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3-way valve creates hole in hull
This is a scary story about a common piece of plumbing that you might have on your boat.
It’s about a valve that has three hoses attached; call them A, B and O. There is a handle that has two end positions; in one of them, it connects A with O; in the other it connects B with O. One application might be to control the source of tap water: you might want it to come from tank A for a while, and then you might want to switch to tank B. On my boat, there is one of these creatures for a different application… it normally connects a seawater intake with a pump for hosing off the anchor and washing the deck, but in emergency situations it can connect the pump to the bilge and thus be used to keep the boat afloat when the normal bilge pump is not sufficient. I've always thought this was a good idea.
I learned 4 scary things about these valves recently.
1) When the handle is on the journey from one extreme position to the other extreme position, the valve passes through a state where all three hoses communicate. In particular, water can flow between A and B. I do not know why anyone would build a valve like this. The preferred behaviour would be to close off the connection between A and O before starting to open the connection between B and O. Usually, people do not want water to flow from one of their water tanks into the other. And very very very commonly people do not want water to flow directly from the sea into their bilge. Nevertheless, the valve I show in the photos allows for exactly that.
2) The valve can fail. The connection between the handle and the internal ball broke on my valve and the handle no longer turns the ball.
3) The valve can fail in the intermediate (unwanted) (illegal) (dangerous) state.
4) The valve can fail in such a way as to be undetectable. The handle on my broken valve still moves smoothly and positively throughout its design range. It feels and operates the same now as it did when it truly did its job. This movement gives one a very secure impression that the valve is operating. If the captain of my sinking ship wanted to know if the valve was in the correct and safe position, anyone who checked for him would assert confidently that it was. Wrongly.
I was just about to launch my boat this week when I decided (for reasons of operability, not suspicion of malfunction) to remount the valve. One of my guardian angels must have inspired me to do this unusual step, because I really don't like random extra work. What I discovered in the process is that the thing was broken and that the boat would have had a FULL-TIME CONCEALED HOLE IN ITS HULL.
I do not know why anyone would bring such a valve to market. I do not know why anyone would purchase such a valve and put it in a boat. I do not know why the ABYC would allow anyone to put such a valve in a boat. I do not know why all the multiple surveyors who have inspected my boat did not catch this vulnerability.
I want everyone to learn the same 4 lessons I just learned. And please make design, operating, and maintenance decisions based on the threat therein.
The first two pictures show the valve with the handle in its two positions. The third one shows the light you can see by looking from port A through to port B. That unfortunate opening is now a permanent feature — regardless of where you move the handle.