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Glass Flow Meter Sealing: Selecting Chemical-Resistant Gaskets for Borosilicate Gas Tubes
Quick Answer: For glass flow meter sealing on borosilicate gas tubes, PTFE gaskets are the first choice for aggressive gas service. EPDM suits clean dry air and nitrogen. FKM suits natural gas and LPG. Always derate torque by 30 percent on glass tube ends and use flat gaskets not O-rings on the sealing face.
Why the Gasket Fails Before the Glass Tube
Most glass tube rotameter failures start at the seal. A gasket swells, hardens, or cold flows. That creates uneven stress on the glass rim. The leak appears small. A technician tightens the packing nut. The glass tube cracks. We have seen this sequence on many customer sites in Southeast Asia.
In gas service the problem gets worse. Gases penetrate small gaps faster than liquids. Dry gas can harden EPDM. Wet gas can attack rubber fillers. A small leak may go unnoticed until a gas monitor picks it up. This is why gasket selection is a safety decision, not just a maintenance item.
Borosilicate Glass Tube Pressure and Temperature Limits
Silver Instruments LZB glass tube flow meters for gas run at low pressure. Typical limits are 0.6 MPa at 20 deg C for DN15 to DN50. For DN65 to DN100 limit pressure to 0.4 MPa. Keep gas temperature below 100 deg C continuous. The glass tube end is ground flat but not always mirror polished. A thin PTFE gasket conforms to this face without high force. A hard rubber gasket needs more compression and that extra force can crack the tube shoulder.
Gasket Material Options for Gas Service
PTFE works from -60 deg C to 260 deg C. It resists wet chlorine, sulfur dioxide, hydrogen chloride, ammonia, and most solvents. Cold flow is a known weakness but the low torque on glass tubes reduces this risk.
FKM works from -20 deg C to 200 deg C. Use FKM for dry natural gas, LPG, and fuel gas with light oil carryover. Do not use FKM with wet ammonia, hot water, or polar solvents.
EPDM works from -40 deg C to 150 deg C. Use EPDM only for clean dry air, nitrogen, and inert gas. EPDM fails fast with mineral oil, gasoline, chlorinated solvents, and strong acids. NBR is not recommended because it hardens after temperature cycles and leaks at the packing nut.
Gas Type Selection Rules
For dry air and nitrogen use EPDM. A customer in Indonesia runs 40 LZB-25 glass tube meters on instrument air with EPDM gaskets. The gaskets last four years before scheduled replacement.
For wet chlorine or chlorine dioxide use PTFE. A water treatment plant in the Philippines had EPDM gaskets crack within six weeks on a DN15 chlorine gas line. PTFE gaskets fixed the leak and the same meter has run nine months without a seal replacement.
For LPG and natural gas use FKM. A distributor in Mexico sells LZB-15 rotameters for gas burner test rigs. FKM gaskets handle the fuel gas and slight oil carryover. Do not use PTFE on very dry fuel gas because it can creep under the compression nut and loosen
For ammonia gas use PTFE. EPDM and FKM are both poor choices with ammonia vapor. Use a thin flat PTFE gasket and tighten slowly.
Torque Limits and Installation Rules
Most technicians overtighten glass tube flow meters because they treat them like metal pipe fittings. Glass does not yield plastically. It cracks. For DN15 and DN25 rotameters, finger tight plus a quarter turn is enough for a PTFE gasket. For DN40 and DN50 use a torque wrench set between 3 Nm and 5 Nm. Never exceed 8 Nm on a glass tube end.
Clean the tube face before seating the gasket. Remove old gasket material from the packing nut threads. Do not use pipe dope or thread sealant on the glass face. Thread sealant belongs on metal threads only.
Common Failure Patterns on Customer Sites
A paint manufacturer in Vietnam had a DN25 glass tube meter on toluene vapor. The EPDM gasket swelled and dissolved into the flow path. The operator tightened the packing nut to stop the leak and cracked the tube. We replaced the tube and fitted PTFE gaskets. That meter has run without a seal failure since last year.
In the Middle East a gas calibration lab used NBR gaskets on dry methane. The NBR hardened after four months and the leak was found during a soap bubble test. We changed the sealing kit to FKM and set the torque at 4 Nm for the DN25 tube. The leak stopped and the glass tube did not crack.
Recommended Sealing Kits for Silver Instruments LZB Flow Meters
For dry air and instrument air order the EPDM sealing kit for LZB-15, LZB-25, or LZB-40. For wet chlorine, sulfur dioxide, and ammonia gas order the PTFE sealing kit. For LPG, natural gas, and fuel gas order the FKM sealing kit. Each kit includes two flat gaskets and two packing nuts.
When you request a quote, send us the meter model number, DN size, gas composition, pressure in bar, temperature in deg C, and flow range. This helps us match the gasket material to your gas stream.
FAQ
Which gasket material is best for borosilicate gas tubes? PTFE is best for aggressive gas. EPDM works for clean dry air and nitrogen. FKM works for natural gas and LPG.
How much torque can a glass tube flow meter take? For DN15 to DN25 use finger tight plus a quarter turn or 3 Nm to 5 Nm. Do not exceed 8 Nm on the glass tube end.
Why do EPDM gaskets fail on chlorine gas? EPDM is not compatible with wet chlorine. It hardens and cracks within weeks. Use PTFE for chlorine and sulfur dioxide.
Can I use thread sealant on the gasket? No. Thread sealant belongs on metal threads only. Do not apply pipe dope to the glass face or gasket surface.
How do I order a sealing kit for a Silver Instruments LZB flow meter? Send us your meter model, DN size, gas composition, pressure in bar, temperature in deg C, and flow range. Contact Silver Automation Instruments. Tel: +86-25-68650347. WhatsApp: +86-25-52155837. WeChat: +86 15365082610.
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