flow rate through a 3/4" bulkhead

[witecap4u]

I know there is not exact amt., but what is the avg. flow rate. The only bulkhead I can fit is a 3/4, so I need to know what size pump to get. Is a mag 9.5 going to be too big. or should I try a mag 12. It will be a straight drop of about 1.5 feet to the wet/dry from the tank.
Thanks,
CS

 

[hairtrigger]

I am not sure about the exact flow rate. But I do know, you want high pressure in the return.... and low pressure in the overflow. So you can base it on that and find a pretty easy common-ground.

 

[broomer5]

witecap4u
Actually there IS an exact amount of water that will flow through a 3/4" bulkhead. Unfortunately - it's very hard to calculate without knowing EVERYTHING about the installation and set up.
Even then - calculating and actual real life flowrates rarely if ever match up exactly. But a 3/4" will flow an exact amount once it's installed. Exact for whatever increment of time you choose.
So ...... averages are used often as you mentioned.
Normally folks use rule of thumbs - which can be dangerous too.
A 1" bulkhead will normally flow 700 gph in most of our installations. That can go plus/minus 100 gph depending on installation/application.
Two 1" can handle 1400 gph
A single 2" bulkhead can handle more than two 1"
The reason is square inch area of the internal pipe. This is what contributes to how much water can flow inside it - along with the head pressure above plus the affects of gravity and air venting.
Square area of various pipe sizes ( pi r squared )
2" pipe = 3.14 square inches of area
1" pipe = .785 square inches of area
3/4" pipe = .44 square inches of area
A Mag12 is WAY TOO MUCH
It puts out 1100 gph @ 4'head, and 1150 gph @ 2'head
A Mag9.5 is still WAY TOO MUCH
It puts out 800 gph @ 4'head, and 900 gph @ 2'head
I would suggest using a Mag5 for your application.
You'll see around 310 gph @ 4'head, and 410 gph @ 2'head
I have dual 1" overflows in my 75 gallon reef tank.
Rating is 1400 gph total drainage.
I run a Mag7 and a Mag9.5 - and I wouldn't think of going any higher than this.
Get the Mag5 and use an oversized ball valve on the returnline. A ball valve that is 1-1/2 times the size of your returnline pipe I.D.
You'll need 2 PVC reducer adaptors for the valve too.
You'll be good to go then.

 

[hairtrigger]

Wow... Broomer... that's deep. Waaaaay over mah hed.

 

[broomer5]

HairTrigger - you can figure it out.
Let's say you're going to order pizza.
The pizza place has large 12" pizza and small 6" pizza.
A 12" cheese pizza is $10.00
A 6" small chees pizza is $5.00
You're really really hungry - and you only have $10.00 in your pocket today.
What should you do - order ONE 12" pizza or TWO 6" pizzas ???
The area of a circle ( how much actual pizza you get to eat ) is calculated by Pi x r squared.
Pi is a fixed number = 3.14
r = the radius of the pizza ( half the diameter )
So if a 12" pizza has a 12" diameter
Then half that = 6"
r = 6
6x6 = 36
pi = 3.14
so 3.14 x 36 = 113 square inches of pizza ( think about 113 little 1 inch squares of pizza to eat )
A 6" pizza has a 6 inch diameter
half that is 3"
So r = 3
3x3 = 9
pi = 3.14
so 3.14 x 9 = 28 square inches of pizza ( think about 28 little 1 inch squares of pizza to eat )
Take the 28 square inches times 2 ( cause you can order two $5.00 small pizzas )
That gives you 56 square inches of pizza.
One $10.00 large 12" pizza gives you 113 square inches of pizza no matter how you slice it.
Two $5.00 small 6" pizzas give you only 56 square inches of pizza no matter how you slice it.
You'd be much better off - being the hungry one that you are - to buy the single 12" pizza pie and spend your $10.00 wisely.
Same thing for the areas of pipes - and how much they can flow.
It's not a linear relationship.
A 12" pizza is more than two 6"
A 2" pipe flows more than two 1"
Now go eat your pizza

 

[hairtrigger]

Ok... I am following you now. That made it a little easier. It gets technical though. Math was never my strongpoint... I am more creative.
I can't wait till the next person asks about this... I can try my hand at helping them. You'd probably have to check my math though.

