Aquarium Hydraulics for Dummies - The Planted Tank Forum
 
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post #1 of 15 (permalink) Old 11-15-2007, 09:01 PM Thread Starter
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Aquarium Hydraulics for Dummies

There seems to be a lot of discussion about this all over the place. I thought I would state some facts as I have learned them studing as a civil engineer. I design bridge for a living now but the concepts are easy enough to remember.

The best way to describe how a pump works is to think about pressure. I pump doesn't push or pull water. All it does is add pressure to the system the pressure does all the work. When you see pumps rated by head all that is really showing is how much pressure a pump can add to the system. When a pump is at its max head there is no flow because the pump can not add enough pressure to move the water.

In a closed system you really don't need a lot of pump head because gravity is already doing 99% of the work for you. It is pushing the water through the siphon, the filter, the pump and all of the pipe. If you leave the pump off the water will go up the return tube until it reaches the water level in the tank that it came from.

The pump only needs to add enough pressure so the water can make it over the lip of the tank. The more you increase the pressure beyond this the greater the flow will be. When ever you have moving water there is always pressure loss from the friction of the water moving inside the pipe and traveling through various turns and bends. Let’s say that a system is setup up with zero losses, not realistic, the plumbing is full of water and the water level in the tank is 1” from the top of the lip. The pump would only have to add enough pressure to over come that 1” to create flow.

If you were to put the pump before the filter you would be adding additional pressure to the canister. As pressure increase so does flow. As flow increase so does loss from friction. By adding pressure to the system before the canister you and creating a bigger pressure loss through filter. Now let’s take that same setup as before but now there is 25” pressure loss through the filter because it got dirty. Now instead of needing 1” of pressure to create flow we need 26” of pressure. The more pressure a pump has to fight the lower the flow.

With the pump after the canister the only real restriction to the pump would be how fast can the water flow through the siphon and the canister before it gets to the pump. In most cases the capacity of the pipe is well beyond what is needed so the filter is what controls flow to the pump. As the filter gets dirty and its flow capacity decreases, it doesn’t matter how big of a pump you have it will only pump water back into the tank as fast as it can get it from the filter. It still only needs to add 1” of pressure to create that flow though.

Either way your flow redecues as the filter gets dirty. The most efficient way is to let gravity do as much work as possible then add the pump as close to the end as possible. This reduces the amount of pressure that the pump sees. In the end you end up with a smaller more efficient pump if it is placed after the filter instead of before. Open system like sumps are a whole different story.

I am guessing that is why on most commercial canister filter the pump is after all the media. They get away with using a small pump then if they place it before. Smaller pumps are cheaper and that means either cheaper prices for us and or more profit for them.

That being said there is one thing that you can do when the pump is before the filter that you can’t do if it is placed after. If you put a pressure gage between the pump and the filter the pressure gage will begin to rise as the filter gets dirty. It will work like a indicator to tell you when it is time to clean out your filter.
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post #2 of 15 (permalink) Old 11-15-2007, 09:31 PM
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Thanks for posting this, I freely admit that I am a dummy when it comes to hydraulics, and learning something new is always nice.

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I desgin brdige for a living now...
That does sound a bit dangerous... Sorry, couldn't resist.

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In a closed system you really don't need a lot of pump head because gravity is already doing 99% of the work for you. It is pushing the water through the siphon, the filter, the pump and all of the pipe. If you leave the pump off the water will go up the return tube until it reaches the water level in the tank that it came from.

The pump only needs to add enough pressure so the water can make it over the lip of the tank.
Let's just assume that the pump made it that far and now we have a completely closed circuit. So if the pump is off no air will enter the tubing anywhere, and there is no head pressure present whatsoever.

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Either way your flow redecues as the filter gets dirty. The most efficient way is to let gravity do as much work as possible then add the pump as close to the end as possible. This reduces the amount of pressure that the pump sees. In the end you end up with a smaller more efficient pump if it is placed after the filter instead of before.
This is where I need to leave all my common sense behind. Because my common sense tells me that gravity doesn't play a role here. Friction, yes. But the water going down the inlet has to go up the outlet to exactly the same height. That's why I have a hard time believing that the pump sees a different amount of pressure depending on its placement. No matter if it is pumping on the end of beginning of the tubing, the restriction (filter media) is the same.

