Section 1. Introduction; Description; Operating Principles - Agilent Technologies VPI Series Instruction Manual

Section 1. Introduction

1.1 Description

The Vacuum Pump Isolation (VPI) valve is a safety valve that protects a vacuum system
in the event of power failure by isolating the vacuum system and venting the mechanical
pump. This avoids oil back up and allows the motor to restart the pump more easily. The
VPI valve is offered with the ISO-KF flange sizes and solenoid voltages/frequencies most
commonly found on mechanical pumps. Since it operates with atmospheric pressure and
activates upon loss of electrical power, the novel design requires no external pressurized
gas source.
The fast acting VPI valve is light weight, constructed of vacuum compatible materials, and
maximizes pumping conductance. Lifetimes in excess of 100,000 cycles are typical, which
translates into years of trouble free use in suitable environments. The opening burst is less
than the critical backing pressure for turbomolecular and diffusion pumps, when
appropriately sized mechanical pumps are used. The closing burst is minimized by the
unique patented buffer volume design.
The VPI valve consumes little power. Installation is simple because standard ISO-KF
dimensions are used, allowing the valve to replace elbows, tees, and crosses of the same
size. Even when the mechanical pump is equipped with an integral anti-suckback valve, a
VPI valve should be used because the pump's integral valve will not vent it.

1.2 Operating Principles

The VPI valve's solenoid valve is connected in parallel with the mechanical pump's
electrical supply, either at its source or at the pump's switch. When the electrical power is
on, the solenoid valve is closed, allowing the pump to keep the VPI valve's body and the
vacuum system evacuated. Interruption of electrical power to the mechanical pump causes
the solenoid valve to open. Air is admitted into the VPI valve causing it to close very
quickly. This isolates the vacuum system from the mechanical pump, while the pressure
differential between the outside atmosphere and the vacuum system provides the force to
maintain the valve in its closed position without electrical or pneumatic power. With the
vacuum system isolated, a series of small orifices admits air to the inlet port of the
mechanical pump until it has risen to atmospheric pressure. When the mechanical pump is
restarted, it evacuates the area above the piston until the pressure is lowered to
approximately that of the vacuum system. The VPI valve automatically opens again
allowing the vacuum system to be pumped at the full speed of the mechanical pump.
Vacuum Pump Isolation (VPI) Valve
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