Maintenance and Troubleshooting | 6-43
Filtration Stages
1. The first filtration stage is a coarse Membrane Filter, which is a thin
membrane with small pores. Gases will pass through the pores in the
membrane with only a small pressure drop. Liquid droplets will stay
on the upstream side because their surface tension is too high to allow
them to pass through the pores. This filter removes particulate and
high surface-tension liquids such as water, alcohols, glycols, and some
amines. Most of the low-surface tension liquids such as hydrocarbons
will also be eliminated. Since this membrane has a low surface area
and volume, the filter will not absorb volatile liquids. In contrast to the
membrane filter, a single-stage, coalescing filter can have problems,
because during upset conditions, these single stage filters become
saturated with volatile liquids and these liquids can only be evapo-
rated slowly over time.
2. The second filtration stage is another Membrane Filter, which has
smaller pores than the first stage. In this stage, small quantities of low
surface tension liquids, such as hydrocarbons, that are not extracted
by the first filter are removed. Most of the pressure drop through the
entire Filterblock Assembly occurs across the second filter stage.
3. The third and final filtration stage is a small Coalescing Filter, which
removes trace quantities of liquid aerosols as small as 0.1 microns.
Should the membranes in the first two stages become ruptured, this
stage will temporarily act as the main particulate and liquid filter.
Vent Flow
• The total vent flow in the Model 933S Analyzer is the sum of the flow
through the analyzer plus the flow from the drain Flow Restrictors in
the three filter stages.
• Maximum 20 NLM (42.4 SCFH) for the High Pressure/Low Range Kit.
• Combined flow through the three drain Flow Restrictors is roughly
proportional to the pressure at the filter inlet.
• Total flow from the three drain Flow Restrictors will increase by about
300 SCCM (0.3 L/min or 0.6 SCFH) for each 30 PSI (210 KPA) increase
in the inlet pressure. Typical values for this combined drain flow are:
- 1.0 L/min (2.12 SCFH) at 100 PSIG (700 KPAG) inlet pressure.
- 2.0 L/min (4.24 SCFH) at 500 PSIG (35000 KPAG) inlet pressure.
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