Siting an anti-siphon loop in the bilge pump outlet

[Ric]

My AWB has the bilge pump outlet at the rear of the topsides, well above the waterline, where there is very little danger of it being submerged. The manufacturer also fitted a check-valve in the outlet hose, near the automatic bilge pump.

The check-valve was presumably fitted to prevent the automatic bilge pump constantly cycling the contents of the relatively long outlet hose. However, this does not work as intended because the check-valve either blocks open, blocks closed, or slowly back leaks (I have fitted and tried several types). I would therefore like to get rid of the check-valve and fit an anti-siphon loop. This will have to be close to the bilge pump in order to prevent the cycling problem. I can mount it inside the saloon settees so that the top of the anti-siphon loop is above the waterline (even when heeled) but it will be below the highest point of the outlet hose. Will this work to prevent back-flow? My thinking is that the water in the tube will run back until the highest level in the tube drops below the top of the anti-siphon loop, then the valve will open and break the lock. The remaining bilge water between the bilge pump and the top of the anti-siphon loop will also run back, but the total run back should be little enough to cause the automatic bilge pump to recycle. Am I correct?

 

[lw395]

Why do you need to use a bilge pump on an AWB?
I can understand having an auto bilge pump 'in case' but in an emergency you won't be worrying about a couple of litres running back.
There should be enough 'hysteresis' in the switch to stop it cycling, i.e. the switch on point should be above the switch on point by several times the volume of water that runs back.
The so-called intelligent pumps which switch on every few minutes to check if there is water by sensing the effort to turn, a seem to be fine in this respect, lots of people use them in open boats like ribs with a sump at the back.
A siphon breaker won't hurt though. Unless the valve sticks because it never gets used.

 

[Tranona]

No need for an anti syphon valve if you can take a loop well above the waterline. Nothing is syphoning back - it is just the water in the pipe, so if you take a loop up, the only water that will run back is in the "up" part. A syphon will only occur if the outlet is submerged. Surely the real solution, though is to find the source of the water ingress so that you don't have water in the bilge. The pump in my last AWB died after 12 years through lack of use.

 

[jwilson]

No need for an anti syphon valve if you can take a loop well above the waterline. Nothing is syphoning back - it is just the water in the pipe, so if you take a loop up, the only water that will run back is in the "up" part. A syphon will only occur if the outlet is submerged. Surely the real solution, though is to find the source of the water ingress so that you don't have water in the bilge. The pump in my last AWB died after 12 years through lack of use.

I reckon despite a loop ALL the water in the pipe runs back, in my case enough from a 25ish foot pipe run back into a small sump to start an auto pump pumping again. As soon as the pump stops water stops coming out of the near-transom outlet, so the water in the downhill exit bit is being sucked back by the siphon effect.

Fitted a non-return valve but it is never 100% effective for long: all it does is to change the bilge pump cycling from 15 seconds to 10-30 minutes. I never leave the bilge pump on auto for this reason.

This is on an AWB too, we get a little (mostly fresh) water in the bilge from two known sources: small amounts occasionally from a hot water PRV (despite two accumulators in the system) and sometimes from the fridge drain. The rest is condensation, spills, splashes, drips from oilies. Condensation is probably the largest source.

 

[prv]

This is on an AWB too, we get a little (mostly fresh) water in the bilge from two known sources: small amounts occasionally from a hot water PRV (despite two accumulators in the system) and sometimes from the fridge drain.

Surely both of those are worth stopping - the hot water valve can easily be piped to a bottle for occasional emptying (about twice a season in Ariam) and the fridge really shouldn't be draining into the bilge for reasons of hygiene.

Pete

 

[Tranona]

I reckon despite a loop ALL the water in the pipe runs back, in my case enough from a 25ish foot pipe run back into a small sump to start an auto pump pumping again. As soon as the pump stops water stops coming out of the near-transom outlet, so the water in the downhill exit bit is being sucked back by the siphon effect.

If that is the case, then i wonder if an anti syphon valve would actually break that syphon. Perhaps an open vent type rather than the valve type would be more effective.

