CDU Loop
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Will your flush cart clean every pipe in the loop?

List the pipe sizes in your technology cooling loop and the cart's flow. The calculator checks each flush step against the Open Compute Project minimum flushing flow, tells you how many branches you can flush at once, and how long the flush volumes take.

Flush cart
gpm
Use the flow the cart actually delivers through the loop at its pressure, not its catalog maximum. Rental carts commonly run 100 to 300 gpm.
Pipes, one row per flush step
"Flushed together" is how many identical branches of that size share the cart's flow in one step, for example two row drops opened at once. Racks and hoses are jumpered out during flushing, so leave them off.
Result
Steps that reach OCP flow–
Largest step needs–
Loop volume–
What each flush step looks like. The cart pumps through the pipes you open for that step and back through its filter; racks are jumpered out. Line thickness is pipe size; green reaches the OCP minimum velocity, red falls short.
Initial recirculation, 3 loop volumes–through 50 to 100 µm filters
Each flush cycle, 8 loop volumes–final filtration 5 µm
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Full loop scan

Equal splits are the easy case. Real loops aren't balanced.

This check assumes the cart's flow splits evenly between the branches you open. On a real manifold the far branches and the long runs get less, and the pipe that misses minimum velocity is the one that keeps the debris. The full loop scan solves the flush hydraulically for every pipe on your drawings, including which valves to close for each zone.

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Why minimum flushing flow matters

Cold plates on current GPU racks have microchannels 25 to 50 microns wide. Weld slag, cutting oil or a flake of pipe scale left in the loop can block one and shut down a node. Flushing removes that debris only if the water moves fast enough to carry it, and in a large header that takes far more flow than a small pump cart can give.

The Open Compute Project's pre-commissioning guideline for technology cooling system manifolds sets a minimum flushing flow for each pipe size, which works out to about 0.9 to 1.25 m/s (3 to 4 ft/s), and recommends 3 to 5 ft/s.

OCP minimum flushing flow by pipe size

The rest of the OCP procedure

  • Initial recirculation of at least 3 system volumes through 50 to 100 µm filters.
  • Flush cycles of at least 8 volumes each, finishing with 5 µm filtration.
  • Final water: turbidity under 5 NTU, and conductivity within 10 to 20 µS/cm of the procedure water or at most 60 µS/cm.
  • Fill at least 3 volumes before sampling; pressure test at 1.3 to 1.5 times working pressure.

Questions

How much flow does it take to flush a 6 inch header?

About 1,430 L/min, or 377 gpm, by the OCP table. That is more than many rental flush carts deliver, so large headers need a bigger cart or two carts in parallel. Shorter sections help only by cutting pressure drop so the cart runs closer to its maximum flow; the minimum flow itself depends on pipe size, not length.

Can I flush several branches at once?

Yes, as long as each one still gets its minimum. A 150 gpm cart covers three 2 inch branches at 39 gpm each, but only if the flow splits evenly. Balance valves or equal runs make that assumption hold; otherwise the longest branch falls short.

Do I include the racks and hoses?

No. Racks are bypassed with jumpers during flushing so debris never reaches the cold plates. Flush the facility piping and manifolds, then connect the racks to clean, tested water.

Is this a substitute for a commissioning plan?

No. It checks flow against the OCP minimum for each pipe size, assuming even splits. Use it to size the cart and plan the steps, then follow the full procedure and your project's specification.

Source: Open Compute Project, Guidelines for Pre-Commission Preparation of Technology Cooling System (TCS) Row Manifolds, v1.0, Tables 3 and 4. Pipe dimensions: ASME B36.10M and B36.19M.

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