It's just a gutfeel though... haven't studied this in depth.


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post #3 of 15 (permalink) Old 11-15-2007, 09:32 PM
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Hmm. Wouldn't having restriction prior to the pump reduce the dynamic pressure on the intake side of the pump? I really appreciate your willingness to discuss this, 'cause I consider myself a logical, physically minded, educated type person, but this topic bends my noodle.
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post #4 of 15 (permalink) Old 11-15-2007, 10:09 PM Thread Starter
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Opps, I fixed the typo. I am not the best person on the keyboard and spell checker, which I didn't use, has ruined my spelling. Math is my language anyway.

Okay, try looking at it like this.

Say you have everything setup and you decide that you have too much flow. To decrease the flow what would you do? You would add a valve of some kind to the system. Now where would you put the valve? I would put it after the pump. As you start to close the valve it increases the pressure that the pump sees and decreases the flow. With the pump before the filter you can look at a filter that is slowly getting dirty like a valve that is slowly closing and increasing the pressure. With the pump after the filter it is only restricting flow, like you are slowly decreasing the diameter of the pipe. The smaller the pipe is the smaller the flow capacity at a certain pressure. The pressure stays the same because the only thing contributing to it is gravity.

Example time:

You take two rubber hoses of different diameters. One is 1" and the other is 1/2". You want to siphon 1 gallon of water out of a tank. Which hose is going to do this faster? The 1" diameter hose is going to be the winner. They are coming from the same tank and going to the same ground level so the pressure is going to be the same in both hoses. The only reason the 1" hose is going to be faster is because there is more volume then in the 1/2" tube.
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post #5 of 15 (permalink) Old 11-16-2007, 12:12 AM
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This reference seems like it could be considered authoritative. I need to read it again, but I believe it says that there's no difference in flow rates regardless of where the restriction is placed. The only consideration with inlet vs. output restriction is cavitation. That, and as you said, the ability to use a pressure gauge where you'd like to. Also whether you have clean or dirty water passing through your pump. Oh yeah, and whether your canister filter is pressurized or depressurized relative to the static head. Depressurized would reduce the chance of my homemade contraption leaking.

Dunno why I didn't find that source last time I was looking for this info...
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post #6 of 15 (permalink) Old 11-16-2007, 03:40 PM Thread Starter
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You are right that there is no difference in flow rates. The difference comes in the size of the pump needed.
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post #7 of 15 (permalink) Old 11-16-2007, 05:35 PM
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It's still hard to grasp for a simple mind like mine. While it is good, I don't see how your example applies to what we are discussing. Your example, gravity pulling down the water, not applicable to closed system canister filters.

Here is an different example. Let's say we have a pump, connected via 1/2 inch hoses to a planted tank. Say we have 6ft of that 1/2" hose, and the pump is somewhere in the middle of it, below the tank for easier priming.

Prime it, turn on the pump, water flows into the hose, through the pump, out of the hose. Say it gives us 300 g/hour flow. Turn it off - no water will flow, equilibrium, etc.

Scenario 1 - move the pump around. Closer to the inlet, closer to the outlet. Will it make any difference? (I think not)

Scenario 2 - let's insert a little piece of 1/8" hose. With the restriction, the gal/hour rate will go down. Will there be a difference in flowrate, and how the pump "sees" the restriction, depending on where it is placed?


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post #8 of 15 (permalink) Old 11-17-2007, 01:13 AM
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I'll take a shot at it:

Scenario 1 - It's not how much hose you have, it's the vertical difference in the location of the pump relative to the outlet, the head. If you have a pump that can pump 300 GPH at 3 feet of head through 1/2 inch tubing, it can pump even more (Say 350 GPH) at 2 feet of head, and less (say 150 GPH) at 4 feet of head. This is the effect of gravity, not the pump or tubing.

Scenario 2 - if the restriction is on the intake you risk putting the pump through cavitation stress. If you put the restriction on the output you just increase the effective head and slow down the rate of water flow. Why putting the restriction on the intake is bad for the pump, it's not that you won't see the same amount of reduction in flow, it's that you put extra stress on the pump by causing the cavitation.

