Circulator Pump Cavitation


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Old 11-04-12, 10:13 AM
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Circulator Pump Cavitation

Hello Everyone,
I've got an outdoor wood furnace that's 150 feet away from the house and the brand new circulator pump in the basement isn't running as quietly as it should. This pump moves water okay, but it also makes moderate cavitation sounds while it operates, even after repeated power purging. There's definitely no air bubbles in the lines.

I believe the cavitation is caused by the fact that the pump is at an elevation that's about 24" above the top of the boiler's water tank (I checked it with a transit), causing the pump to create too much suction at its inlet port. As with most outdoor boilers, this is an unpressurized system, so I can't overcome cavitation by increasing system pressure.

My plan for solving this problem is to install a second circulator pump in the pipe leading from the boiler, as far below water level height as I can, to maximize head pressure at the inlet port. My plan is to us a Grundfos 15-55 Alpha pump for this application. My hope is that by pushing boiler water from the boiler end, it will allow the pump in my basement to operate quietly, as it should.

Is my diagnosis of the cause of cavitation correct? Will my addition of the 15-55 Alphas solve the problem?

Thanks very much for your help! I'm very grateful.

Steve
 
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Old 11-04-12, 11:33 AM
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What you are proposing is adding a booster pump. Should work.

Another approach would be to replace your new pump with one that has a lower net positive suction head required (NPSH). Also, do you have at least 10 diameters of straight suction piping ahead of the pump? Any less can lead to cavitation.

Of course, you need to eliminate the cavitation - besides the noise, it can damage the pump's impeller.
 
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Old 11-04-12, 12:37 PM
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Cavitation

That is quite a long run 125 x2. Is the pipe diameter big enough?
If the pump is at the highest point, all the power purging might not get all the air out. You need a bleeder at the high point. You might be able to loosen one of the flanges just enough to see if there is any air in there.
Sid
 
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Old 11-04-12, 02:06 PM
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Thanks for your confirmation of my plan. There are at least 10 diameters of straight pipe before the pump, so I think I'm good that way. My proposed position for the new pump is pretty well the lowest point in the system, besides being the easiest place to add a pump.
Thanks again,
Steve
 
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Old 11-04-12, 02:12 PM
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Hello Sidny,
Thanks for your insights. The pipes are a full 1 inch diameter inside, so I think they're big enough. The current pump (the one that's cavitating) is at the highest point in the system, but I've done my purging from the lowest point in the system (at the outdoor boiler), with a pretty big pump pushing water through until everything runs clear. This method did quickly solve an air lock problem at start-up, so I don't think the problem is air in the lines. I think it's just that the current circ pump is having to suck too hard, vapourizing boiler water at the pump's inlet port, with those vapourized bubbles collapsing at the impeller and making the noise.
Thanks again,
Steve
 
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Old 11-05-12, 10:44 AM
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Cavitation

Is any or all the piping below ground? And if so do you have a provision for freezing, such as antifreeze or insulation ? Just asking.
Sid
 
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Old 11-06-12, 12:11 PM
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My pipes are Logstor insulated units from Denmark, buried about a foot or so below ground.

Just to close the loop on my original question, I went ahead and installed a Grundfos 15-5 pump out at the boiler and though it reduced the problem, it didn't eliminate the cavitation completely. I swapped that 15-55 for a 26-99 I had inside feeding my DHW tank, and all is well. All pumps are now whisper-quiet.

Thanks, everyone, for you suggestions.

Bye,

Steve
 
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Old 11-14-12, 10:34 PM
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Hello Steve,

I'm new to the forum and this if off subject for this thread but I had a question for you. I googled "sidearm heat exchanger installation" and saw a thread you started about a stainless steel double wall sidearm heat exchanger that you were having problems with. I read your post and it is just about exactly what I could have written about mine. I installed the same kind of sidearm, double wall stainless, and am having about the same results as you did. Did you ever get it dialed in and able to work for you? I'm about at the point I just don't think the dang thing is efficient enough for my set up I'm about ready to order a small 10 plate brazed heat exchanger which I know is way more efficient.

Thanks,
Bruce
 
 

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