3-way valve creates hole in hull - SailNet Community
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post #1 of 17 Old 01-31-2020 Thread Starter
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Exclamation 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.
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post #2 of 17 Old 01-31-2020
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Re: 3-way valve creates hole in hull

You are right, in between positions there is a small passageway between the two sources (in your installation).

http://cdn.conbraco.com/apollovalves.../TS_70-600.pdf

I have a pair of these in what I have rigged to "polish" the diesel fuel.

But in your case, don't you have a seacock on the truhull for the seawater intake? And possibly for the pump discharge.
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Re: 3-way valve creates hole in hull

If cross-contamination could really be a problem, the solution would be a simple Tee and two separate shut-off valves.
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Re: 3-way valve creates hole in hull

Actually, in many applications, there are several good reasons to have both open for a moment. One is water hammer. If the valve is quickly moved from one setting to the other, the pipe can take a hell of a lick, potentially going way over the safe operating pressure. Some times these are called trans-flow valves, for transitional flow. Also switching fuel filters from one side to the other, without starving the engine during the switchover. Most of the time, transflow is better.

Some three-way valves can be ordered either way. Many tree way valves have AB, AC, or ABC options, just by spinning the valve. Multi-port valves are sort of a specialty animal, and there are many variations.

Definitely more complicated than you might guess.


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Re: 3-way valve creates hole in hull

Quote:
Originally Posted by ggray View Post
in your case, don't you have a seacock on the truhull for the seawater intake? And possibly for the pump discharge.
Yes, I have a seacock on the thruhull for the seawater intake. So if we are going to pump the bilge, we could close the seacock first before switching the intake path. But that requires a trained person with a flair for procedure and detail... not a panicky newcomer to the boat. So the arrangement I have can be seen as safe-assuming-competence. But I'm not comfortable with that. I'm looking for another level of goof-proofness, which a better 3-way valve could provide.

Last edited by capnJudd; 01-31-2020 at 10:27 PM.
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Re: 3-way valve creates hole in hull

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If cross-contamination could really be a problem, the solution would be a simple Tee and two separate shut-off valves.
Yes, this also solves the cross-flow problem but also introduces a requirement for training and procedural competence. Not the sort of step backward that I'm in favour of.
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Re: 3-way valve creates hole in hull

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Actually, in many applications, there are several good reasons to have both open for a moment...
Some interesting and subtle points.

Quote:
Some three-way valves can be ordered either way.
That's good news. But all the valves I've found in hardware stores or online have the open-before-close property. Please tell me what the industry lingo is for the alternative close-before-open property.
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Quote:
Many three way valves have AB, AC, or ABC options, just by spinning the valve. Multi-port valves are sort of a specialty animal, and there are many variations.
I found these diagrams elsewhere on the web in a scale big enough to see the innards. Every one of them has the open-before-close property.
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Re: 3-way valve creates hole in hull

The issue is not so much with the valve as the way it was installed. If you think about it, three way valves that can do this are installed on a lot of boats to allow the output of your head to divert between a holding tank and a seacock. The difference is that since the seacock is on the output side of the pump a vented loop/siphon break can be installed. Since the seacock is on the input side of your pump, that is not an option. Your real gripe, as you noted, is with the surveyors for not noticing the problem. The pump never should have been installed this way in the first place. If you feel the need to have two bilge pumps, then install a second bilge pump with its own output line.
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Re: 3-way valve creates hole in hull

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Originally Posted by capnJudd View Post
...I found these diagrams elsewhere on the web in a scale big enough to see the innards. Every one of them has the open-before-close property.
I think that may only be an option on larger industrial valves, where they have "standard" and "transflow."

However, if you know the flow pattern of the plug, you may be able to accomplish the same thing by turning the valve handle in the opposite direction, avoiding the transflo combination. You'll have to change the stops. I've done this before, sort of like wearing the ship instead of tacking.

But it depends on the plug pattern.
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Re: 3-way valve creates hole in hull

Quote:
Originally Posted by capnJudd View Post
Yes, I have a seacock on the thruhull for the seawater intake. So if we are going to pump the bilge, we could close the seacock first before switching the intake path. But that requires a trained person with a flair for procedure and detail... not a panicky newcomer to the boat. So the arrangement I have can be seen as safe-assuming-competence. But I'm not comfortable with that. I'm looking for another level of goof-proofness, which a better 3-way valve could provide.
You have a nice arrangement with deckwash/emercency pump.

If it were mine, I would keep the seawater intake seacock closed all the time when deck wash not being used, and possibly the three-way switched to bilge water as well (depending on access to this valve, and experience of the crew)

Last edited by ggray; 02-01-2020 at 01:09 PM.
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3-way valve , close before open , hole in hull , plumbing hazard

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