CHAPTERS
the burner
in
low fire during ignition. The
modulating control will be calling for higher
input and the burner will move
to
that position
as
rapidly
as
the damper motor can complete its
travel. This rapid heat input can subject the
pres·
sure vessel metal and refractory to undesirable
conditions.
Any control setting must not cause the boiler
to
operate at or
in
excess
of
the safety valve setting.
Settings that do not exceed
90%
of
the valve
setting are recommended, with lower settings
greatly desirable if system conditions permit.
Avoid having the operating pressure
too
near the
valve set pressure, because the closer the
operat·
ing pressure
is
to
the valve pressure, the greater
the possibility
of
valve leakage. Continued
leakage, however slight, will cause erosion
and
necessitate early valve replacement. The control
settings on a hot water boiler must
be
within the
temperature and pressure limits
of
the boiler
as
shown
in
Fig.
2.2
Ideally,
the
burner operating controls should
be
set under actual load conditions. Often, especial-
ly
on new construction, the boiler
is
initially
started and set
to operate under less than full load
req~irements.
As
soon
as
possible thereafter the
controls should be reset
to
provide for maximum
utilization
of
the modulating firing system.
To
accomplish this, and assuming that air/fuel
combustion ratios have been set, make
ap-
proximate adjustments
to
the controls
to
bring the
boiler pressure or temperature
to
meet the load
requirements.
To properly set the modulating control, carefully
adjust it under load conditions, until the load
is
maintained with the burner
fi.ring
at a steady rate.
The firing rate
at
that point may
be
full high fire
or slightly less, depending upon the relationship
of the boiler size
to
the
load.
When the modulating control
is
set
in
this manner
and
if the burner is at full high
frre,
the scale
setting
of
the modulating pressure control
on
a
steam boiler will have a reading that indicates
the
low point
of
the
modulating range. This fixed
differential range
is
described later
in
this sec-
tion. The scale setting
of
the modulating
750-150
ADJUSTMENTPR9CEDURES
temperature control on a hot water boiler will
have a
reading that indicates the midpoint
of
the
modulating range. This also
is
described later.
The operating limit control now should be
ad-
justed and
its
differential established. See direc-
tions later in this section for the mechanics
of
adjusting. In
an
installation that does not require
a very close control
of
steam pressure or water
temperature this adjustable differential should
be
set
as
widely
as
conditions permit, since this will
provide less frequent burner cycling.
The high limit control provides a safety factor
to
shut
the
burner off
in
the event the operating limit
control should fail
to
do so. The setting
of
this
control should be sufficiently above
the
operat-
ing limit control
to
avoid nuisance shutdowns.
The setting, however, must be within the limits
of
the safety valve settings and preferably not
exceed
90 percent
of
the valve setting. This con-
trol
requires manual resetting after tripping.
In the setting
of
these controls, consideration
. must
be
given
to
the time required for a burner
restart. Upon each
s~rt,
there
is
a pre purge period
of
some length, plus the fixed time required for
the
proving
of
the pilot
and
main flame. This,
plus approximately 1/2 minute required for
damper motor travel from low
to
high fire,
may
allow pressure or temperature
to
drop below
desirable limits.
The mechanics
of
setting the controls are:
F.
MODULATING
PRESSURE
CONTROL
(Steam)
Tum the adjusting screw until the indicator
is
opposite the low point
of
the desired modulating
range. Modulated firing will range between this
point and a higher point equal
to
the modulating
range
of
the particular control.
In
0-15
psi
con-
trols
the
range
is
1/2lb.;
in
5·150
psi
controls
the
range
is
5 lbs.;
in
10-300 psi controls
the
range
is
12
lbs.
CHAPTER 5-7
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