Tri-clamp fittings for liquid cooling in US data centers
Many CDUs and prefabricated row manifolds connect with tri-clamp (sanitary clamp) joints rather than flanges. Where those meet a welded stainless header, the ferrules, clamps, gaskets and the spool between them are part of the piping order.
Tri-clamp (sanitary clamp) fittings join a ferrule on each pipe end with a hinged clamp and a gasket. In data center liquid cooling they sit at CDU connections, on prefabricated stainless row manifolds and at rack hoses: Vertiv's CoolChip CDUs, for example, have 2.5, 4 and 6 in sanitary connections. Where a tri-clamp connection meets a welded stainless header, the spool between them ends in a flange or a butt weld fitting: the flange is what we make, in 316L and 304L.
Where tri-clamp joints appear
| Location | Example | Parts at the joint |
|---|---|---|
| CDU primary and secondary connections | Vertiv CoolChip CDUs: 2.5 in (600 kW), 4 in (1,350 kW) and 6 in (2,300 kW) sanitary connections | Ferrules, clamps, gaskets, the spool to the header, a butterfly valve |
| Prefabricated row manifolds | Vertiv describes 304L manifolds with tri-clamp fittings | Matching ferrules and clamps, tri-clamp ball valves |
| Rack hoses | Hose to the in-rack manifold | Ferrule on the branch, gasket and clamp |
| Instrument and service points | Gauges, sample points, temporary flushing connections | Small ferrules, end caps, clamps |
In our sample BOQ for a 20-rack pod, 48 ferrules join the CDUs and the rack hoses to welded Sch 10S headers.
Ferrule, clamp and gasket
- Ferrule: the short weld-on end with the clamp flange. It is welded to the tube or pipe, so it must match its outside diameter and wall, and it is passivated with the spool after welding.
- Clamp: single pin with a wing nut for quick service, double pin or heavy duty bolted clamps where vibration or higher pressure calls for them. The pressure a joint holds depends on the clamp type, the size and the temperature; check it against the loop's test pressure, which OCP puts at 1.3 to 1.5 times the working pressure.
- Gasket: EPDM for water and propylene glycol, which Vertiv recommends in peroxide-cured form for technology cooling loops; PTFE, silicone or FKM where the coolant supplier asks for them. The gasket is a wetted part, so it must be on the coolant's approved list.
The tube size trap
Sanitary sizes are tube sizes. A 4 in ferrule in the inch tube series (ASME BPE, DIN 32676 series C) fits tube of 4.000 in (101.6 mm) outside diameter, but NPS 4 pipe is 4.500 in (114.3 mm). When a CDU's 4 in sanitary connection is piped to an NPS 4 or NPS 6 Sch 10S header, the spool between them needs a reducer, or a ferrule made for the pipe's outside diameter. Ask the CDU maker for the series and the tube outside diameter, not only the nominal size.
Tri-clamp or flange
| Tri-clamp | Flanged, Class 150 or PN 16 | |
|---|---|---|
| Assembly | Fast; one clamp, no bolt torque sequence | Bolts tightened in a cross pattern to a set torque |
| Maintenance | Inspect and retighten periodically (OCP) | Torque checks at commissioning |
| Pressure | Set by clamp type, size and temperature | Rated by the flange class and material |
| Alignment | Tolerant of rotation; needs axial alignment | Bolt holes must line up; a lap joint flange turns to suit |
| Where it fits best | CDU connections, prefabricated manifolds, hoses | Valves, headers, module breaks, future branches |
Most loops use both: tri-clamp where the equipment has it and where joints are opened often, and flanges on the welded headers. The joint types used in technology cooling loops are compared in our secondary fluid network guide.
Tri-clamp fittings on US projects
In the US, sanitary clamp fittings follow the inch tube series of ASME BPE: a 4 in ferrule fits 4.000 in OD tube and a 2 in ferrule 2.000 in OD tube, not NPS pipe. Where a CDU's sanitary connection meets a Sch 10S stainless header, the transition spool carries a ferrule on one end and a weld neck flange or butt weld reducer on the other; the flange end is ours to quote, with its material test report.
Ferrule, sanitary clamp, 4 in, weld end, SS 316L; clamp, heavy duty, 4 in; gasket, 4 in, EPDM; reducer, 4.000 in OD tube to NPS 4 Sch 10S pipe, where the tube series requires it; weld neck flange, NPS 4, Class 150 RF, bore Sch 10S, ASTM A182 F316L, to meet the isolation valve.
Frequently asked questions
What is a tri-clamp connection?
A sanitary joint made of two ferrules, short flanged ends welded to the pipe or tube, held together by a hinged clamp with a gasket between them. It opens and closes without tools other than the clamp's wing nut or bolt, which is why process plants and prefabricated cooling manifolds use it.
Is a 4 in sanitary connection the same size as NPS 4 pipe?
Not usually. A 4 in ferrule in the inch tube series (ASME BPE, DIN 32676 series C) is made for tube of 4.000 in (101.6 mm) outside diameter, while NPS 4 pipe is 4.500 in (114.3 mm). Check the series of the CDU or manifold connection before ordering the spool that joins it to the header.
Which gasket suits a water-glycol coolant?
EPDM is the usual choice for water and propylene glycol; Vertiv recommends peroxide-cured EPDM for elastomers in single-phase technology cooling loops. PTFE, silicone and FKM gaskets are available where the coolant supplier's list calls for them.
Do tri-clamp joints need maintenance?
Yes. The Open Compute Project notes that tri-clamp joints should be inspected and retightened periodically, so place them where they can be reached and seen, not buried above containment or under the floor.
How does a tri-clamp connection join a flanged header?
Through a short transition spool: a ferrule welded on one end and a weld neck flange, or a butt weld reducer, on the other. The ferrule follows the clamp series of the CDU or manifold; the flange follows the header's class, for example ASME B16.5 Class 150 or EN 1092-1 PN 16.
Sources
- Vertiv, CoolChip CDU 600, 1350 and 2300 kW data sheet, SL-80005, May 2026 (sanitary connections)
- Vertiv, Row-based manifolds: the backbone of liquid cooling in modern data centers, 1 December 2025 (304L tri-clamp manifolds)
- Vertiv, Guidelines for Deployment of Single Phase Technology Cooling System, SL-80056 Rev A, February 2025 (elastomers)
- Open Compute Project, Modular Technology Cooling System for "Cloud Scale", version 1.0, 25 August 2025 (tri-clamp to DIN 32676, inspection and retightening, test pressure)
- AWH, clamp ferrule to DIN 32676 inch series C, 4 in size for 101.6 mm tube (read 1 November 2026)