Differences in hydropneumatic tank gas pressure results

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

Problem

The gas pressure results for a hydropneumatic tank node (such as "Gas Pressure (Maximum Transient)") appears to be different than the pressure at the end of the pipe connecting to the tank (as seen in the time history (graph) in the Transient Results Viewer).
 

Solution

When looking at the "Pressure (Maximum, transient)" or "Pressure (Minimum, transient)" result in the hydropneumatic tank compared to the same result in the adjacent pipe, a difference can be due to headloss across the inlet opening. The max and min pressure results for the tank refer to the pressure inside the tank, whereas the max and min pressure results in the adjacent pipe refer to the pressure just outside the tank (in the pipeline). Based on the "minor loss coefficient", "Diameter (tank inlet orifice)" and "Ratio of losses", there will be a headloss across the tank inlet. If the headloss is too large, you can potentially see a large difference in pressure results between the tank and the pipe. You may need to adjust the tank inlet parameters if the restriction is too large and is negatively impacting the effectiveness of the surge protection.

When looking at the gas pressure results - this is due to the two pressure being reported differently (one as gage and one as absolute) plus headloss through the opening.

Hydropneumatic tanks include results for the gas pressure in the tank. The results for gas pressure is available in a number of locations. The Transient Result Output Log includes data for tanks for Maximum Pressure of Gas and Minimum Pressure of Gas. The hydropneumatic tank properties include result fields for "Gas Pressure (Maximum)" and "Gas Pressure (Minimum)". Finally, the Extended Node Data tab in the Transient Results Viewer can display gas pressure over time.

When viewing a time history of pressure in the Transient Results Viewer for the pipe endpoint adjacent to the tank, or when looking at the max/min transient pressure in the properties of the pipe, this is shown as gage pressure. In contrast, maximum Pressure of Gas, Minimum Pressure of Gas, and the other gas pressure results for the tank node itself is displayed as the absolute pressure of the gas. This is done to be consistent with the gas pressure calculations, which are also in absolute pressure. 

The exact method of computing the tank node absolute pressure depends on the elevation type used for the tank. The different methods used to calculate hydropneumatic tank gas pressure are related to the different Elevation types. The following are the ways to calculate hydropneumatic tank gas pressure for different elevation types:

a. Tank elevation type is Fixed:

Gas Pressure = Gas Head (hydraulic grade inside the tank) + Atmospheric Pressure Head – Tank Elevation

b. Tank elevation type is Mean Elevation:

Gas Pressure = Gas Head (hydraulic grade inside the tank) + Atmospheric Pressure Head – Liquid Elevation (Mean)

c. Tank elevation type is Variable Elevation:

Gas Pressure = Gas Head (hydraulic grade inside the tank) + Atmospheric Pressure Head – Water Surface Elevation in tank


Note: Atmospheric Pressure Head is assumed to be 1.0 Atm (10.33 m) for the transient simulation.

Solution - 

Additionally, there may be headloss through the opening, as a function of the entered orifice diameter, loss coefficient and ratio of losses entered in the tank.

See Also

Modeling Reference - Hydropneumatic Tanks

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