Water pressure suddenly drops to zero once it has fallen below 30 psi

[njswede]

Hi guys!

I'm new to this forum and I've seen some similar threads, but I'm not sure they address the same issue. So here's the situation:

A few weeks ago, my very old well pump tank sprung a leak, and since it was a Saturday and I didn't want to spend the emergency repair charges, I decided to replace it myself. Although it was my first major plumbing job, it went well and I have nice water pressure. But I have this strange minor issue.

If I open the centrifugal filter flush, I can drain water than the pump can keep up. The pump kicks in at about 40 psi, but the pressure still drops slowly, since the pump isn't keeping up. However, at exactly 30psi, I can hear a "thud" or a "clunk" from the system, as if a valve opens or closes. The pressure gaguge immediately drops to zero, but water keeps flowing, but only with the pressure provided by the pump. If I close the filter dump valve, the pressure rises to about 30 in about a second.

The pump switch is 40/60psi and the tank bladder pressure is 35psi (a bit low, admittedly). I didn't install an additional check valve, but rely on the one that's on the pump. The pump is probably 20 years old (at least), but works OK. It fills the system from 40 to 60psi in about a minute.

What's going on here? It's really not an issue for everyday use, but I'm worried something more serious may be going on. I'm thinking it could be either a bad check valve that opens at low back pressures or my tank bladder pressure being to low.

Any ideas?

 

[Valveman]

Just means your pressure tank is good. If you have 35 PSI air in the tank, it should drop from 35 to zero and thump when you are using more water than the pump can supply at higher than 35 PSI. If it is falling to zero from 30, that means you only have 30 PSI air in the tank. That would be very common as it is normal for a tire pressure gauge and the water pressure gauge to be 5 or so PSI different. For this reason I use the water pressure gauge and where it falls from for the actual air pressure in the tank.

Also tank bladders break from too many on/off cycles. If the tank has had that many cycles, the pump has been damaged as well.

 

[njswede]

Thank you! That was the answer I was hoping for. My old tank didn't do the "thump", but maybe that just means that the new one is in better condition than the old one. The pump seems fine for now.

One more question: According to the pressure gauge and where I hear the "thump" (at 30psi), I'm about 8psi short of the optimal pressure (2psi below pump cut-in). How much does it matter?

 

[Valveman]

That is plenty close enough for the girls I know. Lol I wouldn't change a thing.

 

[njswede]

Thanks Valveman! You've been extremely helpful.