Page 456 - Compression Machinery for Oil and Gas

436 SECTION III Applications

or discrete excitation with periodic oscillations with high amplitudes at partic-
ular frequencies.
The three types of flow-induced excitation that are analyzed in compressor
systems are acoustic-induced vibration (AIV), flow-induced pulsation (FIP or
flow-induced excitation), and flow-induced turbulence (FIT). API, PTC, and
ISO standard references are needed for the analyses of these phenomena [5],
but only PTC 19.3 currently describes a methodology for performing these ana-
lyses, and that is limited to flow-induced excitation of thermowells. The Energy
Institute (EI) Guideline provides the only widely available reference when per-
forming an analysis of these types of excitations [6]. However, this guideline is
considered by many in the industry to be outdated and overly conservative;
therefore, many engineering companies and operators have developed internal
guidelines when analyzing piping for flow-induced excitation.
The AIV is a vibration phenomenon due to excitation from valves with high-
pressure differentials combined with high-mass flow rates. The flow is choked
at the vena contract creating a high-energy turbulent jet that expands into the
piping creating broadband turbulence. This creates pressure and velocity fluc-
tuations in the piping system that can couple with piping acoustic and MNFs
creating high-cycle fatigue failures at high stress concentration points such
as tee connections or welded supports. The excitation is typically low-amplitude
broadband excitation from approximately 200 to 5000Hz, depending on the cut-
off frequency [7].
API 521, sixth edition requires a screening for the risk of AIV failures in all
vent and blowdown piping systems. Screening procedures for AIV are based on
methodology presented by the EI, Southwest Research Institute (SwRI), Fluor,
and CSTI [8–11]. Most of the screening methods are based on papers by Carucci
and Mueller and Eisinger which present historic AIV failure data [12, 13]. All
methods characterize the source noise, attenuation of noise levels, and a geom-
etry parameter for each connection [14, 15]. Some methods include the devel-
opment and evaluation of mitigation techniques such as stiffening rings,
mufflers or damping wraps, or perform finite element analysis for improved
accuracy. However, no physics-based model of AIV exists that has been vali-
dated with test data.
The FIP (also known as flow-induced excitation) occurs when fully devel-
oped turbulent flow, as defined by Reynolds number regimes, encounters a dis-
turbance in the piping geometry. The most typical disruptions in compressor
piping systems are pipe stub or side-branch connections, instrumentation
inserted in the flow such as a thermowell, and orifice or valve internals. The
fluid excitation takes the form of periodic vortex shedding initiated by an unsta-
ble shear layer of the flow. If the frequency of the pressure fluctuations asso-
ciated with the vortex shedding are close to the acoustic or mechanical
natural frequency of the flow disturbance, energy from the fluid flow can create
a response. The response in turn strengthens the fluid instability and the coher-
ent vortex formation, resulting in a feedback loop between the fluid response