Seawater strainer/pump flow problem

[Slowtack]

I recently installed a standard vetus plastic strainer on my Volvo 2003 with saildrive. Previously the inlet valve was piped direct to the pump, no strainer used. Using the strainer, mounted with cover just over waterline with say3ft total piping length all well clipped, the waterflow is much reduced, at any revs above idle speed. No sign of seal damage or other cause of air leakage, I've pressure tested the unit and pipes. Any suggestions? Flow reverts top normal when the strainer is bypassed. I doubt the added friction losses in the strainer and pipework are a material factor. Any ideas welcome.

 

[roly_voya]

Double check it is sealing, it is possible to get the lid on slightly off centre and it only takes a tiny air leek to reduce the flow. The only other things I can think of concerning the filter are a blockage or kink in the pipe, a crack in the casing or a leek at one of the joints.

Alternatively how far below the waterline is the pump? Could it be that seawater was presented to the pump under pressure but that it is now having to draw up through the filter which could make a diff if the pump is badly worn?

 

[VicS]

If you are absolutely sure there are no air leaks, overtightening the fastening securing the cover is I understand a likely cause, and there are no diameter reductions at the connections to the new filter the only explanation is that it does impose a restriction. What is the filter element? Can you remove it while still leaving just the body in the circuit just to see what happens.

Does your normal engine cooling water flow fall within the recommended flow rate for the filter you have fitted.

 

[VicS]

[ QUOTE ]
Alternatively how far below the waterline is the pump? Could it be that seawater was presented to the pump under pressure but that it is now having to draw up through the filter which could make a diff if the pump is badly worn?

[/ QUOTE ] Since the height of the pump relative to the water line remains unchanged that should not have any bearing.. The only problem would be with priming the filter if it was itself too high or of course if the filter does impose a significant restriction. Vetus recommend a minimun of 15cm from the water line to the bottom of the filter but do not suggest any maximum.

 

[jeremyshaw]

Hmm, I keep meaning to do the same. Further possibilities:

1. Airlock? You are probably making the system work harder if you are taking the water up through a loop, so it's just possible there's an airlock in the hose with only partial water coming past the 'bubble'. You'd think it would pull through, but if you have not changed the impeller lately and it's a bit tired...

2. Conversely, if the intakes on the saildrives are a bit grunged up, and the hose is thin walled, it might be collapsing a bit under suction and thus restricting flow.

Just some more thoughts - it is a puzzle. Do tell when you solve it!

 

[Slowtack]

Filter cover fitted properly, cover only just over water line roughly level with top of exhaust injection elbow. Hoses new and in good condition. Saildrive has been cleaned, used mains water hose to flush from above also, good flow. Impeller new also, but changed it just in case - no effect. ????????????????

 

[davidwf]

Try a smear of silcon grease or vasaline on the seal of the Vetus filter this will help ensure a good seal.

 

[jeremyshaw]

Try this logic:

If as you say "flow reverts to normal when the strainer is bypassed", then the problem is somehow with the filter. Assuming it's the standard Vetus type basket you can just remove the basket as VicS suggested. If that solves the problem then you know the basket is reducing flow. If it doesn't then it's the filter body somehow.

That then begs the question of how much the flow is reduced. Is it just noticeable or enough to worry about? There is generally far more flow than is really necessary. That said, a friend of mine with identical engines has Vetus strainers while I don't, and I've never noticed his flow being different to mine - not that I've particularly studied it.

Finally, take comfort from the fact that my 7 year old cat with two engines has done the Med, Atlantic and Caribbean, and we've never had problems with blockages. So you can always go back to square one!

 

[dickh]

Almost definitely a small air leak where the lid screws on. I had exactly the same problem with my water filter, not Vetus but same principle. Absolutly fine when first fitted but after I'd cleaned it it didn't suck water. I had to grease the screw threads with vaseline and really tighten the lid down. I even made a special wooden spanner to be able to do it. Problem solved! But I do have to be careful so as not to break the plastic lid!

 

[Slowtack]

Thanks for the advice y'all