Page 110 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
108 Chapter Three

All that is written in the previous subsection is relevant but in addi-
tion it is necessary to consider the natural frequency of vibration of the
floor, which will almost certainly be lower than the lowest forcing fre-
quency which the engine produces. To avoid resonance each of the antivi-
bration mounts and its share of the supported mass must have a
resonant frequency well below, say one half of, the natural frequency of
the floor. It will be difficult to ascertain the natural frequency of the floor
and guidance from a structural engineer may be needed. If the deflection
of the floor due to the load of the set can be ascertained, the natural fre-
quency can be calculated using the formula already given. It will be
found that the deflection of the mounts should be at least four times the
deflection of the floor.
To achieve the above, the antivibration mountings may require a sta-
tic deflection of up to 50 mm to avoid any possibility of resonance and
to isolate the movement of the set from the building structure. Very
large forces can be involved during starting, during fault clearance and
during faulty synchronizing, and the freedom of movement can lead to
instability at such times, particularly if the center of gravity of the set
is high. To overcome this problem expensive remedies are sometimes
necessary such as the addition of an inertia block (below floor level) to
lower the center of gravity of the set.
When the antivibration mounts allow a large freedom of movement
it is particularly important that all services to the set pass through
flexible sections. The services include electrical cables, fuel, oil and
water pipes, ductwork, and exhaust pipes.
The location and arrangement of the generating set should always be
agreed with the structural engineer responsible for the building.

Gas Turbine–Driven Generating Sets
A gas turbine has no reciprocating parts and the only out-of-balance
forces that arise are those due to the dynamic unbalance of the gener-
ator, turbine, or gearbox rotating parts. The problem is very much sim-
pler and the use of the correct antivibration mountings is usually
adequate to ensure smooth running. All services must pass through
flexible sections as described for diesel engines.
The location and arrangement of the generating set should always be
agreed with the structural engineer responsible for the building.

Noise
The Units Used in Noise Measurement
The human ear is sensitive to a very wide range of sound intensities. The
lowest sound intensity that the normal ear can detect is 10 12 watts per

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.