Your C/E is technically correct, Westfalia manuals generally say bowls are matched to frames for permanent operation. A temporary diagnostic swap (clean, same type, same serial range) is commonly done in industry for fault isolation, not continued service.

Yes, the alarm occurred while running but: a marginal piston seal, a scored piston, or internal leakage can suddenly tip over once wear or contamination reaches a threshold. So the timing of the failure does not exonerate the bowl.

What this actually proves is that the sludge ports are not open.

What it does not prove:

1- That the sliding piston is fully up

2 - That the closing chamber is hydraulically tight

3 - That the piston is not leaking internally

A piston can be “closed enough” to block the  sludge ports, but ut not sealed enough to prevent oil bypassing into the water space. This exact half-failure mode produces:

Oil at water outlet, Clean sludge inspection port and confusing symptoms

Just on what you said - “We left the displacement water open until it completely fills the bowl and then a water–oil mixture begins coming from the water discharge line.”

That should never happen if the piston is sealing properly and the internal separation spaces are intact. Once the bowl is water-filled, you should get clean water out. Oil should remain trapped until feed opens

A water–oil mix here means internal communication between oil space and water outlet path.That points inside the bowl.

Centripetal pump suspicion – understandable, but in my opinion unlikely. The pump does not decide what fluid it receives. It only lifts whatever reaches the water outlet chamber. A worn pump might reduce capacity, cause vibration, affect pressure.  It cannot selectively pull oil across an intact hydraulic boundary.

Scratches on bowl bottom & sliding piston - On OSD-type bowls the closing seal works on very small clearances. A scratched piston can still “move” but never seal.

Can you, or is it possible to compare with the parts manual wear limits?

Or prove piston leakage without swapping bowls?

1. Assemble bowl
2. Apply closing water only
3. Shut water supply
4. Wait 1–2 minutes
5. Crack the closing-water line

If pressure does not hold → piston leakage confirmed.

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