12volt bilge pumps and non return valves

[idomeneus5]

Hi all, just a query. Over the years of owning largeish trailerable powerboats, I've installed/replaced several bilgepumps (Rule, generally) In the accompanying instructions there is allways a big X on the diagram where a non return valve has been placed in the outlet pipe, I have in fact installed n/r valves and to no detriment, though the outlet pipe was relatively short, maybe 6 feet at most. I've just purchased a Roberts 25, and am in the process of refurbishing her, I will need to install 2 or 3 such pumps (bilge, shower etc) where the "run" will be much longer, and the subsequent backflow, once the pump is stopped, much greater (unless a n/r valve is fitted), Does anyone know or can hazzard a guess as to why this is a NO NO, is it maybe brand related ? I can forsee a problem should a valve jam shut, or in the case of a yacht heeling be "stuck" open, but think this is a relatively rare occurence. Thanks in advance, Peter.

 

[JSYmartini]

Every single point of resistance in the outlet will reduce the capacity of the pump.
Ribs in the pipe, kinks, bends, elbows, valves etc all included.

 

[VicS]

The Rule Pumps FAQs give a few pointers.

Can I install a check valve on the pump discharge?
Check valves are not recommended​

Why doesn't Rule Pumps want check valve on the pump's discharge?
Check valves are prohibited by the American Boat & Yacht Council for use as an anti-siphon device-and with good reason: They're notorious for failing in both the open and the closed position, which respectively leads to flooding or failure to pump. If the valve is close to the pump, the pump may not be able to overcome the weight of the water on the other side of the valve, rendering the pump ineffective​

Why does my automatic Rule Pumps turns off if I install a check valve on the discharge of the pump?
The automatic bilge pump turns on about every two and a half minutes to "check" for high water. If water causes resistance on the pump, it continues to pump until the resistance lowers. With the check valve installed at the pump, it cant feel the weight of the water, and shuts off, allowing the bilge to fill with water!​

 

[Chalker]

I've always felt that the non-return valves would very slowly allow the water to leak back since they are unlikely to be perfect,especially after a couple of years use.

 

[ukmctc]

I have one on mybilge pump with no problems for the past 4 years. I feel safer

 

[davidej]

If the valve is close to the pump, the pump may not be able to overcome the weight of the water on the other side of the valve, rendering the pump ineffective

Sorry to be blunt but that must be bo**olocks.

If the pump can't lift the weight of water it will never pump - n/r valve or not.

 

[parbuckle]

The Rule Pumps FAQs give a few pointers.

Why does my automatic Rule Pumps turns off if I install a check valve on the discharge of the pump?
The automatic bilge pump turns on about every two and a half minutes to "check" for high water. If water causes resistance on the pump, it continues to pump until the resistance lowers. With the check valve installed at the pump, it cant feel the weight of the water, and shuts off, allowing the bilge to fill with water!​

I bought one of these auto bilge pumps this year without knowing how it operated there was nothing on the box to say it would run every couple of minutes to check if it was needed, this turns out IMO to be a very poor idea, as I lay in my bunk this thing droned on and it annoyed enough for me to get up and turn it off and next day I rewired it to the old type floating switch and yes I refitted the one way valve so it is back to the same set up that Ive had for years , peaceful and no drain on the battery.

 

[captainboo]

Non Return Valves

The Whale Super Sub I have just fitted has a sticker on the outside of the package that states "For use with LV1219" - their own brand non-return valve! (Probably helps with marketing.)

The bigger version however, the Supersub 1100, has one built in already.

 

[VicS]

Sorry to be blunt but that must be bo**olocks.

If the pump can't lift the weight of water it will never pump - n/r valve or not.

I'll leave you to tell them .... let us know their response.

 

[ghostlymoron]

There is a recent thread on pump cycling / nrvs. Basically if there's enough water in the discharge line to raise the level in the bilge/sump the pump will cycle continuously so you need a reliable non return valve.

