A question on bilge pumps

[SimonFa]

I have one of those that sits below the water and suck in from the bottom. Its quite a lift to the outlet and when I'm pumping out the bilges and stop the pump I got quite a lot of water flowing back into the bilges.

Should there be some sort of return valve in the system? Or have I go a faulty pump?

Once again, thanks in advance,

Simon

 

[scotts_mist]

I think its normal that the low budget pumps are designed to just move water from the outside side of the pump to the start of the hose. When it turns off, anything in the pipe runs straight back down - at least it does on mine.

More expensive pumps I believe can continue to pump when there's nothing left in the bilge, and are controlled by a timer which switches off after a while.

I think the advice is normally that fitting a non-return valve inevitably restricts the flow a little - and there are times you might need all the flow you can get.

Although a hose full of water running back down is inconvenient, it's not enough to cause any danger and is soon sponged up.

I'm new to this though - so I'm sure someone with more experience may offer some other suggestions.

 

[vyv_cox]

When a centrifugal pump stops rotating there is nothing to prevent water from flowing back through it. Whereas a positive displacement type has a pair of valves that act as check valves. I don't find the cheap plastic NRVs to be very effective, allowing water to leak back over time and, as said above, they increase the system resistance. So the real answer is to install a positive displacement type, either electric or manual.

 

[claudio]

The advantage of the positive displacement pumps is that there is no return flow, however debris in the bilge such as hair or a cable tie can disable this type of pump very easily. The answer might be to use a centrifugal pump for shifting the bilge water and a small positive displacement pump with fine mesh strainer to suck up the water that has run back

 

[cryan]

Not all PD pumps have valves. The Jabsco type won't usually have valves. It is unusual to use centrifugal pumps as bilge pumps as they don't like being sucked dry as air in the volute casing basically disables them and they will not pump. Remember a centrifugal pump can't prime itself. A non return flap can be fitted in the discharge pipe but it must be big enough so that the pump can achieve full flow unrestricted. Basically when open the volume of the area below the flap must be equal to or greater than the volume of a section of the discharge pipe of equal length.
The amount of water that flows back will only be a litre or so despite how much it may look.

 

[vyv_cox]

Not all PD pumps have valves. The Jabsco type won't usually have valves. It is unusual to use centrifugal pumps as bilge pumps as they don't like being sucked dry as air in the volute casing basically disables them and they will not pump.

True enough, but water won't flow back through them. What design does the Jabsco PD pump use?

I have four bilge pumps, two centrifugal and two PD.

 

[cryan]

Jabsco p/ps are generally rotary vane pumps. True that fluid generally wont flow back through unless the rubber vanes are damaged.

 

[lw395]

It should not matter.
Modern boats do not leak if in good order, except when it's very rough.
Then everything is wet anyway.
Wooden boats usually have wet bilges.
On most boats, the bilge pumps are basically for emergency use.
Ribs or open boats are different of course.

 

[SimonFa]

Thanks everyone. I'll add reviewing bilge pumps to my list of things to think about for winter maintenance.

Simon

 

[prv]

It is unusual to use centrifugal pumps as bilge pumps

Really? The majority of electric bilge pumps I've come across in yachts have been what I would call centrifugal types. It's possible I'm misusing the term; I mean something like this:

They don't need to prime themselves as such, because they're positioned in the bottom of the bilge where any water will flow into them naturally.

Pete

 

[cryan]

Really? The majority of electric bilge pumps I've come across in yachts have been what I would call centrifugal types. It's possible I'm misusing the term; I mean something like this:

They don't need to prime themselves as such, because they're positioned in the bottom of the bilge where any water will flow into them naturally.

Pete

I'm not sure if they're centrifugal? I have fitted a fair few of them including a smaller version yesterday but I have never opened one up to see how it works. I always assumed they were rotary vane type or diaphragm pumps?
A rotary vane pump works by moving the fluid into an ever decreasing space thus raising the pressure and forcing it out the discharge pipe. because air can be moved in this manner they will pull a suction and if air is in the casing it will be pushed out. A centrifugal pump works by accelerating along the impeller vanes by way of centrifugal force. Once the fluid leaves the tip of the vanes it continues at a tangent into the volute casing which is shaped a bit like a snails shell. as the fluid moves round the volute casing it moves into a ever smaller volume of space thus raising the pressure and forcing it out the discharge pipe. Because the impeller can not accelerate a gas they can not pull a suction and if the casing is full of air the impeller will just spin in the casing and not pump as it cant get rid of the gas. However if it starts with a positive head of liquid such as bilge water it can continue to pump even after the positive head has gone due to the syphon caused behind the fluid being pumped.
The reason centrifugal pumps are not the preferred option for bilge systems (at least on bigger vessels) Is that they have a nasty habit of, "Gassing Up," where even when there is a positive head of water the gas in the casing can be trapped and the pump will not pump. Also usually there will only be one or two pumps sucking on a system of pipes which a centrifugal could not do. They can move large volumes though so are often used for fire and cooling pumps. Basically PD pumps, including rotary ones, try and squish the fluid whilst centrifugal pumps try and throw the fluid away.
As for the type of pump you show, It could well be centrifugal? As you say it is submersible so, "Should not," get gassed up. Next time I replace one I'll pull the old one apart to find out.

