Any possibility of a Lavac flowing the wrong way?

[Kelpie]

I want to add a saltwater tap at the galley. The easiest place to take water from is by teeing in to the heads inlet.
The heads is a Lavac. Is there any theoretical possibility that the flow could reverse in the pump?
I could add a non return valve after the tee, but not sure that it's really needed. The heads inlet is a very long pipe and the tee would be right after the seacock, so any sort of reverse flow would have to go all the way down the pipe to cause any trouble.
I suppose it's a classic risk assessment situation- small chance of the event happening but severe consequences if it does!

 

[anoccasionalyachtsman]

I'd say that there is a remote chance, but a fairly unlikely set of circumstances which could be minimised by always turning the outlet seacock off.

There is though the probability of a newly-fitted galley inlet picking up water recently pumped from the Lavac - of your boat or the ones upstream.

ps. Would the consequences really be that severe?

 

[PaulRainbow]

The Lavac hand pump cannot possibly reverse its flow. If it was possible (it isn't) seawater would be sucked up and pumped to the bowl, which would simply overflow.

Sea water is sucked through the inlet by the vacuum in the Lavac bowl, caused by the pump. If the galley pump wasn't fully sealed against back flow (it should be) the toilet wouldn't flush as air will be drawn in from the galley pump/fawcet.

The seawater pump will not work very well, if at all, because it will be easier to draw air in from the Lavac than to suck water in from the seacock. You'd need to install valves to isolate one supply or the other or fit one way valves.

The heads inlet is usually fitted close to the outlet, i don't think i'd fancy sucking galley seawater up from next to the outlet.

 

[PaulRainbow]

I'd say that there is a remote chance, but a fairly unlikely set of circumstances which could be minimised by always turning the outlet seacock off.

There is though the probability of a newly-fitted galley inlet picking up water recently pumped from the Lavac - of your boat or the ones upstream.

ps. Would the consequences really be that severe?

Depends how much you like sewage in your galley sink.

 

[geem]

Why would you risk connecting a sewage device to your galley sink? You are effectively connecting the rim of your toilet bowl to the galley sink. Not a great idea

 

[Kelpie]

Thanks for the thoughts.
The heads inlet is the furthest forward skin fitting on the boat. All the other possible candidates are downstream of the heads outlet! There is a good separation between the inlet and outlet.
The plan would be to tee off straight after the inlet, with a self priming foot pump at low level (a fair bit below the waterline) feeding a faucet at the galley.
The branch going to the heads has a high anti-syphon loop and ends at a higher level than the foot pump (but not the faucet).
Based on the comments above I'm thinking about adding a valve at the heads, the idea would be you open that before using the Lavac, and shut it again afterwards.

This all has to be weighed up against the other options, which are to use a different existing inlet (i.e. the engine intake, which is downstream of the heads outlet) or to get the boat lifted out and a dedicated seacock installed. Which means effectively putting the job off indefinitely, as we don't plan to lift out at any point this year.

In terms of risk, it's not as if anybody is going to pour themselves a glass of seawater and drink it. We just want to rinse out pans and the sink itself. But, still, eeeewww...

 

[RJJ]

Thanks for the thoughts.
The heads inlet is the furthest forward skin fitting on the boat. All the other possible candidates are downstream of the heads outlet! There is a good separation between the inlet and outlet.
The plan would be to tee off straight after the inlet, with a self priming foot pump at low level (a fair bit below the waterline) feeding a faucet at the galley.
The branch going to the heads has a high anti-syphon loop and ends at a higher level than the foot pump (but not the faucet).
Based on the comments above I'm thinking about adding a valve at the heads, the idea would be you open that before using the Lavac, and shut it again afterwards.

This all has to be weighed up against the other options, which are to use a different existing inlet (i.e. the engine intake, which is downstream of the heads outlet) or to get the boat lifted out and a dedicated seacock installed. Which means effectively putting the job off indefinitely, as we don't plan to lift out at any point this year.

In terms of risk, it's not as if anybody is going to pour themselves a glass of seawater and drink it. We just want to rinse out pans and the sink itself. But, still, eeeewww...

IMHO the risk associated with a "nearby" outlet is small. Don't flush the loo and rinse your pans at the same time. And if you do, don't do with a #2. And if the water comes out brown, keep pumping, give the sink a wash and start again.

I don't think there's much difference between being a few inches and a few feet apart. And I'm not aware of anyone of the thousands of boats with a saltwater tap ever pumping a poo into their sink.

The inlet pipe, per above posts, isnt' a "sewage device". The outlet pipe is a sewage device and I don't think you're proposing to use that.

 

[eebygum]

Thanks for the thoughts.
The heads inlet is the furthest forward skin fitting on the boat. All the other possible candidates are downstream of the heads outlet! There is a good separation between the inlet and outlet.
The plan would be to tee off straight after the inlet, with a self priming foot pump at low level (a fair bit below the waterline) feeding a faucet at the galley.
The branch going to the heads has a high anti-syphon loop and ends at a higher level than the foot pump (but not the faucet).
Based on the comments above I'm thinking about adding a valve at the heads, the idea would be you open that before using the Lavac, and shut it again afterwards.

This all has to be weighed up against the other options, which are to use a different existing inlet (i.e. the engine intake, which is downstream of the heads outlet) or to get the boat lifted out and a dedicated seacock installed. Which means effectively putting the job off indefinitely, as we don't plan to lift out at any point this year.

In terms of risk, it's not as if anybody is going to pour themselves a glass of seawater and drink it. We just want to rinse out pans and the sink itself. But, still, eeeewww...

Hi, I’ve been going through exactly the same thought process to reduce my freshwater usage on long cruises by using saltwater for washing up/dilution for cooking. I also have a Lavac and ruled out a ‘T’ on the inlet , similarly also considered ‘T’ off engine inlet; but there are arguments against that too.

I’ve decided to stick to the Cheap & KISS principle and use a large 5 gallon saltwater supply I can secure out of the way, then use a little electric pump like the one below to pump in the salt water into the sink/pans.

SYPHON-PUMP-ELECTRIC-BATTERY-OPERATED-WATER-6L-MIN-TRANSFER

It should be good for a few days and then I will just refill in the cockpit.