Anti Siphon Valve function

[gfatula]

When I re-powered my Islander 28 last summer I added a few things. There was no strainer for the raw water intake, there was no AS valve and I added a heat exchanger. When I installed the loop from the strainer to the inlet on the raw water pump I ran it up beside the starboard cockpit wall and put the AS valve at the top of that loop. The loop rises well above the top of the heat exchanger that is being fed by the raw water pump. I am wondering why I have to tape over the opening in the AS valve to allow the raw water pump to start its pickup of raw water. The opening of the AS valve does "suck" air when the pump is opereating. The piece of tape I use to get things started is dimpled in. You can feel the suction with your finger. Is there something wrong with the valve? Apparently it leaks (sucks) so much air it allows the pump to cavitate and not draw water. Is my loop too high?

I have un-mounted the inlet loop so I can pull it down and add a piece of tape when I start the engine. I must be doing something wrong. I guess the loop with the AS valve should be in the hose leaving the heat exchanger for the exhaust? That would remove the pump from the equation.

Thanks,

George

[Don Moyer]

George,

You seem to have answered your own question. An anti-siphon valve cannot work on the suction side of a pump, as the negative pressure inside the hose will continually suck air in through the AS valve.

Don

[gfatula]

Don,

I realized that as I was writing the inquiry but decided to post it anyway.

Thanks.

George

[s/v Dearbhail]

Don- I'm glad you jumped im on this one. My system does not have an AS valve either and the entire cooling system is below thee water line until the wet exhaust rises up and dumps overboard, so I have been reading along. I started my own thread about this topic earlier in the Cooling Section with the subject of Magic?

I found the attached file, a Northern Lights generator that uses raw water cooling and a wet exhaust system that shows several possible installations and the AS valve.

If I understand the drawings correctly the AS valve should be on the outlet side of the water pump. Is there a reason most the installations I see place the AS valve on the outlet side of the engine before the wet exhaust? It seems the AS valve would protect the engine better if it were between the water pump and engine.

[rigspelt]

Thanks for pointing out that article http://www.coastalmarineengine.com/m...t_Drown_Me.pdf. Also has an interesting comment about not using scoop intake on the raw water through hull, so as to minimize risk of forcing water past the water pump impeller into the exhaust system.

[Don Moyer]

An anti-siphon valve will theoretically function OK as shown in your attachment (between the pump and the engine), but they tend to work somewhat more reliably on the downstream side of the engine (between the engine and the exhaust system).

The reason for this improved function between the engine and exhaust has to do with the design and function of the AS valve itself. In its primary role of allowing air into the cooling water hose on engine shutdown, the AS valve has only has a couple feet of negative water head (less than 1 psi) acting on it to open the valve and allow air into the water hose. Conversely, when the engine is started, it has to close (again with very little positive head) or water will flow out of the valve and into the bilge all the time the engine is operating (in worst cases, threatening to sink the boat).

In this near-zero head environment (particularly on the suction side of the AS valve on engine shutdown), the water flowing unimpeded into the exhaust system will be able to impart more of a "mini-surge" to open the AS valve than if it had to flow through the entire engine as in the Northern Lights schematic.

Don

[s/v Dearbhail]

Thanks for explaining that Don. Since I am rebuilding almost every system on the boat I just wanted to get it right the first time and it helps to understand how and why it works.

Shawn, sastanley mentioned in the Magic? thread that his anti-siphon valve had a hose run to the sink. I think we know why now.

[Mark S]

Don,

My exhaust is like the double walled system in Steve Markowski's diagrams in the thread "Water 'drop' muffler-replace with same??" from November. All this talk about anti-siphon valves has me wondering, where is mine? and do I even have one? I don't have a mental picture of ever seeing one. From your knowledge of those systems, do they have anti-siphon valves, and, if so, where are they located?

Mark S

[Don Moyer]

Mark,

My recollection is that Steve Markowski concluded that he didn't need an AS valve (as is the case with most double-walled systems), so if your system really is like Steve's, then you don't need one either.

The easiest way to find an AS valve in a standard wet exhaust systems is to follow the engine cooling water hose from where it enters the exhaust system back upstream to the engine. If you don't see one there, check the hoses back upstream from the engine to the raw water pump (where Steve's AS valve was originally located). If you don't see (or feel) a valve in any of those locations, you clearly don't have one. The AS should never be between the raw water pump and the through-hull.

Don

[Mark S]

Thanks, Don.

My exhaust is a double walled exhaust similar to Steve's diagrams. The exhaust gases exit the manifold and are joined by the cooling water via a hose from the fitting atop the manifold to a fitting in the exhaust pipe. The pipe snakes over to the port side to a vertical stack that rises up to the top of the port cockpit locker. Somewhere out of that stack is a wrapped hose that runs through the locker to the exhaust hole in the topsides just under the transom. I'm positive there is no AS valve anywhere along there and I know there's no valve ahead of the engine. I'll search the site to find Steve's reasoning.

Mark S

[rheaton]

Don, I just installed your fresh water cooling system onto my 1978 Catalina 30. May I ask where you would recommend we have an anti-Siphon value installed with this system.

Take care,
Russ

[Don Moyer]

Hi Russ,

The location of the anti-siphon valve should remain in its original location,
which is just below the top of the galley sink. With your fresh water
system, the valve will be between the sea water outlet of the heat exchanger
and the exhaust system.

Don

[rheaton]

Thank you Don.