Diagnostic Procedures - Siemens Maxum II PD PA AP Service Manual

Valves
4.6 Model 11 Valve
Maintenance work on the Maxum II analyzer should only be performed when the area is known
to be safe for the work to be done.
Note
This procedure must only be executed with the consent and approval of all applicable local
safety personnel and/or the local authority having jurisdiction.
4.6.5

Diagnostic Procedures

Depending on the installation, the following tests can be performed with the valve mounted in
the analyzer. Other tests require the analyzer be shut down and valve ports disconnected.
These diagnostic tests indicate specific areas of the fault or trouble.
Valve Leakage
Vapor analyzers generally have the sample at atmospheric pressure, so any leakage would
be from a carrier-gas port to a sample port within the valve. With the sample inlet flow turned
off, the sample outlet flow should be zero in either the "air off" or "air on" condition. Check for
small leaks by immersing the sample outlet tubing in a beaker of water. Bubbles indicate
internal leakage.
The liquid sample streams may have pressures several hundred pounds higher than the carrier
gas. Leaking between ports will show up on the analyzer chromatogram as base-line shift
when the sample pressure is removed from the valve.
Plugged Valve
Plungers in the valve are pressed upward by air or spring action, but when released depend
on their own weight and sample pressure to drop them to the "open" position. For very small
sample pressures, for example below 5 psi (34 kPa), it may be possible to see problems where
the flow path may not open. This is more likely if the sealing disc has been held against the
cap for a long time (such as a valve in storage). Check for flow across alternate flow paths,
such as air on and air off. It may be necessary to temporarily increase the sample pressure to
get the flow started, and then reduce it to normal after a few cycles. For the Model 11 valve,
samples are typically run at 15 psi (103 kPa).
Ruptured Sealing Disc
To test for a ruptured sealing disc apply air to valve ports, one at a time, while sealing off all
others. Place a small amount of soap solution such as Leak Tec® over the upper control port's
bleed tube air signal manifold (23). Any escaping air at this point indicates a ruptured disc. If
this occurs, proceed with a disc replacement.
If the disc does not appear to be ruptured, remove the valve from service and replace it with
a new valve.
Slow Erratic Piston Switching
Excessive friction on the actuating pistons of the valve can be caused by lack of lubricant, or
dirt or contamination on the O-rings. As a result, the valve may switch erratically, switch slowly
or not switch at all. These conditions can cause a leak port to port, across the sealing disc,
double sampling, or complete closing of flow between two or more ports.
This condition requires disassembly of the entire valve (maxi-maintenance). It is recommended
that the valve be returned to Siemens for service.
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Maxum II Valves and Oven Components
Service Manual, 10/2018, A5E42019844001