High-Purity Weld Fittings: Butt-Weld & VCR

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How a weld joint seals

A mechanical tube fitting seals on a ferrule driven into the tube wall. A weld fitting seals on the weld itself: the fitting end is machined flat and square, and the tube is fused around the circumference, so the joint holds torque and leak rate.

The joint is permanent, so weld fittings suit gas panels, analyser sample systems, dosing skids and high-vacuum or ultra-high-purity manifolds. Removable connections stay at the instrument or regulator needing service. Manual TIG and orbital welding apply, and drawings give flat, square weld ends per size.

Miniature weld connectors

The miniature line covers compact piping: EP straight connector, MR and LR reducing connectors, ME 90° elbow, MEL 90° elbow with one extended leg, MEF 45° elbow, MT tee, MTB cross-plane tee and MCT cross. Full-size LE, LT and LCT use the same naming.

Tube outside diameters run 1/8 in, 1/4 in, 3/8 in and 1/2 in (6, 8, 10 and 12 mm). Part numbers name both ends: MR6S4S joins 3/8 in to 1/4 in, MEL4S-2 is a 1/4 in elbow, and MT10M6MS is a 10 mm run.

Full-size and VCR lines

The full-size series (catalogue 2-09 to 2-14) covers 1/4 in to 1 in (6-18 mm) tube at L 29-33.5 mm. Reducing tees include LT12M6MS and LT8M6MS. Extended-weld bodies are machined from forgings, with uniform wall thickness for weld repeatability.

The VCR face seal uses a metal gasket between flat faces and is leak-tight from vacuum to high pressure. No polymer contacts the media. The male nut has a test hole, female threads are silver plated, and TSG and TLG adapters differ in leg length.

Standards referenced: Pressure ratings follow ASME B31.3 process piping practice, derated for tube size, wall thickness and temperature; Imperial and metric tube outside diameters follow the standard instrument tubing series.

Typical duties: Semiconductor and ultra-high-purity gas distribution panels; Analyser and sample systems in refineries; High-vacuum and UHP lines in pharmaceutical production; Hydrogen, nitrogen and specialty gas skids; Dosing and metering lines on chemical injection packages; Instrument tubing on oil and gas wellhead facilities.

High-purity weld fitting model selection guide

Series / model	Size	Connection	Working pressure	Body material	Notes
MR4S	1/4 in. tube OD x 1/8 in. tube OD	tube weld	5100 psig (351 bar)	not published	micro reducing straight union (英制)
MR6S	1/2 in. tube OD x 3/8 in. tube OD	tube weld	3300 psig (227 bar)	not published	micro reducing straight union (英制)
ME2S	1/8 in. tube OD	tube weld	8500 psig (586 bar)	not published	micro 90 deg elbow
ME4S	1/4 in. tube OD	tube weld	5100 psig (351 bar)	not published	micro 90 deg elbow
ME6S	3/8 in. tube OD	tube weld	3300 psig (227 bar)	not published	micro 90 deg elbow
ME8S	1/2 in. tube OD	tube weld	3700 psig (254 bar)	not published	micro 90 deg elbow
MEL4S	1/4 in. tube OD	tube weld	5100 psig (351 bar)	not published	micro single-end extended 90 deg elbow; MEL4S-1/-2/-3/-4 differ in Lx, B1, F dimensions at the same rating
MEF4S	1/4 in. tube OD	tube weld	5100 psig (351 bar)	not published	micro 45 deg elbow; MEF6S 3/8 in. 3300 psig (227 bar), MEF8S 1/2 in. 3700 psig (254 bar)
MT4S	3/8 in. tube OD x 1/4 in. tube OD	tube weld	3300 psig (227 bar)	not published	micro reducing tee (outlet 1/4 in.) (英制)
MT8S	1/2 in. tube OD x 1/4 in. tube OD	tube weld	3700 psig (254 bar)	not published	micro reducing tee (outlet 1/4 in.) (英制)
MCT4S	1/4 in. tube OD	tube weld	5100 psig (351 bar)	not published	micro four-way cross
MCT6S	3/8 in. tube OD	tube weld	3300 psig (227 bar)	not published	micro four-way cross
LT8S	1/2 in. tube OD	tube weld	3700 psig (254 bar)	not published	tee (英制)
LT12S	3/4 in. tube OD	tube weld	2400 psig (165 bar)	not published	tee (英制)
LT12M6S	12 mm x 6 mm tube OD	tube weld	2902 psig (200 bar)	not published	reducing tee, metric table (公制)
LT12S (reducing)	3/4 in. tube OD x 3/8 in. tube OD	tube weld	2400 psig (165 bar)	not published	reducing tee, 变径三通 table; same value printed for LT12M6S and LT12M8S in the metric table

Selection notes: Working pressure is printed as psig with the bar equivalent in brackets; the catalogue pages used do not state a temperature basis, so the owner should confirm the reference temperature (for example 100 degF) before publishing. Body material is not printed anywhere on pages 2-01 to 2-12; every Body material cell is left as “not published” and the material grade (for example 316 SS / 316L) must be supplied by the owner.

Catalogue reference 2-01, 2-02, 2-03, 2-04, 2-07, 2-08, 2-11, 2-12. Confirm the final rating against the data sheet issued with your order.

See also: Double Ferrule Fittings · Threaded Fittings · Tube Fittings

Frequently asked questions

Can you supply material certificates and dimensional reports?

Yes. Material certificates for the wetted parts are supplied with the goods, and dimensional inspection reports are issued on request. Tell us at enquiry stage which document set your QA department needs, so we prepare it with the shipment.

When is a VCR face seal used instead of a weld end?

Use a VCR face seal where a joint must be reopened. It seals on a metal gasket between two flat faces. Weld ends are permanent and hold torque and leak rate through vibration and thermal cycling.

Which pressure rating applies to my tube size?

The rating follows the tube, not the fitting. A 1/8 in tube with 0.028 in wall is listed at 8,500 psi (586 bar), 1/4 in at 5,100 psi (351 bar), 3/8 in at 3,300 psi (227 bar) and 1/2 in at 3,700 psi (254 bar).