Flow between a pool and a balance tank

[Jhon Stenvall]

Hi, I'm a junior engineer currently working on a water park project. Right now I'm checking the hydraulic design of a small water slide. Basically the system have two tanks: one being the pool and the other being the balance tank. There are two 2HP centrifugal pumps wich have their suction pipe connected to the balance tank and they discharge to the slides, then the water flows down to the pool and back to the balance tank through three 6 inch PVC pipes. As you can see it's a cyclical system. By the way both pumps discharge a total of 100 m3/h (50m3/h each). The two tanks have the same water level when being at static conditions but the pool depht is 1.3 m (dimensions 10mx5mx1.3m)and the balance tank depth is 1.55 m (dimensions 3mx4mx1.55m). My question is the following: As you know there have to be a difference in levels to produce a dynamic flow between the pool and the balance tank. I calculated this head and the result was approximately 6 cm. Now, considering that both tanks have the same initial level, after the pumps are turned on and the cyclic process starts what do you think will happen:
(1) Will the pool level increase 6 cm and the balance tank will keep its initial static level?
(2) Will the balance tank decrease 6 cm and the pool will keep its initial static level?
(3) Will the pool level increse by 3cm and the balance level decrease by 3 cm making a total difference of 6 cm?

Note that there will be water in the pipe system and the slides that will make both the tank and the pool decrease its level but for this question it's not considered. Please take a look at the diagram attached.

Thanks in advance.
JOHN

 

[LittleInch]

Not sure why you really care about 6cm because it's the difference which counts.

I think there will be an issue over the relative volume in each tank.

If the balance tank is significantly smaller than the main pool I would expect it to take the majority of the level decrease, if equal then your 3 and 3 option would apply.

There could also be a timing issue in how long it takes for the water once it has left the balancing tank to appear at the bottom of the slides to set up your steady state flow but that may be just about how both tanks reduce in level when in operation. If the volume of the balancing tank is such that it gets to a 6cm differential before the water starts returning into the main pool then your 1 would apply initially but then as water starts returning to the main pool this would become the point where steady state flow starts.

Interesting thoughts, but maybe a bit academic?





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[mk3223]

The levels of the pool and tank may be depended on the actual water flows from the pump to the slides and the flow through the PVC pipes. If the flow rate of the pump discharges the same as of three PVC pipes, the water level of the pool may be same as the tank. If the less flow through the PVC pipes, the water is to be backed up and raised the level of the pool.

 

[Jhon Stenvall]

LittleInch I guess if I assume that there is no connection between the pool and the balance tank and calculate the time it takes to create a difference in level of 6 cm then the pool will increase its level by 1.16 cm and the balance tank will decrease its level by 4.84 cm. If in that precise time I open the communicating vessels connecting both the tank and the pool then there will be no more changes in levels because the flow through these pipes (communicating vessels) will be the same flow that comes out of the balance tank and return to the pool. Is this assumption right?

 

[Artisi]

Once the system is inbalance, that is flow into the pool equals return to the holding tank, assuming any head loss thru' return pipes is neglected, both pool and tank will be equal.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

I think it is yes.

I still think this is academic but that's up to you.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.