My water pump won't turn off!

[mross]

Probably true.  But it could syphon through the pump if the valves have low resistance.  As you imply, it's a good idea to switch the pump off when you are away.

[Ex Brummie]

6 hours ago, rusty69 said:

Interesting. What pump is it? I can never get our pump to prime unless its low down relative to the tank. 

You name it, over the years, it's probably been fitted. Jabsco, Shurflo, Fliamma. Currently a cheap Chinese one off Ebay 3 years ago.

I always turn off the pump when away from the boat, and in winter drain down if really low forecast, otherwise just open taps and drain multipoint. If there were a freeze after the pump, the resistance through the pump diaphragm would stop syphoning, if the pump body froze and split ( a common occurrence) then it would introduce air to stop the syphon. If a freeze before the pump, then it would drain back to the tank.

[Tony Brooks]

32 minutes ago, Ex Brummie said:

You name it, over the years, it's probably been fitted. Jabsco, Shurflo, Fliamma. Currently a cheap Chinese one off Ebay 3 years ago.

I always turn off the pump when away from the boat, and in winter drain down if really low forecast, otherwise just open taps and drain multipoint. If there were a freeze after the pump, the resistance through the pump diaphragm would stop syphoning, if the pump body froze and split ( a common occurrence) then it would introduce air to stop the syphon. If a freeze before the pump, then it would drain back to the tank.

I don't understand that. The water should stay on one side of the diaphragm unless the diaphragm is punctured. Its the valves that would stop the syphoning and I suspect they only seal with a slight amount of pressure against them being horrid thin plastic things in the main.

[mross]

Perhaps he meant the diaphragm of the valve?  In theory, a siphon could generate almost 15psi differential.  Obviously, on a boat, the height differential would not create this much but it might be enough to overcome a rubber-flap type valve.

Edited July 1, 2017 by mross

[AdamG]

Hi All,

Many thanks for all the advice and comments. Today, I replaced the pump and all is well  Must have been the pressure sensor...

[Ex Brummie]

3 hours ago, Tony Brooks said:

I don't understand that. The water should stay on one side of the diaphragm unless the diaphragm is punctured. Its the valves that would stop the syphoning and I suspect they only seal with a slight amount of pressure against them being horrid thin plastic things in the main.

Perhaps I should have said resistance ACROSS the diaphragm. In the de-energised state, as far as I understand pumps, nothing should pass through the pump.

[Tony Brooks]

38 minutes ago, Ex Brummie said:

Perhaps I should have said resistance ACROSS the diaphragm. In the de-energised state, as far as I understand pumps, nothing should pass through the pump.

Why do you understand that?

I have yet to see a boat water pump that has an electric shut off valve that seals the inlet or outlet when the pump turns off.

There are two valves per pumping chamber so a typical pump will have 3 or more pumping chambers. The more chambers the more valves so the greater chance one or more valve will not seal. Some pumps (bypass pumps) have anther valve between the outlet an dinlet but as they have a decent spring on them to keep them closed until almost at the pump cut out pressure they are far less likely to leak.

I will consider just one chamber. There is an inlet valve that will open by either pressure from the tank side that may be simple head or suction from the diaphragm/pumping chamber side. There is an outlet valve that will open by pressure from the diaphragm/pumping chamber side or suction from the tap side, This suction could be the weight of water leaking out of a burst low level pipe.

With the pump turned off water can still open both valves and pass through the pump providing the tap side can produce enough suction to open the valves. So now the propensity for a pump to pass water when turned off comes down to the head of water in the tank, any suction on the tap side caused by a low level leak, how well the valves seal and the pressure required to open the valves. My view is that the valves are very flimsy and are unlikely to seal long term unless there is pressure forcing them closed. I also feel they will not require much suction or head to open them if they have sealed. That is the bit that is open to question so we can all take our individual view on it but that view should be informed by some experience of such pumps and actually manipulating and handling the valves.

[Ex Brummie]

Tony,

I wont quote your whole post, but my pump is higher than the tank, so with the pump turned off, I don't think any water would pass through the unit with an open pipe on either side of it.

[Tony Brooks]

8 hours ago, Ex Brummie said:

Tony,

I wont quote your whole post, but my pump is higher than the tank, so with the pump turned off, I don't think any water would pass through the unit with an open pipe on either side of it.

I know, you said that a while ago and someone else pointed out that if a ow level pipe burst on the tap side of the pump you could still get a syphon effect. Then it went on from there.

As long as your system works well and primes easily than its all fine. We will not find out who is correct re the syphon effect until it happens but even then if you are proved correct on one occasion a little bit of dirt under a valve or two on another occasion and it might syphon.

If you want to be sure you will not bet any  leaking in the event of low level pipe failure when you are away from the boat then I am afraid the only way to do it is some form of valve on the tank side of the pump.

Edited to add: Of course if all your pipework is also above the tank level then a split pipe on the tap side of the pump could not syphon but then there is the question of how low will the calorifier, if you have one, force the pipe run.

Edited July 2, 2017 by Tony Brooks

[mross]

To illustrate TB's point

This photo shows a typical valve - it's just a thin disk of 'rubber' and would easily be opened by a tiny pressure difference.

Does anyone have a spare pump handy?  They could blow into the pump inlet and see if there is any resistance.  The pump shown is affected by aluminium oxide.  Normally the 'rubber' would be black.