Calculate orifise size given the flow rate

[ghensky]

Say, we have two tanks, Tank A and Tank B. A pump is drawing stock from Tank A to Tank B at a flowrate of 2 liter/sec through a 0.05 m pipe. The discharge pipe bifurcates at some Location 1. One goes back to Tank A and the other continues to Tank B. The system is designed to pump stock at 2 liter/second through 0.05 m pipe. The desired flow rate to tank B is maintained by the valve at Location 2.
Say the valve got broken resulting more flow into the bifurcated line back to Tank A, and the flow rate into the Tank B is now 1 liter/sec. To fix this problem, we decide to install an orifice plate at Location 3 and get rid of the manual valve. The question is how to calculate the orifice flow area so that the flow rate to Tank B is 2 liter/second?

(I think I need to (a) calculate the pressure drop using the flow rate, and then (2) calculate the orifice flow area using the calculated pressure drop. I'll come back later after if I have the answer or need hints. Meanwhile please feel free to add your two cents. Hope whatever answer I come up with will help someone out there. The attached figure is not to scale. Thanks.)

 

[LittleInch]

Why don't you just raise the height of the return line back to tank A?

Is this homework?

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

 

[bimr]

You have several scenarios that will cause this arrangement to fail.

1. You have a siphon going into tank B.

2. The height of the discharge into tank A is greater than the headloss of the piping from A to tank B.

3. As the level in tank A changes, the flow rate from A to tank B will change.

4. The centrifugal pump flow will change with differences in suction and discharge pressures.

5. There is no way to make this work precisely without some type of flow monitor and controller.

Determining the flow through an orifice is the easy part.

http://www.tlv.com/global/TI/calculator/air-flow-rate-through-orifice.html

 

[ghensky]

@littleinch, I can't raise it because it is hard-lined. Not homework.
@bimr, 1. Both tanks are vented. So no siphon taking place. Thanks for pointing out on the figure. 2. The height of the discharge into tank A is less than head-loss of the piping from A to tank B. It is that the figure is not drawn to scale. 5. I am going to get some flow measurements as my next step.

Thanks guys for your comments. So I made two changes in the same figure attached. 1. the tanks are now shown as vented, and 2. The line from Tank A to Tank B is longer than the Circulation line.

I am going to take the velocity readings at upstream and downstream of Location 1 and at Location 3. Nevertheless, I foresee the problem in calculating the orifice diameter. To choose the orifice diameter, one must have the pressure drop, which to my understanding is not possible without actually taking the pressure measurements before and after the orifice plate. Not sure if there is any work around or standard rule of thumb in such situation. Any comment would be appreciated. Thanks again!

 

[LittleInch]

If you take velocity measurements upstream and downstream of location 1 you can work this out.

This gives you flow into the tank ( which you want to be 2 l/sec) so adjust your valve at loc 2 until you get to that value.

Then you can work out the pressure drop in the line from loc 1 to tank B. discharge pressure into tank B is 0 barg, so you can work out the pressure at location 1 - pressure A

With the known velocity (flowrate) into location 1, subtract your 2l/sec and that is the flowrate going back to tank A. subtract the pressure due to the head from loc 1 to the high point of your return piping ( assuming your sketch is correct) - Pressure B. The discrepancy between pressure A and pressure B is the pressure drop being taken by the valve - to be replaced by the orifice. You then have flowrate in the return pipe and pressure drop - Find a liquid orifice plate calculation and there you go.

Clearly everything must stay the same to work ( tank contents, level in tank A) but depending on the accuracy you want of flow into tank B then it might work. It all depends on the numbers, distances etc. If a small change in tank level can make a significant change to the flow then you really need something more sophisticated

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

 

[ghensky]

Thanks Guys for the comments.

At the end I just put a orifice flow meter 75% smaller and it simply worked out! I know it is not engineering but when you see a snake on the floor you don't go for a team meeting or start calculation. You just step on it :/