Tank Recycle Hydraulics

[oilman11]

I have an interesting hypothetical hydraulics problem in order to better my understanding. Please refer to the attached diagram for clarity.

Imagine a closed tank with a roof (not open to atmosphere). It has one inlet, one outlet, one recycle. Inlet line to the tank is through a centrifugal pump and the recycle line also has a centrifugal pump. Recycle pump TDH > Inlet pump TDH. Assume outlet to the tank is blocked and the level in the tank quickly rises until it’s full of water upto the roof. Now since the tank is already full of water, would the pressure at the top of the tank be (Inlet pump suction) + (Inlet pump deadhead TDH) - (Static height losses)? Zero flow through inlet and some flow through the recycle? Neglect friction losses.

My reasoning is that since inlet pump has a lower TDH, it does not have enough head to push the liquid into the tank once the tank is full. Therefore, the tank will see Inlet pump’s head head pressure. And flow through the recycle pump would be according to the driving force of P1 - Recycle Pump Discharge Pressure.

Thanks

 

[oilman11]

 

[akcullin]

I believe your logic is correct.

 

[LittleInch]

For your purposes the recirc pump is irrelevant. the inlet pressure to the pump and outlet pressure into the tank MUST be the same pressure. It is physically impossible to create more pressure in the tank for that recirc pump so just ignore it.

to be pedantic though, it the tank is not open to the atmosphere and you block the outlet, you will have a gas space on top of the liquid. But pressure would still rise to whatever the inlet pump can deliver

Also your "tank" will probably burst...

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[pierreick]

Hi,
Stick to real cases to improve your engineering skill. To me this is no sense. Tank should be equipped to prevent overfill/rupture.
P1@ sky of the tank = Pressure at the inlet of the tank.
My view
Pierre

 

[HTURKAK]

Mr oilman11 (Chemical)(OP),

Let me explain in steps what will happen ..Assume the tank is empty , the tank outlet valve is closed ..When you start the inlet pump ,

- the water level in the tank will start to raise and since the air trapped , the pressure in the tank will also start to increase,
- the tank pressure will eventually reach to the inlet pump shut-off head and the pressure will no more increase,
- the flow at inlet pump will be static and the water temp . inside the pump will start to increase ,
- eventually the water will start boiling and the pump will fail..

Regarding the circulation pump , since the inlet and outlet discharges are equal, ( no water added by circulation pump ) it can not increase the pressure but heat and mix the water ..

However, since tank is not PV, somewhere before step 2 will rupture ..

I hope this answer responds your question










Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[oilman11]

Thanks guys. This confirms my understanding of this problem.

 

[1503-44]

Probably need a vacuum valve for draining. Wouldn't want it to collapse.

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