Pressure and Vacuum Sealed Feed-through Assemblies for Sensors, Probes, Electrodes and Wires

Content Provided by Spectite Ltd.

Sealed feed-through's, sometimes called sealing glands, are used to seal elements under the most arduous conditions and have many applications in a wide variety of industries:

  • Process plant
  • Power generation
  • Vacuum equipment
  • Petro-chem
  • Pharmaceutical
  • Glass production
  • Semiconductor fabrication

The main principle behind the design and application for Sealed Feed-through’s is that they inhibit the leakage of gases or other media and restrain the elements from moving in the assembly because of differential pressure. In some assemblies, elements are also electrically isolated from the feed-through body and from each other.

Feed-through Component Materials

Spectite feed-throughs are made from stainless steel and are designed for mounting on to a process vessel or enclosure. Versions can be specified to seal on both single and multiple elements of different types and sizes.

Spectite feed-through bodies, followers, seats and series EF SS electrodes are manufactured in an Austenitic stainless steel UNS S31603, commonly designated 316L. Equivalent grades are: (UK) BS 316 S11; (Germany) W.-Nr. 1.4404, DIN CrNiMo 17.13.2; (France) AFNOR Z2 CND 17.12; (Italy) UNI X2 CrNiMo 17.12; (Sweden) SS2353; (USA) AISI 316L; (Japan) JIS SUS 316L. The typical chemical composition for this steel is 0.03%C, 16.0-18.0%Cr, 10-14%Ni, 2.0-3.0%Mo, 0.10%N. Caps are manufactured in stainless steel UNS S30300 (303).

When 316L is unsuitable for an application, the ‘wetted’ metal parts of feed-throughs, that come into contact with a process, can be manufactured in other stainless grades or other materials such as Hastelloy® and Inconel® grades, Monel® R-405 or mild (carbon) steel. There may be minimum manufacturing requirements for feed-throughs in ‘exotic’ materials.

Insulators in series WF feed-throughs with 1/8” process connections and in series EF feed-throughs are manufactured in high-purity recrystallised Alumina (Aluminium Oxide Al2O3).

External, single-bore insulators in series WF feed-throughs are manufactured in aluminous porcelain. Internal insulators are manufactured in a high-performance, engineering plastic for use at temperatures up to +230°C or machineable glass ceramic for use up to +870°C.

Electrical conductors for Series EF feed-throughs are made in grade C101, fire refined, tough pitch, high-conductivity copper or 316L stainless steel (as above). Nuts and washers on copper conductors are brass, stainless steel conductors have stainless steel nuts and washers.

The lubricant used on feed-through components is Chlorotrifluoroethylene Polymer (PCTFE). A copy of the Safety Data Sheet is available on request. Spectite feed-throughs should not be degreased before installation.

The cap nut is tightened to a pre-set torque to compress an internal sealant. The sealant provides an efficient pressure seal on the elements without damaging them. At the same time it restrains them from moving. Epoxy sealing is not used.

Feed-through Sealants

The key factors in each application - temperature, pressure and process media - determine the choice of material for the internal sealant in all feed-throughs. Table 1 highlights the most common sealants Spectite employ:

Sealant Material Colour Operating temperature range Material definition and properties
Graftite™ Grey/Black -200°C to +550°C
(to +870°C in a reducing atmosphere)
Graphite 98% purity. Impermeable to gases and liquids. resistant to most media, not "wetted" by molten metals or salts. Asbestos-free. No ageing or embrittlement. Good resistance to thermal shock. Not re-usable.
Neoprene Green -40°C to +90°C Synthetic rubber based on polychloroprene. The elastic properties of the polymer are enhanced by vulcanization. It is much more resistant to heat, light, oxidation, and petroleum than ordinary rubber. Re-usable.
Lava Grey stone -200°C to +870°C Natural Magnesium Silicate (Soapstone, Steatite or Rock Talc). Crushes to a powder when compressed. Porous to light gases and moisture. Hygroscopic. Not suitable for most vacuum applications. Not re-usable.
Teflon® White -200°C to +250°C Polytetrafluoroethylene. FDA approved grade to Title 21 - CFR 17. 1550 and is approved to US Pharmacopoeia Class VI. Has smooth, non-wetting, hydrophobic surfaces that resist biofilm buildup and the lowest coefficient of friction of any solid material. Low thermal transfer. Re-usable.
Viton® Brown -40°C to +225°C
(intermittently to +285°C)
Fluoroelastomer. Resists hydrocarbons, corrosive chemicals and petroleum. Solvent, acid and base resistant. Low permeation rate. Mechanically robust at high temperatures. Re-usable.

