Air valves at hight points

Question

Good morning, 
 While I was modeling a situation, came across a situation where there is a point in the middle of conduct that is above the HGL during the steady state. When I sought information on how to handle the situation, I found the post "Modeling Air Valves At High Points [TN]." After reading it carefully, I got some doubts. Once in my case I am doing a transient analysis, can I admit this situation as it says that happens on this post? "In many cases, the modeler would simply ignore the negative pressure and accept the pump operating point." or is prevalent workaround this situation by applying the air valve? 
 I did the test, and the values of underpressure did not change significantly, but there was an increase on the maximum pressure. The principal change was on the end of the pipeline, once downstream of the air valve the underpressure desepears. So what is the best or most realistic way to model this situation?

Jesse Dringoli · Accepted Answer

Joao, 
 
First, I would double check the information used to build the model to ensure that the elevations in question are correct and that you are accurately modeling the real system. 
 
The increase in maximum pressure when introducing an air valve may be due to a surge occurring after air is expelled from the air valve. A great way to visualize what's happening is to animate the profile path. In the calculation options, make sure "generate animation data" is set to "true", re-run the model if needed, open the Transient Results Viewer and animate. From this, you can see the volume of air and hydraulic grade as they change over time, to get a better understanding of the transient response. 
 
If you do see air expelled too quickly, with a subsequent upsurge, this could be due to an oversized air outflow diameter, or it could be a problem related to the initial air volume used. If no air pocket collapse occurs, the higher max pressure could be due to differences in the way that waves are reflecting. Compare animations of the profile path to help understand. 
 
I would also consider ending the system at that high point instead of using an air valve. This is a common technique in this situation, which is mentioned in a few places including the section titled "What if my air valve is open during the Initial Conditions?" in the below Air Valve modeling technote: 
 
 communities.bentley.com/.../3400.modeling-reference-air-valves-tn 
 
...and in this article: 
 
 communities.bentley.com/.../19855.initial-pressure-less-than-vapor-pressure-at-the-pipe-end-s-the-elevation-s-or-head-s-are-incorrect-solution-500000072593 
 
Basically if you assess the situation and determine that there is indeed an air valve at that high point which is open under normal conditions, with partially-full flow by gravity downstream, then the effects from that section of the model downstream of the high point may not be relevant, since any surges would not propagate past the pressurized part of the system. So, you could model the high point as a Discharge To Atmosphere element, configured to represent the discharge characteristics of the point where the modeled system discharges to part-full flow.