raw water pump priming

[anniebray]

Never had to worry about this on previous boats it just took care of itself but on my Sadler 290 with Yanmar 3GM30 saildrive, I find after dying out (in the yard or on my 3/4 tide mooring) that I have no water coming through the exhaust. I cure the problem by turning off the water inlet cock, removing the strainer cover, & filling the hose to raw water pump inlet with a jug of water. BTW my (Vetus) raw water strainer has been fitted below the waterline !!! when I remove the cover water pours in. I know this is incorrect & certainly unsafe but I would have thought this would help priming. Another thing the pump drive belt appears quite loose & yet I cannot turn the pulley by hand. How tight should this belt be ? Thanks in advance for all advice.

 

[Bilgediver]

Never had to worry about this on previous boats it just took care of itself but on my Sadler 290 with Yanmar 3GM30 saildrive, I find after dying out (in the yard or on my 3/4 tide mooring) that I have no water coming through the exhaust. I cure the problem by turning off the water inlet cock, removing the strainer cover, & filling the hose to raw water pump inlet with a jug of water. BTW my (Vetus) raw water strainer has been fitted below the waterline !!! when I remove the cover water pours in. I know this is incorrect & certainly unsafe but I would have thought this would help priming. Another thing the pump drive belt appears quite loose & yet I cannot turn the pulley by hand. How tight should this belt be ? Thanks in advance for all advice.

Has someone fitted an antisyphon loop before the water pump??? If there is an anti syphon loop it should be after the water pump and before the mixing elbow on the exhaust.

 

[anniebray]

The anti syphon loop is fitted after the pump (downstream) & I'm pleased to say is significantly higher than the water strainer. Also I have recently pressure washed the hull & made very sure of blasting the water inlet on the saildrive leg.

 

[PCUK]

The simple answer is that air is getting into the system somewhere, maybe the pump gasket. It wouldn't necesarily cause a water leak but be enough to stop self priming.

 

[Tranona]

Check that the clips on the hoses are tight. I had one on the inlet to the strainer that appeared to be tight, but actually the thread was partly stripped and enough to reduce clamping and allow air in. Because it was above the waterline, no leak. Also check that the O ring seal on the lid is sound and not distorted. Vaseline helps hold it in place.

 

[Ruffles]

I find that once a salt water pipe has been leaking the only sure way to cure it is to take it off completely and clean both the spigot and the pipe with a pan cleaner. Otherwise calcification from the leak prevents it from sealing. This can be a pain because they are often very hard to get off; how they can leak AND be stuck fast I don't know!

 

[jon711]

Raw water pump priming

If it is a flexible impeller pump (Don't know that engine very well!!) then usual priming problems can be caused by
1) worn impeller
2) gouges on the impeller pump housing end cover (try turning it over!)
3) excessive elbows or restictions on the inlet side of the pump (it has a lot to do to pull through restictions and sometimes cann't cope)
4) shaft seal leaks (although when it is running - would expect to see a water drip from the pump)

My gut feel says that it could be gouges on the endcover, but if possible to test a theory try putting (If possible) the filter after the pump - flexible impeller pumps are fairly hardy and will handle most suspended solids (In industrial applications they are used to pump, amongst other things Baked Beans).

 

[Cliveshep]

Quote: "Another thing the pump drive belt appears quite loose & yet I cannot turn the pulley by hand. How tight should this belt be ? Thanks in advance for all advice. "

Belt should only deflect about 25mm under thumb pressure so quite tight. If you can't turn the pulley by hand two things, (1) belt could be slipping anyway as you say it is loose and (2) something wrong with pump bearings making it too stiff. It should be hand-turnable although you will feel the resistance of the impellor vanes being folded down so don't expect it to spin like an alternator pulley.

These pumps should also be self-priming, check all suction side connections/hoses, no leaks or internal delaminations? If the front cover is worn, reverse it unless that has already been done in the past, in which case apply belt sander and linish one face smooth again and fit with new gasket or replace if you've got spare cash and can source one easily.

 

[VicS]

Belt should only deflect about 25mm under thumb pressure so quite tight

I think you have misread/ mistyped something Clive.

