The below information is for the IX Type boats. I hope will shade some light on the subject.

The starting air system consists of a line from the high pressure air manifold which leads by way of a stop valve, a filter, a 205 to 75 atmosphere (2920 to 1066 psi) reducing valve and a relief valve to the starting air flask with a pressure gauge. The 1066 pound line to the air bottle also has a branch to each engine, which reaches the engine by way of a pressure gauge, a stop valve, the main engine starting valve, a 75 to 30 atmoshere (1066 to 427 psi) reducing valve, a second gauge and the air starting and reversing arrangements on the engine. The reducing valves are similar in principle as those under the low pressure air system.

The starting air flask is normally kept charged to 1066 psi, acts as a volume tank and supplies air which is admitted to the desired engine by opening the related main starting valve. It also serves as an emergency source of air in case air from the high pressure manifold is not available.

Starting air for the diesel compressor is supplied when a starting air flask for the main engines is installed, from a branch on the discharge line from the compressor to the high pressure air maniflod, which in this case acts as a supply line to the diesel compressor, and id fitted with a stop valve, filter, a 205 to 30 atmosphere (2920 to 427 psi) reducing valve and a relief valve. Opening of the stop valve admits air to the air starting mechanism of the diesel compressor.

In lieu of the arrangement described in the foregoing three paragraphs, some vessels of this this have no starting air flask for the main engines, but instead have a line from the high pressure air manifold which leads, via a regulating valve, a filter and a parallel arrangement of two 205 to 30 atmosphere reducing valves (2920 to 427 psi) with stop valve on the high and low side of each reducer, to a common line serving the main engines and the diesel compressor. In this case one reducer acts as a standby for the other, and, if the system is in operation, the opening of the starting valve to an engine or the compressor admits air to the desired unit for starting.

Scott