33.  Q. What is the effect on the flow of air through the fire from opening the door?  What on the burning of the gases?  What on the flues and sheets of the fire-box?

A. When the furnace door is opened, the flow of air through the grate is stopped in proportion to the amount that passes through the door.  The vacuum will be filled from the quickest source and the door is closer than some parts of the grate.  The gases mix with the air from the door and pass out through the flues; no combustion takes place, as the air is not hot enough to unite with the gas.  The flues and sheets of the box will be caused to leak on account of the rapid contraction.

34.  Q. Can the firing be done more effectively if the water level is observed closely?

A. Yes, in order to know how much water there is in the boiler and whether it is necessary to hurry the fire; if the boiler is full, it is possible to prevent the pops opening by delaying the fire.

35.  Q. How should the fire and water be handled in starting from a terminal or other station?

A. The steam pressure should be near the maximum and there should be sufficient water in the boiler to last until such time as the fire is burning well so that the pressure will not be reduced when water is put into the boiler.  There should be a moderately heavy bed of fire well burned and distributed evenly over the grates.  After the fire is burning well, the injector should be started lightly; the feed being gradually increased so as not to cause any decrease of steam pressure.

36.  Q. What is the purpose of a safety valve on a locomotive boiler?  Why are more than one used?

A. A safety valve is used to limit the maximum pressure in the boiler by opening and allowing steam to escape.  More than one safety valve are used as additional protection against excessive pressure; one is set at the maximum pressure and the others are set at two or three pounds above the maximum pressure.

37.  Q. What is usually the reason for steam being wasted from the safety valve?  What can be done to prevent this waste?

A. Careless firing, careless running.  Both engineer and fireman work in harmony to obtain the best results.

38.  Q. What is the estimated waste of coal for each minute the safety valve is open?

A. About fifteen pounds.  The estimated waste of steam when an engine pops equals every second all the heat obtained from a quarter pound of coal.  Safety valves usually remain open about half a minute, resulting in the loss of about eight pounds of coal.

39.  Q. What should be the condition of the fire on arriving at a station where a stop is to be made?

A. On approaching the station where a stop is to be made, firing should be stopped far enough back to allow the carbon gases to be consumed before the throttle is closed, so there will be little or no black smoke from the stack and yet have sufficient fire that it will not be necessary to feed the fire again if a short stop is to be made until the train is started and the engine cut back or nearly to the running cut-off.