Why do I have so many raw water impeller failures?

[Rich Stidger]

I have a Westerbeke generator that uses a Johnson 810B-1 6-vane impeller. This year I put 131 hours on this machine starting with a new impeller at the beginning of the season. Sometime during September I noticed that the discharge water flow seemed to be less volume than normal. No over-heating, or any other symptom of a problem. Absolutely nothing in the strainer, and strong flow into the strainer from the thru-hull. Checking the pump shaft I found no play that would indicate bad bearings. No pump leakage either.

About a month later at the end of the season I pulled the impeller and found one blade missing. I located that piece in the discharge port of the pump.

So I am wondering why I see impeller failures so frequently. Hardly a year goes by without a broken vane. Looking at my maintenance data, 12 out of 16 impeller changes over 2591 hours showed one or more vanes missing. I can attribute 3 of the 12 failing impellers to a severe seaweed plug in one case and bearing failure in the pump for the other two cases.

In contrast, my Volvo auxiliary uses a Johnson 1027B or a Jabsco 1210-0000 12-vane impeller and with 8 impeller changes over 2265 hours, not one impeller has failed.

Any ideas or explanations?

 

[Mark Maulden]

Pulling air from somewhere upstream of pump? Do you sail with it running and thru hull is out of the water? Air in strainer under way? Pull the pump and see if all bronze area that sees rubber is smooth. Verify you have the right impeller p/n? Dont trust the sign on the shelf..its either cavitating, wrong p/n, or something with the pump casting or cam.. Or you could have a fish in the thru hull.....sorry had to say it....but a partial block could also cause problems on the pressure side....you just have to systematicallystart at the thru hull and keep going downstream. Could be a partially block HX..or??

 

[Apex]

2nd the vote on checking the cam, AFTER the fish in the thru-hull. Do you see any signs of wear on the other vanes?

 

[Maine Sail]

It is not uncommon for gensets to burn through impellers due to their location in the vessel usually being well above static waterline.

When you shut it down gravity goes to work and the prime on the pump & hoses is lost bring replaced by air. This causes a longer dry run time than most engines during start up and tends to shorten life...

 

[Rich Stidger]

Pulling air from somewhere upstream of pump? Do you sail with it running and thru hull is out of the water? Air in strainer under way? Pull the pump and see if all bronze area that sees rubber is smooth. Verify you have the right impeller p/n? Dont trust the sign on the shelf..its either cavitating, wrong p/n, or something with the pump casting or cam.. Or you could have a fish in the thru hull.....sorry had to say it....but a partial block could also cause problems on the pressure side....you just have to systematicallystart at the thru hull and keep going downstream. Could be a partially block HX..or??

If this pump is pulling air, I have no idea how to find that issue. Certainly all the clamps are tight and the O-ring on the strainer is fairly new and in good shape. I check the pump cavity for rough spots at every impeller change as well as for any play in the shaft and leaks. There is always excellent flow after impeller replacement and rarely anything in the strainer. The thru-hull pickup is near the center-line just behind the keel- no chance of pulling air. I don't run the generator while underway. This Spring the electrical end failed and I pulled the generator out and replaced the electrical end. While out, I cleaned out the HX, put on a new belt, zinc, and impeller. Also changed the antifreeze. So this Spring everything was up to snuff.

 

[Rich Stidger]

2nd the vote on checking the cam, AFTER the fish in the thru-hull. Do you see any signs of wear on the other vanes?

OK, taking your suggestion I looked closer at the failed impeller. All of the vanes including the broken one appear to have no wear or flat spots. There were no cracks at all in any of the intact vanes. I did see a small vertical notch in the center of every vane - see the attached pictures. When I get back to my boat in the Spring I will look closely at the pump internal area to see if I can find an edge or reason for this notch. There are about 1090 hours on this pump, in service for 8 years.

 

[Rich Stidger]

It is not uncommon for gensets to burn through impellers due to their location in the vessel usually being well above static waterline.

When you shut it down gravity goes to work and the prime on the pump & hoses is lost bring replaced by air. This causes a longer dry run time than most engines during start up and tends to shorten life...

Well that is an explanation that never occurred to me. My generator is at the level of my cockpit while the aux is at the level of the salon floor. But how does the air get in the system? The clamps are tight, the O-ring on the pump is greased, the pump seals are not leaking..... I am mystified. Is there some way I can prove this is the cause? And, more importantly, is there anything I can do about it?

 

[Benny17441]

Irrespective of the cause, the end result is that the generator has proven to be rougher on impellers than the auxiliary engine. My solution would be to shorten the interval of time between impeller inspection/ replacements for the generator. Perhaps try doing it at 100 hours might help. It is not like the impeller is breaking right away pointing a serious mechanical fault but more to a wear issue. You may try an impeller from a different manufacturer to see if it wears any better. I tend to agree with the loss of pump prime when the engine is turned off as the likely culprit but do not think is something that could be improved upon without major changes.