 

[RichardS]

My AWB has the bilge pump outlet at the rear of the topsides, well above the waterline, where there is very little danger of it being submerged. The manufacturer also fitted a check-valve in the outlet hose, near the automatic bilge pump.

The check-valve was presumably fitted to prevent the automatic bilge pump constantly cycling the contents of the relatively long outlet hose. However, this does not work as intended because the check-valve either blocks open, blocks closed, or slowly back leaks (I have fitted and tried several types). I would therefore like to get rid of the check-valve and fit an anti-siphon loop. This will have to be close to the bilge pump in order to prevent the cycling problem. I can mount it inside the saloon settees so that the top of the anti-siphon loop is above the waterline (even when heeled) but it will be below the highest point of the outlet hose. Will this work to prevent back-flow? My thinking is that the water in the tube will run back until the highest level in the tube drops below the top of the anti-siphon loop, then the valve will open and break the lock. The remaining bilge water between the bilge pump and the top of the anti-siphon loop will also run back, but the total run back should be little enough to cause the automatic bilge pump to recycle. Am I correct?

The check valve is a waste of time if it's purpose is to prevent the pump cycling when the water in the uphill part of the outlet runs back under gravity as sooner or later they will all leak back so all you are doing is increasing the time between cycles. All this has nothing to do with syphoning as others have said.

If it's a float switch starting the pump the only solution is to balance the relative heights of the switch and the pump suction inlet (switch higher, pump lower) until the amount of water which will always run back under gravity is not sufficient to trigger the switch again.

If the switch is built into the pump and their relative heights cannot be adjusted then you either have to change the hose routing to reduce the volume of water in the hose or re-wire the pump with a separate switch ..... or stop the water ingress.

Richard

 

[lw395]

I reckon despite a loop ALL the water in the pipe runs back, in my case enough from a 25ish foot pipe run back into a small sump to start an auto pump pumping again. As soon as the pump stops water stops coming out of the near-transom outlet, so the water in the downhill exit bit is being sucked back by the siphon effect.

Fitted a non-return valve but it is never 100% effective for long: all it does is to change the bilge pump cycling from 15 seconds to 10-30 minutes. I never leave the bilge pump on auto for this reason.

This is on an AWB too, we get a little (mostly fresh) water in the bilge from two known sources: small amounts occasionally from a hot water PRV (despite two accumulators in the system) and sometimes from the fridge drain. The rest is condensation, spills, splashes, drips from oilies. Condensation is probably the largest source.

I agree about syphoning back the whole or at least most ofthe hose content, syphons are like that!

Are you running a sauna? I don't believe I'd want to be on a yacht where condensation is enough to pump out.
Yes you can get a surprising amount of bilge water in an AWB, from wet people, sails, spray etc.
Racing is usually messier...
But it takes a lot to even fill the hoses of a bilge pump that would be big enough to take seriously.
A manual pump or electric diaphragm pump that will pump air is better.
But lifting the middle of the cabin sole and wiping up with a j cloth is normally what's needed when putting the boat away.

 

[prv]

But it takes a lot to even fill the hoses of a bilge pump that would be big enough to take seriously.

I've thought about having the normal centrifugal pump with 1-1/2" hose, but then also adding some kind of tiny diaphragm pump that uses small (6mm-ish) plastic tubing and can suck it dry, to extract the dregs.

Pete

 

[rogerthebodger]

I've thought about having the normal centrifugal pump with 1-1/2" hose, but then also adding some kind of tiny diaphragm pump that uses small (6mm-ish) plastic tubing and can suck it dry, to extract the dregs.

Pete

I have done exactly that but with a henderson Mk5 and a bilge strainer like this.

The centrifugal pump has too high a strainer to get most of the water out so I paroled the centrifugal pump with the Mk5 each having a loop (not an anti siphon loop) and then joined just before the outlet that was just above the static waterline.

I have a second centrifugal pump with a Mk5 in series so this will not extract all the water as it uses the centrifugal pump strainer as its extraction point.

 

[prv]

I have done exactly that but with a henderson Mk5 and a bilge strainer like this.

