Why are my maximum transient pressures higher when using an air valve, compared to not using an air valve?Why does using an air valve sometimes make my results worse?
If the outflow orifice is not sized properly and air is released too quickly, the adjacent water columns can 'slam' together when the last bit of air is expelled, causing some of the worst types of surges. This is similar to an instantaneous valve closure but even worse, as the water columns on either side collide together. When using an air valve, it is important to allow fast air inflow on downsurge, but to slowly release the air. This is done by either reducing the outflow orifice diameter or by using a triple acting air valve. A triple acting valve can throttle the outflow orifice size just before the columns collide, to cushion the air pocket collapse.To visualize and confirm this in the model, animate a profile path in the transient results viewer. Make sure the profile passes over the air valve(s). Make note of the air/vapor volume shown at the top of the animation - you may notice a pocket form at the air valve location. When pressure returns to the system (such as after an emergency pump shutdown), notice how fast the air pocket reduces in size. Note also what happens when the air pocket is fully expelled. If the air is expelled too quickly, you'll see a very severe surge emanate from the valve location. A comparison of this phenomenon with an improperly sized orifice versus a triple acting air valve can be seen in the "Modeling Reference - Air Valves" TechNote.
In addition, it is important to note that the air valve in HAMMER will open to let air into the system when zero pressure is reached. A no-protection case could see negative pressures but not see vapor pressure. The correct solution for a case like this will be up to the engineer's discretion, but may be related to the type of system and the nature of the results, i.e., how close the negative pressure is to vapor pressure.
Note also that air valves only protect in the direct vicinity of the air valve location so air valves alone may not be enough for transient protection. You may need to consider other supplemental surge control strategies such as tanks, pump flywheel, bypass, etc. See more here: Negative pressures occur during a transient simulation no matter what protection is used
Modeling Reference - Air Valves
Using Transient Results Viewer animations for visualizing a transient simulation