Tube Wall Temperature Measurement On Steam Reformers - Best Practices

GBH Enterprises, Ltd.

Tube Wall Temperature Measurement
On Steam Reformers
Best Practices

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Operating Manual
Tube Wall Temperature Measurement
It is very important to plant operation that the tube wall temperature readings are
measured accurately and with a high degree of confidence.
If in order to prevent premature tube failure due to overheating, the plant is run
too conservatively, the full potential of the furnace will not be realized. On the
other hand, if the reformer tubes are run to too high a maximum tube wall
temperature, the furnace is likely to experience premature tube failures due to
overheating, resulting in plant down-time and added expense of replacing the
tubes and catalyst.
It is very important that accurate tube wall temperature measurements are taken
and a suitable correction method is employed to give the true tube wall
temperature. This value can then be used to decide on a suitable plant rate and
firing regime. Tube wall temperature measurements can also be used to ensure
that the furnace is balanced with all tubes being run to the same exit conditions.
There are several techniques available for tube wall temperature measurement,
ranging from contact methods such as surface thermocouples and the Gold Cup
pyrometer to the more common non-contact methods, which employ the use of
an infrared, disappearing filament or laser pyrometer.
GBH Enterprises has found over the years that a hand held optical pyrometer
used in conjunction with a suitable correction method for background radiation
results in accurate and trustworthy results.
As with all non-contact temperature measurement techniques, the instrument
relies on measuring the infrared radiation from the target. In practice, such
pyrometers measure the total radiation from the target and the surrounding hotter
surfaces. These instruments cannot differentiate between radiation emitted by or
reflected from the target. They must therefore be corrected accordingly. The
correction method used by GBH Enterprises is discussed in more detail in a
subsequent section.
For confident and sound temperature measurements, the following are
recommended.


1) The emissivity dial on the optical pyrometer should be set equal to 1.0 for
the entire tube shoot. An emissivity correction factor of 0.85 will be
employed manually for correction purposes. If the emissivity setting on the
pyrometer is set at any value other than 1.0, then the measured
temperature will be in excess of the true tube wall measurement thus
inflating the measured temperature readings and imposing an artificial limit
on the plant.
2) For each tube from which a temperature reading is taken suitable
background measurements must also be made. These background
readings should include all hotter surfaces visible to the measurement
tube and likely to radiate to the optical pyrometer. Such surfaces include
side and end wall refractory, furnace roof and flue gas extraction tunnels.
10 – 15 background temperature readings should be taken and used in
the tube wall temperature correction.
3) The number of correction temperature readings taken should be scaled in
such a way as to try and replicate reality. For the bottom temperature
correction, readings for a top fired furnace more allowance should be
made for the flue gas extraction tunnels than the end walls as these are
going to contribute more background heat to the measured tube
temperatures. This should be performed by taking more readings from the
flue gas tunnels than the end walls.
4) Where the bottom peephole is below the tunnel tops, background readings
should be taken from the tunnel walls and the furnace floor. If the
peepholes are above the top of the tunnels then the temperature of the top
of the tunnels should be measured; for every 4 measurements of the top
of the tunnels, at least one should be taken from the furnace floor since
this does contribute to the radiation measured by the pyrometer.
5) When taking any temperature readings either tube or background care
should be taken to ensure that flame temperatures are not inadvertently
picked up thus resulting in enhanced temperatures.
6) Where possible, temperature readings should be taken at 90º to the tube
to minimize the path length and hence the radiation emitted by the flue
gas. By using the shortest path length, the effect will be minimized. For all
measurements the pyrometer should be placed as close to the peephole
as possible to reduce the effects due to ambient surroundings. The
peephole door should be open for the minimum possible period of time,
thus ensuring that the temperature within the furnace will not have fallen
during the tube shoot.

7) Only one peephole should be open at any given time, thus preventing the
possibly of a draft circulation effect occurring.
8) When any peephole is first opened it is recommended that any personnel
stand to one side of the peephole for several seconds, thus ensuring that
the furnace is running at a slightly negative pressure. It is possible for
pockets of higher pressure to exist within the furnace, which may result in
flames being emitted through a peephole.
9) Optical pyrometers have measuring angles of generally 33º. For typical
furnace geometry at distances of greater than about 5m into a furnace a
whole single tube cannot be seen, as it will be obscured by neighboring
tubes. Radiation from these neighboring tubes will be picked-up by the
instrument and mask the true temperature of the target tube. In addition,
optical pyrometers have viewing angles of 7-9º. Therefore, at distances
greater than 1m into the furnace more than one tube will be in the field of
view. This creates problems for the operator of targeting the correct tube.
At distances greater than 10m into the furnace, individual tubes are
virtually impossible to target adequately. These distances should be borne
in mind during any temperature shoot.
10) Temperature readings should be taken from all peepholes in order to get
an accurate representation of the furnace operation. The distance for
temperature readings should be minimized to reduce any sight path
effects resulting from hotter furnace gases and also to avoid interference
from neighboring tubes. On top fired furnaces, this requires that tube wall
temperatures be measured from both ends of the furnace.
11)

It is important that different people who take tube wall temperature
measurements are fully trained so that all tube shoots are conducted in a
systematic method. This allows for sensible and accurate comparisons
between different people’s measurements.


