This paper investigates the flow through a hydraulic pressure relief valve at high levels of operating pressure up to 700 bar (10000 Psi).

Following the flow path from the cold high pressure region before the metering edge to the warm low pressure region behind, the mean viscosity decreases by a factor of 16, the mean bulk modulus decreases by a factor of 2 and the mean density decreases by 6 %. Based on this preliminary considerations, a turbulent single phase flow considering pressure and temperature dependent viscosity, bulk modulus and density is modelled and steady state as well as transient calculations are performed.

The results of this study show that a pressure and temperature dependent viscosity reduces the pressure drop and the spool force by 10 % compared to a simulation with constant fluid parameters. Moreover, it is shown that compressible flow modelling has negligible influence on pressure drop and spool force — nevertheless, it is required to describe the temperature correctly. Due to the effect of volumetric work an incompressible model approach predicts the mean temperature rise 20 % too high. Finally, it was found that the temperature on the spool exceeds 400 °C. Afterwards, this fact is experimentally validated obtaining tempering colors in high pressure tests.

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