Study on Influence of Certain System Parameters on Pressure Pulsations of Multistage Reciprocating Compressor System

Prediction of pressure pulsation magnitude and their harmonic components for a multistage reciprocating compressor, piping and reservoir system depends on the input design and operating parameters. In the present paper, the results of the theoretical study carried out to study the effect of such parameters on pressure pulsations and their harmonics have been presented. The effect of pipe diameter and length has also been included. INTRODUCTION Reciprocating action of piston and intermittent flow of gas induced by suction and discharge valves give rise to pressure pulsations and oscillations. The prediction of the pulsations has become a significant part in the proper planning of a piping system in order to minimise unexpected failures. A digital simulation model involving valve dynamics, flow through valves, pipe, intercooler and reservoir has been developed for predicting pressure pulsations in a piping system of a multistage reciprocating compressor [1]. Some of the parameters which are basic input to the digital simulation program have been suitably selected. It is difficult to quantify precisely the value of these parameters. These parameters are initial force on the valve disc (F;n;J, valve discharge coefficient ( cd1), drag coefficient of the valve disc ( c~), damping ratio of the valve spring (~), polytropic index of compression and expansion (N1 and N2), cylinder pressures at the beginning of suction stroke (P,) and at the end of discharge stroke (P d), cylinder clearance (Cl) and pipe damping factor (a). The parameters such as F;n;, cd1, cd2 and 1; are used in the valve dynamics equation to calculate valve lift. Pd, P, and Cl are used in the equation for flow through valves to determine the volume flow velocity and a is used for calculating pulsating flow from the pipe. The effect of variation of these parameters on pressure pulsations and its harmonics has been theoretically investigated in the present paper. Further the effect of pipe diameter and length has also been studied. SIMULATION MODEL For analysis and formulation of the simulation model, a two stage, three cylinder reciprocating compressor with disc type of valves has been considered as shown in Fig.l. The system is divided into number of subsystems equal to the number of stages, in the present case it has been divided into two sub-systems. The first sub-system begins with the discharge end of the I" stage and ends at the suction of the nnd stage. The second sub-system begins from the discharge of the nnd stage and ends at the stop valve after the reservoir. Each sub-system is further divided along the flow route into elements such as starting end, pipe, volume and the last end. The starting end (section 1) is located near the discharge valve, where the magnitude of pulsating flow velocity is known from the piston speed, the area of cylinder etc. of the compressor. The transfer equations appraoch has been applied to establish the relations of pulsating pressure (P) and pulsating velocity (X) in complex form for each of the elements.