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Thread: Valve names and purpose -I lie awake confused

  1. #1
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    Valve names and purpose -I lie awake confused

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    There seem to be three names for two valves in the Giotto Rocket. Some clarity would be good for accurate reference.

    1. The bypass valve (located adjacent to the pump) I think it to relieve pressure on the pump.
    2. The over pressure valve (opv) I think is to control the pressure of water in the extraction process
    3. The expansion valve I think seems to be applied to both 1 and 2 above??

    Perhaps there is yet another valve!

    Don't bother with this question if it is just too complicated. But I lie awake at night confounded!
    Regarding the opv. There seems to be two values. In various references in these threads 10 bar at the opv seems to result in 9 bar in the group. I would have thought that if the opv opens at say 10 bar that would indicate the pressure in the system and at the group it would be 10 bar. So setting the pressure with a blind filter gauge should be the pressure in the system?

  2. #2
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    An OPV is essentially a pressure control bypass valve - it will maintain a relatively constant backpressure across a range of flowrate, by varying the restriction to flow; thus effectively limiting the maximum backpressure at the pump to the set pressure.

    Regarding the "two pressures":

    The OPV maintains the pressure at the pump discharge (or near to it).

    When there is no flow through the group the pressure at the group will be the same as at the OPV.

    However, when there is flow, some of the pressure energy is lost from the water due (mostly) to friction - this is known as pressure drop.

    The pressure drop will always be roughly equal to the pump discharge pressure, BUT most of it will be incurred across the puck. The 10 barg figure allows for 1 bar pressure drop in the piping upstream of the puck (and 9 bar across it).

    Hope that makes sense?
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  3. #3
    Senior Member sprezzatura's Avatar
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    We use the term expansion valve to describe where we want - or is set - the break point for increased pressure due to thermal expansion (water expands when heated) in the thermosyphon circuit (e.g., in a heat exchanger machine) to break the seal spring tension in the expansion valve and "weep" or drip into the drain box under the drip tray. This may be set at 12bar. Why is this so? Water is constantly added to the circuit regardless whether the pump is on or not. As long as mains pressure is on water will flow through the circuit.
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  4. #4
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    Thank you.
    So we have three valves:
    1. The pump bypass adjacent to the pump to protect it.
    2. Over pressure valve to adjust the water pressure on the puck.
    3. The expansion valve to release general pressure due to heated water expansion. ( I have never seen this release water into the drip tray).

    Re: puck pressure and the opv. I set my extraction pressure to 9 bar using a blind filter pressure gauge. I suppose when I next get into the Giotto Rocket Premium I should reset this to 10 bar as I don't have an scarce (?) gauge.
    Greg Pullman has a wandering gauge that has a measure release valve. I think this would be a better way to set the opv?
    On my Giotto PID the pressure gauge releases at 9 bar. I guess this is because it is measuring at an actual extraction.
    I am beginning to see the light!

  5. #5
    Senior Member sprezzatura's Avatar
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    Hey! Make your own gauge for about $60.00 (3/8" BSB ball valve and 1/8" BSB male threaded pressure gauge). Screw it onto your single portafilter after removing the cup runner.

    For additional $$ buy a DMM with a K or T type thermometer with a bead style pick up. Mine's accurate to 1 Fahrenheit and cost $80.00

    It's not Scace grade but it's not $700.00 either and it works well!
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  6. #6
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    I have most of the bits. I just need the ball valve. I think to set a discharge rate will be good.
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  7. #7
    Senior Member sprezzatura's Avatar
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    You can play around with different pressure too. I have a client who runs his C.M.A. commercial machine @ 7 bar pump pressure and .8bar of boiler pressure. Morgan's Coffee. Very good espresso!



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