Pumps and Cavitation......

[MadDogMe]

If you're getting Cavitation/NPSH you will be getting airbubbles(small) in the system/tubing, this is what made me think I had it. you might not see them until you stop the pump and tap/knock it, then they rise up the tube, why they 'stick' in the pump making noise and not just flowing straight out I don't know, the noise is a very iritating(high pitch?) scratchy~trickling like noise, it got worse when I had slight pressure on the inlet~housing/tube, if you hav'nt then it's wear or a loose impellor...

If it has'nt always made this noise and nothings been changed with the system, then you can gaurentee it's not inlet restriction. impellor noise maybe(glue it if it spins), shaft is worn, or a leak are the most probable IMO, strip the pump down and clean it, the O~ring and mateing surface especialy, look for wear on the shaft (if not ceramic) or the impellor housing shaft/magnet assembly that rubs against the shaft, also check the rubber 'grommets' that hold the shaft in place for degradation if additives have been used...

Whatever happens you got to strip it down to have a look see ...

[myv65]

A lot of the inline pumps I've seen have a threaded inlet that will accept either a male fitting or a hose over the outer diameter of the inlet. Using a fitting always reduces the cross sectional area at the suction, which is generally a bad thing to do.

If you are absolutely convinced you've got cavitation, there are a few things you can do. First is per BillA's suggestion and that's to reduce the resistance at the inlet. If you've got a fitting in there and a larger hose with a clamp could do the job instead, swap it. The other two things that come to mind are not as "nice". One is to reduce the flow rate by throttling the outlet. This has the effect of reducing intake velocity and at some point the suction pressure will be higher than the NPSH required for water. The other is to elevate the pressure in the entire system.

For a given flow rate, you will have a fixed differential pressure across the pump. It's possible to artificially elevate the pressure everywhere in the system by any offset you wish (within the pressure limitations of your system). All that's required is a static column of water with a pressurized air pocket at the top. Engineering sorts would label this an accumulator. By controlling the pressure of the air pocket you add extra pressure everywhere in the system. This obviously works only for those with completely sealed systems. You'll still get some variation as the pressure will vary slightly with temperature. You'll get a whole lotta variation if your system isn't 100% sealed. Heck, even if it is polymer tubing tends to have some slight permeability to gases so you'll lose pressure over the long term.

[Since87]

Originally posted by myv65
It's possible to artificially elevate the pressure everywhere in the system by any offset you wish (within the pressure limitations of your system).

Would connecting the pump intake, to a tall reservoir (say 2" PVC from the pump inlet to four foot above the pump inlet) provide the elevated pressure without the need for sealing?

[myv65]

It's all a question of magnitude. You get ~ one atmosphere of pressure compliments of Mother Earth. This is approximately 33 feet or ten meters of water. If you add an open-topped column a couple of feet high, you're adding less than 10% above and beyond atmospheric. If you're borderline, that may do it. If you've got room for four feet, now you're around 12% of atmospheric. Note that the diameter of the pipe is immaterial (other than having strength to support itself).

[BillA]

be prudent fellows

I think the FAA requires a flashing light if its over 100 ft high

[Since87]

Well, I was thinking of taking some 1/4" tubing and a funnel and climbing to the top of a tree. (The funnel is so that rainwater can keep my pressure source full.)

[bigben2k]

That seems somewhat extreme. Why don't you just seal your loop, and add compressed air in your res? (BTW, that's a good leak test too!).

I put together an air inlet: a barbed tee, with an air tire valve goop'ed in one of the openings. Originally meant for pressure testing only. A buck and a half, and much smaller.

[Since87]

Twas a joke BB.

[myv65]

Originally posted by Since87
Twas a joke BB.

Obviously. If you were serious, you'd have included a heater to convert the snow to water. FAA, heck the guy is in Indiana. I think they need a letter from the governor for anything over 100' including hills. You can darn near see Louiseville from Gary.

