
Applies To 



Product(s): 
HAMMER 


Version(s): 
08.11.XX.XX, 10.XX.XX.XX 


Area: 
Modeling 


Original Author: 
Scott Kampa, Bentley Technical Support Group 

Problem
When computing a model with a gas law exponent greater than 1.0 (for example 1.2), it appears that the initial volume calculations for a hydropneumatic tank is not taking this into account. Why is that?
How is the gas law exponent used in the initial conditions vs. transient simulation?
Solution
The initial conditions calculation uses the isothermal gas law (PV=constant), which assumes a gas law exponent of 1. The isothermal gas law is used because initially nothing is changing (or things are changing very slowly), which works well for the steady state calculations. In particular, the temperature of the gas is constant for the gas law coefficient is not used. Note that this assumption is also made with the calculation of the initial gas volume for a bladder type tank, based on the Preset Pressure  more on that can be found in HAMMERspecific articles such as the ones in the "See Also" section below.
When the gas is expanding or getting compressed rapidly (i.e. during a transient simulation), then an adiabatic process is typically more suitable and the gas law exponent input field is used. This is because when the gas expands or compresses very quickly, heat is lost or gained so quickly that the changes can't be transferred to the surrounding environment.
Note that a constant number of moles / mass of gas in the tank and constant temperature is assumed, so the 'nRT' term in the gas law equation is replaced by a constant, K (PV=K).
See Also
Modeling Reference  Hydropneumatic Tanks
Entering Hydropneumatic tank gas preset pressure based on assumed initial bladder volume