I suspect that so long as your pump is a positive displacement pump (ex: the "vibratory pump" or "rotary vane pump" as described in this webpage about espresso pumps) and is sized to output the flowrate you desire, then you only need a discharge valve or an appropriately designed flow restriction device (ex: a restriction orifice or your "pressurized filter basket").
If you need to reduce flowrate of a fixed-speed positive displacement pump yet maintain a controlled pressure, read on about installing a recycle line.
Controlling mass flowrate and pressure through a pressure vessel (an espresso machine) is possible. I'm not an expert in espresso machines but I am somewhat familiar with the equipment needed to carry out the chemical engineering unit processes of solid-liquid extraction (ex: making espresso); I'm used to larger machinery.
You need a pump that can is capable of providing more than the maximum desired flowrate at more than the maximum desired pressure. A quick search for espresso pumps brings up this page about rotary and piston pumps, both positive displacement types. These pumps tend to output constant flowrate no matter the output pressure (they may still output less due to seal leakage and power limits of the motor).
If your pump isn't a positive displacement type, there are engineering methods that use "pump curves" as Drew described to take into account quirks about other pump types (ex: centrifugal). However, no matter the type of pump used, as long as sufficient pressure and flowrate capacity in the pump is available, I know of three methods for controlling the pressure and flowrate through the system:
- Speed control
- Discharge control
- Recycle control
You will need at least two of the three.
1. Speed control
"Variable Frequency Drive" or, VFD, is a way to adjust the speed of the pump's rotating elements. Slower generally means less flow rate and pressure.
For your application, I imagine this would be an electronic module that adjusts the frequency and voltage of the alternating current electricity that powers your device. In the U.S. it is probably a frequency of 60 Hertz (Hz) at 120 volts of alternating current (VAC). I am not finding many hits on searches for variable speed espresso pumps so perhaps this type of control isn't popular. This doesn't surprise me since adjusting frequency of power requires complex electronics.
Using a VFD is like adjusting your pump size without having to buy a new pump.
2. Discharge control
This type of control involves restricting flow leaving the pressure vessel with an adjustable valve. Generally, when the valve is more closed, an increase in upstream pressure and reduction in flowrate through the system results.
The "pressurized filter basket" you mentioned in a comment on another answer sounds like it may fulfill the role of a discharge control. However you decide to restrict discharge flow, the discharge valve position (or orifice sizes of the basket?) will have to be adjusted in tandem with the other control method you select (Speed control or Recycle control) in order to achieve a desired pressure and flow through the pressure vessel.
3. Recycle control
This method of control involves sending a portion of the flow leaving the pump or pressure vessel back into the inlet of the pump.
In your application, this may be achieved by installing two tees:
- A tee on the high pressure piping at the vessel or pump outlet
- A tee on the low pressure piping at the pump inlet.
Then, you connect the two tees with a length of pipe containing a flow-restricting valve; this is a "recycle line". When the pump runs, some amount of fluid continuously recirculates if the recycle line valve is open. As the valve closes less fluid is permitted to recycle and the flow of fluid out of the system increases (provided the discharge control valve (if present) is somewhat open).
Recycling fluid wastes some energy since fluid is pressurized by the pump and then depressurized at the recycle valve without leaving the system. The wasted energy heats the fluid somewhat. Recycling fluid might be desired if a limited supply of water is available or more extensive leaching of the coffee grounds bed is desired.
Maintaining pressure and flowrate
As I mentioned before, if you have a pump that can output more than the desired flowrate at more than the desired pressure, then two of the three control methods will be necessary to maintain pressure and flowrate within the pressure vessel. This is the result of what in chemical engineering is known as a "degrees of freedom analysis". You will notice this phenomenon if you set up your system in a process simulator (ex: DWSIM (FOSS), VMGSIM (proprietary)) and fix pressure and flowrate of the pressure vessel.
Recycle and Discharge control example
For example, let's say you employ discharge control along with recycle control. Your positive displacement pump runs at a fixed speed. Fluid is pumped from the pump to a the pressure vessel. The fluid then flows out of the vessel to a tee at which point the flow splits: one stream goes to the discharge control valve and one stream goes to the recycle control valve. How much each valve restricts flow depends upon the desired pressure and flowrate setpoint of fluid passing through the vessel.
At this point you could manually adjust the recycle and discharge valves to be more open or more closed and monitor the resulting pressure (ex: with a pressure gauge) and flowrate through the system (ex: with a flowmeter).
A procedure for reaching your target pressure and flowrate might be the following:
- Completely open the recycle valve.
- Completely open the discharge valve.
- Start the pump. Flowrate may be lower than desired and vessel pressure may be lower than desired.
- Slowly close the recycle valve until the flowrate rises to the desired setpoint. This step will cause the vessel pressure to increase.
- Slowly close the discharge valve until the vessel pressure rises to the desired setpoint. This step will cause the flowrate to decrease.
- Repeat step 4.
- Slowly open the discharge valve until pressure falls to the desired setpoint. This step will cause the flowrate to increase somewhat.
- Slowly open the recycle valve until the flowrate falls to the desired setpoint.
- Repeat steps 5 through 8 until both the desired discharge flowrate and vessel pressure setpoints have been achieved.
It is possible to automate what is achieved by the manual control procedure I described by connecting both the recycle and discharge control valves to computer-controlled actuators and connecting the computer to a pressure transmitter and a flowmeter.
The computer would run two "control loops": a pressure control loop and a flow control loop. Often in industry these will be "PID" control loops (short for "proportional" "derivative" and "integral"). A control loop is a computer program that makes a small adjustment to valve position, waits for a certain amount of time, reads the resulting change in a measured variable (ex: pressure), calculates a new small adjustment, repeating the process. Even though each control loop only responds to a single variable, the system as I've described has two degrees of freedom and therefore two control loops will converge upon the appropriate valve positions.
However, I suspect in your application the valve positions will probably be similar from each batch of espresso to the next, especially if the pressure drop caused by the coffee grounds themselves is similar for each batch. If so, then you may not need automatic control to achieve the pressure and flowrate control you desire.
If you found that the quantified flow restriction required by each valve throughout each batch didn't change batch-to-batch, then each valve could be replaced with an appropriately sized "restriction orifice". I imagine this is what espresso machine manufacturers would do to control temperature and pressure while at the same time reducing the cost of each machine.
Examples of piping restriction orifices. Image by NASA released into the public domain.
Speed and Discharge control example
If, for example, you perfectly sized your pump to always output the required flowrate no matter the downstream pressure (which is probably nearly the case for positive-displacement pumps), then you would only need a discharge control valve to maintain the desired vessel pressure and flowrate.
Sizing a positive displacement pump correctly in your application would have the same effect as adjusting the speed of your pump motor to achieve a target flowrate, assuming that pressure drop through the coffee grounds does not change throughout a batch.
I suspect that so long as your espresso pump is a positive displacement pump and is sized to output the flowrate you desire, then you only need a discharge valve or an appropriately designed flow restriction device (ex: a restriction orifice or your "pressurized filter basket").