Bilge pump plumbing

[Robin]

We want to add a small electric bilge pump to a sump area currently served by a Henderson manual diaphragm pump. Can this new electric one be simply plumbed into the pipework to the manual pump or will the internal workings of that prevent through flow? We are not looking at lots of water to shift, mostly just condensate from the cabin air conditioners that accumulates here over time.

 

[William_H]

As I see it to share the pipe work can mean either fitting the electric in series with the manual bilge pump or in parallel. In series would mean that the electric pump would have to force the water through the soft flapper or similar valves of the manual pump. OK doable but some pressure would be lost. On the other hand the elctric pump would have to pull or push water through the centrifugal pump of the electric one. Probably OK.
If the pumps were in parallel it might be easier to have a separate pick up for the additional electric pump. However in plumbing the outlets together and prior to a fairly restrictive (long) pipe to the outside there is the likely hood that the manual pump would find a path back to the bilge via the centrifugal rotor and chamber of the electric pump so might need a one way valve. The valves in the manual pumpn should be no problem when the electric is working.
So only you can try both out in the 2 configurations to see how the performance of both pumps is affected by the connection and if that loss is worth the saving of pipe work. ie an additional pipe to outside. good luck olewill

 

[Robin]

As I see it to share the pipe work can mean either fitting the electric in series with the manual bilge pump or in parallel. In series would mean that the electric pump would have to force the water through the soft flapper or similar valves of the manual pump. OK doable but some pressure would be lost. On the other hand the elctric pump would have to pull or push water through the centrifugal pump of the electric one. Probably OK.
If the pumps were in parallel it might be easier to have a separate pick up for the additional electric pump. However in plumbing the outlets together and prior to a fairly restrictive (long) pipe to the outside there is the likely hood that the manual pump would find a path back to the bilge via the centrifugal rotor and chamber of the electric pump so might need a one way valve. The valves in the manual pumpn should be no problem when the electric is working.
So only you can try both out in the 2 configurations to see how the performance of both pumps is affected by the connection and if that loss is worth the saving of pipe work. ie an additional pipe to outside. good luck olewill

Thanks, you have mde me think harder about this which I should have done beforehand, it just seemed like a simple option easily adopted to avoid extra piping and another through hull, not so easy to install whilst afloat.

I'm thinking now of fitting a shower drain sump box with integral automatic pump such that the aircon condensate drains can be plumbed to that instead of just draining into the bilge sump. The manual pump pickup is in the sump already and although I could replace it with an electric one and float switch I like the idea of retaining the manual pump in case of need after a power outage when offshore. It seems maybe I'm trying to be too clever maybe and am in danger of creating complication when simplicity is best. This particular bilge area and sump is the main sump in the main cabin area and is perhaps best left as uncluttered as possible in case the frightened man with a bucket scenario arises If the aircon condensate drains are fed into a new shower sump box works (these are proprietary gizmos hereabouts, easily available from West Marine and others), the main bilge/sump would remain bone dry most of the time anyway so adding to the existing manual pump would seem to be superfluous.

 

[charles_reed]

Having tried the tandem version, it's unworkable - a centrifugal pump has insufficient head to overcome either intake or output through a manual pump.
An alternative is to fit a Y with one way valves in each arm. These have a horrible habit of jamming, usually closed.
If your bilge pump outlet is above waterline you can put a T junction just by the outlet with a gooseneck on the lower one to reduce backfilling.
By far the best, and very little extra work, is to have two separate outlets and two discrete systems.

 

[CAPTAIN FANTASTIC]

Yes it works; I had a manual pump in the cockpit with a pipe to the bottom of the bilge, draining the bilges; i fitted an electric pump at the bilge end of the pipe and i now have the option of using the manual pump or the electric pump without any modifications to the system; both pumps work very well on the same pipe.

 

[rogerthebodger]

I have 2 manual and 2 electric bilge pumps. 1 set is in series and the other in parallel. Each set had its own outlet. There seems to be little difference in flow but the flow back in the series setup is less/slower than the parallel setup due to the NRV's in the manual pump.

 

[ghostlymoron]

As the Henderson is a diaphragm type pump, it has non return valves built in. A centrifugal type electric pump will pump through it and it will act as a non return to prevent cycling provided the distance between the two is not too long. The output of the electric pump will be reduced however but if, as you say, it's only to mop up condensation, this may not matter. The output of the Henderson won't be affected much.
I had this arrangement on my last boat but I changed it to separate discharges because I wanted the electric pump to maintain a high flow rate.
In your application, assuming I've correctly understood your requirements, it would be fine.

 

[Robin]

Thanks all, food for much thought and perhaps I should just 'suck it and see'