A. Until the main reservoir pressure falls below ninety pounds, when the combined spring and air pressure in chamber “f” will force the diaphragm 28 down, seating the pin valve.  This shuts off the supply of air from chamber “d”, and the air confined in chamber “b” will escape to the atmosphere through the vent port “c”.  The pressure now being removed from above the governor piston, the spring 9 aided by the steam pressure under the valve 5, will force the piston upward, unseating the steam valve 5, allowing steam to pass through the governor to the compressor.

12.  Q. When the steam valve is seated, is steam entirely shut off from the compressor?

A. No; there is a small port drilled through the valve; its purpose is to maintain a circulation in the steam pipe and keep the compressor working slowly; thereby preventing condensation when the steam valve is closed.

13.  Q. With the automatic brake valve in release, running, or holding position, does the maximum pressure head operate?

A. No; as during this time the main reservoir pressure is not sufficiently high to actuate its diaphragm.

14.  Q. Where does the air come from that operates the maximum pressure head?

A. From the main reservoir direct. (See Fig. 1.)

15.  Q. When does the maximum pressure head control the compressor?

A. When the automatic brake valve is in either lap, service or emergency position, also when the main reservoir cut-out cock is closed.

16.  Q. How is the pressure created on top of the diaphragm in the maximum pressure head?

A. By the regulating spring 19.

17.  Q. What is the adjustment of this spring?

A. Spring 19 is adjusted to the maximum pressure desired in the main reservoir usually 130 pounds.

18.  Q. Explain the operation of the governor when the main reservoir pressure exceeds the tension of the regulating spring 19.

A. When the pressure in chamber “a” exceeds the tension of the regulating spring 19, the diaphragm 20 is forced upward, unseating the pin valve, allowing air to flow from chamber “a” to chamber “b” above the governor piston, forcing it down, shutting off steam and stopping the compressor.

19.  Q. How long will the governor remain in this position?

A. Until the main reservoir pressure in chamber “a” under the diaphragm becomes slightly less than the adjustment of the regulating spring 19, when the diaphragm 20 will move down, seating the pin valve, shutting off the flow of air from chamber “a” to chamber “b”.  The air entrapped above the governor piston will escape to the atmosphere through the relief port “c”; this will allow the governor piston to raise, unseating the steam valve 5, again allowing steam to pass through the governor to the compressor.

20.  Q. Is the maximum pressure head cut out in any position of the automatic brake valve?