Losing / poor prime

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There is a small channel in the volute that allows water to run back into the pump basket for self priming. If that is blocked, the pump will not prime if above water level. However, it still does not explain why the pump will lose prime after running a while. That is usually only due to an air leak somewhere. Perhaps there is a crack in the housing.
 
That is the maximum lift the pump can prime so the pump must be less than 2m above water level which I believe it is.
 
Thanks. Makes sense and is approximately the 8ft that is implicitly claimed.

I may have resolved the issue though (famous las words after six weeks...) .

I disconnected the pump to strip it down one last time before junking it. This time I stripped it down behind the wet end and looked at the electrics (about which I understand much more than the hydraulics...). The wiring looked funky to me. Like they'd reversed a phase from the normal. This would effectively reverse the motor. So the turbine would still spin and create some suction but not great.

I put it all back together and put the vfd on reverse and the difference is impressive. The prefilter is now full save for a a big bubble at the top. And the return seems much better. Potentially even enough to power the Polaris without the booster. The odd thing is I have tried the vfd in reverse just after first installing the pump. But perhaps my plumbing was hokey then. I'm not going to over analyse...

So far turning the pump down to half an amp keeps the flow switch latched and maintains prime. Touch wood it was the factory miswiring that is the problem. If so in the morning I will rewire the pump itself (as the vfd defaults to forward and my OCD will kick in if I swap the phases manually) and then try again with the original plumbing in situ.

If that is solved then I can get with cleaning up the deck and building a shed before I have to go into quarantine when I get to the UK for my youngest's 18th and uni start

There's then the fun project I posted about in the SWCG forum and a longer term project of converting a magiline pool to a conventional. How to achieve the implausible.

I'm feeling very relieved at the moment. If the fates are kind I will still feel that way in the morning.

Thanks again all! Will post back tomorrow
 
A reversed centrifugal pump still pumps in the same direction but at much less flow rate so that would explain the initial priming issue.

However, it still does not explain the problem of losing prime after the initial prime. That has to be an air leak.
 
yes indeed.

well, the good news is that this morning, with the direct pipe installed still, after 14hrs of pumping at 0.5A has kept its prime. still a bubble at the top but I wonder whether an hour of full speed might eliminate that.

so no air leak with the bypass stuff. If the prime loss comes back after I put the original plumbing back then it's down to my plumbing joints.
 
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The pressure increased to 22-24 psi at full tilt and with all the returns open.

More to the story today though!

I reinstated the old plumbing. It would not prime beyond 1" from the bottom of the inlet. Being generous perhaps the entire inlet, leaving 3"+ from sight glass. I felt very minor suction from the wall vent where it draws from. Definitely there but not good. No discernible suction from the main drain.

To put that into perspective the distance from the side vent to the pump is 10m.

Contrast with the temporary plumbing of about 4 or 5m and I'd say the suction is enough to be rather dangerous. I had to exert significant effort to remove my hand from the end of the pipe.

But... The pump hadn't primed with the temporary plumbing a good 20min after I turned it on. Above the inlet but still 2"+ from the cover. The pressure was 12psi.

Something's up again (an intervening event is that I backwashed and rinsed the sand filter just in case that was preventing prime. The water was cleanish though.

I will inspect again in an hour to see whether the prefilter is full once more.

As for the old plumbing, I'm in two minds. Either there is a blockage, or there is an air leak or both.

To describe the suction line: the side of the pool has a port in it. At least 100mm diameter. The port goes from the basin into a cylindrical reservoir. 80cm diameter by 1m deep. In the centre of the distance between the port's entry and exit (pool to reservoir) is a pipe which I believe is connected to the main drain. I'd guess it's 50mm but I will check it properly tomorrow.

The bottom of the reservoir has an outlet. I believe that it goes directly into a pipe that ran from the original pump to where my new pump is. The distance from the reservoir to the old pump is about 50cm.

Unfortunately to inspect that line properly I have to remove the old pump which in turn will empty the pool by about 18". But I suspect it is necessary as I don't see another way to snake an unblocker around the elbows to see what's what. I've got a drain king type thing arriving Tuesday so I could wait and try blasting with that first.


And in other news my sewage pump started shouting an alarm this morning so I've spent a particularly fragrant day servicing that. A float switch had given up the ghost. But as a result I've not advanced the build of my ph/orp sensor and doser. A job for tomorrow.
 
four days later and prime is maintained with the same globulous bubble at the top of the pre-filter. Running continuously at 0.5Amps. Loads and loads of dirt being chucked into the pool every night, unfortunately. The perils of living abutting a vineyard (and the harvest was this week). the polaris and vortex3 both pick up a fair amount of the sandy dust but it's an ever challenging and Sisyphean task.

This is still with the temporary plumbing. I have a drain king type thing arriving today and will see whether I can get anything out of the suction line by some reverse pressure. I cannot think what else it can be as there are no leaks in the suction line (no dimpling, no improved priming with wet rags around the unions etc).

Will feed back further when the drain king does its thing.

thanks for the help and engagement. Much appreciated.
 

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got the (ersatz) drain king last Friday. plugged it in to the pipe, the expansion vessel promptly exploded and blocked the pipework. *rse. Clearly not that well made!

a karcher made short work of the blockage but I've not been able to test the blockage theory further. I know I can get about 8m down the line from the pump end. but I can't seem to trace the pipe work from the other end, either with an endoscope or through feel with the karcher drain tool.

However I think this is the wrong approach. I've posted separately in this forum about my latest wheeze to connect flows and returns in reverse. I'd love your views!

But ... everything is now stopped until Wednesday as last night as I was testing the pool the pressure got too great for the phSensor. It flew out of its retainer leaving a 12mm hole. Out of this flowed a healthy jet at 15psi or so. Which hit the wall and splashed straight into my VFD. Which in turn let out the magic smoke (It's on my lab bench now. two big 1W resistors have failed but they wont be the cause. the short circuit is elsewhere, but I've not found it; and I'm not too fussed as these devices are only Eur100 and are readily available. They are not 'supposed' to be used with synchronous motors like pumps. But I've looked at the electronics and it seems to be a duty cycle issue. provided the inverter is oversized for the pump (3HP for a 1.5HP) then I hope it will be ok. In any event, a 300+eur electricity saving vs 100Eur for a new VFD each season is not too traumatic if it comes to that.
 
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