Anti Siphon Loop -- Bilge Pump

[RaymondR]

I'm confused.


It appears that one should never install a check valve in the discharge of a centrifugal pump because it increases back pressure on the pump but it is OK to install a loop which adds head to the discharge (back pressure)


The Folly of Absolutes


"One should NEVER put a check valve in the discharge line of a bilge pump"


It appears that it is better to either:


Put a manually operated valve in the discharge line which might be forgotten and allow the boat to be sunk.


Put an anti syphon loop in the discharge line which has a tiny little rubber check valve in it which might fail to open thereby allowing the anti syphon function to fail and fill the boat with water.


Rather than just fit a check valve and drill a small anti air lock vent between it and the pump discharge to prevent air locking.

[Dockhead]

Originally Posted by RaymondR

I'm confused.


It appears that one should never install a check valve in the discharge of a centrifugal pump because it increases back pressure on the pump but it is OK to install a loop which adds head to the discharge (back pressure)


The Folly of Absolutes


"One should NEVER put a check valve in the discharge line of a bilge pump"


It appears that it is better to either:


Put a manually operated valve in the discharge line which might be forgotten and allow the boat to be sunk.


Put an anti syphon loop in the discharge line which has a tiny little rubber check valve in it which might fail to open thereby allowing the anti syphon function to fail and fill the boat with water.


Rather than just fit a check valve and drill a small anti air lock vent between it and the pump discharge to prevent air locking.

OK, let's try to clear up some of the confusion. In no particular order:

1. Air locking is not the only or even main way that a check valve can put a centrifugal bilge pump out of action.

2. Check valve is fine for a diaphragm pump, which makes an ideal maintenance pump. E.g. Jabsco 36600 Belt Driven Diaphragm Bilge Pump - 12V

3. Anti siphon loop vent failure open will not make anti siphon function fail.

4. Anti siphon function altogether is not important when the boat is moored.

5. A little head from a loop won't kill a centrifugal pump. The head is not calculated to the top of the loop, but to the outlet.

6. A manual valve should be present in any case for all throughhulls, and shutting it off for a high volume secondary pump is no problem. The high volume pump will never save your boat while you're off it, if there's a leak too big for your maintenance pump to handle. When you're on the boat, the bilge alarm will tell you when you need to open the seacock and turn it on.

[RaymondR]

Originally Posted by Dockhead

OK, let's try to clear up some of the confusion. In no particular order:

1. Air locking is not the only or even main way that a check valve can put a centrifugal bilge pump out of action.

2. Check valve is fine for a diaphragm pump, which makes an ideal maintenance pump. E.g. Jabsco 36600 Belt Driven Diaphragm Bilge Pump - 12V

3. Anti siphon loop vent failure open will not make anti siphon function fail.

4. Anti siphon function altogether is not important when the boat is moored.

5. A little head from a loop won't kill a centrifugal pump. The head is not calculated to the top of the loop, but to the outlet.

6. A manual valve should be present in any case for all throughhulls, and shutting it off for a high volume secondary pump is no problem. The high volume pump will never save your boat while you're off it, if there's a leak too big for your maintenance pump to handle. When you're on the boat, the bilge alarm will tell you when you need to open the seacock and turn it on.

1. You are right but if trash can get through the screen most centrifugal bilge pumps have it is unlikely to clog a check valve.

2. Diaphragm pumps generally have two check valves in them as suction and discharge valves, if two of them aint dangerous in a diaphragm pump one aint in a centrifugal pump. The only functional difference is the motivation of the flow.

3. Sure will, it is the air the valve lets into the top of the loop which activates the syphon break.

4. Depends on where the outlet is, if it's close enough to the waterline it could take up backflood if the boat is down on it's lines or heeled.

5. Nope, head is head whether it is generated by gravity acting upon a column of fluid or from friction in a fluid flow. When calculating the pressure regime in an inverted U tube it is head in the up section minus head loss in the down generated by gravitational acceleration of the fluid and the short section between the tip over point and the discharge point does not allow much acceleration to take place.

6. Depends upon the size of the leak, the power source for the pump and the time required to wear the pump out. If you are tied up in the marina and have a big battery charger hooked up and lots of panels it might take a month or two to sink the boat.

Few absolutes in the boating business mate.

KISS is a good practice and if you can keep the systems and fail safes where they run down hill so much the better. But, occasionally the best and simplest solution is just to put in a check valve, drill the small air bleed to prevent air locking and place the discharge well above the water line.

It's a matter of balancing your paranoias. If you are paranoid about check valves don't put one in, find another solution. If you are paranoid about leaving the valve on the auto bilge system closed thereby preventing the auto bilge pumping system from autoing but not about the reliability of check valves - put in a check valve. Etc, Etc, Etc.

[Cheechako]

Originally Posted by RaymondR

Few absolutes in the boating business mate.

