Can anyone in simple terms please shed some light on the impact that an OPV setting has on the performance a vibration pump HX machine. I am currently looking at the Bezzera Domus Galatea which has an OPV set at 11bar (and it is not made to be adjusted).

Does this setting basically mean that the machine will allow up to 11bar of pressure in the group before releasing the pressure i.e. a safety mechanism, or does it actually effect the flow rate on every shot regardless?

Similarly does a machine with an OPV set at 9 bar simply ensure that higher pressures cannot be reached and therefore compensates for barista technique that would otherwise see the brew pressure reach higher levels?

I took a cursory glance of page 11 on this PDF (link). The OPV looks like it should be adjustable, but do you know that it is not? It looks like the entire assembly is sold in 12 and 14 bar versions (for vibe and rotary pump versions respectively). If you can decrease the spring tension (item 2, pg 11) by adjusting the length of the barrel by turning item 1, you should be able to lower the OPV setting. You could also try swapping springs if you had to. It looks like the over pressure release goes to the drip tray.

I had just reread the threads that Dan linked above, hopefully they will answer your question and not confuse you too much. This PDF is linked from one of the threads, and is worth a read.

Many thanks for all the info - think I'm understanding the role of the OPV better now. I'm told that the Galatea's OPV can be adjusted but it is not an easy job - hard to access and locktite therefore needing some force to break the seal.

I think i'll see how the machine performs and decide whether to experiment with altering. One last question....At 11 bar how often would the OPV be activated? i.e. for a correctly dosed and tamped standard double shot of espresso would you expect the group pressure to get anywhere near 11 bar - or does it come more into play with ristretto's/single shots where you are creating more resistance with the coffee puck?

You need the OPV because the flow rate of the pump at 9 bar is too high, on the order of 200ml/minute.
http://www.ulka.it/eng/e4_gra.htm

If dose to maintain the pressure at 9 bar, you better have a VERY large cup.