Running a SWG at 100% and Cell Life

[SoDel]

Yeah I doubt the pump itself will wear out, but like you said the seals will.

I'm not an expert here, but the way these pump seals work is they use a thin film of the fluid they are pumping (water) as the lubricating surface the spinning part of the seal and the fixed part of the seal. Running the pumps dry for too long will damage the sealing surface and lead to leaks. Leaks lead to bearings dying. Saw this happen on my hot tub where a leak on the seal for a long time caused the bearing to die. I could tell it was a long time due to the rust on the pump end of the motor that didn't exist on the other side of the motor.

That said, although I never got around to doing it, this leak just killed the bearing, and with a little clean up work I'd be able to replace the bearing and seal and bring the pump back to normal working condition. I bought a new pump and my plan was to repair the old one to have on hand to swap when the second hot tub pump had a seal leak/bearing issue. Then I could swap it out and fix the old pump at my leisure. I no longer have the hot tub, but I kept the pump. I'll probably find a project to use the motor in at some point in the future.

My gut feeling aligns with you, that you should run the pump on priming speed when you kick it on for a bit in case there was a small leak that let air in, before dropping back down to your normal low speed. In theory nothing has leaked and running at low speed is fine, but if some did than you won't be pumping any water and having air churning up in the pump head could lead to increased seal wear. Still, I don't personally see that leading to pump death at low speed unless the motor isn't air cooled and instead is water cooled, but you also would not be circulating any water which would pool issues over a period of days, especially if you have a SWCG which is no longer producing chlorine due to lack of flow.

Excellent analysis! To sort of zero in on one point though, whether air or water cooled, running a pump without it actually pumping will heat the water in the basket due to friction so there‘s possibly some speed and some degree of suction side leak where it will boil off the water in the basket and run dry (or possibly explode from steam). Also, as you highlight, stop filtering and chlorinating. Very powerful pumps do explode if deadheaded as a result of this type of heating. A pool pump might, at worst, just self-destruct some internals as you mentioned — I don’t know either way (and not willing to find out lol).

(edit — another minute of thought and I don’t see a way for a pool pump to flash water to steam so I’m guessing it can’t explode. Interesting discussion

 

[mgtfp]

Not sure how your setup works. In my case, to he SWG would turn off the pump should there be a no flow situation.

 

[SoDel]

Not sure how your setup works. In my case, to he SWG would turn off the pump should there be a no flow situation.

That seems like a good way to do it. In mine, the SWG turns itself off but the pump will continue to run (at least as best as I can figure; haven’t tried it). I think the pump is smart enough to somehow detect loss of prime and turn itself off, at least at some speeds, based on the sensed versus expected load. Haven’t had reason to test that though.

 

[SoDel]

SoDel mentioned that we my have cells that control amps and although I’m not away of anything specific all the SWG’s I’ve pulled apart have smallish % control knobs which I assume is just volt control. Ive always been under the impression that its a lot harder to control amps then volts and would think that the manufacturers are more likely to do whatever is cheeper.

I’m told the ones that can vary current are common in Australia, but not here in USA. For the type common here, every schematic I’ve seen for the cell supply consists of a honking big transformer (good for likely 10 Amps+), a full wave bridge, and one or two relays. The relays are controlled on, off, reverse polarity on, by the computer which senses voltage, flow, etc.; but it can’t control the voltage or current (other than off or on). The supply is just a dumb, unregulated, linear supply. The knob or automation panel controls duty cycle.

To control current for the Australian type, it is almost certainly a MOSFET H bridge which can be turned on and off very quickly (as well as reverse polarity). The computer then pulse wave modulates the output and the cell itself acts as a low pass filter so even though the supply is really just turning on and off very fast, the cell sees an average voltage based on the modulation. Cell resistance (conductivity of the water) would then result in a current quantum at the voltage “seen” by the cell. This method for controlling a power supply is very common (and the least expensive way to do it).

 

[jseyfert3]

Excellent analysis! To sort of zero in on one point though, whether air or water cooled, running a pump without it actually pumping will heat the water in the basket due to friction so there‘s possibly some speed and some degree of suction side leak where it will boil off the water in the basket and run dry (or possibly explode from steam). Also, as you highlight, stop filtering and chlorinating. Very powerful pumps do explode if deadheaded as a result of this type of heating. A pool pump might, at worst, just self-destruct some internals as you mentioned — I don’t know either way (and not willing to find out lol).

(edit — another minute of thought and I don’t see a way for a pool pump to flash water to steam so I’m guessing it can’t explode. Interesting discussion

Ah yes, good point. All the energy going into the pump motor not wasted as heat in the motor itself from inefficiencies will turn to heat in the water.

I did think of one other scenario. A pump on an above ground pool should not need a priming cycle before running (on a daily basis, initial run would). Due to being lower than the pool water, any leaks won’t lead to air inside the pump, as new water will feed in by gravity. The water could drop in the pool to the point more won’t flow in, but of course by then you have other problems.

Interesting, why. I thought the most wear on the pump was during priming. I've been running my vs pump at some rate 24/7 on that basis...

Just to chime in here, based on my understanding. As I mentioned you’ll get seal wear if you run the pump without sufficient water for the seal to form the lubricating film on the spinning to fixed sealing joint. But running a priming cycle, if there’s no air in the pump, won’t cause any wear that running the pump won’t cause. And running fast to quickly purge any air that may have gotten in would be better than running slow with a bunch of air for a long time, IMO.

I guess in theory you could have slight amount of wear, the same way plain bearings in engines experience wear on start-up before the oil pressure comes up and they start “floating” on a thin layer of oil, at which point there is zero wear if everything is working right. But I’d be surprised if this actually lead to a measurable decrease in life over the normal life of a pump seal with once a day starts.

I mean, the average car engine has what, two starts a day? For like 10-30 years? And now modern cars with stop/start and hybrids are starting the engine dozens or hundreds of times a day. And that’s a much rougher start on the bearings than a pool pump shaft seal . Especially those -15 °F starts. I literally grimace whenever I have to start my engine that cold, thinking of how much wear it’s accumulating. Guess that shows I’m a mechanical engineer.

 

[Jejunum]

Excellent point about modern engines @jseyfert3 . I had no idea that the priming cycle prevented air in the pump. Clearly I did not pay enough (any?) Attention in a misspent chem e degree. I think we need some rigorous testing and a you tube channel.

I've just started skimming this PDF on energy.gov

My pump reads a ludicrously high flow rate at low rpm (750) which drops off rapidly. I assumed that this was just silly math - but am I causing harm by inadvertent cavitation (second and third paragraph pasted out of context below)

'Problems such as cavitation, frequent
energizing and de-energizing of a pump motor,
and valve seat leakage can decrease the length
of time between repairs.

however, under certain
conditions, cavitation damage can occur at low
flow rates as well
Cavitation-like damage can also occur as a resultof internal recirculation. Operating the system atl ow flow rates can establish damaging flowa Patterns in either the suction or discharge regions
of an impeller.