Pulsation Damping Systems for Large Reciprocating Compressors and Free-Piston Gas Generators

Abstract

The pulsation problems inherent in installations comprising large and relatively slow speed-reciprocating compressors or engines, both on the induction and delivery side, have been recognized for many years. The problem of pulsating flow has recently become particularly acute with the large-scale introduction of free-piston gas generators which, because of their unique mode of operation and the fact that they are usually installed in groups drawing from a common air manifold and delivering to a common exhaust duct, are particularly sensitive to such pulsations and where a rigorous approach to the problem is imperative. The present paper is concerned exclusively with intake systems which usually consist of two distinct elements, namely the individual damping system for each gas generator and a common ducting system for the entire group of gas generators, from which the individual damping systems draw their air supply. The individual damping systems take the form of one or two large smoothing chambers or damping capacities connected by Venturi-shaped ducts to each other (when two chambers are used) and to the common ducting system, the gas generators drawing air from these smoothing chambers through pipes which may be arranged to give some ram effect with resultant improved volumetric efficiency. The object of the individual damping systems is to ensure that the magnitude of the pulsations reaching the common ducting system is sufficiently low to prevent harmful pulsations which could cause damage to air filters, or even to property in the vicinity, particularly if resonances are excited in the common ducting system. A typical intake system for an eight gas generator installation is shown in Fig. 1. The paper is concerned solely with the method of analysis applicable to the individual damping systems, not with the external ducting system. The latter should present no difficulties if the individual systems are properly designed. The method of analysis is based on mechanical vibration theory, the governing equations for the damping systems—conveniently referred to as acoustic systems—being identical with those of better known mechanical systems. The equivalent electrical systems are given in an appendix, with a view to facilitating the analysis of damping systems with the aid of analogue computers. The object of the analysis is first to put the design of such damping systems on a sound theoretical basis and secondly to facilitate the calculation of optimum dimensions, given permissible pressure drop and pulsation amplitude outside the system, as well as the form of the suction pulse of the gas generator. The latter is most conveniently represented as a harmonic series using Fourier analysis. The present paper is based on the author's own experience in this field and, though restricted to purely theoretical considerations to avoid excessive length, the material contained in the paper is confirmed by a substantial body of experimental work.