Hydropneumatic Tanks with and without blader (Hammer V8i)

Question

During the modeling of a pressured irrigation network, I encountered the following problem when modeling the Hydropneumatic Tank. 
 When modeling the Hydropneumatic Tank without bladder, I get to the conclusion that I need 1.5m 3 with 0.250m 3 of initial gas volume. 
 The problem starts when I am trying to model the Hydropneumatic Tank with bladder. 
 According to " Modeling Reference - Hydropneumatic Tanks [TN] " at the Transient Simulation Behavior --> With a Bladder it says: 
 
 '.....For example, consider a tank that has been given a full volume of 500 L and the initial conditions pressure head is 50 m. Assume that the pre-charge pressure is 5% of the steady state pipeline pressure. (this is a number that you would know ahead of time) So, the gas preset pressure is set to 2.5 m (50 m times 5%). In this case, HAMMER computes the 'K' constant as (2.5 m + 10.33 m)(0.5 m 3 ) = 6.415. Since K is known now, the initial gas volume for the transient simulation (after the bladder is submitted to pipeline pressure) is computed as V = K/P = (6.415)/(50 m+10.33 m) = 0.106 m 3 = 106 L....." 
 On my model I have the calculated pressure from steady state is 47.67m. 
 If I reverse the calculations, for 1.5m 3 tank with 0.250m 3 of initial gas volume, I get as a result that the "Pressure (Gas-preset)" should be -0.665m, which should not be possible, because with negative pressure, the tank when subjected to the pipeline pressure will be full of water. I feel that I have done some mistake somewhere but I don't know what.

On the following photo you can see the settings/results with the bladder and with Pressure (Gas-preset) set at 5%of pipeline pressure as instructed at Modeling Reference - Hydropneumatic Tanks [TN ]. How do I set up the tank with bladder, in order to have the desired results? And by desired I mean total tank volume 1.5m 3 with 0.250m 3 of initial gas volume that seems to be working on my simulations without the bladder.

Any help on the matter is much appreciated.

Scott Kampa · Accepted Answer

Hello Anthimos, 
 
Depending on the method you use to get the gas preset pressure, it is possible to end up with a negative preset pressure. This will occur if the initial pressure is less than atmospheric pressure. This can be a real situation because a slightly negative gage pressure is still a positive absolute pressure. 
 
Practically speaking, it means that your initial pressure is very low and the bladder would actually be slightly deflated in order to achieve the desired initial gas (bladder) volume. Under the assumption that the bladder fills the containing tank before being submitted to pipeline pressure, this deflated bladder would indeed experience a negative gage pressure in order for it to "stretch" to fill the containing tank. However there are still moles of gas inside the bladder, so it can still be compressed and follow the gas law relationship between volume and pressure. In this situation, check if the initial pressure, elevations and assumed initial bladder size are correct, as it would not be typical for a hydropneumatic tank to experience such low initial pressures. 
 
The following link has addition information on entering this data for a hydropneumatic tank: communities.bentley.com/.../30028.entering-hydropneumatic-tank-gas-preset-pressure-based-on-assumed-initial-bladder-volume . 
 
Please let us know if you have any questions. 
 
Regards, 
Scott