Welded Static Mixer

Product Overview

The welded static mixer is the structural variant built for installations where the mixer is a permanent part of the piping and will not be removed for the life of the unit. It is fabricated as a rolled-and-welded shell with butt-weld ends prepared to ASME B16.25, and the mixing elements are welded to internal supports that are themselves attached to the shell wall. There are no flanges, no threaded joints, and no clamp ferrules — the mixer is welded into the line during construction and stays there.

This construction is mandatory for high-pressure and high-temperature service. The flange and threaded connections both rely on gaskets or thread sealants that have a temperature ceiling around 300-400°C and a pressure ceiling around PN40. Beyond those limits, the welded joint is the only mechanically reliable way to join a mixer into a process line. Hydroprocessing reactors, steam injection headers, and HP polyethylene reactors all use welded mixers for this reason.

The welded mixer is built in carbon steel (SA-516, SA-106, SA-335 P11/P22/P91) for the high-temperature envelope, in SS316L for chemical service, and in low-alloy steels for elevated-temperature petrochemical service. The shell wall thickness is calculated per ASME B31.3 with a 1.5× safety margin on burst, and all longitudinal and circumferential welds are 100% radiographed. The unit is supplied with a full MTR and ASME U-stamp or PED CE-marking where the project requires it.

Design Features

The welded housing is a cylindrical shell with two butt-weld-end preparations machined to ASME B16.25 (30° bevel, 1.6 mm land) for butt-welding into the line. For small-bore units (DN50-DN80), socket-weld ends per ASME B16.11 are available to simplify field installation. The shell is rolled from plate and longitudinally welded, then the two end prep rings are fitted and circumferentially welded. For high-pressure service above PN63, the housing is forged rather than fabricated to eliminate the longitudinal seam.

Inside the housing, the mixing element pack is retained by welded support rings. For SV plate geometry, the plate stack is welded to a central alignment pin and supported at each end by welded ring-stiffeners that are themselves attached to the shell wall with a fillet weld. The element pack is therefore a permanent part of the pressure boundary. For SK helical geometry, the elements are welded end-to-end to form a continuous twisted ribbon, and the upstream end is welded to a retainer plate; this construction tolerates thermal expansion because the helix is free to slide in the axial direction at the downstream end.

For high-temperature service above 400°C, the housing is stress-relieved after welding per ASME B31.3. For service in the creep range (above 500°C, e.g., P91 in steam lines), the housing is normalized and tempered, and the weld procedure includes a pre-heat and post-weld heat treatment (PWHT) at 730-760°C. Hardness surveys are performed on the HAZ of every weld to confirm the 22 HRC NACE MR0175 limit for sour service.

Inspection access for the welded mixer is provided by borescope ports in the upstream and downstream pipe spools. These 1" NPT ports are typically supplied by the customer; on request we will factory-weld a 1" NPT coupling to the housing at 90° to the flow, for periodic visual inspection of the element pack. The welded mixer can also be inspected by RT or UT of the housing welds per the customer's inspection plan.

Technical Specifications

Parameter	Value
Connection Standard	Butt-weld (ASME B16.25) or Socket-weld (ASME B16.11) ends
Pipe Diameter Range	DN50 to DN1000 (custom larger on request)
Pressure Rating	PN100 standard, PN63 (SS316L), PN250 special
Temperature Range	-50°C to +600°C (low-alloy steel) / -50°C to +450°C (SS316L)
Housing Material	SA-516 Gr.70, SA-106 Gr.B, SA-335 P11/P22/P91, SS316L, SS304
Mixing Element	SK helical (default) or SV plate — welded in housing
Wall Thickness	Calculated per ASME B31.3 with 1.5× safety margin
Weld Preparation	30° bevel + 1.6 mm land (B16.25) or socket (B16.11)
NDT	100% RT on longitudinal and circumferential seams
Heat Treatment	Stress-relief per B31.3; PWHT for creep-range service
Design Code	ASME B31.1, B31.3, EN 13445
Certification	ASME U-stamp, PED CE, NACE MR0175 (sour service)

Connection Options

The default connection is butt-weld ends prepared to ASME B16.25, with a 30° bevel and 1.6 mm land to accept the customer's field weld. The weld is typically performed by the piping contractor using a matching filler metal (e.g., E7018 for carbon steel, ER316L for SS316L). The bevel angle and root face are inspected before shipment to ensure the field weld can be made without re-machining.

For small-bore welded mixers in socket-weld configuration (DN50-DN80), the housing end is bored to receive the mating pipe socket, and a fillet weld is applied around the OD. Socket-weld is used where the unit must be removable — the pipe is cut behind the socket, the socket is un-welded, and a new mixer is fitted. It is faster to install than butt-weld, but the pressure rating is limited to Class 6000 (PN100) and the fatigue life is lower because of the crevice at the socket root.

For very-high-temperature service above 500°C, the weld transition is a critical detail. We use a butt-weld end with a 37.5° bevel and a 3 mm root face to give a wider groove that can be filled with a low-heat-input weld procedure. This minimizes the HAZ grain growth that would otherwise reduce the creep life of the joint. Post-weld heat treatment at 730-760°C is mandatory and is documented on the MTR.

For ASME B31.1 (power piping) and B31.3 (process piping) installations, the welded mixer is supplied with ASME U-stamp certification. For European installations, PED CE-marking with category III or IV depending on the design pressure. For sour service, NACE MR0175 compliance is documented with the MTR, and the housing is hardness-surveyed to confirm the 22 HRC limit on the wetted surfaces.

Applications

Hydroprocessing reactors. Refinery hydrocracker and hydrotreater reactors use welded mixers to distribute the hydrogen-rich recycle gas across the catalyst bed. The mixer operates at 150-200 bar and 350-450°C in a hydrogen sulfide environment. The welded SS316L or low-alloy steel housing is the only practical option at these conditions.

Steam injection and attemperation. Boiler feedwater attemperation and steam injection in combined-cycle power plants use welded mixers to blend cold water with the main steam flow. The PN100 / 600°C envelope matches the high-pressure turbine steam conditions, and the welded joint is required because flanges at these temperatures would creep.

HP polyethylene reactors. Polyethylene and polypropylene reactors operate at 1000-3000 bar and 100-300°C. Welded mixers with forged duplex housings are used to blend comonomer, catalyst, and hydrogen into the reactor feed. The mixer is a permanent part of the reactor feed piping and is qualified to the reactor's design code (ASME B31.3 Chapter IX).

Sour crude and gas processing. Inlet separators and amine contactors in sour-gas processing plants use NACE-compliant welded mixers to blend the lean amine with the sour gas. The hard-faced housing and the welded element pack tolerate the H₂S partial pressure without cracking.

Steam-methane reformers. Primary and secondary reformers use welded mixers to blend the hydrocarbon feed with the dilution steam before the catalyst bed. The mixer is mounted inside the reformer inlet piping and operates at 30-40 bar and 800-900°C — well above the limit of any flanged connection.

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