Pilz PSS SB FT, Basic license - Manual (Page 113)

Modular Block System: Burner Management 6-69
Signal cycle of uninterrupted tightness control on a plant without an exhaust valve
If a plant does not have a special exhaust valve, the ventilation of the gas
flues are carried out between both safety gas valves using the safety valve
located directly before the burner chamber.
The cycle is identical to the one described previously.
The function of the exhaust VAV4 is, in this case, undertaken by
valve VAV3.
Caution!
This variation may only be used with the agreement of a specialist
in this field. They must assess whether the small amounts of
fuel brought into the burner chamber are tolerable for the purpose
of the seal check test.
In each case: pre-purge takes place after pressure control,
whether faulty or not.