We build diaphragm seal solutions so your instruments can see process pressure without touching process media. Flange seals give us a direct bolt-on interface to standard process piping flanges, which simplifies installation, shortens maintenance time, and preserves measurement accuracy when the fluid is corrosive, viscous, dirty, crystallizing, sanitary sensitive, or at an elevated temperature. With a flanged seal, the process side looks like any other line flange, while the instrument side is hydraulically coupled through a filled cavity to a gauge, switch, or transmitter. This isolates the instrument wetted parts from the process and lets you choose the right metallurgy, the right membrane design, and the right fill fluid for the job, while keeping your instrument standard and serviceable.
Design and Construction of Flanged Diaphragm Seals
Our flanged diaphragm seals are engineered as two major assemblies. The process connection is a raised face or ring type joint flange per common dimensional standards, and the seal housing contains a thin, flexible diaphragm that deflects in response to process pressure. The small displacement of the diaphragm moves an incompressible fill fluid that transmits pressure to the connected instrument. Because only the diaphragm and process flange contact the media, we can tailor those wetted parts to the service while the instrument remains protected.
We offer common flange patterns to match piping specifications so the seal drops into existing bolt circles without adapters. Typical options include ANSI and ASME B16.5 classes in sizes that match the line size or the branch size at the takeoff. Raised face is common for general service. Ring type joint is used when a metal-to-metal gasket is preferred at higher pressures and temperatures. For hygienic processes, we recommend separate sanitary seal designs, but for utilities and industrial fluids a standard raised face flange is usually correct.
Material selection centers on corrosion resistance, mechanical strength, and temperature capability. 316 stainless steel is a versatile baseline for potable water, many chemicals, and clean steam. When pitting resistance or chloride stress cracking resistance is needed, we can move to higher alloys such as 316L, duplex grades, or nickel-based alloys. For aggressive acids or solvents, we can supply a PTFE-lined or PFA-lined wetted path or use a solid alloy diaphragm with an isolation liner. The diaphragm itself can be metallic for durability or elastomeric for special low pressure ranges. Metallic diaphragms are commonly 316L, Hastelloy, or other corrosion resistant alloys with a surface finish that balances sensitivity and robustness.
Every seal cavity is evacuated, filled, and sealed to remove compressible gases that could degrade response. The fill pathway is then leak-tested. The instrument port is threaded or flanged depending on the instrument. For differential pressure gauges such as our Model 140 or 142, we can provide matched pairs of flanged seals connected to the high and low sides, with capillaries sized to equalize temperature effects. For single pressure instruments, a single flanged seal is sufficient.
Application and Compatibility Considerations
The core reason to use a flange seal is to isolate the instrument from media that could plug small passages, attack wetted parts, or create hygienic or environmental concerns. Slurries, polymerizing fluids, crystallizing salts, heavy oils, caustics, and corrosive acids fit this category. Thermal isolation is another driver. If the line runs hot, the seal allows a cooling extension, a capillary, or a remote mount so the gauge or transmitter sits in a cooler environment.
Correct sizing of the flange and bore ensures minimal flow disturbance and a smooth pressure profile at the tapping point. A seal can be mounted on a dedicated spool piece, a side branch, or a valve manifold depending on layout. Orientation matters. Mount the seal so solids do not settle on the diaphragm and so gases do not collect against it. For vertical lines, mounting on the side rather than the bottom can reduce sediment loading. For horizontal lines, install at the top for gas service or at the side for liquid service to avoid air entrapment.
Gasket selection must match the flange facing and the media. For raised face connections, a spiral wound or compressed fiber gasket is common. For ring type joint, select the correct ring profile and material. Torque should follow the flange class and gasket manufacturer guidance to avoid flange leakage or diaphragm preload. Over-torque on the process flange fasteners does not compress the diaphragm but may distort the body if the mating flanges are misaligned. Use clean, flat faces and correct bolt lubrication to achieve repeatable clamping force.
If the seal will be cleaned in place or steamed in place, the diaphragm alloy and fill fluid must withstand those temperatures and chemicals. If the media can permeate metallic surfaces or if permeation through polymer liners is a concern, choose alloys and liners accordingly. For oxygen service, pharmaceutical solutions, and food contact utilities, we can provide degreased, high-purity finishes and compatible fill fluids with appropriate documentation.
