A pressure drop on the regulated low-pressure gage indicates that the leakage on/off valve is leaking. This completes your leakage check. You must bleed the system by turning the high-pressure regulator to the VENT position until the high--pressure gage reads zero and then open the system bleed valve. This bleeds all remaining lines.
The purpose of the regulated low-pressure nitrogen system is to supply regulated low-pressure nitrogen to the N2 input connection and the in-system leakage rotameters. This system, shown in figure 2-5, has a range of 0 to 180 psig. You adjust the pressure by using the mechanically operated low-pressure regulator. When you open the low-pressure regulator, nitrogen flows through the one-way check valve to the back side of the one-way check valve that protects the high-pressure regulator. As nitrogen enters the high-pressure lines, it flows to the inlet pressure on/off valve, the system's bleed valve, the vent pressure valve, and through the gage guard that protects the in-systems leakage rotameters. (Nitrogen also flows into the rotameters and to the 200-to 230-psig relief valve. However, this is not considered part of the low-pressure system. For the nitrogen to flow to the item under test, you have to open the inlet pressure on/off valve. This allows nitrogen to flow to the input connection inside the chamber where the item under test is connected.
When you pressurize the low-pressure nitrogen system, be sure that the N2 input connection is capped and the supply cylinder valve is open. Turn the leakage selector valve to the HIGH position and the pressure selector valve to the HG position. Turn the inlet pressure and leakage on/off valves to the ON position. Slowly turn the low-pressure regulator clockwise until the regulated low-pressure gage and the N2 input gage indicate 70 psig. (We use 70 psig because this is the pressure used to calibrate the rotameter system.) At this time, return the inlet pressure on/off valve to OFF. There should be no leakage; but if any leakage occurs, the small ball in the high-range leakage rotameter tube will rise.
Figure 2-5. - Model 1172AS100 regulated low pressure N2 system.Continue Reading