Applies To
	Product(s):	HAMMER
	Version(s):	08.11.XX.XX and higher
	Area:	Output and Reporting

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; some are shown as gauge pressure and some are shown as absolute pressure:

• "Gas Pressure (Maximum)" and "Gas Pressure (Minimum)" in pipe properties: gauge pressure
• Adjacent pipe Pressure Time History in Transient Results Viewer: gauge pressure
• Gas Pressure graph in Extended Node Data section of Transient Results Viewer: absolute pressure
• Maximum Pressure of Gas and Minimum Pressure of Gas referred to for the tank in places such as user notifications: 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 10.366 m for the transient simulation. So, gauge pressure = absolute pressure – 10.366
• When using Variable Elevation Curve: Calculated Tank internal HGL = Water Level + Gas Pressure (gauge)
• There may be headloss through the opening, as a function of the entered orifice diameter, minor loss coefficient and ratio of losses entered in the tank.

See Also

Modeling Reference - Hydropneumatic Tanks