Performance, Fill Fluids, and Temperature Effects
The performance of a flanged diaphragm seal rests on three linked elements. The diaphragm must deflect elastically across the instrument range, the fill system must transmit pressure without compressibility or trapped gas, and the installation must control temperature effects that change fill volume.
Diaphragm stiffness is a function of material, thickness, geometry, and effective area. A thinner diaphragm improves sensitivity at low pressures but reduces overpressure endurance. A corrugated or convoluted profile increases area and stroke. For very low ranges, a larger diameter diaphragm helps transmit small differential pressures with minimal error. For high static pressures, a flat, thicker diaphragm resists overload while maintaining adequate resolution.
Fill fluids are chosen for viscosity, thermal expansion, vapor pressure, chemical compatibility, and temperature range. Silicone oils cover broad temperature ranges with stable viscosity. Propylene glycol blends work well for food related utilities. Halocarbon-based fluids are suitable with strong oxidizers. For ultra-high temperatures, specialty synthetic oils protect the instrument while maintaining reasonable response. The lower the viscosity at operating temperature, the faster the response and the smaller the temperature induced error. However, very low viscosity fluids may not be appropriate if permeation or volatility is a concern.
Temperature gradients change fill density and volume, which shifts the zero reading. To control this, we can shorten capillaries, match lengths on differential pairs, add thermal snubbers or cooling elements, and specify a fill fluid with a low volumetric expansion coefficient for the expected temperature band. For hot lines, a remote mount with a cooling element between the process seal and the instrument stabilizes readings. For cold service, prevent wax out or thickening by using a low pour point fill fluid and insulating the seal.
Static pressure effects are also relevant in differential applications. When both high and low sides see the same static pressure, a well matched pair of seals with equal capillaries minimizes common mode error. We factory charge and balance pairs as a set so the zero remains centered at operating temperature and pressure.
Installation, Service, and Documentation
Proper installation protects accuracy and reduces maintenance. Before mounting, verify flange class, bolt pattern, and facing type. Inspect the mating flange for flatness and cleanliness. Install the correct gasket and bring bolts up in a cross pattern to the recommended torque. Use a valve between the process line and the seal when possible so you can isolate the instrument for service without breaking the line. For differential pairs, route capillaries together and secure them to the same supports so both see the same ambient temperature.
Once installed, vent or purge the instrument side only at the instrument ports as directed. Do not attempt to open the seal body. If you suspect trapped gas or damage, return the assembly to us for evaluation. Periodic verification can be as simple as comparing the instrument zero at known ambient temperature or performing a pressure check with a hand pump and reference gauge. If the process media coats the diaphragm, isolate and clean the face with a compatible solvent or cleaning solution. Avoid sharp tools that could score the diaphragm surface.
Documentation supports compliance. We can supply mill test reports for wetted parts, fill fluid certifications, pressure test results, and calibration records when the seal is shipped pre-filled with a specific instrument. For regulated applications, retain the fill fluid safety data sheet and the materials of construction list in your quality files. If you later change the instrument range or process conditions, consult us so we can confirm that the existing seal, fill, and capillary configuration still match the new requirements.
When sealing a differential pressure gauge such as our Model 140 or 142, order both seals and capillaries as a factory matched set. Provide expected ambient and process temperatures, mounting orientation, and line pressure so we can charge the system for minimal thermal shift. For single pressure gauges and transmitters, specify the flange size and class, media, temperature, pressure range, and any cleaning or sterilization cycles so we can select the diaphragm alloy and fill fluid correctly.
Contact Mid-West Instrument Today
Flanged diaphragm seals let us isolate your instrument from difficult media, connect directly to standard pipe flanges, and deliver stable, accurate pressure to your gauge, switch, or transmitter. Correct flange pattern, diaphragm alloy, and fill fluid selection provide a durable seal that resists corrosion, handles temperature, and maintains response across the operating range. Matched pairs and balanced capillaries keep differential measurements centered. Careful installation with the right gasket and torque minimizes leakage and drift. With proper specification and documentation, a flanged seal becomes a long term solution that protects instruments, improves reliability, and supports compliance goals.
Since 1958, Mid-West Instrument has been a leading provider of premium differential pressure gauges. Need help finding the right pressure gauge and equipment for your business? Reach out to us today to speak with one of our experienced professionals.