Air Vessel

Hello Moustafa,

The gas pressure is calculated as per the gas law relationship that governs the transient simulation in the pressure vessel. The maximum gas volume from the snapshots you shared shows slightly above 0.875 cubic meters. What is the tank volume for your air vessel? Is this gas volume greater than the tank volume?

If it is, this indicates that the tank is not completely empty. This is also seen in the water level graph you have shown. This shows that the pressure vessel is almost empty.

We have a detailed article on Hydropneumatic Tanks where this is explained in detail. See the link below;

Modeling Reference - Hydropneumatic Tanks

In the above article refer the note on "pressure at an empty condition";

Let me know if this helps.

Thanks a lot dear Yashodhan.

The tank volume is1 m3. The water level in the tank is still above the base level, while the pressure inside the tank is  -3 m H2O (gauge).

Moustafa, please see the explanation in the wiki article that Yashodhan mentioned, as it addresses this specific question. Here is a copy of the section in question:

A note on pressure at an empty condition: A gas pressure of zero does not necessarily correlate to when the tank becomes empty. The gas in the tank may still be pressurized when the gas volume exceeds the size of the tank, or when the water level in the tank reaches the bottom (if using "Variable Elevation" for the Elevation Type). This also means that you can have a situation where a tank is not yet empty when the gas pressure drops to zero.

If you use the variable elevation table as the elevation type, the water level can be calculated for a more accurate calculation of pressure (based on the water surface elevation instead of based on the tank base elevation), but the situation is still the same, in that the gas pressure is calculated based on the gas law relationship.

For example, consider a case where the model reports a maximum volume of gas that is slightly less than the total tank volume, yet the minimum pressure of gas is reported as 6 m absolute (or -4.3 gage pressure). In this case, the results indicate that when the hydraulic grade drops to the bottom of the tank, the gas volume is still greater than the full tank size and the tank is not yet empty. The HGL then drops below the bottom and the tank is still not quite empty of water. This basically means that there is a negative pressure in the almost-empty tank, which can be imagined as the water column "pulling" at gas pocket inside the tank. In a practical sense, this may mean that the initial gas volume is too small for the given initial pressure. However, if increased, that changes the pressure vs. flow relationship and the tank may become empty.

Thanks a lot  dear Jesse for your help.  I would like to understand more. If a gas pressure drops to zero, is Hammer assumes  a tank is not yet empty (as it unable to model empty tanks)? or is this the situation in field?

Also, what is the problem if the tank is empty? (air entering the pipeline and traveling some distance) i.e. the pressure at the tank reaches -1 or -2 m H2O, as long as the air in the pipeline will be summed again in the tank or at the air valves when the system restarts?

Kind Regards,

Moustafa Darweesh said:

If a gas pressure drops to zero, is Hammer assumes  a tank is not yet empty (as it unable to model empty tanks)? or is this the situation in field?

There is no hard coded assumption; HAMMER simply follows the gas law based on the physical parameters for the tank and the initial conditions. As long as the tank does not become fully empty, this should indeed reflect reality. The gas pressure is not necessarily correlated with how full the tank is, which is the case in the field as well. If there is some water in the tank initially with a low pressure of gas above it, then the tank could certainly still have some water left in it when the pressure of gas drops to zero. 

Here's one way I imagine it - take a bottle and fill it partially with water. Put the cap on the bottle and turn it upside side - there's air above the water like a hydropneumatic tank and the pressure of gas is zero, but the bottle is not empty. Things are not exactly the same in HAMMER since it works with absolute pressure, but the same situation can still apply. 

As the article says, in a practical sense this means that the initial gas volume may be too small for the given initial pressure and you may need to try a different tank configuration.

Moustafa Darweesh said:

Also, what is the problem if the tank is empty? (air entering the pipeline and traveling some distance) i.e. the pressure at the tank reaches -1 or -2 m H2O, as long as the air in the pipeline will be summed again in the tank or at the air valves when the system restarts?

As mentioned above, a negative pressure in the tank does not necessarily mean that it is empty of water. In many cases it does, but the important thing to look at to determine if the tank is empty or not is the calculated maximum gas volume compared to the full tank size. HAMMER does not make any special accommodations if the gas volume exceeds the tank volume, and you'll get a user notification stating that the tank is empty and that you will need to make adjustments. This means that HAMMER assumes you will design the tank so that it does not become empty. It does not model the details of air entering the pipeline so if this situation occurs and you do not want to adjust your tank to prevent it, you'll need to decide if you are OK with accepting the approximated results due to this limitation. You can read more about this in the following article (and you can find many more articles about HAMMER by searching our Wiki): Hydropneumatic tank user notifications: "gas volume exceeds volume of tank" or "fills its containing tank"