 

[witecap4u]

Well, that really wasnt what I wanted to hear, but I guess I have to live with it. I am also looking at about 5.5ft. head too. I have a pondmaster 5 in my pond, I think I will clean it up, and try that to see how it works. I also have a rio 3100 for my skimmer that I will try also. I am also going to keep looking for alternatives for the bulkhead fitting too, maybe I can drill this one out a little, the sidewalls are quite thick. Well, I guess I can look at it this way, I wont have to spend as much $$ on a pump
cs

 

[lordofthereef]

hey broomer5,
you forgot to add tax and tip.

 

[jagermeister]

If you want to try your hand at hydraulic engineering then here's the equation for an orifice flow, which is exactly what a submerged bulkhead is:
Q = CxAx(2gh)^(1/2)
where:
Q = flow rate in cubic feet per second
C = discharge coefficient. Using 0.75 for this value would be conservative. If you have a screen over your bulkhead I'd lower this value to around 0.6 or so.
A = cross sectional area of the bulkhead in feet squared. See Broomer5 explanations above.
g = acceleration due to gravity (32.2 feet/sec^2)
h = distance from the centerline of the bulkhead to the water surface. Also known as the hydraulic head. This value is in feet.
As Broomer5 mentioned, by using equations to calculate flow rates you're not going to come up with an exact answer that matches exactly what's happening in real life. The reason for this in this equation is the coefficient of discharge. No one knows exactly what this number should be and if you ask 10 different hydraulic engineers you're going to get 10 different answers (anywhere from 0.6 to 1.0). This value also varies given the real world conditions of the problem at hand. But, with this equation, you can closely ESTIMATE a flow rate that's pretty close to what's happening in real life.
You have to be careful about units. You'll notice in this equation everything is in feet and seconds and the flow rate is in cubic feet per second whereas everything in the aquarium industry uses gallons per hour.
An example:
Let's say you have an open bulkhead 1" in diameter. It's located in your aquarium/sump/refugium, whatever, so as the distance from the centerline of the bulkhead to the water surface is approximately 5 inches. Now to calculate the flow rate.
C = 0.75
A = 0.005454 feet squared. See Broomer5 post for equation for the area of a circle
g = 32.2 feet/sec^2 (This is a constant)
h = 0.4167 feet (5 inches divided by 12)
Q = 0.75x0.005454x(2x32.2x0.4167)^(1/2)
Q = 0.02119 cubic feet per second
Now we need to convert this flow rate back to something we're more familiar with.
1 cubic foot = 7.4805 gallons
1 hour = 3600 seconds
so multiply your answer by 26929.8 to get gal/hour
Q = 571 gallons per hour
This flow rate is going to change depending on how much head you have above the centerline of the bulkhead.
I hope this helps a little but I'm afraid I'm drastically confusing people. If this is the case then ignore this post and go with the averages that Brommer5 posted. That's much easier!
But if you're an aspiring engineer then go for it!!

 

[witecap4u]

Well, I am an Electronics Technician, not an engineer, so I follow the "put it together and see what happens, then fix it" philosophy.
So here is how it went.
First thing, I couldnt find anything at HD that would thread over the outside of the bulkhead(appears to be 1/1/8 or metric thread), just on the inside. Well, throw that thing to the side for now, I got a couple PVC fittings and basically made a bulkhead. The internal daimeter is about 15/16"(which is about the same as the bulkhead minus the threads) I put 2ft of 1/1/4 flex tubing off the bottom of the homemade bulkhead straight into the wetdry. I put the rio 3100 in the wet dry and connected 3/4ID tubing(about 8ft (5/1/2 total height)coiled coiled cause it doesnt bend/stay straight to well in the cold) and ran that back to the aquarium. The level in the overflow is staying just above the bulkhead, does this mean I can put more flow through it? Also, how much flow am I loosing having the tubing coiled up, since it isnt any higher than what it will be when finished.