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Flow rate from head loss.

by EthanAnderson
Tags: flow, head, loss, rate
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EthanAnderson
#1
Feb6-13, 08:17 AM
P: 15
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.
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EthanAnderson
#2
Feb6-13, 08:26 AM
P: 15
Essentially looking for flow rate from outlet pressure(7.81psi) in a 3" sch80 pipe.
Travis_King
#3
Feb6-13, 09:31 AM
P: 841
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
#4
Feb6-13, 09:43 AM
P: 15
Flow rate from head loss.

Travis, I've got all of the above. 17ft is my total head loss. Just need flow rate at 7psi through 3" pipe..
EthanAnderson
#5
Feb6-13, 09:45 AM
P: 15
Correction, total head loss is 25ft. Pressure is 17psi.
russ_watters
#6
Feb6-13, 10:36 AM
Mentor
P: 22,305
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
#7
Feb6-13, 10:55 AM
P: 15
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.
Attached Thumbnails
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russ_watters
#8
Feb6-13, 11:11 AM
Mentor
P: 22,305
That could work. I'll have to look at it more closely after work when I have more time.
EthanAnderson
#9
Feb6-13, 11:16 AM
P: 15
Thanks, Russ.
EthanAnderson
#10
Feb6-13, 11:19 AM
P: 15
Would you like for me to email you the spreadsheet so you can see the formulas I used in the cells?
Travis_King
#11
Feb6-13, 12:31 PM
P: 841
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
#12
Feb6-13, 01:03 PM
P: 15
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
#13
Feb6-13, 01:10 PM
P: 841
I don't understand. How did you figure out the loss/1000ft if you don't know the flowrate?
EthanAnderson
#14
Feb6-13, 01:17 PM
P: 15
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
#15
Feb6-13, 01:22 PM
P: 15
Used the table on page 3-55 of a Cameron Hydralic Data book.
Travis_King
#16
Feb6-13, 01:39 PM
P: 841
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
#17
Feb6-13, 01:44 PM
P: 15
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.
Travis_King
#18
Feb6-13, 02:04 PM
P: 841
Correct, if the tank is vented, you can assume it is at atmospheric pressure, even with additional volume being added (that is, the caustic soda). The way fluids work is that the system will attempt to balace itself out.

It should be noted that, depending on how much soda is above the railcar discharge, the actual available head could be higher than the 25 psi, as the total pressure at the discharge point will be the pressure of the railcar plus the static pressure of the volume--or more correctly, the height--of soda above the discharge point.
But let's say it's 25 psi. This translates to 37.7 ft.
Your discharge pressure is atmosphere, 0 psig. Tanks that are vented to atmosphere do not put any back pressure on the system, so the pressure at the discharge is 0 psig.
However, you do have static and friction head losses due to elevation and, well, friction which it will have to overcome in order to discharge into the tank.
You say the tank is 10 ft above the tank, so that static head loss leaves us with 27.7 ft to lose by the time it discharges.

As I said, the system will balance itself, you have 25 psi at the railcar and it has to lose that pressure by the time it reaches the tank. You see that it loses some pressure due to elevation change, but the remaining must be eliminated by friction. The way a system controls this (thanks to physics) is: the caustic soda will find equilibrium at a flowrate which results in just the right amount of friction losses so that it discharges at atmospheric pressure.

You'll find that with your railcar pressurized to 25 psi, your flow will be significantly higher than 81.5 gpm.

Hazen-Williams
ANSI Sch. 80 Steel Pipe Info


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