According to the Wshop manual 0.24" or 6mm under moderate finger pressure.

There is a comprehensive (60Mb) workshop manual than can be downloaded from http://www.motoren.ath.cx/

 

[TonyS]

Solution for non priming pump.

Whatever the cause, and there are many possibilities as seen above, the simplest cheapest solution is to insert a non return valve after the pump and before the anti syphon valve. I never found the cause of mine running dry but fitting a 3/4" stainless steel spring loaded non return valve completely solved the problem. Running always with water in the pump stops impeller damage and water seal failure. I bought the valve from RS components and from memory was £5. It needs 2 3/4" spigots at each end for the hose connections.

 

[jon711]

Pump Priming

Whatever the cause, and there are many possibilities as seen above, the simplest cheapest solution is to insert a non return valve after the pump and before the anti syphon valve. I never found the cause of mine running dry but fitting a 3/4" stainless steel spring loaded non return valve completely solved the problem. Running always with water in the pump stops impeller damage and water seal failure. I bought the valve from RS components and from memory was £5. It needs 2 3/4" spigots at each end for the hose connections.

Sorry but you are totally WRONG. Priming is not affected by the outlet of the flexible impelller pump. The only causes of a pump not priming are either an air leak in the INLET system or a failure in the pump (either end cover damage (we have advised turning it over!), or worn impelllers)

I repeat a pump not priming is a problem with the inlet not the outlet!!!!

Tony S: I would that suggest that by changing the pipework as you did solved an air leak that was causing your initial problem..

A non-return valve after the pump will do nothing, - the flexible impeller pump is a positive displacement pump - this means it is a non return valve by it's very design and operation.

I repeat , maybe in different terms, but IF A PUMP WILL NOT PRIME IT IS A PROBLEM WITH THE PUMP OR THE INLET SYSTEM.

Please service your pump and check your inlet pipework for airleaks - remov or replace any anti-syphon valves before the pump -(maybe it's a leaking duckbill valve in your anti syphon loop)

 

[jon711]

Raw water pump priming

Sorry Tony I may have been impolite, but when I used to work for the worlds biggest manufacturer of Flexible Impeller Pumps I used to get a lot of phone calls describing situations (and solutions) similar to that which you have described. The layout in your cooling system has a lack of engineering understanding of hydrodynamics. Can I ask why you chose to use a non-return valve after the pump??? Who advised you to do it, were they qualified marine engineers? The crux of the answer is that ,when you fited the non-return valve on the outlet of the pump, you must have disturbed one of the inlet pipes - by disturbing this pipe you solved the leak and hence your priming problems. I would advise that you check your cooling system NOW..

The only time I would use a non return valve on the outlet of an engine cooling pump would be if it was centifigul (Ie not positive diplacement - not non return). Centrifugal pumps are very unusual on small boat engines...

Firemen use flexible impeller pumps..---- they have a huge "leak" out of the end of thier pipe...... It is the inlet that is the problem not the outlet......

 

[TonyS]

