Captive air tank or pump problem?


Old 08-13-06, 10:55 PM
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Captive air tank or pump problem?

Hello Pump Masters,

Hooked up a new 3/4 hp Craftsman pump a few months ago to a shallow well and got it working good. Only use this well for outdoor use so the connections are simple; 2 ft of piping on the suction side, 3 ft on the pressure side. . . 30 gallon upright captive air tank.

Lots of water, tons of pressure, no problems,


The captive air tank is not filling with water, I can easily shake it, maybe 1 gallon inside. Pump pressure was 60, then 55, then 50 & now 40, it will pump water and reach prime and spray powerful but dies after a gallon, burst of air, pressure drops to 20, picks up prime, does the same thing.

Air tank has 40 lbs of pressure and when I open the air valve only air comes out. This tank is only 3 years old.

Have had a couple burst of rusty water but this has never clogged a pump or air tank in the past so I'm reluctant to open the pump and clean it since it's only been running 3 months, and I've had them running for years and never cleaned impellers or venturi's. Maybe a clog at the entrance of the air tank pipe?

No water leaks on the pressure side, double checked for leaks on suction side, nothing obvious. . .

pump runs, won't shut off, max press. 40 lbs,
captive air tank not filling
no leaks

Just point me in the right direction and I'll fix it,

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Old 08-14-06, 07:32 AM
Join Date: Oct 2001
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There's no water going into the tank because it has 40 psi in it, and the pump will only reach 40 psi.
The main causes of low pump pressure are:
1. Suction side leak
2. Mechanical seal leakage
3. Plugged nozzle/venturi
4. Water level too low
5. Pump worn out
The cause on a newer pump is almost always 1 or 3.
Your tank precharge should be 2 psi less than pump cutin pressure.
Old 08-14-06, 11:28 AM
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Thanks Pumpman,

I can work with those troubleshooting options. One thing though that I forgot to mention and it's important.

When it was working good and the pump would shut off, it lost it's prime after 10 minutes of not running and when it started up it would gain normal water pressure very fast,

So, even back then I kept reminding myself to check for leaks on the suction side, it's really a new pump, so I feel the check valve is ok, and now think there must be some leaks in the piping work I did,

I will temporarily lower pressure in the captive air tank to see if it will fill,

Old 08-14-06, 02:17 PM
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Lowered pressure in air tank to 35 lbs,, ran pump, air tank began filling with water,

pressure in air tank started to climb, when it hit 40, I lowered it again to 35, ,

ran pump 25 minutes, air tank is really filled, maybe 15 gallons, but the air pressure is about 45, lowered it,

meanwhile the pump increased in pressure to 48 lbs, but averaged about 58,

I checked for leaks using soap bubbles on the suction side and could not find anything obvious, really studied it well, checked suction side of pump too, still nothing apperently leaking, definitely no leaks on the pressure side, when the system is off the pump holds its pressure reading,


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