When we add a sprinkler to the database, we define the sprinkler min and max operating pressures - see image below:
When we then place a sprinkler node on the Flowsheet, we can define the Exit Pressure from the Input tab of the Data Palette - see image below. So, if the sprinkler were to discharge into a building, the exit pressure is likely to be 1 atm. However, if the sprinkler is discharging into a pressurised vessel, you have the option of defining the discharge pressure of the sprinkler (tank pressure).
Figure 1 below is taken from the Help file and provides some further background information which might help.
Figure 1