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Old 22-07-2011, 13:44   #1
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Challenge: Check Valves

The mention of a non return, or check valve, in the bilge pump line seems to elicit some strong concerns from my fellow boaters, some for, some against. I've noted that the most common response is "they can get blocked and jammed!" OK, how does that work?

Excluding the manual pumps, very bilge pump I've ever seen has a debris shield in place and, some, even have a course screen in place before the pick up. The check valves that I've looked at that are recommended, and sold, for this purpose all seem to open up close to, if not the full diameter of the discharge hose.

I have a diaphragm check valve on one pump and a swing check valve the shower sump pump (Both Rule 2000's) and have never had a bit of trouble with either one. I'm not saying it can't happen! However, if the debris going through the pump is limited to a size less than 10-25% of the diameter of that of the check valve, how will that debris block the check valve? And what would you say the chances are that it could actually happen?
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Old 22-07-2011, 13:53   #2
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Re: Check valves

I have read the same comments as you The biggest problem I can see it debries keeping the valve from closing
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Old 22-07-2011, 14:01   #3
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Re: Check valves

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Originally Posted by Greg S View Post
The mention of a non return, or check valve, in the bilge pump line seems to elicit some strong concerns from my fellow boaters, some for, some against. I've noted that the most common response is "they can get blocked and jammed!" OK, how does that work?

Excluding the manual pumps, very bilge pump I've ever seen has a debris shield in place and, some, even have a course screen in place before the pick up. The check valves that I've looked at that are recommended, and sold, for this purpose all seem to open up close to, if not the full diameter of the discharge hose.

I have a diaphragm check valve on one pump and a swing check valve the shower sump pump (Both Rule 2000's) and have never had a bit of trouble with either one. I'm not saying it can't happen! However, if the debris going through the pump is limited to a size less than 10-25% of the diameter of that of the check valve, how will that debris block the check valve? And what would you say the chances are that it could actually happen?

It's not just about debris, in fact debris is the least of your worries with a check valve. These valves quite often stick shut especially if the bilge pump does not cycle often. The biggest draw back is the added head pressure that the pump can not overcome. This additional head is caused by the standing water in the discharge hose, and thus, they simply cavitate and do nothing but make some neat little bubbles in the bilge. Fill a grain silo with corn and then try to push a door open into it...... Pretty tough. The check valve creates the same type of resistance to the bilge pump, Centrifugal pumps deal horribly with any added head pressure..

On top of all that the largest maker of centrifugal bilge pumps, Rule, specifically says NOT to use them.

They can be a danger if used on a system with centrifugal bilge pump.
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Old 22-07-2011, 14:18   #4
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Re: Check valves

If not using 2 pumps sharing a common line, why would you want a check valve in the first place?
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Old 22-07-2011, 15:05   #5
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Re: Check valves

A couple of things. It is my understanding that a pump must lift the water to the highest pointin the discharge system be it through head loss or elevation, and will do so unless the pump is weak (or very near its shutoff head), but it has nothing to do with a check valve holding that water in the discharge line. If a check valve is located some distance away from the pump and the portion of line from the pump to the check valve drains back into the sump one may have a priming issue. A check valve does require additional head ussually comparable to several additional feet of discharge tubing/pipe in head loss. A failed check valve (stuck closed) will cause a pump to reach its shutoff head-not pump anything. A failed check valve (stuck open) will cause the discharge line to drain back into the bilge, and possibly cycle the pump, or set up a syphon if the discharge is below the water level.
It is not typically not good for a pump to spin backwards (this is what happens when a submersible pump quits, and the water within the discharge line drains back through the pump, so a check valve holds the water in the discharge line as opposed to allowing it to drain back to the bilge (and possibly cycling the pump on and off).
Thanks
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Old 22-07-2011, 15:39   #6
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Re: Check valves

Rule Installation Instructions:

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Old 22-07-2011, 15:46   #7
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Re: Check valves

There's a powerboat here on the hard at this boatyard that was built in 1939. Several of the thru hulls have rubber flappers on the outside of the hull fastened with a bronze strip at the forward edge. Simple, effective and very cool looking.
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Old 22-07-2011, 16:06   #8
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Re: Check valves

The reason most people put check valves in a bilge pump discharge is because the hose can hold enough water to cause the pump to turn back on after the water in the hose has drained back into the bilge. The check valve is supposed to hold the water in the hose to prevent the pump from cycling off and on. Sometimes (but not often) they fail and the pumps cycle until the batteries die.

