17.12.1 Anhydrous ammonia may cause stress corrosion
cracking in containment and process systems made of carbon-manganese
steel or nickel steel. To minimize the risk of this occurring, measures
detailed in 17.12.2 to 17.12.8 shall be taken, as appropriate.
17.12.2 Where carbon-manganese steel is used,
cargo tanks, process pressure vessels and cargo piping shall be made
of fine-grained steel with a specified minimum yield strength not
exceeding 355 N/mm2, and with an actual yield strength
not exceeding 440 N/mm2. One of the following constructional
or operational measures shall also be taken:
-
.1 lower strength material with a specified minimum
tensile strength not exceeding 410 N/mm2 shall be used;
or
-
.2 cargo tanks, etc., shall be post-weld stress
relief heat treated; or
-
.3 carriage temperature shall be maintained, preferably
at a temperature close to the product's boiling point of -33°C,
but in no case at a temperature above -20°C; or
-
.4 the ammonia shall contain not less than 0.1%
w/w water, and the master shall be provided with documentation confirming
this.
17.12.3 If carbon-manganese steels with higher
yield properties are used other than those specified in 17.12.2, the
completed cargo tanks, piping, etc., shall be given a post-weld stress
relief heat treatment.
17.12.4 Process pressure vessels and piping of
the condensate part of the refrigeration system shall be given a post-weld
stress relief heat treatment when made of materials mentioned in 17.12.1.
17.12.5 The tensile and yield properties of the
welding consumables shall exceed those of the tank or piping material
by the smallest practical amount.
17.12.6 Nickel steel containing more than 5% nickel
and carbon-manganese steel, not complying with the requirements of
17.12.2 and 17.12.3, are particularly susceptible to ammonia stress
corrosion cracking and shall not be used in containment and piping
systems for the carriage of this product.
17.12.7 Nickel steel containing not more than
5% nickel may be used, provided the carriage temperature complies
with the requirements specified in 17.12.2.3.
17.12.8 To minimize the risk of ammonia stress
corrosion cracking, it is advisable to keep the dissolved oxygen content
below 2.5 ppm w/w. This can best be achieved by reducing the average
oxygen content in the tanks prior to the introduction of liquid ammonia
to less than the values given as a function of the carriage temperature T in the table below:
|
T (°C)
|
O2 (% v/v)
|
| -30 and below
|
0.9
|
| -20
|
0.5
|
| –10
|
0.28
|
| 0
|
0.16
|
| 10
|
0.1
|
| 20
|
0.05
|
| 30
|
0.03
|
Oxygen percentages for intermediate temperatures may be
obtained by direct interpolation.