Nozzle/Branch pipe sizing

[PantheraX]

Hi all, I'm quite new to piping engineering, and I've been tasked to determine the sizing/diameter of a branch pipe for a reducing tee junction.

Are there any equations that I can use to compute this?

I've seen some of the ASME B31.3/31.4 and I am not sure as how it is governed. for example, the use of the header-branch diameter ratio.

Thanks all in advance.

 

[BigInch]

K(v^2)/2g is not the D'arcy (or Darcy) equation, or the Churchill equation

What would you be doing, if you knew that you could not fail?

 

[PantheraX]

hmm, but i came across it in some piping articles that suggested using the K (obtainable from Miller chart) instead of f(L/d) to obtained additional losses due to fittings such as that of tees. Am I not supposed to do that?

Also, your emphasis on knowing the flow rate for my design is that the velocity would be required, is that right?

 

[BigInch]

Don't do that. K values are for calculating pressure drops at relatively short valves and fittings.

Three primary variables are,
Flow
Inlet pressure
Outlet pressure

knowing 2 of those, you can calculate the one missing variable.
Flow divided by pipe area = velocity

What would you be doing, if you knew that you could not fail?

 

[PantheraX]

ok i understand, since K is just a modification of the f(L/D) term. Better to go back to basics.

But now i'm stuck with a problem, how do I calculate pressure of a point in a flowing pipeline?

It has static and dynamic pressure components?

Say, for an underground buried pipe?

 

[BigInch]

That is derived beginning with the Bernoulli equation, include a head loss coefficient (based on D'arcy's method, or some other one) for friction due to the flow between any two points. With Bernoulli, if you know the energy at one point, you should be able to calculate the energy at another point.

Since two of the three variables I gave you above are pressure and you must know two of the three, at least one known variable must be a pressure.

If you know the upstream pressure, you can calculate downstream pressure, or visa versa. If you know both pressures, calculate the flow.

What would you be doing, if you knew that you could not fail?

 

[PantheraX]

ok. I understand it now.

Thank you so much BigInch. I felt i have gained a lot from your comments. Thanks stanier as well.

 

[BigInch]

Good. You're off on the right track then.
Come back when you have more questions.

What would you be doing, if you knew that you could not fail?

 

[PantheraX]

Ok, I have managed to create an Excel template to help me compute.

BigInch, do you know of any pressure drop limits for large diameter pipelines? It seems like my pressure drop value is quite small though.

 

[BigInch]

What's your pipeline diameter, length and the product inside? Gas or liquid? Some pipelines can have flow friction losses of up to 10 psi/mile, especially if pump power costs are low. Once an initial pipeline diameter has been found, usually based on liquid velocities of 3 to 10 fps, a potential increase in that diameter can be seen worthwhile if pumping costs drop at least as much as the cost of the larger pipe diameter plus the supposed construction cost increment. Most of my pipelines seem to fall into 50 to 100 mile distances between pump stations, depending on products, water, diesel, gasoline, kerosene, natural gas, etc. and, if a liquid product on the topography of the pipeline (elevation profile). Pumping over hills or a mountain range will roughly shorten the distance between stations by the static pressure of the change in elevation / psi per mile pressure loss.

What would you be doing, if you knew that you could not fail?

 

[PantheraX]

my pipeline diameter (run diameter) is about 7ft, run length about 5ft , branch diameter is 4ft with the branch length of 2ft and it carries water (liquid). My initial pressure at inlet is 230psi and a relatively slow flow of 1.6 fps.

Based on what I calculated, it seems the pressure loss over the length of branch pipe is around 0.07psi.

I'm not quite sure if such a small value of pressure loss would be practical as I have no dealt with such pressure loss calculations thus far.

Would your experience justify such a value for the above mentioned dimensions?

 

[BigInch]

It's probably correct for the pipe. The pressure drop with normal flowrates for pipes that short would be less than the pressure loss of fluid exiting the Tee fitting where they start. The Tee losses are probably 15-20 x the loss of a foot of branch piping.

If you have a 1.6 fps velocity the size is OK. The trick may be holding enough backpressure in the downstream connection to keep the branch pressure near the 229.93 something psi that you will need to keep your actually developed flowrate hanging around that calculated value. Without sufficient backpressure at the branch outlet in the downstream pipeline, flow in the branch will tend to accelerate. You need to verify that the downstream pipe pressure at that point is nearly 230 psig, or try to live with the increased flowrate, if it is lower, or for that matter, the decreased flowrate, or backflow, if it should be higher. If it's backflow, put in a check valve to stop backflow when the pressure is high. If the downstream pipe has a very low pressure, you will need to install a backpressure control valve on the branch to keep the flow from increasing.

What would you be doing, if you knew that you could not fail?

 

[racookpe1978]

"my pipeline diameter (run diameter) is about 7ft, run length about 5ft , branch diameter is 4ft with the branch length of 2ft and it carries water (liquid). My initial pressure at inlet is 230psi and a relatively slow flow of 1.6 fps."

7 ft diameter x 5 ft run, branch = 4 ft dia x 2 ft run?

That describes a single Tee fitting, not a pipeline. What are the lengths of the whole assembled run?

 

[SNORGY]

If your objective is to treat the fitting in isolation and assume that the delivery pressure at the end of the branch pipeline is the same as the delivery pressure at the end of the main run pipeline (thus equating the dP in the run and branch downstream of the tee), then I think your flow split will end up being:

(Qb/Qr) = [(Lr/Lb)*(Db/Dr)^5]^0.5

I apologize if I got that wrong and made myself look like a fool.