Cavitation - it can happen to you!

hot_rod

Again, you've made it easier for the minds eye to see something that is extremely hard to explain. Wonderful job. Keep up the excellent work!

ME

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Mark J Strawcutter

doesn't take much to cause it

1970's era Burnham boiler (actually, Glenn sez it's a re-branded American Standard) installed according to the I&O manual from that time. Gravity conversion with 2.5in mains, 1in to 1st fl rads (qty 3), .75in to second fl rads (qty 5).

B&G 100 pumping toward the boiler, fresh water feed at low point between circ and boiler, non-diaphram expansion tank and relief valve on separate top-of-boiler tapping.

This fall I put Danfoss ra2000 TRVs on the 2nd fl rads. Ran out of warm weather before I could switch to pumping away and the other things I'd like to do.

The original valves look like they had a Cv of at least 50 :-) and the ra2000s are 2.5-3.0 if I'm reading the chart right.

Fired the system up and noticed vibration in the return piping strongest right before the circ. Not "gravel" like I've heard cavitation described, just a vibration. It in turn created some interesting harmonics in various segments of system return piping. It wasn't that cold and TRVs were throttled down pretty far. Here's my guesses as to what was/is happening and how I've attempted to correct:

Differential pressure from circ is all manifest as decrease on the inlet side. The TRVs are causing enough pressure drop that cavitation starts. So, I pop off the TRV heads and manually open them up full. Helps enough to notice.

Next, I bring the static pressure up from 12psi to 20psi. Helps some more. But as expected the noise gets worse when the water temp gets high - which of course it's been doing for the past couple of weeks :-) But as long as it stays in the 20's or above outside the noise is hardly noticable.

So, here's a practical example of the benefits of pumping away. And an example of why it pays to check the Cv of valves.

Mark

RB_2

RB_2

Great example!

The attached 212 kb file gives a pictorial description of what Mark described above...was convertewd from a PowerPoint file so excuse some of the goofy graphic layovers.

RB_2

Great example!

Attached 212 kb .pdf file, (21 pages) is a pictorial representation of what Mark explained.

High speed internet takes about 10 seconds to download.

(excuse the goofy graphic overlays...file was converted from our PowerPoint Training disk and sometimes the animation stuff just gets fouled up in the conversion)

RB_2

Glad to have you back proffesor!

Have some toys coming to you soon...early 2003 Christams present.

RB