Pump Cavitation

[Solutions Guy]

Hi, would appreciate any info on a rather strange problem one of my customers has. The pump cavitates, or in other words, gets inundated with air. This was the situation as of last night. Initial problem was upstairs rads quite warm, downstairs cold. 1st thoughts would be pump, right? Right! Changed pump, worked well for a few days then gradually went back to pre new pump. I'm thinking air lock now, so fiddle with the pump and rads for an hour or so to release the air lock. Things better but not as expected. Hmm! (Scratch head), fully drain system and fill again. System drains like a dream and fills even better. Hmm! Hmm! (More head scratching) Can't be airlocking if the system fills that quickly. Anyway, I run the system cold until the pump is running with a gentle hum. Fire up the boiler, primary flow heats up PDQ and the pump starts to cavitate. Right now you're thinking kettling, right? Wrong! Boiler is running whisper quiet. So now I'm thinking maybe need to power flush as there could be a blockage on the primary return, but it doesn't seem to fit the bill. Any suggestions would be most welcome

 

[kevplumb]

keep bleeding the pump may just be a stubborn air pocket
check the f&e pipework

 

[Solutions Guy]

Bleeding of the pump was done when I went back after fitting new. Re: stubborn air pocket - wanted to make sure all air was out of the system hence running it cold. Pump is in direction of flow, problem only seems to occur when system is heating up. Thought about cf & v, but system would have drained and filled a lot slower than it did. Also, F&E tank emptied ok (eliminate cold feed). Partially blocked vent a possibility. but system has a standard "H" design 150mm centres, so pump is pulling equally on cf & vent. Thanks for reply.

 

[kevplumb]

could it be an underfloor dead leg full of air ?

 

[Solutions Guy]

Possible. Hopefully when I go back to power flush it, it should resolve it. I dont like the idea of flogging a dead horse or barking up the wrong tree. If power flushing doesn't resolve it, I may have to look at converting to a sealed system. Customers says the system has never been overly fantastic. It may even turn out to be something obscure likne an old flux brush someone put in a pipe when the house was being built, just for a laugh. Hmm! Yeah! Got me with that one, huh? lol.

 

[brumylad]

It might not be air causing the noise, it might be debris?

see if a magnet will stick to the pipe work in any of the usual debris collecting places (I presume its copper pipework)

Keep us updated if you have the time

 

[PEDANTICVINDICTIVEMAN]

Are you sure it's not pulling air in around the pump unions (mini leak).

Stick a Myson airjec in, that will get rid of any air.

Had a similar problem with a floor mounted Glowwworm boiler, I fitted all new rads and t/valves, no change to pipework, fully pumped system, only a short run to the cylinder in airing cupboard 1.5 metres, blooddy air went round and round and round and round, spent a whole morning fitting air vents to top of boiler kept venting and venting and venting, eventually left it to it's own devices and it cleared after about 3 weeks, the customer admitted that the system had been in 20 years and never drained and it could have been like that when first fitted but couldn't remember, thing is there was no logical explanantion for alll the air or why it eventually went on it's own

 

[Andy Velcro]

Hello

I'm no plumber but have had experience of pumps, all be it large ones in industry. Cavitation in my field is when a pump trys to pull water through it which isn't there ie low flow on the inlet side. This causes low pressure within the pump. When water is placed in a low pressure enviroment it vapourises i.e boils at room temperature creating bubbles. These bubbles are then accelerated along the pump vane imploding at the vane tip which gives you the sound of gravel running through your system. If you haven't got an airlock then perhaps your pump is pulling more water than is there. Like I said I'm no plumber!!!!!!!!

Andy

 

[wilhelm]

dont matter what you are, thats quite informative. i like it.

 

[ChrisR]

Yer that's why you shouldn't have hot water going into a shower pump too hot. It's often said that 65 is the limit, though I don't think there's any critical temperature.

Can you raise the header tank, which would increase the pressure and raise the boiling point a bit. What's the static head now?

 

[croydoncorgi]

Although 'cavitation' is frequently used to describe pump noise, it's actually quite unlikely on low-speed circulator pumps - the impeller ain't going fast enough to create the right conditions. Of course if there is a lot of air dissolved in the water, it will come out of solution in low-pressure areas in the pump and 'cavitate'. Mostly, though, the air is already in the form of bubbles entering the pump.