Chemical Engineering May 2012 - 64

Feature Report
Engineering Practice
A Method for Quantifying
Pipe Vibrations
Simply supported pipe
A technique to quantify vibration forces
can help prevent pipe failures due to
vibration-induced fatigue
S. Saha
Reliance Refinery
F
ailures of piping due to vibrationinduced
fatigue are a serious
problem in the chemical process
industries (CPI) and a matter of
concern for the safety and reliability of
plant operations. Due to the complexity
of flow-induced vibrations in pipes, no
closed-form design solutions - those
that can be expressed in terms of wellknown
functions - are available.
In this article, we present a method
for quantifying vibration forcing functions
for the optimal design of metal
piping systems in the CPI, as well as
an example of its use. The method is
an analytical technique based on the
theory of vibrations in the frequency
domain (Inverse Theory of vibrations).
The method can be easily adopted by
practicing engineers.
VIBRATION MEASUREMENT
Piping systems experience various
vibratory loads throughout their lifecycles.
If not controlled, these pipe vibrations
will lead to fatigue failures at
points of high stress intensity and can
even damage pipe supports. These failure
scenarios could result in plant outages
or in more severe consequences,
such as fire or loss of human life. Thus,
it is imperative that piping systems be
safeguarded against such failures.
To avoid fatigue failures in piping
systems, engineers carry out dynamic
analyses of vibrations during a design
adequacy check for a piping system. The
major difficulty in dealing with the vibration
problems lies in estimating the
forcing function. If the exciting forces
MO
U
X
L
FIGURE 1. The span of pipe between two supported
points can be measured for vibrations
acting on the pipe can be
quantified precisely, the
system response can be
determined with great
accuracy by the existing
analytical methods. But
unfortunately, this is not
readily possible in most
cases, since the vibrations
in an operating pipeline
are flow-induced.
The complexity of flow
patterns and the mechanism
of force-coupling
render the determination
of the forcing function extremely
difficult. In such a scenario,
data - in the form of field vibration
measurements in conjunction with
analytical methods - can provide a
basis for estimating the dynamic force
and stress [1-3].
In our method, we analyze the problem
in terms of the theory of vibrations
in the frequency domain. We present a
simple numerical technique that can
be easily built into any of the common
spreadsheet computer programs with
the help of macros.
Current vibration approaches
The current practice for exploring
pipe vibrations is the vibration screening
criteria method. In this method,
vibration response parameters, such
as velocity or displacement, are measured
in situ and compared against
some established acceptance criteria,
usually in the form of graphs known
as vibration severity charts [4]. In the
46 CHEMICAL ENGINEERING WWW.CHE.COM MAY 2012
Vessel
nozzle
Compressor
discharge piping
Compressor
nozzle
A
Region of failure
FIGURE 2. The mid-point between two supported
points on a pipe is often where vibration failure occurs
petroleum refining and petrochemical
industries, these charts are used extensively.
However, they are typically
found to yield conservative estimates.
Another widely used tool is the
ASME OM Code [5] - a standard followed
for piping in the nuclear power
industry. Here the vibration velocity
for a piping span between two nodes
is the criterion. The limiting value for
pipe-vibration velocity is determined
by an empirical relationship, which
involves coefficients that depend on
several parameters, such as weld arrangements,
mass lumping, and others.
When the peak value for the velocity
is less that 12.7 mm/s, it may be
assumed that the piping has sufficient
dynamic capacity. If the vibration exceeds
this level, however, the ASME
guide recommends reviewing the vibrations
with more information on the
potential causes and taking steps to
reduce vibration levels.
B
ML
http://WWW.CHE.COM
Chemical Engineering May 2012

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