be turned over to an owner.
The intent is to turn equipment over so that the owner
gets a fully functional piece of equipment which should
operate for at least its expected equipment life. This
is also one of the basic tenets for the use of baseline
vibration testing when commissioning buildings and
equipment. I previously worked with a franchise of a
major HVAC manufacturer, and I had the opportunity to
balance hundreds of fans in the field. The majority were
very easy to balance, allowing the S&V CP to come out
as the hero, whether deserved or not.
The next step seemed to be vibration analysis of larger
rotating equipment. When the company I first worked for
with vibration as a service started down this path, it was
to perform analysis on large centrifugal chillers. Pulling
large AC units down to inspect bearings on a periodic
basis was the norm in the early 1980s. Recovering large
quantities of refrigerant and pulling machines apart was
very expensive to do, and is even more so now. If it could
be determined what problems existed in a unit (usually
bearing problems), then documented justification for
pulling a unit apart for repairs is present. You can also
allow a unit to operate for an extended period before
a unit needed to be shut down for maintenance repair.
These steps have the potential to save the customer
money versus a periodic teardown of equipment.
When I first started analysis of this type in the early 80s, it
required 8 to 16 hours of hard copy print outs of each point
of vibration. This was then followed up with 16 to 32 hours
of calculations to determine where the vibration problems
existed to couple with determining what was critical to
machine operation to prevent catastrophic failure.
Remember earlier about what I said about those rocket
scientists (aka engineers- the people we love to throw
under the bus when things do not work the way we expect
or want them to), doing things we do not understand and
run away from? Well, those people have made our lives
better, especially in my world of vibration. Their math skills
have allowed equipment and vibration analysis tools to
become accesible to those of us who are willing to read
beyond the Sunday comics, and appear to be smarter and
faster than we really are. I do not subscribe to the idea that
just anyone” can or should get into vibration work, just
that if one is willing to apply themselves and be detailed
in their work, observations, and analysis, the mechanical
world will be better off, as would building owners.
Vibration equipment manufacturers put out many types
of great equipment which can perform many different
functions, provided you are willing to purchase all of
the functions. This is capable of making all of us appear
way smarter than we actually are (I say this from
experience- I have been given credit where none is due).
Many manufacturers sell software that will perform the
analysis for you and assist in report writing. 90-95% of