166 5977B Series MSD Troubleshooting and Maintenance Manual
6 Vacuum System
Vacuum Seals
Vacuum seals are shown in Figure 32 on page 167.
Several types of Viton elastomer O-ring seals are used to prevent air leaks into
the analyzer chamber. All these O-rings, and the surfaces to which they seal,
must be kept clean and protected from nicks and scratches. A single hair,
piece of lint, or scratch can produce a serious vacuum leak. Two of the O-rings,
the side plate O-ring and the vent valve O-ring, are lightly lubricated with
Apiezon-L vacuum grease.
Face seals
A face seal is an O-ring that fits in a shallow groove. The sealing surface is
usually a flat plate. The manifold side plate and end plate O-rings fit into
grooves around the large openings in the analyzer chamber. The side plate
swings into place against the side plate O-ring, and must be held in place when
the MSD is turned on for pump down to ensure a good seal.
The front and rear end plates are screwed onto the manifold and should not
need to be removed. The GC/MSD interface fastens to the manifold with three
screws.
The calibration valve assembly is fastened onto the front end plate by two
screws. The vent valve knob threads into the front end plate. Small O-rings in
grooves in the front end plate provide vacuum seals.
The diffusion pump baffle adapter has a groove for its O-ring. The baffle
adapter is clamped to the manifold with four claw grips.
KF (NW) seals
Most of the seals for the high vacuum pumps, foreline gauge, and foreline
pump are KF seals. KF seals have an O-ring supported by a centering ring. The
centering ring can be either on the inside or outside of the O-ring. The clamp
presses two flanges against the O-ring, making a seal. KF clamps must not be
overtightened.
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