Frequency limits for gas flow pulsation damping criteria

Abstract

The accuracy of most flowmeters is seriously affected by pulsations, and these must be damped before reliable measurements can be achieved. Damping criteria based on the Hodgson Number have been used for many years in flow measurements made with differential pressure-type flow meters. The objects of this paper are to demonstrate the validity of such criteria in gas flows and to specify their frequency limits. A damping criterion derived from a theoretical analysis applicable to any pulsation waveform is presented. Results of experimental tests carried out with both piston- and siren-type devices, producing pulsations of sinusoidal and non-sinusoidal waveforms in the frequency range of 5-500 Hz, are shown. The results verify that the damping criterion is valid on the Surrey University air flow rig at various pulsation amplitudes and damping volumes at frequencies below 50 Hz. It is suggested that these results are applicable to any gas flow system provided the limiting frequency is such that: (i) The dimensions of the damping chamber are not greater than one-tenth of the pulsation wavelength. (ii) The length of pipe between damping tank and flowmeter should not exceed one-fifth of the pulsation wavelength.