Page 47 - Reliability and Maintainability of In service Pipelines

36 Reliability and Maintainability of In-Service Pipelines

where s is pipe slope, u is velocity of stream (m/s), j is pH-dependent factor for
proportion of H 2 S, DSŠ is dissolved sulfide concentration (mg/L). A concrete
½
pipe is made of cement-bonded material, or acid-susceptible substance, so the
acid will penetrate the wall at a rate inversely proportional to the acid-consuming
capability (A) of the wall material. The acid may partly or entirely react. The
proportion of acid that reacts is variable (k), ranging from 100% when the acid
formation is slow, to perhaps 30% 40% when it is formed rapidly. Thus, the
average rate of corrosion (mm/year) can be calculated as follows

c 5 11:5kΦð1=AÞ ð1:18Þ
where k is the factor representing the proportion of acid reacting, to be given a
value selected by the judgement of the engineer and A is the acid-consuming
capability, alkalinity, of the pipe material, expressed as the proportion of equiva-
lent calcium carbonate. The A value for granitic aggregate concrete ranges from
0.17 to 0.24 and for calcareous aggregate concrete, A ranges from 0.9 to 1.1
(ASCE No.60, 2007). Substituting Eq. (1.17) into Eq. (1.18):
b
3=8
c 5 8:05k 3 ðsuÞ j DSŠ 3 ð1:19Þ
½
P A
0
Therefore the reduction in wall thickness in elapsed time t, is:
b
3=8
½
dtðÞ 5 c t 5 8:05k ðsuÞ j DSŠ 3 0 t ð1:20Þ
P A
1.6.2 DETERIORATION OF FERROUS PIPES

Ferrous pipes, including cast iron, wrought iron, ductile iron, and mild steel, all
hold a similarity in the sense that they are all derived from metals. Each of these
pipes hold varying structural and physical properties, and show differences in
strength.
However, the different types of ferrous pipes all show the same deterioration
mechanism, even though corrosion rates may differ depending on the various con-
ditions these materials present. The various conditions include the presence of fac-
tors that favor corrosion, or pipes holding particular structural characteristics that
are not able to withstand high levels of stresses and can readily lead to failure.
Therefore, the degree of corrosion occurring in these different types of ferrous
pipes and the severity of the damage caused as a result would depend on where
these pipes are used, the environments they are subject to as well as differences
in the structural composition, (i.e., carbon content) and design methods. For
example, the corrosion rate in ferrous pipes carrying freshwater compared to
wastewater can differ, due to a number of given various conditions, including