A. Usually twenty pounds.

8. Q. With the spring adjusted at twenty pounds, what will be the total pressure on the upper side of the diaphragm?

A. Twenty pounds, plus the pressure in the feed valve pipe.

9. Q. With the feed valve adjusted at seventy pounds, and the regulating spring at twenty pounds, what pressure will be had in the main reservoir when the governor stops the compressor?

A. Ninety pounds.

10. Q. Explain the operation of the governor in controlling the compressor when a main reservoir pressure of ninety pounds is reached.

A. When the main reservoir pressure in chamber "d" slightly exceeds the pressure on top of the diaphragm it will move upward, carrying the pin valve with it. The air in chamber "d" pa.s.ses by the unseated pin valve through port "b" into chamber "b" above the governor piston, forcing it downward, seating the steam valve 5, thus shutting off the steam to the compressor.

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

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 pa.s.s 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 c.o.c.k 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 pa.s.s through the governor to the compressor.

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

A. No; as the air that operates this head comes direct from the main reservoir, therefore is not controlled by the brake valve.

21. Q. Is the excess pressure head cut out in any position of the brake valve?

A. Yes; as the air that operates this head comes through the automatic brake valve, and when the handle is moved beyond holding position, the port in the rotary valve seat, through which the air flows to chamber "d" is closed, thereby cutting out this head, leaving the compressor under the control of the maximum pressure head.

22. Q. What is the object of the duplex or double head governor?

A. By use of the duplex governor the main reservoir pressure may be controlled at two different predetermined pressures; as when running along the excess or low pressure head controls the compressor, at the low pressure--usually ninety pounds--this being sufficient to keep the brakes released and fully charged; whereas, in lap position, as following a brake application, the maximum or high pressure head controls the compressor at the maximum pressure used--generally 130 pounds--this for a prompt release and quick recharge of the brakes. From this it will be seen that the compressor has to work against the high pressure only during the time the brake is applied.

23. Q. In what position should the automatic brake valve handle be placed when adjusting the excess pressure head? The maximum pressure head?

A. Running position for the excess pressure head; lap position for the maximum pressure head.

24. Q. If, with the automatic brake valve handle in running position, the brake pipe and main reservoir do not stand twenty pounds apart, where would you look for the trouble?

A. Would first learn if the maximum pressure head was properly adjusted, and if it were, would then look for the trouble in the adjustment of the regulating spring in the excess pressure head.

25. Q. What should be done?

A. The regulating spring should be properly adjusted.

26. Q. How should the adjustment of the regulating spring in either pressure head be made?

A. By removing the cap nut 25 or 17 and s.c.r.e.w.i.n.g the regulating nut 26 or 18 up or down as may be required.

DEFECTS OF THE GOVERNOR

27. Q. What would be the effect if one or both of the pin valves leaked?

A. Would cause a delay in opening of the steam valve after the pin valve had seated; and if air leaks by faster than it can escape through the relief port "c", pressure will acc.u.mulate in chamber "b" and force the governor piston downward, so as to partially or wholly close the steam valve 5.

28. Q. How can you tell if the pin valves leak?

A. Leakage past the pin valve in the maximum pressure head will cause a constant blow at the relief port in all positions of the brake valve; leakage past the pin valve in the excess pressure head will cause a blow in the first three positions of the brake valve only.

29. Q. What would be the effect if the relief port "c" stopped up?

A. The compressor will not start promptly after the pin valve seats.

30. Q. What would be the effect if the drain port "W" were stopped up?

A. Steam leaking into the chamber under the governor piston will form a pressure and prevent the piston being forced downward to close the steam valve; the compressor will therefore continue to work until the main reservoir pressure is about equal to boiler pressure.