Hi,
My model is attached.
Pipeline.zip
It contains two scenarios :-
Scenario 1 : With AV-1 in which Air Valve AV-1 is active
Scenario 2 : Without AV-1 in which the valve AV-1 is inactive.
Reverse to what logic says, lot of negative pressures are observed near AV-1 in Scenario 1 which are not observed in Scenario 2.
Scenario 1 : Red Dots means transient pressure (min) below minus 6.50
Scenario 2 : No Red Dots meaning all pressures above minus 6.50
What could be the reason ?
Hello Martin,
The air valve does seem to have an impact of the results here. This is probably easiest to see in the time history graphs. When the air valve is active, an air pocket is generated because negative pressure occurs. As this air pocket starts to diminish, this corresponds to when the pressures at the downstream junctions are a little lower.
When the air valve isn't in place, the instability starting around 35 seconds isn't as large. Ultimately, what is likely happening is that the change in flow in the system caused by the air valve is interacting with the pressure waves already in the system, causing slightly larger transients to occur.
One thing to keep in mind with air valves is that they will add air to the system as soon as negative pressure occurs. If the air valve is not active, you will see negative pressures at the same point, but since you are not seeing vapor pressure, no vapor is forming. This link has some additional information on this.
Regards,
Scott
Hi Scott
The link provided by you mentions about increase in maximum (positive) transient pressure due to air valve, which is quite logical because water column will slam with inhaled air causing higher transients.
Also, you have mentioned about "change in flow in the system caused by the air valve is interacting with the pressure waves already in the system, causing slightly larger transients to occur.", which also means that positive pressure must increase.
Whereas, in my case, the negative pressure in increasing. What could be the reason ?
The positive pressure is increasing as well. Starting around 35 seconds, the flow related to the air valve is interacting with other pressure waves in the system. Basically, you have two pressure waves that are interacting at the nodes downstream of AV-1. This interaction is cause a relatively small upsurge and downsurge in pressure.
One thing I would recommend trying is computing both scenarios and looking at a time history graph, such as one for P-13:J-100. If both scenarios are computed, you can click the create a graph that includes results for both scenarios. This link has information on how to do this. In doing this, you will see some pressure transients starting around 35 seconds with no air valve. These same transients are present with the air valve as well, only compounded with the flow related to the expansion and contraction of the air pocket at AV-1.