Prime question

[All U Get]

Hi all,

We have had an issue ever since we left last fall with our raw water pump on the generator. When we move the pump seems to lose prime or not pump water to the Hx so I shut it down, remove the hose at the pump, and then give it a short crank. The water runs fine.
I then replace that end and remove it from the Hx. This section also has the vent loop. It will run fine here also, but not if I start here and go backwards. I've flushed this in both directions and yesterday pulled the vent loop apart to flush through there again. It acts like a block when we need it to work but not when I'm flushing it.
Now if we sit at an anchorage it will work fine for a month. Once we pick up and move, all bets are on loosing the prime. All raw water pumps self-prime so this one has me stumped. A fellow cruiser said he turns the seacock off on his when he travels. We never had this issue before so wondering what's up. I've replaced all hose on the suction and get free flowing water all the way to the pump. Now that we have a little time staying at a marina I'm determined to get this figured out.

All U Get

 

[woodster]

first thing i would check is the pump impeller you may have one of the ears broken or completely gone...when they are all good they act as a sort of seal for the path but when one is weak or damaged it may be causing the water to go back out the hose.......karma ...for messing with the toilet habits

 

[Gunni]

Maybe not much help, but here is my configuration: My generator does not have a vent loop. The entire unit sits above the waterline. I would not assume that my raw water pump could pull air and water into the pump and pressurize the loop and possibly a restricted heat exchanger. In fact, after a series of over-heats related to impeller failure and forgetting to open the through-hull I bought myself A LOT of hot-water circuit de-scaling to remove the precipitated buildup in my exchanger intake fitting and tubes. It happened rather quickly. So I would look for buildup in the exchanger circuit restricting flow sufficiently to overcome your pump head pressure.

I also shut off my genset thru-hull when not in use. Just one less thing to worry about...unless you forget to open it. I now use a lock-out-tag-out system to remind me to open the thing prior to firing up the genset.

 

[Capt jgw]

I have the same problem with my sea water galley pump. It's a centrifugal type so not self priming, but it's well below the water line and should fill with water when the seacock is open. But it doesn't. After sailing or motoring a while it won't pump. I believe the boats motion through the water sucks the water out of the intake.
Rubber impeller pumps are supposed to self prime to a point, but of they're full of air that can take a while. Also the pump body will wear after time especially of its been run in silty water and that reduces the pump efficiency a lot. Grooves worn in the cover plate can also do that, but that's an easy fix. On many you just turn the cover over to give a new surface.

 

[All U Get]

All good replies and right on target with what I have done since leaving last fall. New raw water pump with speed-seal cover, checking the new impeller is easy and I have kept an eye on it. I was starting to think like Gunni that the Hx was restricting the flow, but when it runs at anchor there's no problem. Today we ran it after our sail from the Solomons to Annapolis and it ran fine, first time since I blew out the vent loop both directions and blew through the vent on top. Chessy water doesn't taste that bad. When we move again I follow up if it looses the prime.

All U Get

Somebody tell Woodster he can shower now.

 

[woodster]

Somebody tell Woodster he can shower now.

wow i was beginning to wonder if i was in line for a GI party......

 

[JamesG161]

All raw water pumps self-prime so this one has me stumped

Well not so much. The best flex vane pump for boating I have seen might "lift 6 inches", if the discharge has an easy air venting path.
Is the outlet of your pump below the water line? If yes, roughly estimate how many "inches".

My point is there is NOT a lot of spare pressure drop available to loose.

My wild guess is you have CAM wear ( the bulge on inside of pump that causes the vane to bend) or face plate wear ( pump vane bypass).

A boat in forward motion can create low pressure spots along the hull. Example: the bow pushes water out of the way (high pressure) as the your boat "makes a hole" in the water and the stern allows water to fill the hole your boat made (low pressure). A possible reason a moving boat may cause a sensitive flex vane pump to lose prime versus anchored.

Is the intake, through hull, forward or astern of midships? My guess is yours is forward of midships.

BTW a self priming pump MUST have a easy path to vent the air as the water primes (fills) the pump. Oops we have NO exit vent, if the muffler is full of water.
Jim...

