The refractory-lined effluent piping needs to be monitored to ensure that an
internal refractory failure does not lead to a pipe wall failure. Even a small crack
in the refractory will increase wall temperature. A major refractory loss will
rapidly heat the wall above its allowable maximum. To monitor for this, the
outside wall of the transfer line is painted with temperature-indicating paint,
normally changing color if the pipe wall exceeds about 500
F. This provides a
margin between maximum allowable temperature and actual.
Conversely, it is important that the steel wall of the refractory-lined header be
maintained above the water condensation temperature. The SMR effluent, if
condensed, will form carbonic acid. That acid will corrode the low-alloy steel.
The outside wall of the refractory lined pipe normally runs about 250–350
F to
meet all the requirements.
• Safety – Any fired heater needs a safety system, and an SMR furnace is not an
exception. The complexity of the SMR means that the safety systems around
these units are correspondingly more complex. These systems will not be
comprehensively covered, but a few specific considerations in SMR furnaces
are highlighted:
– Loss of draft control – Because an SMR fires at such high rates, loss of draft
control in the firebox creates a potentially hazardous situation. This can occur
with loss of the forced or induced draft fans, damper or register control issues,
PSA trips, excessive changes in fuel gas rates, high or low fuel pressures, and
many other causes. Safety systems for these furnaces generally trip on high or
low draft. These systems are backed up by excess oxygen monitoring at the
firebox outlet.
– SCR problems – Where a furnace has an SCR, the SCR may be tripped on
high or low flue gas temperature or on loss of ratio control in the vaporizer
skid. These trips are intended to avoid potentially explosive mixtures or
deposits in the vaporizer and/or flue.
– Fuel gas, pilot gas, or PSA off-gas high/low pressures – Depending on the
type of failure, there will generally be trips on several fuel stream pressures
because these can cause other issues. Fuels are tripped out of the furnace to
avoid accumulating an explosive mixture. Generally, pilot gas is maintained
unless draft is lost.
– Firing protection – In modern reformers, the safety systems contain a number
of interlocks to ensure the heater is fired safely. Typical interlocks include
verifying that all fuels are isolated before allowing a purge, ensuring that a
purge is completed, verifying that pilots are lit before allowing main fuel to be
admitted, and verifying that the main fuel is on before allowing PSA off-gas.
Factory Mutual (FM) cock valves and nitrogen pressurization of fuel headers
have both been used to verify fuel isolation before allowing a purge to start.
– Process trip – If the process side of the furnace trips for any reason, the heat
load on the furnace is drastically and immediately reduced. The furnace must
be cut back very quickly, and this can lead to the furnace tripping on draft or
fuel pressure. Generally, however, a process trip does not require a furnace
trip if the furnace can be reduced back to pilots with control.
598
S.A. Treese
internal refractory failure does not lead to a pipe wall failure. Even a small crack
in the refractory will increase wall temperature. A major refractory loss will
rapidly heat the wall above its allowable maximum. To monitor for this, the
outside wall of the transfer line is painted with temperature-indicating paint,
normally changing color if the pipe wall exceeds about 500
F. This provides a
margin between maximum allowable temperature and actual.
Conversely, it is important that the steel wall of the refractory-lined header be
maintained above the water condensation temperature. The SMR effluent, if
condensed, will form carbonic acid. That acid will corrode the low-alloy steel.
The outside wall of the refractory lined pipe normally runs about 250–350
F to
meet all the requirements.
• Safety – Any fired heater needs a safety system, and an SMR furnace is not an
exception. The complexity of the SMR means that the safety systems around
these units are correspondingly more complex. These systems will not be
comprehensively covered, but a few specific considerations in SMR furnaces
are highlighted:
– Loss of draft control – Because an SMR fires at such high rates, loss of draft
control in the firebox creates a potentially hazardous situation. This can occur
with loss of the forced or induced draft fans, damper or register control issues,
PSA trips, excessive changes in fuel gas rates, high or low fuel pressures, and
many other causes. Safety systems for these furnaces generally trip on high or
low draft. These systems are backed up by excess oxygen monitoring at the
firebox outlet.
– SCR problems – Where a furnace has an SCR, the SCR may be tripped on
high or low flue gas temperature or on loss of ratio control in the vaporizer
skid. These trips are intended to avoid potentially explosive mixtures or
deposits in the vaporizer and/or flue.
– Fuel gas, pilot gas, or PSA off-gas high/low pressures – Depending on the
type of failure, there will generally be trips on several fuel stream pressures
because these can cause other issues. Fuels are tripped out of the furnace to
avoid accumulating an explosive mixture. Generally, pilot gas is maintained
unless draft is lost.
– Firing protection – In modern reformers, the safety systems contain a number
of interlocks to ensure the heater is fired safely. Typical interlocks include
verifying that all fuels are isolated before allowing a purge, ensuring that a
purge is completed, verifying that pilots are lit before allowing main fuel to be
admitted, and verifying that the main fuel is on before allowing PSA off-gas.
Factory Mutual (FM) cock valves and nitrogen pressurization of fuel headers
have both been used to verify fuel isolation before allowing a purge to start.
– Process trip – If the process side of the furnace trips for any reason, the heat
load on the furnace is drastically and immediately reduced. The furnace must
be cut back very quickly, and this can lead to the furnace tripping on draft or
fuel pressure. Generally, however, a process trip does not require a furnace
trip if the furnace can be reduced back to pilots with control.
598
S.A. Treese