Agilent Technologies VPI Series, Инструкция по эксплуатации

Страница: 15 / 38

Vacuum Pump Isolation (VPI) Valve
15
Figure 1 (a): Normal Operation – Open Position
Power is on to the mechanical pump and
the VPI valve. The mechanical pump, inlet side of the VPI valve, and the VPI valve are all
under vacuum at approximately the same pressure. The spring is holding the VPI valve
open, and the vacuum system is exposed to the mechanical pump’s full pumping speed.
Figure 1 (b): Power Interruption The VPI valve solenoid loses its power, since it is
connected in parallel to the mechanical pump. The solenoid opens to the atmosphere. Air
enters the VPI valve through the solenoid and creates a higher pressure above the piston
than exists below it. The piston closes rapidly (30 ms). During this time only the inside of
the piston (buffer volume) is being filled, isolating the vacuum system from any introduction
of air. This figure shows the piston in transit downward.
Figure 1 (c): Valve Closed – Mechanical Pump Vented The piston is fully down and the
VPI valve is fully closed. The higher pressure above the piston forces it against the valve
seat, where the Viton
®
O-ring makes a vacuum tight seal. Air from the higher pressure area
above the piston continues to flow through the small orifice in the top of the piston into the
inside of the piston, and through a second small orifice in its side into the mechanical
pump’s inlet port. This vents the mechanical pump to atmospheric pressure gradually.
When the pump is fully vented, the pressure above the piston, inside the piston, and in the
mechanical pump are the same (atmospheric). The pressure below the piston, in the
vacuum system, is still lower and the piston remains down (closed), maintaining the
vacuum system under vacuum.
NOTE
Given enough time, the pressure in the vacuum system will rise
due to outgassing, leaks, etc., or venting by the operator, and
when the pressure is high enough, the spring will open the
piston and the vacuum system will be at the same pressure as
the mechanical pump (atmospheric).
Figure 1 (d): Power Restored
The solenoid coil is energized causing the solenoid to close,
isolating the inside of the VPI valve from the outside atmosphere. The mechanical pump
evacuates the inside of the piston through the small orifice in its side and the area above the
piston through the small orifice in the top of the piston via the inside of the piston. When the
pressure above the piston is approximately equal to that in the vacuum system, the spring
pushes the piston upward, fully opening the VPI valve. The vacuum system is now back in
normal operation and exposed to the full pumping speed of the mechanical pump.
NOTE
In Figure 1, the mechanical pump is connected to the port on
the right side and the vacuum system to the bottom port.