 

[capta]

Are you using proper suction hoses? A softer hose will narrow as the pump sucks water through it. Even a short length of softer clear hose, possibly added to see water in the line, will cause a rapid burn out of impellers.

 

[Rich Stidger]

Are you using proper suction hoses? A softer hose will narrow as the pump sucks water through it. Even a short length of softer clear hose, possibly added to see water in the line, will cause a rapid burn out of impellers.

Yup. All of the hoses are wire-reinforced water hose. All are in good shape- no softness

 

[rardiH36]

I seem to recall from many years ago a discussion about certain impeller brands failing prematurely. Did you by chance obtain several years of inventory all in one purchase? If an old or bad manufacturing lot, that might also be a cause. Maybe for your next purchases, try a different manufacturer if you can find a cross reference. Just a thought.

 

[woodster]

ol desert peet use to say ya have to prime the pump and you must have faith and believe

 

[woodster]

Well that is an explanation that never occurred to me. My generator is at the level of my cockpit while the aux is at the level of the salon floor. But how does the air get in the system? The clamps are tight, the O-ring on the pump is greased, the pump seals are not leaking..... I am mystified. Is there some way I can prove this is the cause? And, more importantly, is there anything I can do about it?

yes you can mount a large bronze groco strainer 1 in or 1 and 1/4 in up on the same level as your gen set and when the gen set is off immediately close the sea cock and while you are spending all this money had a ball valve in between the strainer and your seacock at the strainer and just shut it off it should hold the water in the strainer till you start her up the next time

 

[Stu Jackson]

ol desert peet use to say ya have to prime the pump and you must have faith and believe

you've got to give of yourself, if you're worthy to believe...

 

[woodster]

you've got to give of yourself, if you're worthy to believe...

you know them guys too lol

 

[Jerry Clark H356 SV Persi]

Rich: Look into a Speed Seal Life. The have a Teflon washer that allows the impeller to run with much less friction on a slotted brass plate. They claim they will run 10 minutes dry befor failure. I know they will, because once I changed mine on my Northern Lights 5kw generator and forgot to reopen the thru hull. 10 minutes later, it shut itself down!

Since putting on both my Yanmar 3GM30F and the Northerm Lights I haven't experienced blade separation, where I usually did on both before with sometimes 100 hours. A couple of times, I have looked at the impeller and left it in. The life has been extended fo me. I now go 200 hours before change out.

 

[bigpapaporsche]

I would suggest that you temporarily replace the suction/inlet hose to the WP with a Clear HD hose of the same diameter. Then start the motor and watch for any air inside the hose. I would think you might see some initially bubbles, but after a few seconds it should clear up. If there is an air leak you will see a continual stream of air bubbles coming from downstream. Also when you stop the motor, watch the water in the hose. Does it drain back from the source or stay in the hose? However, I can't imagine that any air, unless it extremely heavy, would cause a premature failure, without also causing over heating issues. Is the Cooling System a "Closed System" or a Raw Water System?

 

[Gunni]

Jerry, I agree the Speedseal Life is a great product and will protect an impeller from the heat and failure caused by friction with the cover plate BUT looking at Rich's photos I see none of the tell-tale heat stress cracking from that kind of failure. There do appear to be cracks at the base of each impeller blade, like it is flexing too much. Mystery to me. Maybe try those Globe urethane impellers...

 

[Rich Stidger]

I seem to recall from many years ago a discussion about certain impeller brands failing prematurely. Did you by chance obtain several years of inventory all in one purchase? If an old or bad manufacturing lot, that might also be a cause. Maybe for your next purchases, try a different manufacturer if you can find a cross reference. Just a thought.

This problem of broken vanes has always plagued me. Over 18 years of use, I have used genuine Westerbeke impellers (they are Johnson 810B-1 BTW), Johnson brand impellers, and Jabsco brand impellers. I have not seen any of these brands outperform the others. I once thought that the problem might be an old stock impeller since I buy 3-5 at a time, but I discovered that at the time the problem was a bad bearing that allowed the shaft to wobble ever so slightly.

 

[Rich Stidger]

I would suggest that you temporarily replace the suction/inlet hose to the WP with a Clear HD hose of the same diameter. Then start the motor and watch for any air inside the hose. I would think you might see some initially bubbles, but after a few seconds it should clear up. If there is an air leak you will see a continual stream of air bubbles coming from downstream. Also when you stop the motor, watch the water in the hose. Does it drain back from the source or stay in the hose? However, I can't imagine that any air, unless it extremely heavy, would cause a premature failure, without also causing over heating issues. Is the Cooling System a "Closed System" or a Raw Water System?

I can do that next year and check for a stream of air bubbles and drain-back. The cooling system is a closed loop with antifreeze, but the impeller is in the raw water path.