# canister filter flow restriction - bad?



## Hoppy (Dec 24, 2005)

Flow restrictions on the outlet side of a canister filter are no problem, but the same flow restrictions on the inlet side can cause cavitation in the filter pump.


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## oldpunk78 (Nov 1, 2008)

I'm planning on running a few things in-line on the out flow side. Would that off set any cavitation issues?


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## andrews02r (Nov 16, 2010)

I think Hoppy already answered that question.


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## oldpunk78 (Nov 1, 2008)

well... it's only going to pull in what it can push out, right?


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## VeeSe (Apr 16, 2011)

oldpunk78 said:


> well... it's only going to pull in what it can push out, right?


or it can push out the same amount with more pressure in smaller diameter hose ("feels" like it's coming out harder if you put your hand in front of it, etc.)


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## m00se (Jan 8, 2011)

http://www.mcnallyinstitute.com/01-html/1-3.html

Cavitation is pretty well understood - when you subject a liquid to less than it's STP it will "boil" or outgas. This is the essence of cavitation.


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## Chlorophile (Aug 2, 2011)

oldpunk78 said:


> well... it's only going to pull in what it can push out, right?


I'm not sure if its actually possibly, but if the inflow is restricted the pump might be capable of pumping out more than that can fill the canister in a given time, and it would make a vacuum resulting in an air bubble. The air bubble would collapse and fill with water very quickly but it would cause a lot of damage to the rotors. 

Search "Time Warp - Bottle" on YouTube for a visual representation. 
In that scenario the bottle moves away from the liquid with enough force to create an air bubble at the bottom of the bottle, when water rushes back into the bubble it is somewhat of an explosion, shattering the bottle. (That is my take on it at least)


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## Hoppy (Dec 24, 2005)

Canister filters don't "pull in" water. They are fed by siphon action, which is why they are supposed to be mounted below the tank. The pump doesn't generate much pressure at all. All the pressure it needs to generate is enough pressure to raise the water about 2-3 inches over the rim of the tank. The pump is a "water mover" not a pressure generator.


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## oldpunk78 (Nov 1, 2008)

Thanks guys, I just couldn't get mu head around that one, lol.

I really appreciate the explanations. However, no one directly answered my question. Have any of you successfully run 13mm pipes with a xp3?


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## jasonpatterson (Apr 15, 2011)

Hoppy said:


> Canister filters don't "pull in" water. They are fed by siphon action, which is why they are supposed to be mounted below the tank.


I dunno about the siphon. If it's a closed system drawing water from the tank and returning water to the tank it shouldn't siphon at all, as the inlet and outlet are at the same height (or more likely the outlet is slightly higher.) Unless you mean that it will siphon until the canister fills and the outlet hose fills with water up to the water level of the tank. 

It really shouldn't matter where you put the canister, as long as the seal on it is strong enough to hold the water pressure created by its placement (the reason that those crappy zoomed filters recommend that you put them at water level, and the reason that they leak so often when people don't.)

Agreed entirely about the remainder though.


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## xmas_one (Feb 5, 2010)

VeeSe said:


> or it can push out the same amount with more pressure in smaller diameter hose ("feels" like it's coming out harder if you put your hand in front of it, etc.)


it's actually less pressure when you have the same flow with an increase in velocity..


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## VeeSe (Apr 16, 2011)

xmas_one said:


> it's actually less pressure when you have the same flow with an increase in velocity..


oops. you're right.


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## Chlorophile (Aug 2, 2011)

jasonpatterson said:


> I dunno about the siphon. If it's a closed system drawing water from the tank and returning water to the tank it shouldn't siphon at all, as the inlet and outlet are at the same height (or more likely the outlet is slightly higher.) Unless you mean that it will siphon until the canister fills and the outlet hose fills with water up to the water level of the tank.
> 
> It really shouldn't matter where you put the canister, as long as the seal on it is strong enough to hold the water pressure created by its placement (the reason that those crappy zoomed filters recommend that you put them at water level, and the reason that they leak so often when people don't.)
> 
> Agreed entirely about the remainder though.


What if you put the canister above the tank, would you have the same flow rate? I'm positive the siphon action is helping the pump be more powerful, so having the canister above the tank _should_ make things harder for the pump since gravity is trying to take water out of the canister instead of push water into the canister. Although its would probably still run fine, it would be hard to prime!
I agree though that level or below the filter shouldn't make a difference. Inflow water is falling into the canister, pushing outflow water out to the tank - they will reach equilibrium without a pump, but with the pump the water is pushed into the tank. 
I imagine if the outflow was higher than the inflow, we would see a reduction in flow, actually I'm fairly positive we would see a reduction in flow.


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