 

[capta]

I would check the intake hoses. I thought I'd get fancy an put a clear section of hose before the gene s/w pump to see the water flow and had the same problem. You need wire reinforced suction hose from the sea cock to the sea strainer and then to the pump. If any of your hoses are weak, they can collapse when suction begins. Also check for bad seals at any joints before the sea water pump, as letting in air when running or draining the line when underway and not running, could cause your problem.
I'm a little unclear where this vented loop is, but it should be in the exhaust line after the heat exchanger, not on the intake side of the cooling system. If properly placed on the exhaust side of the system it should not affect the intake side at all.

 

[All U Get]

I would check the intake hoses. I thought I'd get fancy an put a clear section of hose before the gene s/w pump to see the water flow and had the same problem. You need wire reinforced suction hose from the sea cock to the sea strainer and then to the pump. If any of your hoses are weak, they can collapse when suction begins. Also check for bad seals at any joints before the sea water pump, as letting in air when running or draining the line when underway and not running, could cause your problem.
I'm a little unclear where this vented loop is, but it should be in the exhaust line after the heat exchanger, not on the intake side of the cooling system. If properly placed on the exhaust side of the system it should not affect the intake side at all.

The vent loop is after the raw water pump and before the HX. All new hose on the suction side. I thought I had it licked with many flushes and probing, but this morning after a four-hour trip to the C&D Canal from Balt. it acted up again. I just removed the hose at the HX and ran the gen for 5 seconds to flush water through the hose. It did. Put it together and it ran fine. We're running from Cape May to Block Island so I'll try it again in two days.

All U Get

 

[All U Get]

Well not so much. The best flex vane pump for boating I have seen might "lift 6 inches", if the discharge has an easy air venting path.
Is the outlet of your pump below the water line? If yes, roughly estimate how many "inches".

My point is there is NOT a lot of spare pressure drop available to loose.

My wild guess is you have CAM wear ( the bulge on inside of pump that causes the vane to bend) or face plate wear ( pump vane bypass).

A boat in forward motion can create low pressure spots along the hull. Example: the bow pushes water out of the way (high pressure) as the your boat "makes a hole" in the water and the stern allows water to fill the hole your boat made (low pressure). A possible reason a moving boat may cause a sensitive flex vane pump to lose prime versus anchored.

Is the intake, through hull, forward or astern of midships? My guess is yours is forward of midships.

BTW a self priming pump MUST have a easy path to vent the air as the water primes (fills) the pump. Oops we have NO exit vent, if the muffler is full of water.
Jim...

New pump two years ago. Midship and didn't have a problem for 30 years. Exhaust blows out the remaining water in the muffler. ????
The only difference is we had the bottom stripped and barrier coated.

All U Get

 

[capta]

The vent loop is after the raw water pump and before the HX. All new hose on the suction side. I thought I had it licked with many flushes and probing, but this morning after a four-hour trip to the C&D Canal from Balt. it acted up again. I just removed the hose at the HX and ran the gen for 5 seconds to flush water through the hose. It did. Put it together and it ran fine. We're running from Cape May to Block Island so I'll try it again in two days.

All U Get

About the only other thing I can think of is that the raw water pump is shot. Perhaps where the impeller sits is just too worn to bend the impeller blades sufficiently to create the suction necessary. Or if there is any distance between the impeller and the cover plate the pump won't function properly. There should actually be wear marks on this plate from the impeller spinning against it. These pumps are not terribly expensive if you go to a pump supply like http://www.depcopump.com/index.php, http://www.pumpagents.com/ or http://www.pumpvendor.com/. Of course, that is if you have an Onan MDKD early 80's, as we do. The pump should be an Oberdorfer OBN 202M-11 for the Onan and quite possibly the same for the Westerbeke.
At this point I would recommend against trying to rebuild the old pump until you are sure that the pump isn't too far gone.
To the best of my knowledge my intake line doesn't drain of water, but since the pump is less than a foot above the waterline, it really doesn't have too much head to get the water circulating, even if it is empty. Am I right in assuming that your gene is on a platform to port of the main engine, as mine is?
Of course, you could just turn around and sail down here and we could get this all sorted out in a jiffy!

