API Spec 20H - Heat Treatment-20H_e1-20150424.pdf
8.Heat Treatment Equipment Temperature Uniformity Survey (TUS) and Calibration
8.1 Calibration
Calibration
Equipment used to record heat treatment or other equipment necessary to control heat treatment
operations shall be identified, controlled, calibrated and adjusted at specified intervals to maintain the
accuracy required by this standard. Calibration shall be performed in accordance with documented
instructions, which are consistent with nationally or internationally recognized standards specified by the
heat treatment supplier. Records shall be maintained.
8.2 HSL-1 and HSL-2 Furnace TUS and Instrumentation Calibration
8.2.1 The temperature at any point in the working zone of a furnace used for austenitizing,
normalizing, annealing or solution annealing shall not vary by more than ± 25°F (14°C) from the furnace
set-point temperature after the furnace working zone has been brought up to temperature.
8.2.2 Furnaces used for tempering, ageing and/or stress-relieving shall not vary by more than ± 15°F
(8°C) from the furnace set point temperature after the furnace working zone has been brought up to
temperature.
8.2.3 For furnaces used for both of the above heat treatment operations, the heat treatment supplier
shall define the temperature range for each operation. The furnace temperature tolerance shall conform
to the requirements specified in 8.2.1 or 8.2.2, as appropriate.
8.2.4 TUS Method for Batch-Type Furnaces
8.2.4.1 A TUS within the furnace working zone(s) shall be performed on each furnace at the maximum
and minimum temperatures for each range for which the furnace is being used.
a) A minimum of nine thermocouple test locations shall be used for all furnaces having a working zone
greater than 10 ft3 (0.3 m3
). For each 125 ft3 (3.5 m3
) of furnace working zone surveyed, at least one
thermocouple test location shall be used, up to a maximum of 40 thermocouples. See Figures 1 and
2 for examples of thermocouple locations.
b) For furnaces having a working zone less than 10 ft3 (0.3 m3
), the temperature survey may be made
with a minimum of three thermocouples located either at the front, center and rear, or at the top,
center and bottom of the furnace-working zone.
8.2.4.2 After insertion of the temperature-sensing devices, readings shall be taken at least once every
3 minutes to determine when the temperature of the furnace-working zone approaches the bottom of the
temperature range being surveyed.
8.2.4.3 Once the furnace temperature has reached the set-point temperature, the temperature of all
test locations shall be recorded at 2-minute intervals, maximum, for at least 10 minutes. Then, readings shall be taken at 5-minute intervals, maximum, for sufficient time (at least 30 min) to determine the
recurrent temperature pattern of the furnace-working zone.
8.2.4.4 Before the furnace set-point temperature is reached for austenitizing, normalizing or solution
annealing, none of the temperature readings shall exceed the set-point temperature by more than 25°F
(14°C). Before the furnace set point temperature is reached for tempering or aging, none of the
temperature readings shall exceed the set-point temperature by more than 15°F (8°C).
8.2.4.5 After the furnace control set-point temperature is reached, no temperature reading shall vary
beyond the limits specified.
8.2.4.6 When a furnace is repaired or rebuilt, a new temperature survey shall be carried out before the
furnace is used for heat treatment except for minor repairs as noted below.
Minor repairs or replacement of damaged or malfunctioning components or scheduled preventive
maintenance that restores the furnace to its original condition and are not expected to affect the
temperature uniformity characteristics of the furnace shall not require repeating the TUS. Examples
include, but are not limited to the following:
a) Replacing a burner or tile with another duplicate burner or tile,
b) Refractory repair using material with similar thermal properties,
c) Replacing a control or monitoring sensor to its previously documented location,
d) Replacing heating system components (e.g. gas regulator, valve, metering device, heating elements,
etc.),
e) Restoring original combustion pressure settings and tuning constants,
f) Replacing a controller with an identical controller with the same tuning constants,
g) System accuracy test failure,
h) Correction of furnace pressure control problem,
i) Repair of furnace door seals.
8.2.4.7 All furnace repairs shall be documented.
The organization responsible for quality assurance
shall make the determination whether an additional TUS is required based on the repairs made and the
particular furnace configuration before returning the furnace to service.