Both of those are far larger than I had in mind - I was thinking more the sort of kit you'd see tucked behind a domestic aquarium. This is just to dry up drips, where a full-size bilge pump is like trying to use a JCB to scoop cereal out of the packet

Pete

 

[lw395]

I've thought about having the normal centrifugal pump with 1-1/2" hose, but then also adding some kind of tiny diaphragm pump that uses small (6mm-ish) plastic tubing and can suck it dry, to extract the dregs.

Pete

I have a big syringe which is good for accessing little pools of water.

 

[Ric]

If that is the case, then i wonder if an anti syphon valve would actually break that syphon.

This is exactly what I was wondering myself, before thread-drift set in. I can't fit the anti-siphon valve both close enough to the bilge pump to prevent cycling, and higher than the through hull outlet. So when the pump stops pumping, water in the pipe will definitely run back until the highest head is at the level of the anti-siphon valve. At this point, I don't know what will happen. If the anti-siphon valve opens, then only the small amount of water from the valve to the pump will run back (and the remainder will be trapped between the through-hull and the valve). However, if the valve does not open, then it will serve no purpose and all the water will run back. I just don't know the mechanics of how the valve is controlled to know which of these is correct.

 

[Tranona]

It won't do anything if it is not higher than the outlet as the water will always run downhill. So it is not syphoning that is the problem but gravity. It will only work if there is a downhill run from the syphon break to the outlet. Then the water between the pump and the syphon will go back into the sump and the water downstream of the syphon will go to the outlet. If the pump is always pumping uphill - that is the outlet is higher than any part of the run it will always run back into the sump.

 

[prv]

I have a big syringe which is good for accessing little pools of water.

Yes, I keep one in the little coffer-dam I built around the log fitting.

Pete

 

[Ric]

If it's a float switch starting the pump the only solution is to balance the relative heights of the switch and the pump suction inlet (switch higher, pump lower) until the amount of water which will always run back under gravity is not sufficient to trigger the switch again.

The relative height between pump and switch makes no difference - the variables which determine whether it cycles or not are volume of water in the pipe, the debit rate of the pump, and the timing hysteresis of the switch. The pump has to carry on running for long enough after the switch turns off to clear a volume greater than that stored in the outlet pipe - otherwise the water in the pump runs back and switches the pump back on.

If you make the switch higher as you suggest, all that happens is that you store more stagnant water in your bilge.

 

[prv]

The pump has to carry on running for long enough after the switch turns off to clear a volume greater than that stored in the outlet pipe - otherwise the water in the pump runs back and switches the pump back on.

A centrifugal pump won't blow air up the pipe hard enough to push water out. So once it runs out of water in the bilge, the pipe stays full however long you run the pump for. Then as soon as it stops (assuming no valves), the water in the pipe falls back down through the pump and into the bilge. The point of raising the switch is so that a pipe-full of water isn't enough to reach it and turn it back on.

Pete

 

[VicS]

A centrifugal pump won't blow air up the pipe hard enough to push water out. So once it runs out of water in the bilge, the pipe stays full however long you run the pump for. Then as soon as it stops (assuming no valves), the water in the pipe falls back down through the pump and into the bilge. The point of raising the switch is so that a pipe-full of water isn't enough to reach it and turn it back on.

Pete

But if you raise the switch are you not raising both the cut in and cut out levels . What you need to do perhaps is to keep the cut out level as low as possible but raise the cut in level.

Won't his will require either separate switches or a switch for which the difference between cut in and cut out levels can be widened ?

 

[rogerthebodger]

Both of those are far larger than I had in mind - I was thinking more the sort of kit you'd see tucked behind a domestic aquarium. This is just to dry up drips, where a full-size bilge pump is like trying to use a JCB to scoop cereal out of the packet

Pete

For the final drips I use a wed & dry vacuum cleaner anyway.

 

[prv]

But if you raise the switch are you not raising both the cut in and cut out levels

Ric's scenario assumes a timer that will run the pump for a fixed time after the switch cuts off. He wants this to clear all the water right out of the boat, but of course with a centrifugal pump it won't because they can't throw the water all the way up the hose.

Might work with a diaphragm pump.

Pete