White Paper: Guidelines to Rack Manifold Requirements and Qualification

Overview

The paper gives recommendations on manifold design, manufacturing and testing for standard 19-inch racks and ORv3 racks, with the aim of a common, stock-item product that can be interchanged on similar-size racks regardless of brand or origin. Its scope runs from concept to productisation, with contributions from manifold suppliers and liquid-cooling solution providers. It is a recommendations white paper, not a formal OCP specification. Key performance indicators include pressure (MAWP and burst), mechanical resistance (shock and vibration) and certification (ISO, CE, RoHS, UL and others).

The form, fit and function chapter lists minimum physical parameters: server loop spacing follows the 44.45 mm (1.75 in) RU for EIA-310-D racks and the 48.0 mm (1.89 in) OpenU for ORv3 racks; cross-section may be square or circular; CDU connections are 3/4, 1 or 1.25 in hose ID depending on rack flow, and server loop connections 1/4, 3/8 or 1/2 in. Safety features are a pressure relief valve (or auto air vents) and a drain valve, and control features are shut-off or bypass and flow control valves. For terminations, a straight SAE thread with O-ring boss per ISO 11926-1 is recommended at minimum; NPT may be used on thick-walled machined manifolds but is discouraged because of sealing difficulty on tapered threads.

Functional requirements call for geometry that supports the full rack cooling load with uniform flow to each server loop: high-point bleed or vent valves on top remove trapped air, and a low-point drain valve at the bottom removes sediment and drains the loop. The operating-condition table requires a maximum operating pressure (MAWP), burst pressure of at least 3x MAWP, velocity below 1.5 m/s per the ASHRAE guideline, an operating range of 17-65 C (based on ASHRAE W45 with a 2 C CDU approach and about 13 C delta-T), a shipping range of -40 to 75 C and a surface treatment. A deep rack (for example 1200 mm) is recommended so rear plumbing does not clash with power and network cabling and PDUs.

The test chapter recommends pressure tests at 100 psig and 300 psig per ASME B31.3, high and low temperature tests, flow resistance, leakage, salt-spray corrosion and fully assembled shock and vibration tests. For durability, at least 5,000 pressure cycles (for example 0 to 150 psig held for 3 seconds) are applied, and all performance requirements must be met at time zero and at the 5,000th cycle. Shelf life is 5 years and service life 10 years. Manufacturing methods described are machining from billet, building from standard pipe and welding (TIG, followed by descaling and pickling or passivation); O-ring seating surfaces must be Ra 63 microinch (1.6 micron) or better.

Passivation is required for stainless steel, while pickling and electropolishing are recommended. For post-production cleaning, washing with soap and water is required and pressurised steam washing is recommended; compressed-air drying alone is not adequate and the air must be filtered. The wetted materials table gives examples for Dow DOWFROST LC 25/55 and Huntsman JEFFCOOL ISF-25/55 propylene glycol coolants, covering copper, brazing fillers and elastomers such as EPDM and Viton. The appendix presents floating mechanism examples from Southco and Ingrasys for aligning blind-mate UQDB connectors in ORv3 and standard racks. The paper carries no publication date and cites 2022 references.

Key points

1. Server loop spacing follows the 44.45 mm (1.75 in) RU for EIA-310-D racks and the 48.0 mm (1.89 in) OpenU for ORv3 racks.
2. CDU connections are typically 3/4-1.25 in hose ID and server connections 1/4-1/2 in.
3. Manifolds must have high-point bleed/vent valves at the top and a low-point drain valve at the bottom.
4. Minimum burst pressure must be 3x MAWP, and flow velocity should stay below 1.5 m/s for uniform distribution.
5. Required operating temperature range is 17-65 C (ASHRAE W45, 2 C CDU approach, about 13 C delta-T); shipping range is -40 to 75 C.
6. Recommended pressure tests are 100 psig and 300 psig per ASME B31.3, plus temperature, flow resistance, leakage, salt-spray corrosion and shock/vibration tests.
7. Durability calls for at least 5,000 pressure cycles (for example 0 to 150 psig held for 3 seconds), with requirements met at time zero and at the 5,000th cycle.
8. Shelf life is 5 years and service life is 10 years.
9. A straight thread with O-ring boss per ISO 11926-1 is recommended for terminations; O-ring seating surfaces must be Ra 63 microinch (1.6 micron) or better.
10. A deep rack (for example 1200 mm) is recommended so rear plumbing does not clash with power and network cabling and PDUs.

Using it in design

• Write MAWP with at least 3x burst pressure, the 17-65 C operating and -40 to 75 C shipping ranges and the 1.5 m/s velocity limit into the manifold specification; because the range is based on ASHRAE W45, ask suppliers for a higher rating if a warmer-water strategy is planned.
• Prefer a deep rack (for example 1200 mm) and place the manifold at the rear so it does not clash with power distribution, PDUs and network cabling; request the rack mounting kit for single manifolds at low load and double manifolds at high load.
• Ask suppliers for a qualification package with 100 and 300 psig pressure tests, 5,000-cycle durability, salt-spray and shock/vibration reports, and require the same acceptance criteria at time zero and at the 5,000th cycle.
• Specify a bottom drain valve, a top vent valve and ISO 11926 O-ring boss terminations for hoses and adapters, and ask for a justification if NPT tapered threads are proposed because of leak risk.
• Add manufacturing and cleaning requirements (passivation, soap-and-water washing, filtered-air drying, first article inspection) to the purchase contract and confirm the wetted materials list with the fluid supplier.

Who should read it

Engineers who design, manufacture and procure manifolds for liquid-cooled racks, and rack integrators.

Document contents

1. Introduction
2. Nomenclature
3. Key Performance Indicators
4. Form/Fit/Function
5. Features and Dimensions
6. Terminations
7. Functional Requirements
8. Performance Requirements
9. Operating Condition Requirements
10. Testing Requirements
11. Durability Requirements
12. Shelf and Service Life Requirements
13. Manufacturing Methods and Surface Treatment
14. Post-production Cleaning and Inspection
15. Assembly and Rack Integration Considerations
16. Wetted Materials Requirements
17. Conclusion
18. References
19. Appendix: Southco Floating Mechanism using UQDB04
20. Appendix: Ingrasys Mechanism for ORV3

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Publication details

Authors

Berhanu Wondimu; Edward Kung; David Zhou

Publisher

Open Compute Project Foundation

Year

2023

Document language

English

Source type

Industry body

Access

Open licence — downloadable

Licence

CC BY-SA 4.0 — share with attribution under the same licence

Licence evidence

www.opencompute.org

Citation