 

[ghostlymoron]

The rule 1500 is definitely centrifugal. If you've not had to take one apart to clear a blockage, you've been very lucky. It's usually done by twisting the motor off to expose the impeller.

 

[prv]

I'm not sure if they're centrifugal? I have fitted a fair few of them including a smaller version yesterday but I have never opened one up to see how it works. I always assumed they were rotary vane type or diaphragm pumps?
A rotary vane pump works by moving the fluid into an ever decreasing space thus raising the pressure and forcing it out the discharge pipe. because air can be moved in this manner they will pull a suction and if air is in the casing it will be pushed out. A centrifugal pump works by accelerating along the impeller vanes by way of centrifugal force. Once the fluid leaves the tip of the vanes it continues at a tangent into the volute casing which is shaped a bit like a snails shell.

In that case, these pumps (absolutely standard for bilge pumps on yachts and motorboats) are centrifugal. They're certainly not diaphragm pumps - wrong in every way from shape to sound to spares. Your description of a rotary vane pump sounds like the typical Jabsco or Johnson used for engine cooling water, with the rubber impeller (I know there are larger versions with sliding metal vanes too). It's certainly not one of them either.

I'm puzzled that you have installed several of them but never "opened one up". The usual method of installation is to remove the red cage and screw that to the structure of the boat, then clip the rest of the body back into it. The impeller - clearly a centrifugal type - is exposed on the bottom of the body when you remove the cage; indeed it needs to be as that is also the method of clearing a blockage. The body itself is a sealed unit in order to be waterproof and submersible.

It's worth remembering that the big ship practice you're clearly quite familiar with often bears little relation to how things are done in yachts .

Pete

 

[cryan]

When, "Installing them," It has only really been to run wires and power supply to them in open boats that originally were only installed with a hand diaphram pump. It was for the Sea Cadets and I left it up to them to final place the unit when the boats were back in the water so as they could see where the trim of the boat would set the deepest part of the bilge which they were unsure of when the boat was in the yard. Commercial boat practice/leisure boat practice is all the same. There is only good practice and not so good practice. I personally think that having at least one PD pump is essential as centri pumps do sometimes fail to pick up suction. Which was the jist of my first post. Some people might not think thats as important but then some people don't think VHFs are important? Having faced flooding and pump failures, I tend to place more thought into pump selection.

 

[NormanS]

When, "Installing them," It has only really been to run wires and power supply to them in open boats that originally were only installed with a hand diaphram pump. It was for the Sea Cadets and I left it up to them to final place the unit when the boats were back in the water so as they could see where the trim of the boat would set the deepest part of the bilge which they were unsure of when the boat was in the yard. Commercial boat practice/leisure boat practice is all the same. There is only good practice and not so good practice. I personally think that having at least one PD pump is essential as centri pumps do sometimes fail to pick up suction. Which was the jist of my first post. Some people might not think thats as important but then some people don't think VHFs are important? Having faced flooding and pump failures, I tend to place more thought into pump selection.

I would say that if you are having anything to do with the installation of bilge pumps in any kind of boat, or ship, one of the most important considerations is how a choke can be cleared. I can't imagine installing a pump without looking at that.

 

[cryan]

I would say that if you are having anything to do with the installation of bilge pumps in any kind of boat, or ship, one of the most important considerations is how a choke can be cleared. I can't imagine installing a pump without looking at that.

Well yes, but I'm not in the habit of dismantling other peoples equipment when all they asked me to do was install some wiring.
I'm currently without a boat of my own but previously I had two pumps. One jabsco pump clutched to a belt drive off the engine and the other a hand pump. That way I knew I had a means to pump out the bilge even if I lost power/batteries died etc. Bilges should be kept as clean as possible to avoid blockages in an emergency. You don't want to get to the point that your flooding and the pump blocks due to poor housekeeping.

I am a little perplexed by the hounding I seem to be getting here? The OP asked for advice, I gave my advice based on experience in my professional and leisure life. If the OP chooses to ignore that advice that's fine, no hard feelings, its just an internet forum.