My original statement back in the other thread was that the pump does not react well to having the restriction on the intake. Theoretically showing that the same amount of water flows in either case doesn't change the fact that in one situation the pump will burn out on you faster.

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post #9 of 15 (permalink) Old 11-17-2007, 02:03 AM
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if you want more math you could look up the bernoulli equation regarding fluid dynamics

this is a good info
http://physics.bu.edu/~duffy/py105/Bernoulli.html
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post #10 of 15 (permalink) Old 11-17-2007, 02:26 AM
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Cavitation ain't going to happen with aquarium temperatures and pressures.
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post #11 of 15 (permalink) Old 11-17-2007, 04:55 AM
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Scenario 1 - It's not how much hose you have, it's the vertical difference in the location of the pump relative to the outlet, the head. If you have a pump that can pump 300 GPH at 3 feet of head through 1/2 inch tubing, it can pump even more (Say 350 GPH) at 2 feet of head, and less (say 150 GPH) at 4 feet of head. This is the effect of gravity, not the pump or tubing.
That's not what I meant... both in and out of the hose are in the same tank. No head pressure. Just friction.


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post #12 of 15 (permalink) Old 11-17-2007, 04:11 PM
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It doesn't matter if the inlet and outlet are at the same level, you still have to provide lift to push the water uphill. There is head pressure.

Lets say the pump is even with the surface of the water, and as you say friction is the only factor. You would need a lot of hose to provide enough friction to slow down the water flow. If you started adding elbows then you would add turbulence to the flow which has a much greater impact that friction from the hose.

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post #13 of 15 (permalink) Old 11-17-2007, 06:44 PM
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Quote:
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Let's just assume that the pump made it that far and now we have a completely closed circuit. So if the pump is off no air will enter the tubing anywhere, and there is no head pressure present whatsoever.

This is where I need to leave all my common sense behind. Because my common sense tells me that gravity doesn't play a role here. Friction, yes. But the water going down the inlet has to go up the outlet to exactly the same height. That's why I have a hard time believing that the pump sees a different amount of pressure depending on its placement. No matter if it is pumping on the end of beginning of the tubing, the restriction (filter media) is the same.
Break the system down to static head (pushing against gravity) and dynamic head (resistance to flow.) If there is no "head" there is no flow ie a canister with no power.

My understanding of centrifugal pumps is that the "blow" better than they "suck" so you would want to restrict the outflow. Perhaps this the the root of the cavitation thread already in progress?

Moved to Tucson.
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post #14 of 15 (permalink) Old 11-17-2007, 08:38 PM
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Cavitation ain't going to happen with aquarium temperatures and pressures.
Cavitation will occur, because water has gases dissolved in it. The concept of vapor pressure that you read on your linked resource* is replaced with Henry's law and the dissolved gases will come out of solution at pressures well above the vapor pressure of water. The reduction of flow, and hence pressure, to the impeller will lead to the frothing of the water in the pump chamber, this is cavitation.

*see SUCTION THROTTLING.

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post #15 of 15 (permalink) Old 11-25-2007, 10:07 PM
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OH so true.....laws of physics with gravity can not be pushed aside. Anything being elevated will develope a certain amount of gravity weight being ascerted against the instrument used to lift the load.

Simple example: a rope, trough a pully, 10' off the ground, lifting 100lbs.
The lifting tool is your hands, back and arms. With only 1 pully on the rope, you are going to lift 100lbs of dead weight.

Now, add two pullies with four grooves on each pully. You thread the rope around the 4 grooves and tie the loose end to the bottom of the upper pully. You have now just made you lifting device 4 times the ability to lift the 100lbs. Which in turn, you will only excert 25lbs of force to lift the 100lbs 10 feet off the ground.

This very same principle is applied to fluid dynamics in moving volumes of fluid up hill. Drawing the fluid has very little effect on the pump being used. Your actual pressure differential is 3:1 in force being excerted by the pump. Your pump will experience 3 times the force lifting the water, compared to the force used in drawing the water from out of the tank.
Thus we have head pressure, for every 10 feet above or below sea level, you will experience 14.7lbs of berometric pressure, per square inch.
This is the law of gravity. Otherwise known as atmospheric pressure.
The higher up in elevation, the less pressure being ascerted, the lower you go at sea level, the greater the pressure will increase.

Maximo
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