That may be your view but in my opinion it is you that simply don't understand pipes and water. If you fill a metre length of hose with water and put your thumb over the end you can raise that pipe to the vertical and no water will come out. The reason being that nature abhors a vacuum - O level physics.
Now to be practical. My Johnson pump and many others of the 20X0 type, certainly do not provide a seal. I have clear PVC pipes and you can see the water level pass below the pump with air entering through the Vetus anti syphon valve. The reason I concluded on my boat, and it had done it from almost new, was the Bernouille effect of water rushing past the sail drive inlets and causing a negative pressure.
The first thing I did 3 seasons ago was to fit a flow switch which showed my pump was often dry and could be for up to a minute as the pump struggled to drag water in from an empty system. I then bought a solenoid valve to close the inlet when the engine was off. At the same time I bought a non return valve. The non return valve has worked perfectly from day one, and I never fitted the solenoid valve. I am still using the impeller that I fitted then with no sign of a crack. Before fitting some lasted not more than a month and I fitted a Speedseal to make changing easier. I asked on this forum and many were having the same experience. If you have rubber pipes and no flow switch you cannot tell when the pump is empty except in very severe cases as in the cause of this thread. Sometimes you think the pump is pumping water but this is just the residual water in the exhaust. Incidentally, I was worried if the non return valve would affect the anti syphon valve but as the engine is switched off you can see the water level drop to the inlet of the heat exchanger within a few seconds of turning the engine off.
This subject has concerned me a great deal and I have discussed it endlessly with Volvo, Johnson etc and many others at SIBS. Johnson say their pumps and impellers are designed for continuous use for more than 1000 hours. Volvo are not really interested saying the impeller is a wear component and will need changinging often. They also say the pump is designed to prime from dry. Their agents say the water seal only lasts a few hundred hours as well.
I have also discussed this with Fairview Sailing who run many yachts like mine. They say that my pump impeller experience of failures is like theirs but you cannot get the seal to last for more that 800 hours.
My seal has just started leaking at 1050 hours.
Owners with bigger engines that fit a pump with a higher number of vanes never experience these problems. My thought is that these pumps do in fact seal.
So although you may work for Volvo ( probably interested in selling lots of exensive spares) and be a hydodynamic expert, you hypotheses simply do not agree with my facts. My advice to anyone is fit a non return valve and a flow switch. You will save a lot of money and also save your engine. Perhaps with the recent post on water in oil you should have the flow switch after the syphon valve but I have not thought of that.

 

[alan_d]

... the flexible impeller pump is a positive displacement pump - this means it is a non return valve by it's very design and operation....

That's what I always thought, but when I go to turn the seacock off after I have shut down my engine I find that both it and the pipe connecting it to the pump are warm to the touch, presumably due to reverse flow of cooling water after shutdown. This has persisted despite a change of impeller, and I have had no problems with pump priming or the engine overheating.

 

[TonyS]

That's what I always thought, but when I go to turn the seacock off after I have shut down my engine I find that both it and the pipe connecting it to the pump are warm to the touch, presumably due to reverse flow of cooling water after shutdown. This has persisted despite a change of impeller, and I have had no problems with pump priming or the engine overheating.

You will find that your original thinking is in line with 99% of people. It seems completely logical that the pump seals. But in the case of small pumps, or some initial wear, that this is not so.
There is no harm in fitting a non return valve and it will keep the water in the pump and thus prevent impeller wear and water seal wear. Incidentally, it is easy to remove the pump seal in about 10 mins without removing the pump, but if you install a non return valve it should be the last time!

 

[Bilgediver]

A non-return valve after the pump will do nothing, - the flexible impeller pump is a positive displacement pump - this means it is a non return valve by it's very design and operation.

Actually you are quite wrong.. I think that possibly there is a problem with the water draining back and as the pump is possibly above the water line and slighty worn then it is rfusing to prime. If the air leak is the antisyphon valve then the non return valve after the pump will work if there are no other airleaks though I agree its best location is at the sea cock.

I would also suggest putting the antisyphon valve beteen the engine and the exhaust mixing elbow as this might also reduce the draining back.

It sounds like a combination of things that have come togather to cause the problem.

 

[TonyS]

Position of non return valve

Actually you are quite wrong.. I think that possibly there is a problem with the water draining back and as the pump is possibly above the water line and slighty worn then it is rfusing to prime. If the air leak is the antisyphon valve then the non return valve after the pump will work if there are no other airleaks though I agree its best location is at the sea cock.

I would also suggest putting the antisyphon valve beteen the engine and the exhaust mixing elbow as this might also reduce the draining back.

It sounds like a combination of things that have come togather to cause the problem.

I agree with you, of course, except for the position of the non return valve. It is my opinion that if it is at the sea cock, which in the case of a sail drive is deep down, it is not easily accessible. The fewer breaks in the pipe below the water line the fewer potential places to leak (possible sinking). It works best before the anti syphon and after the pump and just as efficiently as at the sea cock. It can also easily be cleaned if that became necessary. If there was an air leak at the filter then the pump would still remain full of water and therefore primed.