Development of an accumulator system for a pulsating Pelton nozzle

Abstract

Hydro Power Technologies Inc. (HPT) aims to develop a Pelton turbine configuration where a pulsating nozzle shapes the water into projectiles, one for each passing Pelton bucket. The scope of this thesis is to develop and test hydraulic damper systems for attenuation of harmful pressure amplitudes caused by the pulsating Pelton nozzle.

Initial analytical calculations have been carried out to find the size of the expected water hammer near the nozzle. Laboratory tests of the Pelton turbine rig have been conducted without the turbine installed to study pressure amplitudes and their effect on the water jet shape. All tests were run for a specter of nozzle pulse frequencies, ranging from 2,5 Hz to 25 Hz. A continuous water jet was found to be the most ideal for the intended use.

A 50-liter bladder accumulator was tested for four different pressure charges. Pressurization above system pressure gave results which were found to be unsuited for amplitude damping. With charge pressure at system pressure and lower, the accumulator showed a damping effect of 12-45\% compared with the test rig without damper. This was at pulse frequencies below 20 Hz.

A Helmholtz resonator was sized, designed and produced based on a suggested operational frequency of the nozzle. For a wide range of frequencies, the Helmholtz resonator proved excellent effect. Upstream the resonator, a damping of 70-94\% was observed for all frequencies except for the two tested natural frequencies. Downstream the resonator, a damping of 60-90\% was found, for a pulse frequency below 20 Hz.

The Helmholtz resonator nor the accumulator were able to damp the amplitudes at the two hydraulic natural frequencies sufficiently. Especially the second harmonic, near 24 Hz, was not damped significantly.

The most ideal jet shapes were apparent between the two natural frequency modes, for this case 17,4 Hz and 19,9 Hz, with the Helmholtz resonator installed. Operating the nozzle frequency near the open valve natural frequency resulted in a jet with a rippled shape. This is not desirable for a Pelton turbine.