Hydropneumatic tank effectiveness

I have split your post to a new thread since it is a new subject. I answered the question about the PSV in the other thread since it is related to the original discussion about the high point.

Ali Khalfan said:

I have placed 1 hydropneumtic tank in the pumping station but it does not help me in negative pressure prevention at the end of the system as shown below even by increasing gas volume. i tried to put another tank close to the high point but no improvment

Looking back at your screenshot, I am not surprised that you are having trouble keeping the pressure positive near the high point. You will have a low initial pressure there, so even a small downsurge wave could quickly cause it to drop below the vapor pressure limit. 

First, I would strongly recommend animating the standard Transient Results Viewer profile path (rather than just looking at the envelope), so you can see the waves propagating and understand the transient response better. 

It is possible that you could improve the pressure near the high point by using different configurations of the other parameters of the hydropneumatic tank (not just the initial gas volume or size. See more suggestions here: Modeling Reference - Hydropneumatic Tanks

However, a downsurge wave may still make it to the high point (again, animate the profile to see this), causing negative pressures. If the pipes and joints are strong enough, some negative pressure acceptable with your design standards, and the pressure is not near the vapor pressure limit, you might accept this. Otherwise, you might try adding more air valves upstream of the high point where you experience some negative pressure problems. Coupled with the hydropneumatic tank, this may improve the negative pressure problem. You'll want to make sure to size the air valve outflow orifices such that a severe upsurge is not caused when the air pockets are eventually expelled. You may need to run a pump shutdown + startup in order to see the impact of the air being released back out of the air valves.

Ali Khalfan said:

my second consern is in regards gas & air volume curves in side the surge vessel they do not look reasonable!!

What specifically does not look reasonable about the graph? It indicates that the gas volume starts at 5 m^3 and after the pump shuts down, the gas pocket expands, and continues expanding even to the end of the simulation. If you seeing the gas volume increase beyond the full tank volume, it is important to note that HAMMER doesn't have any special handling if a hydropneumatic tank becomes empty. It follows the gas law relationship based on the initial pressure and initial gas volume. If the gas volume increases beyond the full tank volume, it still continues to expand and you'll get a user notification about the tank being too small (and that the results could be consider invalid in such a condition). This means that you need to try a different tank configuration. You can read more about this limitation in the aforementioned modeling reference article for hydropneumatic tanks.

Secondly, with such a long pipeline (and based on the pipe wave speed), you may need to run the model for longer than 60 seconds to see the conditions "settle". Again, I recommend animating the profile path to get a better idea of whether the simulation duration needs to be extended. Ideally it should be long enough to observe the transition from the initial steady state, to its final steady state.

Lastly, if the pressure drops to zero at the D2A at the downstream end of the system (or at a downstream air valve), HAMMER has limited ability to track the air pocket location and by default does not track the air-liquid interface. This means that as long as the pressure remains below zero at the high point, the air pocket will continue to increase, yet the HGL will remain equal to the node physical elevation. This can impact upstream results. See "air pocket formation in the D2A Technote, and this article: 

Assumptions and limitations of tracking air or vapor pockets in HAMMER

You are correct. I reduced the report time just for quick results only that time. normally more than 500 seconds i put

negative pressure is not acceptable at all.Minimum pressure of 0.25 bar  shall be maintained all the time please refer to below graph with negative pressure at the end. I tried to place same AVs but still the problem is there

any suggestion to enhance the pressure in that part ??

Is a dipping tube hydropnumatic tank can help??

thanks & Regards

Hello Bushra,

Air valves tend to mainly help at the point where they are placed. Including a surge tank (to add additional head to the system) could help. If the hydropneumatic tank is becoming empty, adjusting the parameters of the tank, or trying other settings or locations, may improve the results.

Regards,

Scott