I have a little problem with a waste water lift station. The lift station is comprised of 2 pumps which take turns opperating. These pumps are sometimes cavitating, and I have an idea why, but just wanted to see what some more experienced people had to say about it. The wet well is approximately 5 feet from the well inlet to the centerline of the pumps intake (they pump in parallel and are approximately 7 feet apart). The owners are telling me cavitation is happening because of vortexing. They think this because of the fact that the inlet effluent 'waterfalls' into the wet well. It falls probably about a foot or so into the well. There is a steel grading to help disrupt the flow, but there are still reports of cavitation. I have visited the site a few times and watched the wet well for a while, never once saw any vortexing. So I am suspicious of vortexing. Another reason why I do not think it is vortexing is because 3 weeks ago they had major flooding in the wet well. Water levels doubled. I was not there when it happened, but the pump operator said the pumps still did cavitate a bit (whatever 'a bit' means, I'm not sure, that is just what he told me, 3 times after I questioned it) even with all the increased head. This leads me to think that it may indeed be obstructions at the suction? However I am fairly unfamiliar with how much obstructions can affect the pressure into a suction inlet. Theoretically I can see cavitation being an Chemical pumps issue if there is enough obstruction because in order to maintain flow, pressure would drop below vapour pressure in the suction. But I am just unsure if this is actually possible on this scale. Yet it seems to be really the only explanation. Is there something I am missing? Do both pumps indicate the same problem when they run?During the period of "flooding" was the inlet still free falling into the the wet well?Vortexing is not always a surface phenomonen, it can be from the sump wall or floor or even from the steel grating you have installed. It could be pre-rotation of the flow, or the pumps running too far out on their curve, have you actually checked flowrates against headThe inlet was not free-falling during the flooding. Since the water level rose about 6 feet, the water would have backed right up into the incoming line.I have done preliminary calculations and found that the head is right around where it should be (50 ft.) and am working on preliminary averages and numbers to estimate flows (both the suction and discharge pressure gages are shot on both pumps).One other tid-bit to note: I was looking over the drawings for the wet well and the drawings indicate that the suction virtually rests on the ground in the wet well (hard to confirm if this is actually how it was built). This seems to me like a terrible design, or is this normal? 

2011-09-23

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