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What Does “Head (HGL)” Mean for Submerged Pumps and Exit Pressures?
In AFT Fathom and AFT Impulse, it is possible to model a submerged pump where a short and possibly frictionless suction pipe for the pump’s inlet does not need to be modeled. When modeling a submerged pump, there are two options available for specifying the system inlet boundary condition at the pump suction. As shown in Figure 1 below, the Submerged Pump’s Suction Pressure can either be specified as “Head (HGL)” or “Pressure”.
Modeling a submerged pump is not the only time where the “Head (HGL)” or “Pressure” choices will arise. If an Exit Valve (i.e., a valve that discharges into ambient conditions with no downstream pipe), Exit Orifice, Spray Discharge Nozzle, Exit or Inline Exit Relief Valve is modeled, then an “Exit Pressure” value like that shown in Figure 2 will need to be specified with the same type of “Head (HGL)” or “Pressure” choices. This blog will clarify how to specify either option appropriately.
Consider the system in Figure 3 where a pump is submerged in a large tank. The pump is at an elevation of 75 meters (246 ft), liquid surface elevation of the tank is 100 meters (328 ft) and is open to the atmosphere, therefore, the liquid column above the pump suction is 25 meters (82 ft).
Modeling this system in AFT Fathom is very simple. Figure 4 contains three identical systems on the same AFT Fathom Workspace. The top system includes a reservoir junction that represents the tank from Figure 3 as well as a short, frictionless connector pipe. The top system serves as a reference to help determine the inlet suction pressure for the middle system and allows for a consistency comparison amongst the three systems. The middle system models a single pump junction with no suction pipe and represents the submerged pump in Figure 3. The middle system uses the “Pressure” option for the Submerged Pump Suction Pressure option while the bottom system uses the “Head (HGL)” option.
After running the model, the pump inlet pressure in the top system is determined to be 3.458 bar (stagnation). This value can then be entered into the middle system when the “Submerged Pump” feature is chosen with the “Pressure” option selected for the Suction Pressure. The suction pressure can also easily be calculated from the relationship shown below the image in Figure 5.
The bottom system in Figure 4 uses the “Head (HGL)” option for the Suction Pressure specification on the submerged pump. One may be tempted to enter the value of the liquid column height above the pump suction. However, this is incorrect. When specifying the Suction Pressure with the “Head (HGL)” option, the liquid surface elevation of the reservoir or tank in which the pump is submerged is what needs to be specified.
When the liquid surface elevation of the reservoir in which the pump is submerged in is specified, then AFT Fathom and AFT Impulse will be able to determine the amount of liquid head above the pump suction based upon the pump’s inlet elevation and the “Head (HGL)” value. This will provide the correct boundary condition at the inlet of the pump, as shown in Figure 6.
After running the model for the three systems in Figure 4, you can see from the results shown in Figure 7 that the systems are identical. This should help clarify that the “Head (HGL)” value that needs to be specified is the liquid surface elevation of the reservoir or tank that the pump is submerged within, NOT the height of the liquid column above the pump suction.
If the pump is submerged into a pit which is below zero elevation, the same concept applies, just be sure to specify the elevations correctly with the appropriate negative values. The pump suction is 170 m (558 ft) below sea level, therefore the “Inlet Elevation” for the pump will be -170 meters. However, the liquid surface elevation of the pit in this case is 150 meters (492 ft) below sea level, therefore, the “Head (HGL)” that is specified will be -150 meters. This establishes the 20 meters (66 ft) of liquid column above the pump suction.
As mentioned previously in this blog, the “Head (HGL)” and “Pressure” options can exist for other junction types such as exit valves, spray discharge nozzles, etc. This is the “Exit Pressure” that must be defined for the case of an Exit Valve that was shown in Figure 2. The “Exit Pressure” is simply the ambient conditions in which the fluid is discharging into through the valve, spray discharge nozzle, etc. If the fluid is discharging directly into the atmosphere, then it is easiest to choose the “Pressure” option for the “Exit Pressure” and then simply enter the atmospheric pressure value. However, if the valve or spray discharge nozzle is submerged into a tank or reservoir and that is the medium in which the fluid is discharged into, then it may be easier to use the “Head (HGL)” option. Again, specify the elevation of the component in the junction property window with reference to the zero point of the model. Then for the “Head (HGL)” option, specify the liquid surface elevation of the tank or reservoir in which the component is submerged.
In conclusion, this discussion should now have clarified that the value that needs to be entered for the “Head (HGL)” option for the Suction Pressure on a submerged pump or Exit Pressure for an Exit valve is the liquid surface elevation of the tank/reservoir in which the component is submerged. NOT the liquid column of fluid above the component. Based upon the junction’s inlet elevation and the “Head (HGL)” that is entered, AFT Fathom and AFT Impulse will properly take into account the liquid column of fluid above that component.