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?