Volvo Penta MD 2020 D. - Shredding Impellers

[jwfrary]

Nitrile Impellers are for bilge water or oil/diesel. They are harder and fail quickly I'f run dry. A neoprene impeller is far more tolerant in all cases except for when faced with oily water in which case they break down.

 

[PetiteFleur]

I had impellers broken every year with my Johnson pump using genuine Johnson spares (which I believe are Nitrile). I changed to Jabsco replacements and they last several seasons. I understand they are Neoprene - they feel far more flexible in the hand.

.

Indeed - I know about Nitrile v Neoprene and one is more suitable for these pumps and I did make a note but can't find it....

 

[Boater Sam]

A very clever little pump.
It prompts two questions in my mind:
1. Is it possible to pump in too much water for the exhaust pressure to eject?
2. Many pumps have solid vanes, like this one , washing machines and C/H circulation pumps. Why do the pumps on marine engines need flexible vanes?

A flexible vane pump positively displaces water and to some extent air so it will self prime i.e. suck. The down side is friction.
A rigid vane circulation pump will not displace water because the vanes do not seal against the casing, it will not self prime. It has however no rubbing friction.

 

[Fairweather Sailor]

Thank you to everyone who contributed, some very interesting theories.

 

[Daydream believer]

The reversed vane could be due to the propeller rotation as the engine is turned off whilst in gear. The current or forward motion of the boat could have rotated it back until it reaches compression point

 

[philwebb]

I think the doubled over vane is probably normal behaviour. There is a video on my website showing that rubber vane pumps reverse themselves readily without any problem.
However, I wonder if the cam is correctly located/damaged/the right one. It looks a bit severe in the photos.

I think Vyv Cox has the answer. I’ve got a 2010 which has a similar pump. The part which he calls the cam, I’m sure that it’s thinner on my pump. From recollection it looks like crescent moon. I’ve had no problems with mine other than being supplied with the wrong size impeller as there are two very different pump sizes.

 

[freespiritdavid]

Have a chat to Re power in Penryn. I was told about volvo oem ones doing this, they recommended changing to an alternative make with no further problems. Not my boat or engine , just chatting with the engineer.

 

[Fairweather Sailor]

Thank you again for all the contributions, suggestions and advice on my VP MD 2020 D.
So here's an update. While flushing out the heat exchanger, we went looking for the missing paddle from the impeller. We found it as usual in the boot before the heat exchanger. We then decided to remove the heat exchange cylinder for cleaning, so removed the rear boot attached to the exhaust elbow. Inside that boot was what looked like some sort of restricter, but on further examination it popped out, and I attach photo of same.
It looks hard, but will break with a little finger pressure. We have no idea what it is, or how it got there. Is there any reason why an engineer would place such a restricter in the boot just before the exhaust elbow, or is it a build up of some form of dirt/shale etc., ?
Thanks again.

 

[Daydream believer]

What is the condition of the exhaust elbow?

 

[Fairweather Sailor]

We checked to see if it was a corroded piece from the top of the exhaust elbow, it's in reasonable condition, thanks.

 

[Boater Sam]

its a spent circular bibendum ring from the automatic chortle valve, ignore it but dispose of it safely.

 

[Daydream believer]

We checked to see if it was a corroded piece from the top of the exhaust elbow, it's in reasonable condition, thanks.

This is only a theory & you can shoot me down. Others will certainly try.
It seems to me that water is meeting gases sooner than it is meant to & the carbon deposits are forming earlier than one would normally expect. due to early cooling. That would suggest some sort of back pressure.
Is there any flap etc in the exhaust line to prevent backwash of water coming in the stern that might be stuck?
To check for back pressure start the engine & when running on a fast tickover try to squeeze the bellows. They should not be much harder to squeeze when running than when stopped & just with residual water in them. If they are fairly ridgid then there is back pressure. That is a simple test for a blocked exhaust.
However, you say that is clear. But I might question that you can actually look into the pipe considering the position on the engine. Did you take it off? or did you just poke a screwdriver down it? My last elbow lasted 3 years. It was my third replacement in 17 years from new. The hole is quite small to start with & does not take much to restrict flow. It caused the rear bellows to eject.
Apologies if I am suggesting something that you really have checked properly.

 

[Beneteau381]

Thank you again for all the contributions, suggestions and advice on my VP MD 2020 D.
So here's an update. While flushing out the heat exchanger, we went looking for the missing paddle from the impeller. We found it as usual in the boot before the heat exchanger. We then decided to remove the heat exchange cylinder for cleaning, so removed the rear boot attached to the exhaust elbow. Inside that boot was what looked like some sort of restricter, but on further examination it popped out, and I attach photo of same.
It looks hard, but will break with a little finger pressure. We have no idea what it is, or how it got there. Is there any reason why an engineer would place such a restricter in the boot just before the exhaust elbow, or is it a build up of some form of dirt/shale etc., ?
Thanks again.View attachment 151322

A build up of carbon and salts, causing a restriction which then makes the positive ish displacement pump work harder and this then fatigues the blades. I suspect you have found the problem!