Technical
Instructions
SKPx5.xxxUx
Electro
Ͳ
hydraulic
actuators
Document
Number
CC1N7643us
December
10,
2018
Page
12
Siemens
AG
Building
Technologies
Operation
continued
(see
Figure
3)
SKP55.xxxUx
During
the
burner
pre
Ͳ
purge
period,
when
the
gas
valve
is
closed,
only
the
air
pressure
difference
acts
on
the
regulator
causing
the
air
diaphragm
to
move
and
close
the
regulating
hydraulic
bypass
valve.
When
the
actuator
is
powered,
the
gas
valve
begins
to
open.
The
downstream
gas
pressure
difference
immediately
begins
to
increase
until
the
gas
pressure
difference
is
in
balance
with
the
air
pressure
difference
(in
accordance
with
the
pressure
ratio
adjusted
on
the
regulator).
The
bypass
valve
is
now
partially
open
so
that
the
oil
flow
supplied
by
the
pump
is
identical
to
the
return
flow.
If,
for
example,
heat
demand
increases,
the
air
damper
opens
further
increasing
the
differential
air
pressure.
The
SKP55.xxxUx
air
diaphragm
moves
and
causes
the
bypass
valve
to
close
and
the
gas
valve
to
open
further.
The
opening
of
the
gas
valve
increases
the
differential
gas
pressure
moving
the
gas
diaphragm
to
the
right
until
balance
is
restored
and
the
flow
supplied
by
the
pump
is
once
again
identical
to
the
return
flow
through
the
regulator
bypass.
Unlike
conventional
direct
acting
regulators,
the
SKP55.xxxUx
servo
operated
regulating
system
displays
virtually
zero
drop
(offset)
across
the
turndown
range.
)
NOTE:
The
SKP55.xxxUx
/
VGxxx.xxxU
is
a
1:1
differential
pressure
air/gas
ratio
controller.
This
means
that
the
control
adjusts
the
same
pressure
difference
on
the
gas
side
as
it
senses
on
the
air
side.
Any
other
gas
to
air
ratio
adjustment
will
require
a
modification
to
one
of
the
restrictions
or
the
installation
of
an
adjustable
orifice
(this
is
normally
an
adjustable
metering
orifice
in
the
gas
line).
For
this
purpose
the
VGxxx.xxxU
gas
valve
with
manual
operation
kit
AGA61
may
be
used.
See
gas
valve
Technical
Instructions
N7636us
and
N7631us.
)
NOTE:
1.
For
optimal
performance,
both
the
air
damper
and
SKP55.xxxUx
/VGxxx.xxxU
gas
valve
must
be
installed
upstream
of
its
pressure
differential
orifice.
For
other
layouts
please
consult
your
authorized
Siemens
AG
Building
Technologies
Division
Products
sales
representative.
2.
To
avoid
oscillation,
do
not
oversize
the
VGxxx.xxxU
valves
(see
Technical
Instructions
N7636us
and
N7631us).
Many
burner
designs,
because
of
reduced
mixing
energy
at
the
low
fire
level,
require
increased
excess
air
at
low
fire
in
order
to
maintain
optimum
combustion
parameters.
To
accommodate
this
requirement
the
SKP55.xxxUx
incorporates
a
bias
adjustment,
which
allows
the
characteristic
of
the
regulator
to
be
displaced
either
towards
excess
air
or
reduced
air.
Pressure
fluctuations
in
the
combustion
chamber
do
not
influence
the
performance
of
the
SKP55.xxxUx
air/gas
ratio
controller.
There
is
no
need
for
any
compensation
circuit.
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