How do submersible pumps work?

[JohnW2]

The "Vanes" on the impeller (lets call them that for now) are angled to move water in a certain way and whilst I can`t quite see from the photo on yours often the housing is not central to the impeller but is set up to be nearer the housing and just past the outlet therefore by closing the gap the water is pushed out. I spent 5yrs in pump R&D before computers were doing this stuff and by moving housings around you get a BIG difference however the wear life on bearings and and other parts have to be considered, well that and the end user of course.

Yes, I suppose I realised that the explanation must have been along those lines but, when I think about pressures and flows, I'm still extremely surprised. Given that the inlet and outlet orifices are roughly the same size (and even if their were negligible resistance to flow presented by the pipe connected to the outlet), the fact that there is a considerably higher flow rate though that outlet than (if any) back through the inlet, this seems to imply very different pressures in two parts of that chamber which are no more than a couple of inches apart.

Whatever, they obviously work (and work very well), so my 'surprise' is clearly not really justified!!

Kind Regards, John

 

[just pumps]

The square box (float switch) contains a large ball bearing and a pair of contacts at the cable end of the box. If the box is raised up by increasing water level, the box floats cable end lowest, the ball drops inside the box and shorts the contacts out which causes the pump to run until the box falls again, as the water is pumped out. You can hear the ball inside rattle, if you shake the float switch.

The ball hits a microswitch, otherwise spot on.

 

[JohnW2]

The "Vanes" on the impeller (lets call them that for now) are angled to move water in a certain way and whilst I can`t quite see from the photo on yours often the housing is not central to the impeller but is set up to be nearer the housing and just past the outlet therefore by closing the gap the water is pushed out. I spent 5yrs in pump R&D before computers were doing this stuff and by moving housings around you get a BIG difference however the wear life on bearings and and other parts have to be considered, well that and the end user of course.

Yes, I suppose I realised that the explanation must have been along those lines but, when I think about pressures and flows, I'm still extremely surprised. Given that the inlet and outlet orifices are roughly the same size (and even if their were negligible resistance to flow presented by the pipe connected to the outlet), the fact that there is a considerably higher flow rate though that outlet than (if any) back through the inlet, this seems to imply very different pressures in two parts of that chamber which are no more than a couple of inches apart.

Whatever, they obviously work (and work very well), so my 'surprise' is clearly not really justified!!

Kind Regards, John

 

[just pumps]

Yes, I suppose I realised that the explanation must have been along those lines but, when I think about pressures and flows, I'm still extremely surprised. Given that the inlet and outlet orifices are roughly the same size (and even if their were negligible resistance to flow presented by the pipe connected to the outlet), the fact that there is a considerably higher flow rate though that outlet than (if any) back through the inlet, this seems to imply very different pressures in two parts of that chamber which are no more than a couple of inches apart.

Whatever, they obviously work (and work very well), so my 'surprise' is clearly not really justified!!

Kind Regards, John

Don`t forget the different heights and shapes of inlet & outlet.

 

[Harry Bloomfield]

Whatever, they obviously work (and work very well), so my 'surprise' is clearly not really justified!!

There are lots of pump designs, which design works best for which application, depends on the pressures needed and the volume/ flow.

The Archimedes pump, is still very much used, to raise fluids a short increase in height. I put one in a Middlesborough station 40+ years ago of around ten foot diameter. From memory it raised the water around fifteen feet and rotated at around 15 RPM.

 

[JohnW2]

The 'swirl' shaped lips on the bottom of that impeller, once it begins spin, will fling the water out to the sides of the impeller through centrifugal force, then out via the discharge port. That will cause a low pressure in the middle of the impeller which draws in more water from the base. The don't produce much water pressure, but shift a large volume.

You say that, but the one I'm using claims to be able to lift water 7 metres - that's about 0.7 bar, far from an insignificant pressure. So, given that water is not being forced back through the inlet, I remain (obviously 'incorrectly'!) very surprised that there can be such a pressure difference between two points just a couple of inches apart in the the chamber.

The square box (float switch) contains a large ball bearing and a pair of contacts at the cable end of the box. If the box is raised up by increasing water level, the box floats cable end lowest, the ball drops inside the box and shorts the contacts out which causes the pump to run until the box falls again, as the water is pumped out. You can hear the ball inside rattle, if you shake the float switch.

Indeed - but, in my case, I have that float permanently 'tied up', simply because, for historical reasons (which I've never bothered to change), there is a separate float switch controlling the power to the pump.

Kind Regards, John

 

[JohnW2]

Don`t forget the different heights and shapes of inlet & outlet.

They are both circular, and both about 1" diameter, and as I've said, the output is about 2 inches above the inlet.

Kind Regards, John

 

[just pumps]

They are both circular, and both about 1" diameter, and as I've said, the output is about 2 inches above the inlet.

Kind Regards, John

One is a flat inlet whilst the other is a chamber John.

 

[JohnW2]

OK, it seems like I have two pumps for the tip. I've just similarly dismantled the second one and, in this case, the motor (or, at least, the impeller) is totally seized, and can't be rotated at all.

Ah well, it was an interesting little exercise!

Kind Regards, John

 

[JohnW2]

One is a flat inlet whilst the other is a chamber John.

Hmmm - but the pump (when working!) still works fine (and shoots water out of the side for goodness knows how far!) even if the outlet is left 'open', which isn't all that much different from a 'flat inlet', is it?

Kind Regards, John

 

[Harry Bloomfield]

Indeed - but, in my case, I have that float permanently 'tied up', simply because, for historical reasons (which I've never bothered to change), there is a separate float switch controlling the power to the pump.

A common thing to do, when the pump needs to pump down to a much lower level than the built in one permits.

 

[Harry Bloomfield]

The ball hits a microswitch, otherwise spot on.

Thanks, I never opened one up.

You can also get mercury versions of float switches of a similar type, but they are required to operate via a contactor.

Did you ever come across the level sensing, using fixed metal rods and low voltage ac relays?

 

[conny]

If you take the impeller off you may find a rotary seal behind it with one part fitted on the shaft and the other half in a housing in the next 'cover'.
Do not attempt to take this section apart because there is a high degree of damage likely to occur and they are not very cheap to replace. They are also very delicate and easily damaged by chips or scratches in the seal surfaces.
The only 'repairs' a novice can do to these units is to gently clear and debris around the impeller and ensure the inlet/outlet are not clogged with detritus. Oh and don't run them dry for more than a few seconds to check they are working or you will burn out the seals.

 

[SUNRAY]

I got a cheap one from LIDL or ALDI and I find I have to prime it when first placed into water, or, initially run it without a discharge hose to start it.
So counter intuitive it hurts my head.
Once working it can sit there starting and stopping, working without issue as long as it doesn't run dry - the float switch stops it well before that point.

I have found that the discharge hose makes a massive difference, for a long time I used it with a standard 1/2" domestic hose and it would empty a 250L butt in about 30minutes however it took about 15 minutes to empty a pair of butts using a 1" hose of similar length and it runs a lot quieter, I suspect the limitation of the smaller hose creates cavitation within the impeller cavity. Also the flow rate is astounding, enough to create a gash in the lawn directly from the end of the unrestricted 1" hose, much more than mains water pressure from a 1/2" hose.

 

[JohnW2]

A common thing to do, when the pump needs to pump down to a much lower level than the built in one permits.

Yes, I can imagine that. Once upon a time, one could buy these pumps with or without float switches (the latter being slightly cheaper), so I always got the latter. However, more recently they all seem to come with float switches as 'standard', so I've had to invest in a piece of string to old the thing up!

Kind Regards, John