Flow Control Question - Group Head Pressure Exceeding Pump Pressure?

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dmau9600

#1: Post by dmau9600 »

Flow control gurus - installed FC on my Quick Mill Vetrano (rotary pump, tank water).

I have my pump pressure set to around 9bar, but I've noticed I can exceed that by 1-2bar on the gauge at the group head, particularly if i quickly ramp up pressure on a finer grind.

Is that normal? Should I reduce pump pressure a bit to combat this?

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Jeff
Team HB

#2: Post by Jeff »

Unless you've got "impossibly high" flow rates where inertia dominates, water will flow from higher pressure to lower pressure.

I am guessing that one or both of those gauges aren't well calibrated. 5% of full scale would not surprise me.

A quick way to check at least at one point is to compare the two gauges with a blind basket in place, once the pressure equalizes.

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mrgnomer

#3: Post by mrgnomer »

Pump pressure is set for 9 bar but the OPV is usually factory set for 11bar. The buildup might be before the OPV opens up.
Kirk
LMWDP #116
professionals do it for the pay, amateurs do it for the love

dmau9600 (original poster)

#4: Post by dmau9600 (original poster) » replying to mrgnomer »

Interesting. On my last machine, Elizabeth, I would adjust the OPV as a means of regulating pump pressure.

Should I be adjusting the OPV in the Vetrano at the same time in adjusting the pump pressure? Doesn't seem like that's the manufacturers intent since the pump pressure is accessible external and available to the user to easily adjust.

Am I just overthinking all this?

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Jeff
Team HB

#5: Post by Jeff »

With a rotary pump, you set the pressure with the bypass valve on the pump. "9 bar" is a common choice.

The "OPV" is sometimes called the "expansion valve". It's main purpose in a rotary-pump machine is to keep the pressure generated by the expansion of the hot water in the boiler from getting too high. It typically is set above the pump bypass, maybe at 11 bar or so.

dmau9600 (original poster)

#6: Post by dmau9600 (original poster) » replying to Jeff »

This is very helpful, thank you. So, should I be concerned that I am seeing 10-11bar at the group head? The fact seems to be that I was always getting those pressures at the group head, I'm just able to see them with the gauge provided with the FC.

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mrgnomer

#7: Post by mrgnomer »

Very good question. I don't know what's normal but my experience with my plumbed in single boiler HX Vetrano was the line pressure and the pump pressure gauge climbed pretty high on a cold start up. Up to 9 bar and sometimes beyond. I adjusted the pump pressure to 9.5bar and messed with the OPV, finally adjusting it with a blind portafilter to a standard 11bar. Didn't affect the up and down HX line pressure other than the OPV stayed closed more. I watched a YouTube Rocket and saw the same high line pressure on the gauge.
When the pressure is high I would lift the brew lever to the line pressure preinfusion point. That would be the point that opened the grouphead flow but didn't trigger the switch that turns the pump on. Pressure dropped to supply line pressure right away. After a while my normal around 2 bar line pressure stabilized to around 4 bar in the HX. A big flush would see the pressure roller coaster around a bit. I put that down to expanding cold water being heated to above boiling point pressure. I had my pressure stat set to 1.5 bar boiler pressure for more steam so it ran hot.
Kirk
LMWDP #116
professionals do it for the pay, amateurs do it for the love

lukeap69

#8: Post by lukeap69 »

I remember this was discussed somewhere else. I have a Bianca, the pressure reading at the head is usually 0.5 bar to 1 bar higher than the pressure gauge for the pump. I can't remember the explanation now but it is there if you search other forums.