VBM Domobar Super HX: no flow through puck

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peafarmer

#1: Post by peafarmer »

Hi, I have a VBM domobar with a vibration pump that I have been troubleshooting. It will reach 10 bars with the blind filter, but when I load a puck with coffee I get no flow.

I did the test that I found for the Expobar where you measure the return flow to the reservoir. The video says that you need to get 1 ounce return in 12 seconds at 10 bar. I am getting 2 ounces in 12 seconds.

I am wondering if the OPV might be the problem. I have seen the thread by Ixo on April 8, 2014 where he rebuilds the OPV. I have removed the adjustment screw from the OPV but have not completely taken it out and apart. It seemed to be in decent shape. The machine is old, but has been used gently. Anyway, how likely is my problem the OPV? If so, do all the parts in the OPV come out easily? When I had the adjustment screw out I looked into it with a flashlight and did not see how to get the other parts out other than the spring.

Or, is my problem more likely the pump? Or, something else?

Thanks,
Andy

JRising

#2: Post by JRising »

I may be misunderstanding what you're expecting, but I think you're saying "With full flow going over the OPV at 10 Bar backpressure" so your pump is fine. What about with no portafilter? Is there no flow over OPV with low pressure? (The OPV shouldn't be allowing water out of the brew circuit until the pressure exceeds a set limit, usually at least 8.5, often higher with a vibe pump to try to get proper pressure-drop across the puck with a good flow rate for extraction).

peafarmer (original poster)

#3: Post by peafarmer (original poster) »

Thanks for responding. With no portafilter, no back pressure at all, I am getting 8 ml in 20 seconds (24 ml/min) through the bypass. With the blind I am getting 10 bars of pressure and 60 ml in 12 seconds (360 ml/min) through the bypass.

I just looked closely at the lines and I see a bubble in the line off the de-aerator that jiggles when the blind filter is in. The line from the de-aerator goes to a T. The T gets flow from both the OPV bypass and the de-aerator, and flow goes back to the reservoir.

peafarmer (original poster)

#4: Post by peafarmer (original poster) »

I just looked at the diagram, and this valve is called the "plastic autoprime valve". Mine seems to be brass, though.

peafarmer (original poster)

#5: Post by peafarmer (original poster) »

I took an even closer look late last night, and can see flow from the autoprime check valve on startup, but then the flow stops after pressure develops.

So, I think the autoprime check valve is working correctly, which leaves the OPV.

Will taking the OPV apart and cleaning it correct the excessive bypass flow?

Thanks,
Andy

peafarmer (original poster)

#6: Post by peafarmer (original poster) »

Was surfing a UK coffee site and found a thread about a boiler autofill failure (Parker two-way valve). In this case the autofill valve was not closing completely, and was allowing flow into the boiler rather than the brew circuit. I think this could be my problem: pump creates pressure and flow, but the flow does not make it to the group head.

How common is a failure of the auto-fill solenoid valve?

JRising

#7: Post by JRising »

peafarmer wrote:I took an even closer look late last night, and can see flow from the autoprime check valve on startup, but then the flow stops after pressure develops.
So, I think the autoprime check valve is working correctly, which leaves the OPV.
Will taking the OPV apart and cleaning it correct the excessive bypass flow?
Thanks,
Andy
Correct.
The priming valve will stop passing flow back to reservoir once there's pressure at the pump outlet (Once the air bubbles are out, the flow through that valve will create enough pressure).
If there's still flow over the OPV at that very low pressure with no portafilter in place, then yes, your OPV is definitely being held open by something. The OPV shouldn't allow any flow until it is overcome/opened by a pressure higher than 8 Bar (Or maybe higher).