Instabilities in a flow-control valve

This paper is about some instabilities that have been observed during the testing procedure of a flow-control valve. It was observed that the complete closure of the valve was very often reached after a transient state with oscillating behaviour of its mobile element whose displacement was recorded by means of proximity sensors, facing on opposite sides the moving body of the valve. The outputs have been collected by two signal conditioning devices and sent to a dual channel spectrum analyser. Measurements have also been performed through a servo-accelerometer which has been fixed to the pipe. The influence of the pipe length on the behaviour of the system was studied. The oscillations recorded by the proximity sensors are fairly well reproduced by a series of square waves, with a number of low amplitude, strongly damped, higher frequency contents. During a single test the frequency of the oscillations is not a constant but increases in its central part. The phenomenon cannot be related to one of the natural frequencies of the piping system, that are much higher. The observed frequency depends on the length of the adduction pipe.