Rectriction Orifice (RO) calculation 2

[process1907]

Hi guys,
Need advice from you all.
Refer to attached sketch;
After installing new line (line no 4), the header pressure was dropped from 5.0 barg to 4.0 barg (as expected due to additional opening)
Line no 1 to 4 are at all at same level & orientation. Hence equal distributed flow is expected.
The pump speed has been increased from 1300 rpm to 1500 rpm but still unable to get 5.0 barg pressure.
Based on pump spec, rated pressure at 1500 rpm is 7 barg.
It is crucial for us to maintain the pressure at 5.0 barg. The only option have is to install flow orifice (FO) at new line (line no 4)
My question;
1) How to size this FO since involves branches of pipes?
2) Which flow (Q) that I need to use. Is it flow to point no 4 or total header flow?
3) In term of flow distribution, can I assume flow to point 1 - 4 & recycle based on pipes surface area ratio?

Your kind input are very much appreciated

 

[Homayun]

Process1907,
Increasing the pump speed doesn't result in higher head (pressure). It simply results in higher flow, which I suspect would give you the opposite effect, less pressure.

To size the RO, you would first have to determine how much flow is need going to point 6. Then when you know that, you can determine the pressure drop from the RO to the end point, let's call it DP6. Then the orifice pressure drop whould be 5 - DP6, 5 being the header pressure that you are ineterested to maintain.

Remember, if you don't have any restrictions in your system like a control valve, the flow thorugh each bracnh is governed by the total system and static pressure drops in that particular branch.

Hope this helps..

 

[process1907]

Thanks Homayun..

Firstly, my apologies.... the sketch should be as attached below.
If i not mistaken, according to pump laws, pump speed has a direct impact to volumetric flow & pump head. Thus I guess this will also help to increase the discharge pressure. Pls correct me if I were wrong.

V1 x(n2/n1) = V2
Hp x(n2/n1)^2 = Hp2
V = volumetric flowrate
n = speed of pump impeller
H = head developed by pump

Any rules of thumb to best estimate the pressure drop from RO to the end point (DP4)?

Can you clarify further on relation of total system & static pressure drops with the flow distribution across branches of pipes? What is the formula use?

Thanks.

 

[kkimic]

process1907, I suggest you to get a copy of "Crane Technical Paper No. 410", you can buy it here:

http://www.flowoffluids.com/tp410.htm

The book is really cheap compared to others in the market and will help you to solve your problem, basically there are many similar examples to the problem you are trying to solve. Really really worth the money.

For RO sizing, there is also a chapter in the book with the formulae/graphs, otherwise I have sometimes used the following:

http://www.flowmeterdirectory.com/flowmeter_orifice_calc.html

In the link above you can also see the theory behing it, should you be interested in it.

Regards.