How to calculate fluid velocity at multiple points in a piping system

[MRLatLSC]

I have a piping system consisting of a pig launcher with a fill line flowing into it and a vent line flowing out of it. The 2" diameter fill line pulls crude off a mainline operating at 1017.1 psi and a flow rate of 633 Kbpd. The crude flows from the mainline to a pig launcher via the fill line. The length of this fill line is 454". A 2" diameter vent line is attached to the pig launcher so that when the launcher is full, oil is forced out and into a sump tank, rather than over pressurize the launcher. The vent line and sump tank are open to atmospheric pressure. I am trying to find the volumetric flow rate (and hence velocities) so this is a steady state operation. FYI for simplicity sake I am ignoring minor losses in the fill and vent line as they have not yet been designed; the lengths were estimated from a drawing, and I do not have reliable elevation changes and or bends in them, so for the purpose of this analysis they are straight and at the same elevation as the launcher and mainline pipe. Can anyone offer any insight?

 

[bimr]

It would seem that you may develop a spreadsheet to arrive at the velocities. You know the flow and the pipe diameters. Velocity is Q/A.

 

[LittleInch]

This appears on the face of it to be crazy. You seem to be saying you have in effect a 2" hole in your main pipe and a mere 11.5 m of 2" pipe between 100 psig and atmosphere. The flow rate will be huge and completely against any practice I have ever seen for filling a pig trap.

You need to control the fill rate using some sort of control valve (manual plug or globe valve usually to fill at a much slower rate than simply steady state open ended.

You say "over pressurise the launcher". How?? If you are cross connecting two different pressure rated system you need much more than just an open pipe!

Please explain your situation more clearly as there is something fundamentally wrong here in your current description - a sketch would be most useful.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MRLatLSC]

Is there a more convenient way to attach a file or sketch rather than using html format?

 

[LittleInch]

Go to the bottom of the post box and click on where it says "...or upload your file to engineering.com" and follow the instructions, basically browse to your file(s) and up load and don't forget to click the link at the end of the process. The attached file then appears under your post as a link.

That's how I upload any additional data and it works fine for me.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MRLatLSC]

On my first attempt I did get a velocity that was way too high (around 600 m/s!) and knew I did something wrong or made a faulty assumption. A second attempt provided more believable results but I am still concerned about some of my assumptions and the way I simplified things on my model. I will double check but I don't believe we have specified the length of the vent and fill lines at this point. Those numbers were attained from using the measuring tool in Adobe on a drawing.

Thanks again.

 

[LittleInch]

The real problem here is that you're trying to get an answer based on an unrealistic situation.

Filling a pig trap is a controlled operation taking as long as you want it it take, but generally in the order of 5 to 10 minutes at a carefully controlled flow.

This flow is controlled by a manual valve designed for that purpose - usually either a globe valve or a plug valve such that the vast majority of the pressure drop is taken by the valve and not by the piping. Neither in your description or your sketch is such a valve indicated - this is your main faulty assumption IMO.

I am struggling to understand why you want to calculate this anyway. I have seen on occasion people put an orifice plate in this fill line to reduce the flowrate / velocity to an acceptable level in the event some dolt just opens up the line full blast but not by using the pipe to do this. if you explain why you're trying to calculate something no one else ever does, it might clear the fog of confusion a bit (a least for me)

As I said earlier, your pig trap will be (or should be) at the same or higher design pressure than your connected system, so your comment about over pressurising the pig trap is also fundamentally incorrect.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MRLatLSC]

The reason I am doing this is to get an idea of what velocities will be seen at some level flow sensors installed at these vent lines. I am under the impression that this will aid the procurement and electrical folks in determining what flow switches they need. You are correct-I did not draw the valves on the fill lines and vent lines or vent lines.

Pig trap comment was wrong-I have another project on my plate regarding a frac tank and ensuring it can vent properly and I confused the two.

 

[LittleInch]

You probably need to ask the operations guys how long they normally take to fill the trap and then work a volumetric flow rate from there. Doing it this way there's no real way of calculating it as you don't know how far they crack the valve open.

your configuration with the vent line and tank is a little odd. Normally the operator watches the vent which is free standing over a bucket or open tank and as soon as he/she sees liquid they turn off the vent and let the trap pressurise to line pressure. I've never seen any instrumentation at all on the vent line so not sure why you're doing this??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MRLatLSC]

Thank you for the insight. I will consult with the operations crew. I am not sure if it is a client requirement or what? I have recently been added to this project group and don't have the full details.