Page 266 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 251

Ref [19] discuss how moderate levels of torsional vibration can correspond to
high levels of lateral vibration at the torsional resonant frequencies (and
1 harmonic) as a result of lateral-torsional coupling within a compressor.
Compressors operating with torsional vibration are not rotating at a constant
angular velocity, which is typically assumed for inertial rod load calculations.
The nonconstant angular velocity creates additional inertia rod loads that pri-
mary respond at 1 harmonic from the torsional vibration harmonic. These
rod loads couple into the main bearings and react on the frame. The nonconstant
angular velocity also creates vertical forces at the torsional vibration harmonic
and 2 harmonics that react in the guides. These shaking forces can be calcu-
lated with simple rigid body kinematics at nonconstant angular velocity. The
frequency shift is due to cosine/sine multiplication at different frequencies
(i.e., 1 and n ) in the acceleration terms of the force calculations.
The forces caused by the torsional oscillation are proportional to the rotating
inertia, and importantly, to the square of the torsional frequency. For high-speed
reciprocating compressors with heavy pistons and moderate torsional vibration,
these forces can be on the same order as the primary unbalanced inertial forces,
which can be several thousand pounds of dynamic force.
While lateral-torsional coupling can and does exist in many reciprocating
compressors, elevated lateral vibration levels caused by this phenomenon often
pass undiagnosed. In addition, lateral-torsional coupling in reciprocating com-
pressors is typically not analyzed on a regular basis in the oil and gas industry, as
opposed to other topics, such as torsional vibration or pulsation studies, which
are very common. As these force calculations require the torsional response to
be calculated and can only be used in a design approach three mechanical study,
the absence of API methodology to address them is a current design gap in the
industry.

References
[1] These descriptions of diaphragm compressors were provided by PDC Machines, Inc.
[2] NEMA MG1 21.36.1.
[2a] ANSI/NEMA MG 1-2016 (NEMA MG1 21.36.2), Motors and Generators. https://www.
nema.org/Standards/ComplimentaryDocuments/ANSI_NEMA%20MG%201-2016%
20CONTENTS%20and%20FOREWORD.pdf.
[2b] API 618, Reciprocating Compressors for Petroleum, Chemical, and Gas IndustryServices,
Fifth Edition, December 2007, API Publishing Services, 1220 L Street N.W., Washington,
DC, 20005, United States.
[3] GMRC, Guideline and Recommended Practice for Control of Torsional Vibrations in Direct
Driven Separable Compressors, Section 3, GMRC, 2015.
[4] G. Phillippi, Basic Thermodynamics of Reciprocating Compression, in: Turbo Symposium
Tutorial, 45th Turbomachinery and 32nd Pump Symposia, Houston, Texas, September
12–15, 2016, 2016.
[5] Gas Machinery Research Council (GMRC), Guideline and Recommended Practice for Control
of Torsional Vibrations in Direct-Driven Separable Reciprocating Compressors, 2015.