Shaft torque and corresponding stresses in a motor driven - direct coupled - reciprocating compressor system, are significantly influenced by the system’s mass-elastic properties. Uncertainty in the system’s mass-elastic properties will therefore translate into uncertainty in the calculated system stresses. Three case studies provide the reader with an appreciation for the importance of defining uncertainty bands in the mass-elastic properties of a torsional system. The case studies are followed by a theoretical discussion section. The reader is introduced to the concept of torsional resonance, uncertainty in calculated system torsional natural frequencies is defined, and the relative influence that specific system mass-elastic properties have on the overall system is discussed. A full compliment of system mass-elastic uncertainties is presented.

While new gas compression in pipeline service tends to be dominated by centrifugal machines, reciprocating compressors still have a significant place in the industry. Specific dynamic design is required to ensure reliable and efficient operation of all reciprocating compressor installations. This requirement is particularly significant in pipeline installations, because the compressor is intended to be in service for many years, and because high efficiency is important for economic reasons. It is widely recognized that the design of these types of installations should include a “pulsation study”. A pulsation study involves analysis of the proposed installation to predict pulsation, vibration, and stress levels. Further, a pulsation vibration control scheme is developed as part of the overall design. The objective is to ensure that predicted pulsation and vibration levels meet guidelines while limiting associated pressure drops and horsepower losses to acceptable levels. Various guidelines have been used in these studies, but the most commonly used standards are in API 618. While this standard was not originally intended for pipeline service, in reality it represents the best design standard available for high specification reciprocating compressor installations in any application. Recently, work has been done to upgrade the API 618 design standard. One of the changes in the proposed new 5th edition is the addition of unbalanced force guidelines to the existing pressure pulsation guidelines. Much discussion occurred regarding the need for and the advisability of making the addition. Real examples show designs in which a reduction of pressure pulsation is accompanied by an increase in unbalanced forces, illustrating the need for an unbalanced force guideline. It is also shown that problems can occur due to unbalanced forces in parts of the piping system not currently addressed by the pulsation guidelines in API 618. The paper compares the current 4th Edition versus the draft 5th Edition. Comments are made on the applicability of the various guidelines. While API 618 is the best available design document, the addition of force guidelines will help API 618 do a better job for industry.

This paper discusses and illustrates how the application of analytical tools, if properly applied, during the design process ensures a design that is both reliable and economic. Emphasis is on the dynamic behavior of centrifugal and reciprocating compressors. Illustrations deal with both types of compressors. The key requirement is that pulsations, vibration levels and dynamic stresses must be low enough, that there is minimal impact on performance or reliability, while maintaining an economical design. To achieve this result, it is necessary to understand and control forcing functions, natural frequencies, mode shapes and dynamic stiffnesses. These considerations apply to rotor dynamics, piping vibration, torsional vibration, skid and foundation vibration. In addition, similar considerations apply to gas pulsations, where the interaction of the piping geometry with pressure pulsations, (arising from the uneven flow of gas through the suction and discharge nozzles), can produce significant forces and stresses for both reciprocating and centrifugal compressors, as well as degradation of performance. Computer modeling can be used to avoid problems that reduce reliability, degrade performance and increase maintenance. Considerations for determining the appropriate level of analysis are outlined. The probability and cost of events in the absence of suitable design are discussed. The cost of these events is compared to relevant design costs. The practical implications are illustrated with three cases where adequate design modeling and optimization was not done. One case involves a lateral critical and a structural resonance in a centrifugal compressor installation. The second involves a piping failure in a reciprocating compressor installation. The third involves a torsional failure in a reciprocating compressor installation.

Experience with compressor valve modelling has shown that reciprocating compressor performance can sometimes be improved by subtle changes in valve design. Modelling has led to a better understanding of the physical behaviour of valves and of the compression process. Three compressor valve studies presented here demonstrate the benefits of valve modelling. Case 1 challenges the commonly held assumption that reducing the lift of a compressor valve will reduce the efficiency of the compressor. The capacity of this compressor is increased by reducing the valve lift. A plot of BHP/MMSCFD versus valve lift shows an inflection point that assists the analyst in optimizing the design. Case 1 also presents a method of calculating the economic effect of improvements in valve performance. Case 2 demonstrates the effect of inadequate flow area through the valve. Pressure in the clearance volume cannot decrease fast enough if flow areas are inadequate; the result is late valve closure, and therefore decreased valve life. Case 3 shows the importance of considering the design of the cylinder casting in addition to that of the valves. Here, insufficient cylinder flow area constricted gas flow. Since these cases were simulated, the analyst had the opportunity to evaluate the proposed solution over the entire range of operating conditions. He was able to select a valve which solved the immediate problem and be confident that it would perform adequately throughout the specified range of conditions.