View Full Version : A Hydralics "Word Problem" I'd Like To Ask An Engineer.

2010-Jun-08, 11:00 PM
I'm sure it's fairly simple if you have the equations. :)

One time, back in my professional fishkeeping days, I suffered a serious lapse of "critical thinking" during a moment of amazment.

The guys were working on quieter overflow/sump configurations for large coral reef tanks. I came in two hours earlier than everybody else to do the preopening tank inspections. 250 tanks, only 30 of which were twenty gallons or less. Which is why I had to come in two hours before opening everyday, to make sure everybody was doing okay. A lot easier to deal with untoward occurances if you are not up to your armpits in gawkers.

Now in the back, where the tinkering was done, I saw a marvelous set up. The guys had been working on it for days, but as I got off two hours earlier than closing time I didn't see them finally complete it.

When I turned the light's on I saw two 180 gallon aquariums, full of water but bare. The top tank had a three inch drain that ran from the bottom of the tank to about three inchs below the surface of the bottom tank, and the bottom tank had a inverted 2.5 inch "J" place so the crook was just below the surface, attached to one of our bigger pumps and matched the down flow so well that the only sound was a low rumble, mainly felt through the feet.

And as I took a closer look, what did I think was a good idea?

Well, stick my hand in there to see what the flow was like.

As Jed Clampett used to say, "Whew doggies!". The mistake became immediately apparent but fortunately I'm large and strong and the danger signal only had to go as far as the "lower spine response"*, which I guess is the reason it's there. But I consider myself fortunate that I got my hand back.

So the question is, what sort of force did I put my hand into?

*What's the real name for that?

mr obvious
2010-Jun-08, 11:11 PM
Perhaps someone else can visualize your setup better but I'm having trouble figuring it out. I can see the drain from the bottom of the upper tank flowing into just below the surface of the bottom tank. However, an inverted J with the crook just below the surface appears to have both ends submerged in the bottom tank. So it seems to me the bottom tank is recirculating water and should overflow from the water being introduced by the top tank. Obviously, there is flow from the bottom to the top tank (even if indirectly) but I don't see how it's set up.

I will say that to get a rough idea of the forces, we probably need the heights of the various water levels and the distance between the top and bottom tank.

2010-Jun-08, 11:16 PM
Oh, two and a half feet from the bottom of the top tank to the top of the bottom tank. I saw them measure. The inverted J was attached to an external rotary pump that weighed as much as I did.

Jeff Root
2010-Jun-09, 02:04 AM
It sounds to me like you stuck your hand just a couple of inches
under the surface of the water in the bottom tank, underneath
the opening in the short side of the "J", into which the pump was
sucking water and (potentially) baby whales.

Is that correct?

If so, I don't think you were in danger of losing your hand. It may
well have felt like it was going to pull your hand off, or maybe pull
the flesh off your bones, but I think the pressure would only have
been about one atmosphere, 14.2 pounds per square inch. I'd
guess that it could have given you a really huge blood blister, as
long as your hand stayed mostly outside the tube. My guess is
that your hand was too big to go in. If it went in, you'd have had
a Kittenger.

-- Jeff, in Minneapolis

2010-Jun-09, 06:31 AM
A 3" diameter pipe has an area of around 7 square inches. Times 15.925 psi (14.7 psi for sea level, and another 1.225 for the 2.5 feet of head) gives you 111.5 lbs of pressure.

Not enough rip flesh from bone, but certainly enough to get your attention!

2010-Jun-09, 03:22 PM
Mugs! You're back!