This paper addresses the issue of understanding the dynamic phenomena in piston type accumulators; and how this leads to improved numerical accumulator models. Developing a numerical model has two challenges. First; the unsteady heat transfer between the accumulator gas and the wall has to be described. Secondly; a suitable real gas model had to be identified and employed. To verify the model; a series of experiments were conducted at Fritz Schur Energy in Glostrup; Denmark. The experiments were designed to investigate gas dynamical properties at various precharge pressures; maximum pressures; and ambient temperatures. These parameters are varied to obtain parameter independent conclusions. During controlled piston movements; hydraulic and gas pressures are measured together with the piston position and the gas temperature.
It was found that the simple thermal time constant approximation by Rotthäuser was suitable and stable for the application. It was also found that the Soave-Redlich-Kwong equation; was overall best suited with experimental data. The Soave-Redlich-Kwong equation is as much as six times faster than the widely used Benedict-Webb-Rubin equation; independent of the ambient temperature; maximum pressure and precharge pressure.
This project concludes that the Soave-Redlich-Kwong equation should be used in simulation of piston type accumulators. It is noted that the pressure available after expansion is some 5-10 bar lower than predicted by any model. Therefore; it is suggested that more research is conduced to obtain an improved model for the heat transfer. finally; measurement quality was confirmed by comparing measured pressure data by pressure calculations based on measured temperature<
Keywords: Hydraulics; Accumulator; Thermodynamics; Modelling; SIMULINK; Matlab; Verification; Thermal time constant
13th Scandinavian International Conference on Fluid Power; June 3-5; 2013; Linköping; Sweden
 Pourmovahed A. Otis; D. R. An Algorithm for Computing Nonflow Gas Processes in Gas Springs and Hydropneumatic Accumulators. Journal of Dynamic Systems; Measurement; and Control; 107:93–96; 1985.
 A Pourmovahed and D. R. Otis. An Experimental Thermal Time-Constant Correlation for Hydraulic Accumulators. Journal of Dynamic Systems; Measurement and Control; 112:116–121; 1990.
 S. Rotthäuser. Verfahren zur Berechnung und Untersuchung hydropneumatischer Speicher. Fakultät für Maschinenwesen der Rheinisch-Westfälischen Technischen Hochschule Aachen; Essen; Germany; 1993.
 D. R. Otis. New Delvopment in Predicting and Modifying Performance of Hydraulic Accumulators. National Conference on Fluid Power; 1974.
 G. Soave. Equilibrium Constants from a Modified RedlichKwong Equation of State. Chem. Eng. Sci.; 27:1197–1203; 1972.