Temperature Measurement Technique
Top – Fired Reformer
Figures 1 and 2 below will be used to explain the method used by GBH
Enterprises for tube temperature measurement within a typical Top-Fired
reformer.

Figure 1

Tube temperature measurement

Tube Temperature Measurement
i.

For each tube the temperature at the centre of the peephole height
should be taken with the pyrometer held at 90º to the horizontal.
Care should be taken to ensure that the pyrometer is held in a
constant position for the entire tube shoot with all readings being
recorded at a uniform level

ii.

Half the tube temperature readings should be taken from one end
of the furnace, with the remainder being from the opposite end thus
minimizing the length of radiation measurement.

iii.

This procedure should be repeated for all peephole levels, in order
to ascertain the degree of balance within the furnace.


Background Temperature Measurement

Figure 2

Background temperature measurement

i.

For each row of tube temperature measurements taken it is of extreme
importance that an adequate number of suitable background (correction)
temperature readings are also recorded. For both Row 1 and 3 it will be
necessary to take 10 – 12 sidewall readings, half from either end of the
furnace. It will also be necessary to take 3-4 end wall readings from the
opposite end furnace peephole. All measurements should be taken at the
same level as the tube temperature readings.

ii.

For top level peepholes it will be necessary to take some roof
temperatures were possible, while for bottom level peepholes it is of vital
importance that tunnel temperature readings are recorded as they
contribute a considerable percentage to background radiation due to their
very high operational temperature. The number of coffin temperatures
measured and used in the correction technique should equal the number
of sidewall correction readings.

iii.

For Row 2 where it is not possible to see the sidewalls corrections should
be made using the adjacent tube rows 1 and 3 in the same way that the
sidewall was used for the outer rows. The end wall, roof and tunnel
temperature readings should be taken as described in ii.

All the above recorded tube temperatures should be used in the correction
method described later.

Side – Fired and Terrace Wall Reformer
Figures 3 and 4 below will be used to explain the method used by GBH
Enterprises for a typical Side fired and Terrace Wall reformer.

Figure 3 Tube temperature measurements

Tube Temperature Measurement
iv.

For each tube the temperature at the centre of the peephole height
should be taken with the pyrometer held at 90º to the horizontal.
Care should be taken to ensure that the pyrometer is held in a
constant position for the entire tube shoot with all readings being
recorded at a uniform level.

v.

If sidewall peepholes exist these should be used where possible to
minimize any sight path effects, Figure 3 shows the method of tube
temperature measurement to be used from the sidewall. If it is not
possible to use the sidewall peepholes then the tube temperature
survey should be carried out using the same method as that
employed for the top-fired furnace.


Background Temperature Measurement

Figure 4
iv.

Background temperature measurements

For each row of tube temperature measurements taken it is of extreme
importance that an adequate number of suitable background (correction)
temperature readings are also recorded. For end tubes it will be necessary
to take 4-6 sidewall readings, half from either side of the furnace. It will
also be necessary to take 6-8 end wall readings from the end side
peepholes. All measurements should be taken from the same height as
the tube temperature readings.
v. For top level peepholes it will be necessary to take some roof
temperatures were possible, while for bottom level peepholes it may be
possible to take some floor temperature readings.

vi.

For Centre tubes where it is not possible to see the end walls, corrections
should be made using the sidewall temperature readings, each sidewall
temperature being measured from the opposite side peepholes as shown
in Figure 4 above.

All the above recorded tube temperatures should be used in the correction
method described later.


Safety Considerations
It is very important that all temperature measurements are carried out in the
safest possible manner.
Fire resistant clothing should be worn where possible and at a minimum gloves
with a high resistance to heat. When making temperature measurements with the
optical pyrometer normal cotton gloves are usually not adequate to prevent
radiation burns to the hands and suitable heat resistant gloves should be utilized.
During IR pyrometer surveys the use of green shaded IR furnace glasses is
recommended to prevent any eye damage which may occur when viewing the
inside of a reformer furnace for an extended period. These glasses also help
when looking for hot banding since they make the hot bands stand out from the
normal tubes.
To prevent dehydration adequate supplies of water should be available on the
furnace itself. For a large furnace regular breaks should be taken during the
temperature shoot.
For Foster Wheeler and Side fired reformers where 2 cells run in parallel, the
time spent between the cells should be minimized as this area tends to be very
confined with little or no breeze, hence increasing the possibility of thermal
fatigue occurring.


Tube Wall Temperature Measurement On Steam Reformers - Best Practices

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