Vortexscience.com: Bubble bubble, toil and trouble

Bubble, Bubble, Toil and Trouble

                      The most important discovery in Hydraulics in the last 100 years, since the observation of Cavitation is: The Elimination of Cavitation!

The Observation

                      As screw propellers began to replace the paddle wheel as the more efficient means of ship propulsion in the latter half of the nineteenth century, a mysterious phenomena began to plague Hydraulic Engineers. For some strange reason the propellers "raced", the efficiency deteriorated and the destruction of the prop increased as the shaft revolution speed was increased.

                      Osborne Reynolds, as early as 1873, attributed racing to what he described as the admission of air to the propeller which would interfere with the supply of water to the blades.

                      The source of the problem was finally identified before the Institution of Civil Engineers in 1895 in a paper by J. I. Thornycroft and S. W. Barnaby in which they described "cavities being formed in the water"... which... "were the source of the great waste of power and of other difficulties..." such as a high degree of propeller erosion.

                      This new phenomena was called "Cavitation" by R. E. Froude, director of the British Admiralty's ship model testing laboratory.

                      Reynolds experimented with creating cavitation in a tube containing a constricted section and demonstrated the "boiling of water in an open tube at ordinary temperature" and the formation of "cold steam".

                      Cavitation is defined today as the process of formation of the vapor phase of a liquid which has been subjected to reduced pressure at constant temperatures.

The Mystery

                      A complex and illusive process, Cavitation has continued to puzzle Engineers. This local boiling within a liquid that has not been heated is thought to begin with presence of minute gas-or vapor-nuclei. However, the source of the nuclei, and just why they persist are still unresolved questions.

                      Once a nucleus starts growing into a full-fledged bubble, the resulting flows become very complicated.

                      It is believed that all vapor appears on an impeller vane surface in the low-pressure zone. The rate of vapor production depends upon the vane area in the low-pressure zone.

                      The average life of the bubble depends upon the velocity of the flow through the impeller channel and the length of the low-pressure zone. Thus, the average life of the bubble depends upon the impeller design, or its suction specific speed.

                      An accurate determination of the maximum pressure developed at the collapse of these bubbles is difficult since the rise of pressure to its maximum value occurs in a time of the order of microseconds and over an extremely small area. Nevertheless, experiments have shown that spherical bubbles in water can produce pressures at collapse of at least 12,000 atmospheres.
 

The Curse

                      What is known about these unwanted bubbles is that they have continued to plague the designers of hydraulic equipment and marine propellers because of the increased speeds at which these devices operate. The creation and collapse of these tiny devils brings surface damage to the device's impeller and housing, head loss, pressure fluctuations, vibration and noise.

                      Cavitating regions radiate noise of high intensity, and such noise produced by cavitating propellers was a principle method of detecting ships during the Second World War.

                      It is easily seen that the collapse of bubbles of such pressure as mentioned above, near an impeller blade or upon impact with a vane or paddle can dig away at the metal surface and in a short time make the blades look like lace-work curtains.

                      Head loss which is a measurable effect of cavitation when pumping liquids other than water, is caused by the obstruction of the suspended vapor bubbles to the relative flow of liquid through the impeller channels. This can be visualized as a sudden local acceleration of the flow in the low-pressure zone, and then a more gradual deceleration beyond the low-pressure zone.

                      Pressure fluctuations downstream from the pump can split a line in half, tear a heat exchanger apart and can be serious damage in a petro or chemical plant. It is almost impossible to pump chemicals of low boiling temperatures in a conventional pump withotu cavitation.

                      Plant Engineers have found they must labor under certain limitations when using cavitating pumps or propellers or when pumping cavitating liquids. To insure appreciable savings in pump costs and downtime and to observe certain safety requirements, they have tolerated a "controlled cavitation operation" for a safe plant operation.

                      So this trouble maker - Cavitation - demands, in the end, a lower overall performance efficiency for the devices and the plant operations, a great loss of time and a great expense.
 

The Cure

                      A new principle of operation and a revolutionary design has completely eliminated cavitation and impingement in the unique Possell Bladeless Pumps.

                      What do these pumps mean to industry? The Possell Bladeless Pumps can perform the most difficult or "impossible" pumping requirements where conventional lifting surface pumps cannot function at all.

                      These pumps can pump a gas, a liquid and a solid TOGETHER, or any combination of these three and none of the materials ever touch the pump because of the protection of the naturally occurring "boundary layer" created between the surface parts (discs) and the spiralling medium.

                      This means these pumps make it possible to pump concrete slurries, extremely corrosive chemicals, dry materials such as mine ore concentrates, dry grains, sand and things that have never been pumped before.

                      Elimination of all lifting and impact surfaced eliminates all pressure fluctuations within the pump, therefore cavitation bubbles DO NOT FORM within the pump.

                      It is a field-proven fact that these pumps can pump at high speeds and elevated temperatures, chemicals which tend to cavitaty and "boil" because of the pressure differentials of other pumps. The Possell Bladeless pumps do not have to operate at low efficiency "controlled cavitation operation".

                      Two outstanding advantages which come with the elimination of cavitation are: the elimination of a noise and vibration factor and severely lowered maintenance costs and down time. Since these are no lifting surfaces and no impact occurring within the pump, there is very little noise or vibration. Since the two major destroyers of pumps (cavitation and impingement) are eliminated, maintenance is minimal.