Hydropneumatic tank user notifications: "gas volume exceeds volume of tank" or "fills its containing tank"

Applies To 
Product(s): HAMMER
Version(s): V8i, CONNECT Edition
Area:  Modeling
Original Author: Scott Kampa, Bentley Technical Support Group


What do the following user notifications mean and how can I resolve them?

"Maximum calculated gas volume (____) exceeds the volume of the hydropneumatic tank (____). A larger tank should be used."


"The bladder-type hydropneumatic tank fills its containing tank"


These  message indicate that the tank has become empty during the transient simulation. The message about the "bladder-type" occurs when using a bladder in your tank (has bladder = true). In order to assure that the tank is providing adequate protection, such as making sure that the pressure doesn't drop too quickly due to the pressure/volume relationship and headloss, you will want to make sure the tank doesn't become fully empty. HAMMER assumes that you will design the system in this manner, so if the tank becomes empty and you see this notification, the results may be invalid as the calculated gas volume will exceed the size of the tank. 

Note also that an empty tank (gas volume = total tank volume) does not necessarily mean a gas pressure of zero. When the tank is empty, the gas may still be pressurized. Conversely the gas pressure may reach zero before the tank is fully empty. For more on this, see the section "A note on pressure at an empty condition" in the main Hydropneumatic Tank Technote (link in "see also" below).

Preventing the tank from becoming empty can sometimes be a difficult task because there are many factors that impact the effectiveness of the tank, including the total tank size, initial pressure and gas volume or preset pressure (for bladder tanks, which determine the relationship between change in pressure and change in volume), headloss through the inlet, etc. It is therefore often a trial-and-error process to fix this, keeping in mind other constraints such as cost and space. The basic steps to take include adjusting the tank parameters, running the simulation, checking the transient envelope and max gas volume, adjusting as necessary, and repeating the steps again. 

Try to make adjustments not just to the tank size, but the other parameters as well. Pay attention to other user notifications as well, as there may be some other issue unrelated to the hydropneumatic tank. For more information on modeling hydropneumatic tanks, please see this link. There is also a Hydropneumatic Tank example model located in the Samples folder within the HAMMER installation folder.

Note that you might encounter a situation where the gas volume in the hydropneumatic tank continues to increase no matter how long the simulation is extended. This can occur in situations where there is no other boundary condition (water source like another tank or reservoir) at a higher elevation than the tank, so as long as the pump remains off, the hydropneumatic tank will continue to drain. You may need to consider how long the pumps will remain off and potentially model a pump shutdown followed by startup.

But what if I want to model the tank becoming empty?

When a hydropneumatic tank becomes empty, additional factors come into play which cannot be properly simulated. For example if the tank has a bladder, HAMMER does not have enough information to know what would happen when the bladder completely fills the tank (would it stretch into the pipeline? Would it stop and essentially act as a junction? Would it break? Something else?). If the tank does not have a bladder, HAMMER cannot simulate gas/air entering the pipeline and traveling some distance, as air/vapor pockets are only modeled locally, as explained here.

If other transient analysis software claims to be able to model empty tanks, any assumptions that they make in this empty condition are likely not any better than HAMMER's, as the results will still be skewed. HAMMER will still model the surge pressure wave reflections when the tank is empty (nothing changes at the empty threshold point - the volume simply continues to increase and the hydraulic grade continues to follow the gas law) but the point to focus on is that you will want to design the system so that this does not happen, and that is HAMMER's assumption.

See Also

Modeling Reference - Hydropneumatic Tanks