Why Those Troublesome "Rubber" Impeller Type Pumps?

[rardiH36]

I've often wondered but haven't been able rationalize an answer why the seawater pumps for our small diesel engines tend to be the flexible Neoprene or Nitrile flexible impeller type. Rather than pumps with centrifugal impellers such for our car's cooling systems. Or our bilge pumps for that matter.

Protocol is to partially disassemble our seawater pump every season and replace the nitrile or neoprene impeller. Or at a minimum at least inspect the impeller for tears and cracks. And if one gets distracted from the normal start-up protocol and fails to open the raw water seacock, by the time the engine overheat alarm sounds, the impeller is likely to have been "dry run" too long and will have disintegrated into pieces that can wreak further mayhem as they lodge in the heat exchanger.

So why aren't centrifugal type pumps assigned to this duty? Modern alloys and plastics I would think would be suitable for years of service? No need for annual inspection. And while it is never good protocol to let the engine overheat, if the seacock is left unopened, at least the pump and heat exchanger don't need servicing.

 

[LeslieTroyer]

Centifugral pumps tend to NOT be self priming

Les

 

[Maine Sail]

Centifugral pumps tend to NOT be self priming

Les

Bingo...! Centrifugal pumps are best for closed cooling systems, hydronic heating systems, where no priming is necessary, or high volume bilge pumping until it entraps air.... When self priming is necessary then nothing really beats the old flexible vane impeller...

 

[rardiH36]

Good point! Priming efficiency did occur to me at some juncture.

But what if the pump is located at or even below the water line? Wouldn't the pump either have an easy time of priming ... or already have a steady source of water along with some pressure in which case self priming isn't really important?

 

[Maine Sail]

Good point! Priming efficiency did occur to me at some juncture.

But what if the pump is located at or even below the water line? Wouldn't the pump either have an easy time of priming ... or already have a steady source of water along with some pressure in which case self priming isn't really important?

Centrifugal pumps deal quite poorly with head pressure. Your exhaust system can have a lot of head especially if the strainer begins to plug or the exhaust elbow starts to plug....

 

[rardiH36]

Maine Sail:

Thanks for the insights. Got to keep my jabsco 2760 functional!

 

[kloudie1]

Another biggie is that the pressure generated by a centrifugal pump varies with the square of the speed of the pump.. so to have enough pressure at idle would require a pretty big impeller which would make too much pressure at higher rpms.. A flex vane pump makes pretty much the same pressure at different rpm .. its volume of flow increases with rpm which is fine since ya need more cooling water at higher power generation..

 

[capta]

By the way, the fresh water circulation pump on your diesel is a centrifugal pump.
We get something like 800 hours on our generator sea water pump impellers, but it gets used every day, several times a day. Even though we must completely remove the start solenoid and bracket and the pump to change the impeller, I don't consider this to be one of our maintenance headaches. I wish everything on the boat was on a 400 day cycle; I'd have plenty of free time.

 

[Skipper]

Also a flexible impeller tolerates and passes debris better.

If you catch a pebble or shell in a centrifugal bilge pump, it might just trip the breaker.
If you catch it in an engine-driven rigid vane, you could chip teeth or snap a shaft or pin.

 

[Bill Roosa]

one great way to reduce the life of your impeller pump is to use antifreeze during winterization. Makes the rubber brittle and the vanes crack off.

 

[Calif. Ted]

And there are rotary vane pumps that do not use a flexible impellor but the complexities of a mechanical vane would be less suitable for seawater use.