Why does the OPV squeal?

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BaristaMcBob

#1: Post by BaristaMcBob »

Can someone please explain what exactly is happening in the OPV valve when it starts to squeal? I noticed this when trying to reduce the pressure. I was able to drop the OEM pressure from 13 to 11 bar. Anyting beyond that causes a squealing sound - which seems like something to avoid.

JRising

#2: Post by JRising »

It is common in machines with OPVs with rubber/silicon valve-faces. It's just able to vibrate harmonically because of the spring's position, the hardness of the valve-face and the flow:pressure-drop ratio, like squeaky car brakes with the rotor passing through under pressure at the right pressure and speed, or a violin string with the rosined up bow passing over at the right pressure and speed.
Change a variable, flip the spring backward (if same diameter on both ends) or replace the valve face with a newer, softer one, if it's obtainable. I am assuming you're talking about a simple OPV common to most "E61 knock-off" machines with vibe-pumps... I don't know Bezzeras well.
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another_jim
Team HB

#3: Post by another_jim »

That was a good explanation. I'd like to add I've had luck ending the squeals by just disassembling and reassembling.
Jim Schulman

BaristaMcBob (original poster)

#4: Post by BaristaMcBob (original poster) »

So if I understand correctly, the squealing is more a matter of harmonics rather than the valve being on the verge of popping off the threads. So if I drop the brew pressure a bit more will the squeal go away?

Pressino

#5: Post by Pressino »

If their explanation is correct, changing the pressure (either way off the resonance point) should do it. :)

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cafeIKE
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#6: Post by cafeIKE »

Pressino wrote:changing the pressure (either way off the resonance point) should do it
Maybe, maybe not. Sometimes it can be dead quiet with the covers off and start squealing two days later, only to quiet again with the cover off. Sometimes rotating the adjuster in and out and back to original position can work. Also some HiTemp silicone grease in the spring cups doesn't hurt. [Lasts longer when the OPV is in the cold side]

Depending on OPV design and age, flipping the rubber puck to put a new face on the seal sometimes works. If the puck is toast, a replacement may be available.

JRising

#7: Post by JRising »

What's worse is the fact that sometimes a new rubber valve face makes it worse than the old hard one it replaces. Sometimes the green rubber ones that are supposed to be used when the OPV is dual-purposing as the expansion valve are worse than the black rubber ones.