17.13.1 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 17.13.2, 17.13.3, 17.13.4, 17.13.5, 17.13.6, 17.13.7 and 17.13.8 should be taken as appropriate
17.13.2 Where carbon manganese
steel is used, cargo tanks, process pressure vessels and cargo piping
should 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 should also be taken:
-
.1 lower strength material with a specified minimum
tensile strength not exceeding 410 N/mm2 should be used; or
-
.2 cargo tanks, etc., should be post weld stress
relief heat treated; or
-
.3 carriage temperature should 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 should contain not less than 0.1% w/w
water.
17.13.3 If carbon manganese
steels with higher yield properties are used other than those specified
in 17.13.2, the completed cargo
tanks, piping, etc. should be given a post weld stress relief heat
treatment.
17.13.4 Process pressure
vessels and piping of the condensate part of the refrigeration system
should be given a post-weld stress relief heat treatment when made
of materials mentioned in 17.13.1.
17.13.5 The tensile and
yield properties of the welding consumables should exceed those of
the tank or piping material by the smallest practical amount.
17.13.6 Nickel steel containing
more than 5% nickel and carbon manganese steel not complying with
the requirements of 17.13.2 and 17.13.3 are particularly susceptible
to ammonia stress corrosion cracking and should not be used for containment
and piping systems for the carriage of this product.
17.13.7 Nickel steel containing not more than 5% nickel may
be used provided the carriage temperature complies with the requirements specified in
17.13.2.3.
17.13.8 In order 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 Tin the table
below:
|
T (°C)
|
O2(%v/v)
|
| -30 and
below
|
0.90
|
| -20
|
0.50
|
| -10
|
0.28
|
| 0
|
0.16
|
| 10
|
0.10
|
| 20
|
0.05
|
| 30
|
0.03
|
| Oxygen percentages for intermediate temperatures may
be obtained by direct interpolation.
|