The concensus on this forum seems to be that it is better to mount a very small pump in the lowest part of the bilge with a hose so small that it won't hold enough water to cause the pump to cycle. You then mount your larger pump just a little higher and it will only come on if the incoming water is too much for the small pump to handle.

Sailboats should either mount the discharge high enough that water will not come in when the boat is heeled or put a vented loop in the discharge line like you would on a toilet discharge.
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Old 22-07-2011, 16:47   #9
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Re: Check valves

Mainsail,

Good point about them sticking shut if not operated frequently, although that is not a problem in my boat. Both my pumps get activated with some frequency. The smaller one, set up to take care of the runoff from the anchor chain, much less frequently, but have never had a problem with any of them, and they have been in use for at least the last six years.

Checking the Installation / Owners Manual for my Rule 2000 pumps, I find no reference at all to check valves, which would indicate to me that it is not an issue for this model. But perhaps they have a newer version?

In my setup the discharge hose runs from the center of the boat outward and towards the stern, then up above the waterline, then to the through hull at the stern. This setup means that I will have quite a volume of water returning back to the pump once it quits pumping. While the bilge will handle this, the shower sump, which also acts as a back-up bilge pump, will not. Thus the check valves in each line. I know there might be better setups, but this is what I have, it works and has worked for the last six years. That said, I'm always open to suggestions.

Thanks,
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Old 22-07-2011, 16:53   #10
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Re: Check valves

The cycling problem (water runs back from hose to bilge, causing pump to start again) can be resolved with a larger spacing between "on" and "off" water levels (i.e. tweak the float switches), by running the pump dry for 15 seconds (some modern Rules do this automatically) or by using a small pump for normal dewatering, and a bigger one at higher elevation for emergencies.

Anti-siphoning seems to be the other main reason for these things, and I've yet to come across a way to put a check valve in a bilge pump hose that offers less added resistance than a (far more reliable) vented loop. Even a really good check valve is a major flow restrictor.
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Old 22-07-2011, 17:38   #11
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Check valves can stick. I installed a check valve on a previous boat, despite warnings, and a few years later the valve stuck. I took it out of the system that day.
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Old 22-07-2011, 17:41   #12
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Re: Check Valves

Bash,

Stuck in the closed position, I presume? And under what conditions, used often or rarely?

Thanks,
Greg

PS - What type, brand and size if you remember.

Thx - G
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Old 22-07-2011, 18:41   #13
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Re: Check Valves

A frozen check valve can also sink a boat; my neighbor did.
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Old 22-07-2011, 19:51   #14
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Re: Check Valves

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A frozen check valve can also sink a boat; my neighbor did.

Again, as I have them in my boat, I would be very interested in make, model, age and circumstances of the indecent.

In a recent flooding incident, involving a fellow boater that I know, THE major problem was that the bilge that was so full of crud and debris filters on the pumps were clogged instantly. After the first minute there was nothing getting through the pump, let alone any check valve. Afterwards I asked him if he had tasted the water to determine if it was fresh or saltwater. He said no, he wasn't going to taste that slimy s**t! The check valve was the least of his worries. (turned out it was fresh and all ended well) My bilge is very clean, to the point of pressure washing it every now and again, plus I have lock down bilge boards so I guess the check valve, for me, is the greatest concern. Thus the question.

Thx - G
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Old 22-07-2011, 19:57   #15
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Re: Check Valves

Not to muddy the water, but....
Bilge pumps on our cat came with check valves. Where the discharge hose must travel, it would be difficult to fit a anti siphon loop ( I do have a loop, just not vented). 10 years and never had an issue, other than water seeping back and recycling.

So I did the change to the smallest pump with the smallest hose, along with a horse of a pump ( another hose, switch and thru hull, of course). Still with check valves. Once one of the small pumps did exhibit what must be the objection to check valves on bilge pumps: I guess what would be called an airlock. The pump was spinning, but not pumping. I loosened and drained the hose above the check valve, and then it could pump. I think the solution will be to install the check valve higher in the loop.

True, a check valve is a restriction, but for the same installation, there should be less static head than a vented loop.
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