Draining a Pipeline System
Draining a Pipeline System
(OP)
So, I have this problem. There are 3 Scenarios. All 3 scenarios have the same geometric layout:

The user can choose the elevations and pipe diameters and roughness's to drain the pipe out the pipe with or without an air valve at A.
I want to know why (using Bernoulli) if I make the pipe diameter smaller between the nodes B and C the Q versus time does this (time x-axis):

This happens while the total head of the system does this (time x-axis):

I have checked, re-checked and re-checked my maths and my Bernoulli assumptions several times and my colleagues got the same answer. Any ideas if this is correct or not?
This occurred whether there was an air valve or not.

The user can choose the elevations and pipe diameters and roughness's to drain the pipe out the pipe with or without an air valve at A.
I want to know why (using Bernoulli) if I make the pipe diameter smaller between the nodes B and C the Q versus time does this (time x-axis):

This happens while the total head of the system does this (time x-axis):

I have checked, re-checked and re-checked my maths and my Bernoulli assumptions several times and my colleagues got the same answer. Any ideas if this is correct or not?
This occurred whether there was an air valve or not.
RE: Draining a Pipeline System
Another way to put it, water can't drain out of the last pipe faster than water can get into it, but once the smaller middle pipe is fully drained, there is no such restriction.
All of this of course assumes the last pipe has full flow. If there is a significant restriction caused by the middle pipe, the last pipe might not achieve full flow.