Chizer & Bunoi 67 gallon

[sphelps]

Originally Posted by midgetwaiter

You assume that the volume of water coming over the weir is constant but it's not. Having a larger volume of water over the drain makes adjustment easier because it evens it out.

First it is constant unless your pump is fluctuating which it shouldn't be and second volume has nothing to do with static head pressure.

[midgetwaiter]

Originally Posted by sphelps

First it is constant unless your pump is fluctuating which it shouldn't be and second volume has nothing to do with static head pressure.

Possibly I am using the term "Static Head " incorrectly, I'm more a wrench bender than an engineer. However I can show that the height of the water above the drain determines the velocity of the water draining by applying Bernoulli's Principle. There's all sorts of handy calculators on the web that help figure this out, like this:

http://www.efunda.com/formulae/fluids/draining_tank.cfm

It has been my experience that this increase in velocity makes adjusting a gate or ball valve on the drain MUCH easier. Determining the reason for this goes well beyond my math skills but it's very easy to demonstrate. I think it's because you end up with a full pipe before the drain valve which ends up giving you a more even flow rate. Regardless it translates into easy, repeatable and reliable adjustments.

[sphelps]

Originally Posted by midgetwaiter

Possibly I am using the term "Static Head " incorrectly, I'm more a wrench bender than an engineer. However I can show that the height of the water above the drain determines the velocity of the water draining by applying Bernoulli's Principle. There's all sorts of handy calculators on the web that help figure this out, like this:

http://www.efunda.com/formulae/fluids/draining_tank.cfm

It has been my experience that this increase in velocity makes adjusting a gate or ball valve on the drain MUCH easier. Determining the reason for this goes well beyond my math skills but it's very easy to demonstrate. I think it's because you end up with a full pipe before the drain valve which ends up giving you a more even flow rate. Regardless it translates into easy, repeatable and reliable adjustments.

You're still way off, the velocity at the valve and in the pipe will be the same regardless (delta z is equal for both cases), the formula you linked doesn't take an internal pipe into account, reason being is that it doesn't matter. Yes with the pipe extended the input velocity is shifted from the bulkhead to the top of the extended pipe but this won't effect flow and the velocity will be constant throughout the pipe until after the valve. Even if you want to break out Bernoulli's equation all you'll end up proving is the pipe friction from the extra 6" of straight pipe makes no difference, and even if it did it would still give constant flow and not result in anything constantly changing.

The issues you've experience are likely from the pipe being too close to the surface creating a partial or occasional vortex like I've mentioned a few times now. You need enough space from top of the pipe to the water surface to eliminate any vortex effects but you certainly don't need to remove it all together.

I assure you the typical Herbie has an extended pipe to prevent the overflow from draining and it doesn't cause issues. This you should have noticed with the problem already being corrected with an extended pipe.

If you wish to continue learning about fluid mechanics PM me, I'll happily discuss it with you but don't post incorrect information in this guys build thread.