Sump Pump Thumping Noise/Vibration (video)

[cfabe]

I'm having a very strange problem with my newly installed sump pump and I home someone here can maybe offer some advice.

The pump works fine as far as removing water, but right after the pump shuts off, I get a repeating thumping noise that increases with frequency. This thing is under our master bedroom and it's driving us nuts! I am guessing I have some sort of pressure wave/fluctuation in the outlet pipe that is making the check valve flutter, but I can't think of what to do to fix it. I thought about removing the check valve but the water in the pipe fills the pit back up almost all the way when it rushes back in. It's a 1/2hp Simer pump pumping through about 70 feet of 1.5" PVC with only about 3 feet of head. It discharges to a low spot in our backyard.

I made a video of the problem so you can hear what it's doing with captions to explain. Video is here:
http://fabiens.org/ckblog/SumpPump.wmv
The splashing the water makes is kind of loud, but if you listen closely you can hear the thumping. From upstairs the thumping is much louder than the splashing.

[agmantoo]

The video and my computer refused to accept each other. I would think the problem is as you stated in that the checkvalve is not seating and the sound (gurgling) is a consequence. It should be simple, provided you can access the line in the yard, to add a second checkvalve at the yard to see what impact that would have. The sump storage area/tank is vented isn't it? If it is not and you are pulling a vacuum as the pump functions all kind of strange things could happen. I forgot that you are in cold country so leaving the checkvalve exposed would be a disaster but you could observe the impact of the second checkvalve then take it back off. PS .... another thought, do you think the water once pumped then is trying to siphon the water out of the sump tank? That also would make a noise and the checkvalve would let that happen.

[cfabe]

That's an interesting idea to add another check valve. I had thought maybe it was trying to siphon but when I disconnect the line in the pit after pumping water rushes back in, if it were trying to siphon I think I would get suction instead.

[fishhead]

My first guess is that water is draining from the line on the outside and allowing gulps of air to enter the pipe. There must be a low spot in the pipe that prevents the line from draining fast.

My second guess would be that siphoning action is causing air to enter the line through the pump or a loose connection and as it escapes to the outside the water in the pipe is lifting and dropping.

[Harry Chickpea]

Go to the outlet end of the pipe and add an extension that goes up about four feet, so the water exits at a higher level. That should stop it. Check valves are generally installed vertically to allow positive back pressure. Some have adjustable springs. By having the long run, the slug of water doesn't stop immediately so you are getting a bit of ram pump action where the valve opens briefly in response to the reduced pressure, allows in some water and air, the pressure differential drops and the valve closes, and so on. Once you stop the forward movement of the slug of water in the outlet pipe, the valve can operate properly. Having it turn and go up a pipe will impede the flow and the weight of the water in the pipe will press back down, closing the valve. Looking at the video, it also appear that you have a bad seal between the pipe and pump. That allows air in the pipe rather than water, exacerbating the problem.

[cfabe]

Well I got it fixed today. On the suggestion from a friend I replaced the check valve with a spring loaded PVC check valve and that seems to have fixed the thumping. The old valve was the cheaper "flap" type which apparently flutters quite easily. The spring valve has enough resistance that it doesn't do it.