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Old 16-06-2007, 18:01   #1
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Flexible Impeller Pump Question

I think I know the answer but ------.

I have a Jabsco clutch pump which is engine driven off a pulley. If I use it for salt water washdown with a hose and shut off nozzle on the end (for anchor and chain cleaning especially), what happens when I shut off the water at the nozzle? Do the impeller vanes just bend a bit more and allow the pressure to dissipate? No problem? Or, does the pressure build and build until an internal plumbing pipe bursts?

I believe the impeller would just bend a bit more and release pressure but it certainly would be good to know before I really put it to the test.

Kind Regards,

JohnL
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Old 16-06-2007, 18:04   #2
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Hmmm, be sure to let us know how it works out...<gr>
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Old 16-06-2007, 18:57   #3
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Wow, good question. Ever considered a pressure switch to cut power to the clutch?
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Old 16-06-2007, 22:25   #4
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It doesn't work Pat. The clutch cycles in and out to fast. I guess if you had a decent size accumulator tank in line, the cycling would reduce.
I don't know the answer either. That's because rubber impellor pumps were designed to be shut off. A centrifugal pump is designed for that and is one reason why centrifugal pumps are used as washdown pumps. One thing I do know, you won't build enough pressure to burst a pipe of hose. But you may eventually break impellor blades.
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Old 16-06-2007, 22:30   #5
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Aloha Monday,
This is a manual clutch pump. You have to manually throw the lever. I guess running back down below after you get done with the chain and anchor cleaning isn't a really big deal but I'd like to know how long I can keep it engage without ruining the vanes.
JohnL
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Old 16-06-2007, 22:38   #6
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The pump impeller would cavitate and burn up. You need a bypass with a pressure relief valve or just let it run until you shut off the clutch.

Most washdown pumps are a diaphragm pump with a pressure valve to stop it when it exceeds max pressure.
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Old 16-06-2007, 23:47   #7
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Quote:
Originally Posted by SkiprJohn
I think I know the answer but ------.

I have a Jabsco clutch pump which is engine driven off a pulley. If I use it for salt water washdown with a hose and shut off nozzle on the end (for anchor and chain cleaning especially), what happens when I shut off the water at the nozzle?
Yo John,

Del is right. You are defeating yourself by adding the shutoff nozzle. Once you engage the clutch on your "washdown" pump, let it run freely until you can get below to disengage the clutch. These pumps are lubricated and cooled with water. Do not run them dry!

best, andy
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Old 17-06-2007, 11:37   #8
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Aloha Delmarrey,

By cavitating the impeller would have too much air in the chamber and not enough water to lubricate it? That makes sense. I can just leave the nozzle off the end of the hose and let it free flow until I get below to shut if off.

Terra,

It wouldn't run dry because it would still have water entering from the intake side. It just wouldn't be spitting it out on the outlet side. I think you are right though. That wouldn't be good for the vanes on the pump to just circulate in the same water and cause cavitation.

Thanks for the answers.

JohnL
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Old 17-06-2007, 12:23   #9
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Quote:
Originally Posted by SkiprJohn
Aloha Delmarrey,

By cavitating the impeller would have too much air in the chamber and not enough water to lubricate it? That makes sense. I can just leave the nozzle off the end of the hose and let it free flow until I get below to shut if off.


JohnL
To get into more detail;
At first it's not cavitation! When a pump get plugged it creates volocity, in turn creates cavitation.

Quote:
Cavitation occurs in liquid when bubbles form and implode in pump systems or around propellers. Pumps put liquid under pressure, but if the pressure of the substance drops or its temperature increases, it begins to vaporize, just like boiling water. Yet in such a small, sensitive system, the bubbles can't escape so they implode, causing physical damage to parts of the pump or propeller.
http://www.wisegeek.com/what-is-cavitation.htm

So what it comes down to, is you don't want to restrict the inlet/outlet flows more then a pump can handle (permissable flow rates) for very long. A rubber impeller has a very short life span in those conditions.
..............................._/)
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Old 18-06-2007, 03:10   #10
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Operating against a closed discharge valve will cause a rotary pump to continue to build pressure until it either overloads the motor, or damages a component.

From Johnson Pump:

”... Do not run dry for more than 30 seconds. Lack of liquid will burn the impeller and damage the seals ...”
http://www.johnson-pump.com/JPMarine...-404_F2P10.pdf

”... This pump cannot run against a closed outlet. Risk for overheating ...”

http://www.johnson-pump.com/JPMarine...410_F4B-11.pdf


Dead head is the head produced by a pump at zero flow.
Pump dead head/shutoff head is where all power is delivered to no net flow out of the pump, so all power is being input only to the fluid contained within the pump itself. The high pressure fluid spills back past the impeller clearance into the intake area again. Look at the pump's efficiency at zero flow and you see it is very low, signifying that almost all the energy spent is being converted into heating up that little bit of trapped fluid. It will usually overheat quite rapidly, build up temperature and boil the fluid, dry seals and bearings.... and pretty soon you're smoked. Continued operation below 20% of BEP flow will usually start heating up the fluid, below 10% you will usually get some pretty fast temperature buildups to damaging levels.
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Old 27-06-2007, 22:58   #11
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Aloha All,
I started taking the clutch pump apart last week to replace bearings and seals. My pump has the ceramic seal which is on the shaft. I would like to replace the seal which fits between the ceramic and the bearing. It is called a GACO Part No SP 2701 48 and has B 31536 on it as well.
Does anyone know who I should call first to find such a seal? I did find the bearings at MaGuire.
JohnL
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