Airbus A320 family has two APU option, whether APIC APS 3200 or Honeywell 131-9A.
Both are almost similar but have some different sub system, for example the bleed air system.
When APU is running, the load compressor will also run and produce compressed air.
If the air is un-used, then it should be discharged overboard, otherwise the bleed system will be over pressure.
The un-used air is discharged through APU exhaust.
In APS 3200 (Above picture), bleed air system uses one valve named bleed control valve to perform two function, user supply and discharges un-used air overboard.
If bleed air is un-used, means the user line is closed and surge line is open, vice versa.
The valve is powered by fuel pressure.
While in Honeywell 131-9A (Below picture), it uses two valve to perform the same function.
Load control valve for user supply and surge control valve to discharge un-used air overboard.
Load control valve is powered by electrical while surge control valve is powered by fuel pressure.
Author – Antonius Prasetyo ( Senior Instructor for Maintenance Type Training (A320/A330 CEO/NEO) at Lion Group Training Center)