Reliability analysis of a hydraulic on/off fast switching valve

This paper presents an analysis of a hydraulic on/off valve from a reliability point of view. The objective is to clarify the potential pitfalls of the current valve design, while identifying the component(s) exerting the most significant risk of failure during the lifetime of the valve. Specifically, the mechanical topology of Fast Switching hydraulic Valves (FSVs) are of interest since these undergo operating cycles in the gigacycle regime in theirs functional lifetime. Application of these FSVs is relevant in e.g. digital displacement units, which for the specific design considered, experience pressure differentials of 345 bar at flow rates of 120 L/min and impact velocities around 1 m/s. All of which introduce a concern about the severity of long term fatigue and wear. Answers to these concerns are sought by first establishing a fault tree analysis of the FSV. Hereafter, the failure rates of different components are elaborated and expressions for such failure rates are established. This is done by a pseudo-analytic approach, where the expected reliability of the FSV is based upon statistical data. Results of two different cases are highlighted, namely failure with a low and high allowed internal leakage across the valve seat (0.0005 and 0.5 L/min). The study shows that the probability of a low leakage failure is close to 100% before 5 years of operation and 10% risk of high leakage failure after 25 years of operation. Valve seat failure is therefore a potential threat to limit the functional lifetime of the FSV. The applied reliability models involve a high amount of uncertainty, but there is a clear indication that failures will evolve around the flow passage. Allowing additional leakage prolongs the expected lifetime, and further work involves design of a framework to simulate design factors influencing the valves performance and lifetime.