Dropping Water Pressure to Marzocco GB5


(Nathaniel Millar House) #1

Hey Folks,

I am trying to figure out a way to drop water pressure in the groups to around 5-6 bar. According to the techs we use, it’s not possible because the pump needs to pull in a minimum pressure to the machine otherwise it will burn out.

Is there a way to create a lower pressure without putting the pump at risk?

Thanks!
Nate


(Troy) #2

A pump does not need to “pull a minimum pressure” for safe operation - but there might be a minimum flow required to prevent overheating. These are not the same thing.

To clarify, a pump doesn’t provide pressure. A pump moves the water. Everything else downstream of the pump resists that movement. It is that resistance which determines the pressure.

A pump can only do a given amount of work (limited by its power output) - so there is a trade-off between moving the water and overcoming the resistance. This is illustrated by the pump curve, which I’ve posted about previously (in this thread).

There are two options to reduce the pressure at the group:

A. Reduce the resistance in the puck (i.e. grind coarser, dose less) - which will increase the flow.
B. Reduce the flow to the group.

Since when most people talk about reducing their group pressure, I think they really want to reduce the flow, that leaves us with option B - reduce the flow to the group. There are at least three ways to achieve this:

1. Reduce the speed of the pump:
This changes the pump curve relationship and is likely to be expensive and complex, and not necessarily useful when you have multiple groups.

2. Divert some of the flow elsewhere:
This is a common method for controlling pressure/flow on espresso machine pumps. It’s usually implemented in the form of a valve which opens above a certain pressure, diverting flow back to the pump inlet. I believe most rotary pumps used in espresso machines have these inbuilt. Smaller domestic machines usually use a separate overpressure valve (OPV).

Another method is to install a needle valve to divert some of the flow from the pump discharge to the pump suction, which provides an alternative path for the flow. The amount of flow which is diverted depends on the resistance of the valve, relative to the resistance of the group(s). The upside is that you can adjust the flowrate on the fly, however it may be unreliable on a multi-group machine (if you have one pump). I have modified my Bezzera BZ99S in this way, and have been using it for a year or so.

Another way to achieve this is to add some resistance upstream of the puck. You could do this by installing a needle valve or a restriction orifice (aka gicleur) between the pump and group. The advantage of a needle valve is that it is adjustable.