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Originally Posted by khoj Trolling the internet and wikipedia etc etc is not my cup of tea. One does not know how much of the information is genuine and how much is coming from an armchair expert. |
I agree, these days research and doing a google search have become synonymous. Never the less, there is a huge wealth of information out there on the net, but indeed the trick is to figure out whether it is genuine.
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Originally Posted by khoj The very reason I posted here was to find out what type of pumps are being used in vehicles. |
Mostly PD pumps, on regular cars nearly always PD in a wide array of different versions and models
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Originally Posted by khoj Meanwhile outside of a car's bonnet the World moves on. Industrial production floors are replete with self priming centrifugals, that too without the help of any external priming aides and I am not talking about submerged pumps either. |
Sure, self priming centrifugal pumps have been around for decades. I was taught about them when I went through naval academy in the mid '70. The first ship I sailed on, built in the early 60's had them too. A few years ago when I was on holiday on the west coast of the USA I visited a Liberty vessel from the second world war and even they had them.
Lets not quibble over semantics. For me a centrifugal pump is not self priming and it would need some sort of external help to prime, be it vacuum or positioned below the liquid level etc. A self priming centrifugal pump is different. I haven't kept myself up to date but in my days, they did require liquid to be present in the pump house. To put it differently, they did not prime "out of the box'. If they leaked or the liquid in the housing evaporated they would not prime. Maybe there are self priming centrifugal pumps that do not require the housing to be filled with liquid? I would be interested to update myself on those.
I've worked on vessels and rigs where the fire pumps were self priming centrifugal pumps, but they always had external means of creating a vacuum as well just in case.
I'm not sure how well suited those sort of self priming pumps would be for automative application. The design is more complex (thus more expensive) and you still need to ensure its (partial) filled with liquid.
For this who want to see it all in action:
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Originally Posted by khoj Dozens of pump overhauls later if you feel cavitation is a minor issue compared to frothing then obviously you have not seen a PD pump cavitate. When I said it is like a bomb going off I meant it literally. A cavitating PD pump such as a twin/multi screw, especially a gear pump can not only self destruct (in a matter of minutes) but also take down a fair amount of plumbing with it. The foaming as you called it is destructive for the pumpage rather than the pump. Frothing still happens but now a days these episodes are less frequent compared to how often one sees cavitation. This because more often than not the effects of changes in viscosity on a pd pump's performance are ignored amongst other overlooked criterion. Again before this is generally applied to vehicles, I am stating this w.r.t. industrial applications. |
I am no expert but I don't think cavitation by itself will destroy a pump. It has to be in combination with pressure surges.
I have read a bit about cavitation in PD pumps and actually by and large it was found to be not that harmful in experimental/laboratory setup. Never the less, manufacturers are careful and several standards exist to ensure correct suction pressure is maintained to avoid any possible damage.
Don't believe everything you find on the internet, but I do believe this to be a genuine and very enlightening, fairly recent, article on cavitation with PD pumps.
http://turbolab.tamu.edu/proc/pumppr...nt%20Pumps.pdf
Cavitation does occur more often than foaming. If anything because temperature (thus viscosity) plays a big role for the foaming to happen in the first place.
That first ship I mentioned earlier spend the first fifteen years of its life in tropical waters. Nice and warm. But when I joined her, we went to Japan for the first time, during winter and we had all these problems with our hydraulic cranes, hydraulic winches, hydraulic deck hinges etc.
Jeroen