A vent valve is provided in some applications to allow gas
that may leak past the first valve to escape to a safe point
of discharge.
Vent valves are not used on propane fuel
that is heavier than air or fuel that may be toxic.
A gas pressure regulator is used to provide a constant
supply pressure to the gas train and butterfly control
valve. This constant pressure through a variable orifice
in the gas control valve obtains consistent gas flow rates.
The regulator must be capable of operating through
the full range of flows and pressure with consistent and
steady pressures. The regulator may be located up-
stream of the safety shutoff valves or integral with the
second safety shutoff valve.
The high and low gas pressure switches are used to
detect improper gas pressure and will prevent the burner
from firing under these conditions. The low gas pressure
switch is located near the supply of gas to the gas train,
to detect a loss of supply pressure. The high gas
pressure switch is located before the metering valve to
detect a surge in pressure to the burner.
The gas train is designed to work with the pressure avail-
able at the job site. This supply pressure generally refers
to the pressure available at the entrance to the gas train,
which is the pressure supplied to the gas trains shown in
Figure D-1. The supply pressure may vary depending on
the operation of the unit, in which case a minimum and
maximum pressure are needed to define the supply pres-
sure. The maximum pressure is the static pressure, or
the pressure in the line when there is no flow. The lead-
ing components of the gas train are selected to operate
up to these pressures. The minimum, or dynamic pres-
sure is that pressure available when the unit is operating
a full rate, or the reduced pressure due to the flow in the
line. The gas train is sized to this pressure, so that it can
deliver the required flow to the burner with this available
pressure. The job site supply pressures must be consis-
tent with the pressures listed on the burner material list.
The regulated gas pressure is that pressure required to
overcome the pressure drops in the piping, firing head
and furnace pressure to deliver the required flow at high
fire. Usually, one of the first steps in setting up gas com-
Page 30
Section D - Fuel and Electrical Systems
JBE(X) Manual
bustion is to adjust the regulator to get rated capacity.
This regulator is usually at the beginning of the train, but
in some cases, it can be integral to the second shutoff
valve. The pressure drops and regulated pressures will
be different in these two designs.
The manual valves are provided to lock out the fuel flow
during off times and during initial startup checkout. They
provide added safety and can simplify maintenance.
The gas piping can play a critical role in the operation of
the system. Throughout the system, the piping must carry
the required flow without significant loss of pressure. If
the drop is too high, there may not be enough pressure
to operate the burner a full capacity. This is especially
true between the gas train and the burner, where the
pressure is lowest. The piping between the train and
burner should have a minimum number of elbows and/
or turns to prevent high pressure drops.
2. Gas Pilot
Figure D-2 show typical gas pilot systems. Like gas
trains, they have the common components of a pres-
sure regulator and shutoff valve. The gas line connects
upstream of the gas pressure regulator in the gas train.
The gas pilot is positioned behind the diffuser, with the
pilot flame passing through the diffuser. The flame must
be large enough to pass in front of the scanner tube for
the Flame Safeguard to detect the pilot flame and allow
the burner to start. This also insures that there is
sufficient pilot flame to ignite the main fuel.
The gas pilot can operate with either natural gas or pro-
pane. Different internal orifices are used to regulate the
gas flow and maintain the same pilot size with the two
different fuels.
3. Pressure Atomized Oil System
Figure D-3 shows a pressure atomized return nozzle oil
system. This system uses pressure atomizing oil nozzles
that have a built-in return flow path. When oil is allowed
to flow into the return path, flow through the nozzle is
reduced. This modulates from low to high fire.
Shutoff
Valve
Gas Pressure
Regulator
Customer
Gas Supply
IRI >12,5000 MBH
Application
For all Applications
Except IRI >12,5000 MBH
Burner
Manual 1/4
Turn
Valve (Canada
Only)
Manual 1/4 Turn
Valve (Canada
Only)
Customer
Gas Supply
Manual Ball
Valve
Manual Ball
Valve
Shutoff
Valve
Shutoff
Valve
Gas Pressure
Regulator
Vent
Valve
Figure D-2
Gas Pilot Train Arrangements
Burner
