Baslton Sample Filter Application Recommendations
Acid Plant Stack Gas
A frequently encountered sampling requirement is to analyze the gas composition in the exhaust from absorbers or scrubbers in acid manufacturing plants. The exhaust gas invariably contains droplets of dilute acid, which must be removed from the sample before it enters the analyzer. The recommendations are similar to those for natural gas sample filtration: Grade DQ or DX filter tube, inside-to-outside flow, and two stages of filtration if slipstream sampling is required. Depending upon the composition of the suspended liquid, housings may be stainless steel, PTFE (Model 95T), Monel (Model 95M), or PVDF (DFU 8822-11).
Sampling Ambient Air or Other Atmospheric Pressure Gas
The filtration requirement for ambient air samplers is usually to remove solid particles or liquid droplets which could deposit on analyzer optical surfaces or cause other calibration problems. Grade DX or DQ filter cartridges are recommended. For low flow rate personal samplers, the compact and lightweight DFU 9933-05-DQ is often used. For higher flow rates, the Model 90 filter holder with Grade DX or DQ filters is recommended.
Ambient air sampling systems are often under negative pressure, induced by the sampling pump. If it is necessary to drain coalesced liquid from the system, the external reservoir is often the most convenient method (see Figure 2).
Most water analyzers are well protected against the damage or calibration drift caused by solid contamination if a 10 micron (LP Grade 30) filter cartridge is used. If long filter life is desired in a system with high solids loading (including most tap water, well water, and cooling water), a two stage LP cartridge system is recommended: LP Grade 10 followed by LP Grade 30.
Sampling Liquid Effluent Streams
Liquid effluent analyzers usually deal with aqueous streams having a high solids content. In addition, the analyzers are often located in remote areas of the plant and are infrequently serviced. Therefore, the sample filter system must have long life between filter cartridge changes, even in a high solids situation. The general recommendation for this requirement is a two stage filter system, LP Grade 10 filter cartridge followed by LP Grade 30 filter cartridge. The filters should be oversized as much as possible without causing excessive lag time. Plastic filter housings are usually a good choice.
Measurements of steam and condensate conductivity, specific ion concentrations, and feedwater additive concentrations are often required in high pressure boiler systems. In a continuous sampling system, the high pressure steam or condensate is cooled to below 100°F (38°C) and then the pressure is reduced to near atmospheric pressure for metering to the analyzers. Filtration is required upstream from the pressure reducing valves, to prevent pitting of the valve seats by suspended particles and to eliminate variations in flow rate to the analyzers.
A stainless steel filter housing with the appropriate pressure rating and Grade DX or DQ filter cartridge is recommended. Since the analyzer system is often located some distance from the sampling point, slipstream filtration is usually required. Figure 9 shows a sampling system in operation at a nuclear steam generating facility
On-Line Process Analyzers
The variety of filtration requirements for on-line process analyzers precludes making general recommendations above for the required filtration functions. The filter housings most frequently used for process analyzer applications are the Model 95S6 and Model 91S6, which provide the corrosion resistance of Model 316 stainless steel (complies with NACE specification MR-01-75), a pressure rating of 5000 psig (345 barg), have full slipstream sampling capability, and minimum internal volume.
To protect gas composition analyzers from liquids and solids, Grade DX or DQ filter tubes are recommended, with inside-out flow direction. If both slipstream sampling and coalescing are required, a two stage system must be used, as described on page 75.
The Model 85, 5000 psig (345 barg) rating and Model 37, 4000 psig (276 barg) rating housings comply with NACE specification MR-01-75. For lower pressure applications, any stainless steel housing of appropriate flow capacity may be used.
Stack Gas Sampling
The Model 30 housing with Grade DH filter cartridge is used for quantitative determination of solids in stack gas, as described on page 68. The Model 30 may also be used as a beginning-of-the-line filter at stack gas temperature up to 900°F (95°C), to prevent solids from entering the gas sample line. Grade DH is used for this purpose. After the sample is cooled, a coalescing filter with Grade DX tube is used to remove suspended liquids before the sample goes to the analyzer. Flow direction is inside-to-outside. Model 33G or 45G housings are often used in this application to permit a visual check on the liquid level in the filter housing. Since there often is a considerable amount of liquid present at this point, positive steps must be taken to drain the housing to ensure that liquid does not build up and carry downstream to the analyzer.
The coalescing filter should be located as close to the analyzer as possible to minimize the chance of condensation between the filter and the analyzer. Additional precautions which can be taken to avoid downstream condensation are to cool the sample below ambient temperature upstream from the coalescing filter, and to heat the line.