Flow rate from head loss.

EthanAnderson

I have head loss (25.95ft) and converted that to pressure (17.19psi). I am trying to find flow rate in 3" sch 80 pipe coming from railcar at 25psi. So outlet pressure is 7.813psi. The fluid is 50% Caustic soda.

EthanAnderson

Essentially looking for flow rate from outlet pressure(7.81psi) in a 3" sch80 pipe.

Travis_King

It matters a great deal how long the pipe is, how many elbows, what kind of valves and how many, and what the positive or negative elevation change is between the railcar connection and the discharge connection.

EthanAnderson

Travis, I've got all of the above. 17ft is my total head loss. Just need flow rate at 7psi through 3" pipe..

EthanAnderson

Correction, total head loss is 25ft. Pressure is 17psi.

russ_watters

I'm not sure you understood Travis's point: you need all of that information to calculate the flow rate. What you have provided isn't enough.

Ideally, if you know the pressure drop across a single device, with a well known resistance (CV), that can substitute as an accurate flow meter. Using an entire system is much more difficult, if the piping arrangement is complex.

EthanAnderson

Sorry for misunderstanding guys. Check the attached snip shot. That's all the info I have. Length of pipe is 283ft from tank to furtherest unloading station. Tank is 30ft tall, top of railcar is 20ft.

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russ_watters

That could work. I'll have to look at it more closely after work when I have more time.

EthanAnderson

Thanks, Russ.

EthanAnderson

Would you like for me to email you the spreadsheet so you can see the formulas I used in the cells?

Travis_King

Quick question, is this system venting to atmospheric pressure (i.e. an open tank) or to a closed system/vessel?

If the former what we've got here is this (correct me if I'm wrong)

You have a measured pressure at the railcar discharge (20ft) of 25 psi (37.7 ft).
(If the pressure is being measured at a lower or higher elevation, you should indicate this)

You have a measured pressure of 7.8 psi (11.8 ft) at the discharge. (Or is this the value you arrived at through your calculations?)

Either way, what you'll want to do is take your measured inlet pressure (head) and subtract from it the known static head (that is the elevation between the inlet measurement instrument and the discharge pressure measurement instrument). This will give you your friction headloss.

Using this, you can use the Hazen-Williams calculation/approximation to get a decent estimate of your flow.

EthanAnderson

I am a co-op. I have not taken fluid mechanics yet, sorry if I am having trouble understanding, and if I am stating the obvious. The railcar is closed. We'll force air into the vessel to push out the caustic. There are no measurement instruments. I have 283ft of straight pipe and a number of valves and fittings. I found equivalent lengths of straight pipe for the fittings and valves resulting in a total 333ft of straight pipe.

Caustic loses 28.9ft/1000ft. Total head loss is 25.95ft including 20% safety factor. My mentor stated the railcar vessel is at 25psi. Then he converted the head loss from 25.95ft to 17.18psi. He said outlet pressure was 25-17.18= 7.8psi . I have to find what the flow rate will be through a 3" line.. at 7.8 psi? ( I'm assuming). Again, sorry if I've been redundant- This is my first project.

Travis_King

I don't understand. How did you figure out the loss/1000ft if you don't know the flowrate?

EthanAnderson

We got a recommended flow rate of 81.5gpm from a process engineer. Used kinematic viscosity of 50.97 centistokes to interpolate between 43.2 and 65.0 centistokes at 81.5gpm. Giving me ft loss/1000ft. Hope that answers your question..

EthanAnderson

Used the table on page 3-55 of a Cameron Hydralic Data book.

Travis_King

No problem, ask away.

Yea, but all you did there was find out the expected ft loss/1000 ft at 81.5 gpm. Basically, you assumed a flow of 81.5 gpm in your effort to calculate the flow...

Is the caustic soda discharging into a tank that is open to the atmosphere or into a process?

EthanAnderson

The soda will discharge into a tank. The tank is covered, but with exhausts- basically atmospheric pressure, right? The tank is only used for storage.