 

[Stu Jackson]

My generator does not have a vent loop.

This ^^^^^^^^^^^^

Odd to have VL between a rw pump and a HX.

 

[JamesG161]

Exhaust blows out the remaining water in the muffler. ????

Yes!
But not at point the pump needs to prime.
With a new pump, then the impeller or a slight suction air leak, after heat from genset warms the enclosure and expands some fittings. I have used Lewmar gear grease or a sealing caulking to assure no vacuum side leaks at the hose coupling.

Start with this scenario....
1) All lines are empty and the muffler seal loop is full of water.
You are relying on the pump vanes to pump the volume of air from water line up to the vanes. Since most genset engines are running at a near constant rpm.

2) The pump is lifting and compressing the air to discharge it (making way for the water) and the engine is creating exhaust back pressure to "burp" the muffler seal loop.
Only for an instant does the exhaust pressure over come the seal, but at max back pressure.

3) Water is lifted to the pump housing and the pump blasts away with non compressible water.
Yeah baby! Now sea water refills the muffler seal and burp, slosh, burp, slosh....

Now if you loose any of the pump's lift power (suction leaks), or get any clogging of the inlet thru hull bulk strainer, the pump will cavitate and lose prime. (Did they paint over that strainer?)

Thank goodness my Kohler water pump is below the water line and needs very little suction lift (only friction loss).

It only takes a small difference in the seal between the vanes and the cover plate to loose lift power. When did you change your impeller and does it use a gasket or O-Ring to seal the plate?
Jim...

 

[All U Get]

After all the hose flushing I had done on the generator, I blew through the hose this morning and got a piece of vane in the bucket. Back when we were in Key Biscayne and the strainer plugged with grass, that impeller and water pump fried so I put in the new pump. I had found all but one vane at that time so we've been carrying it around for three years. Hope that's it.

BTW the water from NYC is a little oily.

All U Get

 

[JamesG161]

I blew through the hose this morning and got a piece of vane in the bucket.

WOW! Next time you are in low winds, you know what to do!

Levity aside, which hose? Pump discharged or suction? Describe the size of the piece, pea size?

This reason I ask, is you still may have more debris. My 1 coffee morning brain, is trying to understand the "at anchor" observation. My intake is just aft of the mast. They could have put the thru hull directly under the pump but chose to move almost 4' astern. My guess is the genset weight (forward) is use to balance the boat, the thru hull is at a "way on" hull pressure neutral point, a bit more aft.

I would love hear back if your persistence, paid off!
Jim...

 

[woodster]

my guess it was in the discharge side with my 3 cups of coffee and 1 shower........ i can't help but think that his pump does not backfire.....

 

[Gunni]

The most common place for the broken vanes to go is downstream toward the HE. However, when I was having flow problems after an impeller failure (and getting no joy in a remedy). I tried blowing back to the thru-hull, first with my lung power, and then with my dinghy foot pump. The results indicated I had a problem in the suction side. Upon dissasembly of the entire suction side about 4 impeller vanes shook out. They were likely back-washed by the raw water after genset shutdown.

 

[JamesG161]

my 3 cups of coffee and 1 shower

You probably need that reversed. "3 showers and 1 cup of coffee"

my guess it was in the discharge side

Mine too, but here, debris size is important.

@All U Get remember if you pressurize from pump discharge forward to muffler, you have at least 1 engine exhaust valve open to blow debris/water into.
Jim...

 

[JamesG161]

They were likely back-washed by the raw water after genset shutdown.

If you lose 2 or just 1 vane in right position, you have no water seal and from suction to pump discharge. Gravity still works, well at least this morning it did.
Jim...

PS: My 3 cup brilliance now, but shower at gym

 

[Gunni]

If you lose 2 or just 1 vane in right position, you have no water seal and from suction to pump discharge. Gravity still works
Jim...

Exactly. Which illustrates the theorem that whatever time and money you save by not replacing that impeller which spent the winter in your raw water pump is inversely proportional to the time, effort and frustration of fixing an impeller failure during the sailing season.