1961 West-Side Drive, Rochester 1, New York
OF ENGINES, GOVERNORS AND NOMENCLATURE
Dear Editor:
It has occurred to me that some of the readers of your fine
magazine might be interested in an introductory discussion of the
ordinary flyball governor as used on the steam engine and in the
terminology of control instrument engineering. To accentuate the
control instrument point of view, I have capitalized terminology
common to the process control field.
For the same governor setting, it is well known that within the
limits of engine operation with the governor valve being neither
fully opened nor practically closed, the heavier the load, the
slower the speed. Speed is the CONTROLLED VARIABLE in this PROCESS.
When a LOAD CHANGE occurs, say the load is increased, the speed
falls because the governor must open the steam valve wider to
permit the passage of more steam to handle the extra load; and the
steam valve will be opened wider by the governor only as the speed
is reduced.
Conversely, a reduction in load will cause the speed to be
increased to a higher value. With the system IN BALANCE so the
speed is again steady, the deviation of the new value is known as
OFFSET. Should the exact same speed be required with the different
loading, the operator would have to ‘touch-up’ the governor
setting. The control instrument trade would call the means of
‘touching up’ the setting MANUAL RESET.
This throttling type of control in which the CORRECTIVE ACTION
(repositioning of the governor actuated steam valve) is
proportional to the DEVIATION OF THE PROCESS (speed change) from
the CONTROL POINT SETTING (governor adjustment) is known as
PROPORTIONAL CONTROL. In proportional control, there is no single
exact speed for one setting of the governor. Instead, there is a
CONTROL BAND which consists of all engine speeds that the engine
will assume, as dictated by the loads, within the limits of the
governor-controlled steam valve being neither fully opened nor
practically closed.
Incidentally, if the control band is relatively wide, the action
of the governor is sluggish and it is said to have LOW SENSITIVITY.
If the band is narrow, the action of the governor is quick and the
engine is ‘up on the bit.’ A governor so designed is said
to have HIGH SENSITIVITY. However, if the control band is too
narrow with resultant too high sensitivity, there will be excessive
correction of the steam valve position for the change in speed
incurred and there will be a tendency to HUNT. In others words, the
engine will tend to alternate between running too slowly and too
rapidly and we have INSTABILITY.
When the load is so heavy that the engine cannot maintain the
speed corresponding to the fully opened position of the governor
valve, the process is said to be OUT OF CONTROL. The process is
also out of control whenever the engine speed is higher than that
corresponding to the practically closed position of the governor
valve as has occurred in descending steep hills at too high a
speed. The governor valve would be closed because of the speed, and
no steam would be available for reverse braking. The engine would
run down the hill, literally out of control a situation that would
evoke something like ‘quick John, off the governor
belt!’
* Project Engineer for The Taylor Instrument Companies at
Rochester, New York; in charge of pressure standards and author of
‘The Engine Wasn’t There,’ ‘The Boiler Is Leaking,
Dad’ and other articles.
