Bentley HAMMER " negative pressure "

Question

Hello Sir, 
 
 I need your kind input fir the inquiries below 
 First: What is the scientific reason for negative pressure ? 
 Second: What is the difference between "wave speed" and "pressure wave speed" and how can i determine both? 
 Finally , what are the solutions to neglect or eliminate this negative pressure -- can I use Air Valve instead of hydropneumatic tank or air valve has nothing to do in Hammer ? 
 
 Thanks & regards, 
 Hesham

Jesse Dringoli · Accepted Answer

Hesham, 
 
Are you referring to negative pressure during a transient simulation in HAMMER, during the initial conditions, or both? Try doing a search for "negative pressure" in the search bar on the Hydraulics and Hydrology Community. This is a subject that has come up quite a few times so you should be able to find some advice there on the practical/theory side. For some general information on negative pressures in the initial conditions, see the below Support Solution: 
 
 communities.bentley.com/.../10830.negative-pressures 
 
The solution to eliminate this depends on a few factors. If you're seeing it during the initial conditions and do not expect it, you'll need to think about what's different between your model (or the model assumptions) and what happens in the real system. For example if you have an air valve at a high point that prevents negative pressure, you'll need to add one. See the below support solution for using air valve during the initial conditions for high points: 
 
 communities.bentley.com/.../2662.modeling-air-valves-at-high-points-tn 
 
If the initial conditions results are expected but the transient event being simulated in HAMMER results in negative pressures during the transient simulation, then there are two main things to consider. First, ensure that the negative pressure (or the "downsurge" that causes it) is expected. For example if you see this happen in the first transient timestep yet your transient event doesn't happen until say, 10 seconds into the simulation, then you know there's a problem that needs to be fixed. (try focusing on the area where the unexpected surge starts) 
 
If the negative pressures occur after the transient event, it could be expected due to the nature of the event (ex: valve closing too fast, pump spinning down too fast, etc) in which case you'll need to closely examine the results to decide what action needs to be taken. First, ensure that your transient calculation options are set to save animation date, then animate one or more profile paths in the Transient Results Viewer. This is a great way to visualize what's happening during the transient event. From here, you might notice for example that an air valve opens when the pressure at a high point reaches zero, admits air into the system, but then when pressure rises again, the air is released too quickly, resulting in a severe positive "upsurge" that reflects off the end of the system, returning back as a severe downsurge wave, causing even more severe negative pressure. Or, maybe you don't already have any devices in place and you notice that the pressure drops below zero at a location after your transient event. Based on your observation and the options available to you, you can try a number of different surge mitigation devices and strategies. The powerful Scenario functionality can be used to try and compare different options in the same model. 
 
Some additional advice on mitigating negative pressures at a high point during a transient event can be found here: 
 
 communities.bentley.com/.../8420.surge-mitigation-for-systems-with-intermediate-high-points-experiencing-negative-pressure 
 
To answer your question about surge mitigation - both the air valve and the hydropneumatic tank (among other element types) can be used during the transient simulation to address transient issues. Not all devices or strategies will work in all situations, so you'll need to use the model results along with some engineering judgment to decide the best approach. You can read more about some of these devices in the Help and by searching on the Hydraulics and Hydrology Community. For example our Support Solution section contains several longer Technote articles on devices including Air Valves and Hydropneumatic tanks. See links below: 
 
 communities.bentley.com/.../modeling-reference-_2D00_-air-valves.aspx 
 communities.bentley.com/.../modeling-reference-_2D00_-hydropneumatic-tanks.aspx 
 
Regarding "wave speed" - as far as I know, this is synonymous with the terms "pressure wave speed" or "celerity". You can find the definition and more information in the Help and by searching on the Hydraulics and Hydrology Community, but it's basically the speed at which a pressure wave travels in a given pipe. A wave speed calculator is provided with HAMMER (Tools menu) which can assist in assigning wave speeds to your pipes based on factors including the material and anchoring. 
 
Hope this helps,