[jtroutma]

Just now thinking of it.....I may just have a slight air leak somewhere. Either that or more proof that my pump is cavitating. I seem to recall that my coolant is actaully a bit "merky" of sorts with a white haze. Thought it was just from filling the system and the WW reacting but it has yet to truely go away.

I will try pinching BOTH the inlet and the outlet tonight and see if I can hear any differences (of course not both at the same time )

Also I have taken that pump apart several times looking for anything that might be causing it to be so noisy. Impeller looks great (no wear at all), shaft is clean, gromits are nice and fit wonderfully (can hardly get the shaft out of them sometimes), and the blades are solid on the impeller (AKA no slack between blades and magnet).

I think that potentialy the largest size tubing that I can go up to is 5/8" MAYBE 3/4" for the inlet of the pump and I can get rid of the elbow between the res. and pump. I just figured that gravity was going to be doing most of the work getting the water to the pump so it really didnt matter. Guess I got a stronger pump than I thought

Ok thanks for all the ideas. Hopefully soon I can have this thing working correctly and not keeping me up at night :shrug: :shrug:

[jtroutma]

WEll I just tried pinching off the lines and here are the results:

Outlet line closed off; no change, actually got a little bit louder (higher pitch)

Inlet Line closed off; a bit quieter but could hear that the impeller was NOT happy.

Both lines closed

But now looking at the system again, there is far too many air bubbles in the WHOLE loop. My water is now WHITE there is soo much air in the loop. There has to be a leak some where. I think I even know where it is too.

This Sedra does not have a screw down inlet housing. It has a latch type setup where you twist the housing CCW to release two latches and then pull apart. There is an O-Ring that is suppose to keep a seal between the parts but I suspect that it is not keeping a very tight seal.

What can I do to help seal off that part of the pump? Vasoline/Petroleum Jelly? Teflon?

Next time I take the pump apart I will also redo all the fittings on the pump inlet with more teflon to make sure that they are not leaking as well.

I guess I should also bite the bullet and get some new fittings and 5/8" vinel to put on the intel side too

[MadDogMe]

Exactly the same place as mine!, do you have any pressure on the tube there?, 'levering' it up/down?, however slightly!. I did'nt think I had, but I did'nt dismantle it, just moved it around abit and it went away :shrug: . check where the barb screws into the housing as well. might want to check the O~ring for 'nicks' or grazes...
I would'nt put anything on the O~ring, if anything I'd degrease it and the housing with some isopropal alchohol...

[myv65]

Originally posted by jtroutma
Outlet line closed off; no change, actually got a little bit louder (higher pitch)

Just figured I'd mention that this is normal (for both fans and pumps). Load increases with flow rate. Speed decreases with load. By closing the outlet you actually drop the motor's output energy resulting in a small speed increase. You'd find exactly the same result if you covered the inlet or outlet of a fan.

[UnloadeD]

I bet the blue/green you are talking about is oxidation from the copper in the heater core. Be sure and get it all cleaned out while you have your system apart as its probably pretty abrasive. I doubt it has anything to do with your noisy pump, but it'll cause extra wear&tear on everything. Also dont use any petroleum based products (vaseline) on your orings, it will cause them to degrade. One final thing, when you replace the tubing on your intake side, try to get something thick walled, I've seen some collapsed flat from the suction 8/

peace.
unloaded

[jtroutma]

Damnit! Now I KNOW that I have a leak somewhere.....My coolant looks like MILK!!!!!!!!

Hopefully I will be able to take the system down today and get the leak fixed.

Intresting though....with soo much air in the water, the pump actually has gotten a bit quieter :shrug:

[MadDogMe]

Just figured I'd mention that this is normal (for both fans and pumps). Load increases with flow rate. Speed decreases with load. By closing the outlet you actually drop the motor's output energy resulting in a small speed increase. You'd find exactly the same result if you covered the inlet or outlet of a fan.

I seem to remember an argument going on at OC'ers about whether pumps draw more currant(or producing heat maybe?) when underload(pressure) or running free involving Since87 ...