.....
It's a matter of balancing your paranoias. If you are paranoid about check valves don't put one in, find another solution. If you are paranoid about leaving the valve on the auto bilge system closed thereby preventing the auto bilge pumping system from autoing but not about the reliability of check valves - put in a check valve. Etc, Etc, Etc.

Yep. It's boating. One size does not fit all.
The devil is in the details. A heavy Bronze check valve mounted vertically may have much more resistance to flow than a high loop. A plastic/rubber check valve may have little resistance to flow. A bilge pump with a good screen may have no risk of clogging a check valve, one that allows bigger stuff may....

[Sailor Doug]

Centrifugal pumps deliver high volume low pressure (low head hight) check valve add restriction so do long hose length. Best pump choice if you can get well above back flood hight. A anti syphon valve adds do restriction but any back flow valve will decrease pumps volume capacity and is not fail safe for back flooding. A loop is a loop with no function without a hole in it, where to drain the hole?


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[Cheechako]

This whole discussion is insane. A loop does have a function as stated earlier a couple times. It keeps water from starting a siphon if your exit is above the waterline. The water would have to go up and over the loop probably 3 ft above the water line to start.... any time the fitting comes out of the water for a second or two the siphon is broken. It's the most often used solution. Basic stuff.

[Terra Nova]

Originally Posted by Dockhead

OK, let's try to clear up some of the confusion...

Good luck with that.

There will always be the uninformed. And also those who, even with all the the information at their fingertips, will just never get it, and who will be stupefied when one of their crew is injured or lost because of their negligence.

Minimum safety requirements are published that reflect the body of knowledge that has accumulated ever since the first boat sank. Boats and lives have been lost due to the failure of the bilge pump system with added check valve. It isn't rocket science. But lives may depend on it. So let's use our heads.

[RaymondR]

Originally Posted by Cheechako

This whole discussion is insane. A loop does have a function as stated earlier a couple times. It keeps water from starting a siphon if your exit is above the waterline. The water would have to go up and over the loop probably 3 ft above the water line to start.... any time the fitting comes out of the water for a second or two the siphon is broken. It's the most often used solution. Basic stuff.

No, if the loop is anti syphon and not just to defeat backflow when heeled, in the case of athwart ship discharges, or in following seas, for transom situated discharges, it will have a small, rubber, duckbill check valve at the highest point of the loop. When the loop is under pressure the duckbill valve stays closed ensuring all the fluid is pumped through the loop, when the flow stops or slows down and the back pressure on the anti syphon valve goes to vacuum, the duckbill valve allows air to enter the loop and the legs of the loop to drain and consequently no syphoning.

There are three subjects one can rely upon being introduced into any superficial argument; the race card, global warming and safety. Global warming and race card have yet to appear.

[pappa gasket]

Originally Posted by jongleur

Does anybody have their bilge discharge
into their cockpit? Would act as a siphon
break and let you know if the bilge pump
is running.

I have a center cockpit boat with a fairly high floor. The bilge pump tees into one of the cockpit drains. It's near center, always vented

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[Terra Nova]

Originally Posted by pappa gasket

I have a center cockpit boat with a fairly high floor. The bilge pump tees into one of the cockpit drains. It's near center, always vented...

Bad idea. Could wind up with flooded cockpit flooding bilge.

[pappa gasket]

Originally Posted by Terra Nova

Bad idea. Could wind up with flooded cockpit flooding bilge.

Perhaps you have a good point. Not my design. There's 2 cockpit drains to separate thru hulls. I'll have to do some worrying now to if I have a problem with this.

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[stillbuilding]

Originally Posted by pappa gasket

I have a center cockpit boat with a fairly high floor. The bilge pump tees into one of the cockpit drains. It's near center, always vented

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Not highly regarded I know but I had this on a racer and it worked perfectly. My cockpit drains x2 both had ball valves as did the bilge T.


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[Franziska]

Btw. How about adding a large mobile bilge pump with a large diameter hose to the kit.
Add 2-3 reliable connectors and you can plug it in at strategic spots.
Can double act as fire pump...

At least that's what I have on my cat.


www.ladyrover.jimdo.com

[Dockhead]

Originally Posted by Franziska

Btw. How about adding a large mobile bilge pump with a large diameter hose to the kit.
Add 2-3 reliable connectors and you can plug it in at strategic spots.
Can double act as fire pump...

At least that's what I have on my cat.

An excellent idea. As I wrote about earlier in the thread, I have a massive 230v trash pump with fire hose which pumps more than 40,000 liters per hour. Besides saving your own boat, such a rig could be used to save another boat, with it or use it as a fire pump.

The big advantage of this is that you don't need to install any discharge pipe, which would be daunting or impossible with such diameter. Just unroll the fire hose and throw out the cockpit.