Water Pump Not Priming

Feb 5, 2004
3,739
Tartan 3800 Westport, MA
I'm not sure it's a "not priming" problem, but, when I switch from an empty water tank, as indicated by the pump running continuously without building pressure, to a full tank, the pump doesn't successfully resume its duties: it just runs and runs, and never builds water pressure.

It's a Groco 36800-1000 pump, 20 to 40 psi automatic pressure switch. It's sucking on a line that runs down to the tank manifold about 4' down and 10' away horizontally.

It feeds a 2 gallon Jabsco accumulator, a water heater, and the cold distribution line.

I have found a way to resolve this, every time without fail, and that is to loosen the output line of the pump. What I do is shut off the accumulator, which can be isolated via a ball valve, and then unscrew the polypropylene connector on the output side of the pump. Some water spurts out, but then a lot of air, and then the tone of the pump changes and it starts pumping water again. I hurriedly reconnect it, open the accumulator valve, and all is well; until next time.

I'd really like to know why this happens! It's a Royal Pain to empty the cockpit locker, and dive in to do this.

I'm considering a couple of solutions. But, if there's a basic system problem here, I'd prefer to rectify that.

Ideas:
  1. install a bleed valve remotely, perhaps in the head, that I can open to bleed the air when this happens;
  2. install an air eliminator, such as are used in home plumbing systems.

All analysis and ideas are welcome.

BTW, original plumbing is polybutylene tubing with Parker® O Ring/Grab Ring Tube Fittings. For new work I'm using the same fittings (they're great) and Tamco® RT-55D LLDPE Tubing.
 
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Jan 22, 2008
1,464
Hunter 34 Alameda CA
I wasn't sure if after you switch from the empty tank, you leave the faucet on while running the pump. That is how mine picks up the prime again. If I leave the faucet closed, the pump will run forever and not prime and shut off.
 
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Feb 5, 2004
3,739
Tartan 3800 Westport, MA
I have tried opening a faucet, but I've left them open a long time to no avail. I'm thinking I'd have to leave it on long enough to drain the accumulator, then the air could be pushed out the tap.
 
Jan 22, 2008
1,464
Hunter 34 Alameda CA
I have tried opening a faucet, but I've left them open a long time to no avail. I'm thinking I'd have to leave it on long enough to drain the accumulator, then the air could be pushed out the tap.
That's possible. i don't have an accumulator and it takes a while if I have to pull from the stern tank which is pretty far from the pump. Good luck.
 
Jan 22, 2008
7,041
Beneteau 323 Annapolis MD
That's possible. i don't have an accumulator and it takes a while if I have to pull from the stern tank which is pretty far from the pump. Good luck.
would a check valve near the hose business end be of help?
 
Sep 25, 2008
5,424
Alden 50 Sarasota, Florida
I’m not sure from your explanation - if you leave the empty tank valve at the manifold open, the pump may suck air from it rather than a tank with water in it.
 
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Apr 3, 2019
275
Macgregor 26M P Cub Boo Mobile AL
Can you move the pump closer to, and below the manifold? If so, you could install an auto bleed and take care of it.

Put level indication in the tanks and changeover before the suction line fills with air.

You could tee into your pump discharge line and install a priming valve to flood the suction line - since you have the accumulator.

Sorry I'm not more help. Don't know that I have an exact image of your system in my head.
 
Feb 5, 2004
3,739
Tartan 3800 Westport, MA
I’m not sure from your explanation - if you leave the empty tank valve at the manifold open, the pump may suck air from it rather than a tank with water in it.
Don, I don't know to whom you are directing your question, but for me, the answer is yes. I don't understand it, but if I leave both tank valves open leading to the manifold, the pump will suck air. But that's solved by only having the full tank's valve open, and is not the problem for which I started this thread.
 
Feb 5, 2004
3,739
Tartan 3800 Westport, MA
Can you move the pump closer to, and below the manifold? If so, you could install an auto bleed and take care of it.

Put level indication in the tanks and changeover before the suction line fills with air.

You could tee into your pump discharge line and install a priming valve to flood the suction line - since you have the accumulator.

Sorry I'm not more help. Don't know that I have an exact image of your system in my head.
The pump sucks water just fine once the tanks are switched, it seems that the the line after the pump becomes air-bound, and that stops it from building pressure. At least that's how it seems to me. Once I bleed the output side of the pump of air, it works fine, immediately.
 
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Apr 3, 2019
275
Macgregor 26M P Cub Boo Mobile AL
What I'm thinking is, the suction is air bound and your pump is trying to pump air into the water on the discharge side which your accumulator has under pressure. When you remove the water back-pressure, the pump can then pump the air out of the suction line.

That's just the impression I get based on your description of the component relative locations. I could be off my rocker... Any way you can verify that the suction line has water in it between the manifold and pump? Temporary clear hose maybe?
 
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Apr 3, 2019
275
Macgregor 26M P Cub Boo Mobile AL
One last thought. Could you have a fitting/component in the suction line that's leaking air under vacuum?

The Jabsco pump with that model number has a dry suction lift limit of 5 ft. The pump simply may not do it in the current configuration.
 
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Feb 5, 2004
3,739
Tartan 3800 Westport, MA
One last thought. Could you have a fitting/component in the suction line that's leaking air under vacuum?

The Jabsco pump with that model number has a dry suction lift limit of 5 ft. The pump simply may not do it in the current configuration.
Yes, but...it works fabulously well when it's working, and once I bleed the output side when it does this bad thing.

You have a point, though, it could well be that the pump is trying to pump air, and reaches the limit to which ic can compress the air before it reaches the pump switch cut-off pressure, which is currently 30psi.

I'm thinking I can solve this by installing an air eliminator on the pump output line, like those home plumping air vents. This way the pump could push the air out on its own, and when it's pumping water the air vent will close. make sense?
 
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Feb 5, 2004
3,739
Tartan 3800 Westport, MA
It also makes sense that the pump is pumping air, since once I open the output line, in a second or two the tone of the pump changes and it's pumping water.
 
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Apr 3, 2019
275
Macgregor 26M P Cub Boo Mobile AL
I'm thinking I can solve this by installing an air eliminator on the pump output line, like those home plumping air vents. This way the pump could push the air out on its own, and when it's pumping water the air vent will close. make sense?
Maybe. They only let air out that has made it to the bleeder. Can you get it close enough to the pump to make sure you're getting air? It won't relieve the discharge pressure (unless you and I are talking about different devices). We used the auto bleeds at high points in HVAC hydronic systems. Never tried one in this application, but if you can get air to it, it will let it out of the piping. A tee off the pump discharge just upstream of a discharge check valve would let you manually purge it. You could pipe the discharge out or wherever you wanted to. If you can't visualize it, I could sketch it for you.
 
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Apr 3, 2019
275
Macgregor 26M P Cub Boo Mobile AL
Thanks Jim. I have that manual.

Are you making a point I'm not getting?
Probably. I don't understand myself half of the time.

Vert. Dry Suction lift 5 ft.

In the lab, the pump could only suck water up 5 ft through a dry tube. You indicated around 4 ft. You're close to the limit of pump operation.
 
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Feb 5, 2004
3,739
Tartan 3800 Westport, MA
Thanks, Jim. I can visualize it. I have been contemplating that, I think I mentioned it in the first post. I think an air eliminator would do the same thing, automatically.