 

[lw395]

I think pump manufacturers don't want non-return valves for a couple of reasons.
1) they are unreliable, and any system which depends on them is likely to fail
2) they present back pressure, which reduces the performance of the pump. The smaller sizes of Rule-type pump do not generate much pressure in the first place
3) If the valve is holding back a column of water, the pump may not be able to start pumping, if there is initially air in the pump. The pumps centrifugal action does not work well when there is air in it, as will happen sometimes when the bilge slowly fills.

Centrifugal pumps are good and cost effective, but they have limitations. If you want the bilge actually dry, you want something like a diaphragm pump.

 

[lw395]

There is a recent thread on pump cycling / nrvs. Basically if there's enough water in the discharge line to raise the level in the bilge/sump the pump will cycle continuously so you need a reliable non return valve.

Or you need a smaller discharge pipe, or move the float switch up, so that the bilge holds a bit more water.
What really matters is the 'hystersis' in the float switch, it needs to switch off at a significantly lower level than it switches on. The Rule ones are OK, I think there is about an inch of difference? So an inch of depth in your bilge sump needs to hold more than the discharge pipe. Then the water in the pipe can flow back without re-starting the pump.

 

[JimC]

I have one on my bilge pump with no problems for the past 4 years. I feel safer

+1

My auto bilge pump's outlet is on the transom about 10" above the waterline. If (heaven forbid) the boat was holed and filling then, in the absence of a n/r valve, it would fill all the faster when the pump outlet went underwater.

 

[VicS]

+1

My auto bilge pump's outlet is on the transom about 10" above the waterline. If (heaven forbid) the boat was holed and filling then, in the absence of a n/r valve, it would fill all the faster when the pump outlet went underwater.

It is not recommended, in fact not allowed by the ABYC, to rely on a NRV in this way.
ITYWF that an antisyphon loop and vent valve is the preferred option ... if appropriate for to your installation.

 

[lw395]

+1

My auto bilge pump's outlet is on the transom about 10" above the waterline. If (heaven forbid) the boat was holed and filling then, in the absence of a n/r valve, it would fill all the faster when the pump outlet went underwater.

If you have enough water in the boat to sink it 10 inches, it has all gone horribly wrong.
Most NRV's seem to cause enough back pressure that you'd be much better off with an anti-siphon loop.
On my last boat, I had a rule pump and an NRV for the shower drain. I was very pleased to replace it with an impeller pump, which would suck and blow air as well as water.

 

[fisherman]

My pump is about 120mm diameter and 200mm high, whatever capacity that is. I have a bronze NRV near the outlet, plus anti syphon loop, all in 1" pipe about 5ft above the pump. No problems at all.

 

[ghostlymoron]

Or you need a smaller discharge pipe, or move the float switch up, so that the bilge holds a bit more water.
What really matters is the 'hystersis' in the float switch, it needs to switch off at a significantly lower level than it switches on. The Rule ones are OK, I think there is about an inch of difference? So an inch of depth in your bilge sump needs to hold more than the discharge pipe. Then the water in the pipe can flow back without re-starting the pump.

Agreed but reducing the size of the discharge line will significantly reduce the discharge rate of the pump which could be important if dealing with a significant inflow.

 

[Seajet]

I'm about to get a Supersub 1100, any feedback anyone please, especially if it's 'Don't go near ! ' ?

It has a built in nrv, but I will fit it emptying into the outboard well via a closeable seacock as backup in case a change of trim - ie damage & flooding - should put the outlet underwater.

 

[lw395]

Agreed but reducing the size of the discharge line will significantly reduce the discharge rate of the pump which could be important if dealing with a significant inflow.

Yes, you have to know what the pump is for.
If it is just to deal with rain, or water that gets below in the ordinary way of things, automatic operation is more important than capacity.
If it's for emergency use, the subtleties of automation go out the window.
The small ones are so cheap, you can justify having one in addition to an emergency big one on many boats. If the small one is set up to work below the level where the big one stops, that can work well.

Increasing size of pipe is diminishing returns. Avoiding sharp bend also helps.

An AWB should not need an auto bilge pump anyway.