Sedra 5000 Needle Wheel Loses Prime

[miwoodar]

Please help...I'm out of ideas. My skimmer is a DIY knock off of a Euroreef RC135 (http://www.marinedepot.com/Euro_Ree...in_Skimmers-Euro_Reef-ER1613-FIPSISNW-vi.html). I'm running it with a Sedra 5000.

Two weeks ago I took the pump off and gave it the big vinegar bath. The next day I came into the living room and it had lost it's prime...the impeller was banging around inside the volute making a racket. Hmmm, took it back apart, inspected the inside, reinstalled, and didn't think much about it. Welp, it's happened 3x since then and I'm at a loss to understand how.

Fast forward to yesterday. I walk into the room and glance at the skimmer - all was fine. I went into the other room and grabbed some fish food. Upon my return it had lost it's prime again. The power didn't flicker. The water level in the sump and skimmer didn't change. I did notice that the venturi muffler had about a 1/4 cup of water in it (noticed this on prior events as well but didn't think much of it so I can't say it happens every time). I pulled off the pump, made sure all the connections were sound, then plugged it back in. This time nothing happened. The NW had no tension on it and didn't even try to spin. 15 minutes later I plugged it in again and it fired right up.

The only modification done to this pump is one very small piece of mesh tied into the needles. I did this a few years ago right after I got it.

I've lined up a replacement motor block JIC that might be the problem. This is a short term fix as I want to build a larger skimmer in the not-too-distant future. I can't do it now though as I'm soon to go away for a few days and won't have time to figure out the tuning on a new skimmer before I leave.

 

[Wicked Color]

IME~ a dry run pump will cavitate, heat up, and seize, sometimes the seize is only temporary while the impeller itself is hot and expanded, once cooled it shrinks and is freed again (this will only happen to some pumps that have just enough clearance to run cool) If this happens enough times the impeller becomes warped or disfigured and will cause additional friction in the pump, and thus additional heat.
My first approach would be to attempt to re-prime and start the pump, listen for new noises from the pump.
Since this is an air induction pump the likely solution is a new impeller, or maybe just removal of the mesh mod.
I would suspect the origin of the problem is the mesh mod keeps too much air in the chamber to achieve the proper draw, allowing additional RPM's, and less lubricant and cooling from less water flow.

 

[miwoodar]

Thanks WD.

It doesn't sound like it's cavitating but these pumps aren't exactly silent so I can't say for sure. It sounds like it always has. Have you ever seen a pump cavitate all the way to the point of spontaneously losing it's prime?

The impeller was definitely spinning freely when the pump wouldn't start. Do you know if these pumps have an auto shutoff for heat? Perhaps it got hot after I shut it off since I took it out of water immediately?

I'll pull the mesh off tonight. I really don't think it's the problem but the mesh is so small I doubt it's doing anything anyways. It's about the size of a quarter and sits in the middle of the wheel (doesn't overlap with the needles). I put it on right after I got the pump years ago but never took it any further and never did a full mesh mod.

 

[miwoodar]

OK, I've decided not to change anything this close to my departure. (Thanks Richard L for sourcing me your motor block...sorry it didn't work out.) I've instructed my house sitter to simply unplug the skimmer pump if it loses prime.

I figured out what is happening. If the power flickers, the volute fills with air...the impeller bangs around and doesn't catch prime. If the power stays out, the air in the volute gets slowly displaced by water...when the power comes back on all is well. I don't know what has changed for it to behave this way.

Ideas?