Pressure Ratings of Spectite Feed-throughs

The pressure and temperature ratings and typical leak rates quoted are given for guidance only. Pressure tests have been undertaken at 20ºC using elements appropriate to the feed-through under test, i.e., Mineral insulated cable in probe feed-throughs, bare or insulated wire in wire feed-throughs, etc. Ratings vary with temperature and sealant used. With an increase in temperature, a reduction in the maximum pressure rating can be expected. Contact Spectite for further details. The BSPT and NPT mounting thread pressure rating is the same or is higher than the feed-through pressure rating.
Spectite feed-throughs have been designed to provide an efficient seal on the elements and restrain them from moving under pressure and vacuum. It is good installation practice to provide additional, mechanical restraint to the elements when differential pressure exceeds 75% of the feed-through pressure rating at 20ºC. (Not applicable to series EF feed-throughs with integral electrodes). Consult Spectite for further guidance on pressure ratings.

Feed-throughs with Graftite, Neoprene, PTFE and Viton sealants are suitable for vacuum applications to 5 x 10-6 torr (6.67 x 10-4 Pa or 6.67 x 10-6 mbar). Lava sealants are not suitable for vacuum applications.

Leak rates

A typical leak rate for Spectite feed-through assemblies with Graftite, Neoprene, PTFE and Viton sealants is better than or equal to 1 x 10-6 scc/sec. under 1 Atm. He @ 20ºC. (1 Atm. = 9.87 x 104 Pa or 987.2 mbar).

Process connections

Feed-through bodies can be specified with a choice of threaded process connections. Feed-throughs with the common tapered thread-forms, BSPT (Conical gas thread or ‘R’ thread) to BS21, DIN 2999 ISO 7/1 & JIS B0203; and NPT (National pipe tapered thread) to ANSI/ASME B1.20.1 are generally stocked items.

Feed-throughs with parallel threaded process connections, BSPP (parallel gas thread or ‘G’ thread) to BS2779, DIN ISO 228/1 & JIS B0202; and ISO metric to DIN13, may also be specified. Feedthroughs with parallel mounting threads need an ‘O’ ring or a gasket seal (not supplied) to prevent leakage at the process connection.

Feed-throughs without process connection threads may be specified for welded mounting and with a fitted flange from a range of styles including ISO-KF and -CF types for the vacuum industry and general applications as well as Triclover® and triclamp types for the food and pharmaceutical industries.

Custom engineered assemblies can be designed and made to meet customers’ specific application requirements.

Caps
Plain, hexagonal caps are available for all sizes of feed-through. Additionally, caps with a threaded extension for a conduit connection are also available. Specifiers may choose NPT, BSPP (parallel gas thread) or BSPT (conical gas thread). These caps can be specified for feed-throughs with 1/4”, 1/2” & 3/4” process connections. Cap threads are the same size as the corresponding feed-through body process connection.

Pressure Equipment Directive (PED)
Spectite sealed feed-throughs have been classified as ‘Piping’, satisfying the requirements of the category of Sound Engineering Practice (SEP), according to the European Pressure Equipment Directive (PED) 97/23/EC. The PED does not require the ‘CE’ symbol to be identified on Pressure Equipment that is categorised as SEP. Caps are marked SPECTITE on one of the hex. faces.

Products:

Series PF – Feed-throughs for single elements
These feed-throughs are designed for sealing single elements, usually sensors, probes or tubes, where they penetrate a pressure or environmental boundary. Common applications include sealing sheathed thermocouples and resistance thermometers, small-bore tubes and other types of sensor where they enter a process enclosure.

Series MF – Feed-throughs for multiple elements
A single access port into an enclosure or process vessel is all that is needed to allow multiple probes, sensors, etc., to pass through an environmental or pressure boundary using a single feed-through assembly.

Series PSF and MSF - For single and multiple elements
Series PSF (single element) and MSF (multiple elements) feed-throughs are similar to series MF feed-throughs, but have split internal components.

Series WF - For multiple wires & probes
When multiple wires must pass through a pressure boundary, series WF feed-throughs can provide an efficient seal without recourse to epoxies or other non-adjustable fixture methods.

Series EF - high voltage/current electrode
The integral electrode mounted in these feed-throughs enables specifiers to provide high-voltage, high-current supplies in process enclosures, autoclaves, vacuum furnaces and reactor vessels to power heaters, electric motors & other devices needing high power supply.

Series HF - High density, insulated wire, sealed tube assemblies
A PTFE-lined, stainless steel tube is swaged over multiple, insulated, single-core copper and/or thermocouple-material wires to make a continuous wire, high-density, sealed feed-through tube.

For more information on Spectite feed-throughs, Please contact Spectite Ltd.

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