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Failure Analysis Case Histories
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Improper Installation
of Iron Piping in a Fire Sprinkler System
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ENVIRONMENT:
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Office Building |
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EQUIPMENT:
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Fire Sprinkler System |
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MATERIAL: |
Iron Pipe |
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FAILURE MODE:
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Improper Installation |
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Summary |
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Based on our evaluation of the
samples sections of an iron pipe in a fire sprinkler system, the pipe corroded
due to water leaking past the threaded joints. The escaping water continuously
wetted the outer surface causing corrosion of these sections. Tarnished threads
and salt deposits in the threads evidenced that the threads were leaking for a
considerable period. The greater buildup of corrosion products on the outside
bottom of the pipes demonstrated that free water was leaking around the threaded
joints and flowing downward due to gravity. The fact that the leaking joints
were not fully threaded, unlike those that were not leaking, was consistent with
improper fit-up of the sprinkler piping system.
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Background |
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CTL visited the site of the
client to observe corrosion deposits on both the inside and outside of the
piping for a fire sprinkler system located in a new building. The sprinkler
system was installed during the construction of the building five years ago.
CTL examined the sprinkler system in the building and
collected samples of corrosion products from the outside of the pipes
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Description of Material |
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CTL received two elbows joined by
a short section of pipe (nipple), Figure 1. CTL also receiver a T-section of
piping, Figure 2, and a water sample drained from the fire sprinkler system.
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Figure 1.
Piping Overhead. |
Figure 2. Corrosion on T-Section. |
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Findings |
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Black scrapings from the inside
surface (ID) of the pipe elbow assembly, Figure 1, were reacted with 10%
hydrochloric acid solution. A definite sulfide order was detected indicating
that some anaerobic sulfate reducing bacteria were active in the pipe.
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The conductivity reading of the
water sample was 250 mmhos/cm3
that indicated a moderate amount of dissolved salts in the water. The water
sample was tested for a microbiological activity using an agar growth medium for microbial
organisms for monitoring microbial activity in process water. After immersing
the slide in the received water sample, the slide was incubated at 36°C
for 48 hours. Examination of the slide revealed
approximately 100 bacterial colonies, corresponding to a bacterial count of
roughly 1´105 bacteria per
milliliter of water. This qualitative test indicated significant
microbiological activity inside the pipe. The ideal microbial limit would be
1x103.
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| The relative amount of outside
surface (OD) corrosion deposit on the T-section was noted to be greater on the
bottom of the pipe, Figure 3, than on the rest of the pipe. |
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Figure 3. Bottom
view of the T-section pipe. |
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The pipe was sectioned and a
metallographic mount was made of the pipe threads. Considerable salt buildup
was noted on the all of the threads, Figure 4. A large amount of salt was also
found in a broken thread and at the entrance to the threads from the elbow,
Figure 5.
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An elemental analysis of
corrosion products and plumber’s putty was performed using energy dispersive
x-ray spectroscopy. The results were tabulated in Table 1.
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Figure 4. A
salt deposits in a broken thread on the T-section of the pipe. |
Figure 5. Large salt deposits at the entrance
of the OD of the threads on the T-section of pipe. |
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Table 1.
Analysis of
Deposit and Plumber’s Putty
Quantitative EDS Analysis (wt.%)
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Element |
Deposit |
Plumber’s
Putty |
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Aluminum |
11.0 |
15.3 |
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Calcium |
24.4 |
43.5 |
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Iron |
34.2 |
0.9 |
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Magnesium |
1.3 |
1.2 |
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Oxygen |
14.3 |
18.0 |
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Silicon |
13.8 |
19.4 |
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Sulfur |
0.4 |
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Figure 6. Elbow C showing the relative
condition of the threads on the right and those on the left. |
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Titanium |
0.5 |
0.9 |
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| The side of the elbow with OD corrosion was only engaged about
half the distance of the uncorroded elbow side. The threads on the side of the
elbow with the corrosion were tarnished and had corrosion products in them,
Figure 6. |
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Discussion |
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The OD corrosion of the fire
sprinkler system in the building was isolated to selected fittings, elbows, etc.
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The build up of salts in the pipe threads demonstrated that a
considerable amount of water passed through these threads over time. The
analysis of the plumber’s putty showed it to be composed of calcite, alumina and
silica with small amounts of other metals. The material in the threads contains
significant amounts of iron (i.e., rust), as well as those elements present in
the putty. The tarnished threads provided additional evidence that water was
leaking through some of the threads.
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The leaking joints were only threaded into the joint half as
far as the non-leaking. The corrosion on the outside of the pipes was limited
to the close proximity of pipe at the leaking joint. The larger buildup of
corrosion products on the bottom of the pipes is also consistent with a
mechanism that excess water ran to the bottom (i.e., via gravity) of the given
pipe section and more corrosion occurred there.
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The presence of a large amount of rust in the
threaded joints and the tarnishing of the threads was evidence that water leaked
for a considerable period of time. The presence of microbiological
activity in the water and the sulfate buildup inside the piping confirms that
microbial corrosion was active on the inside of the pipes.
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The corrosion on the outside of some of the pipe sections is
consistent with the fact that some of the joints were leaking and that these
selective areas were wetted for a considerable time period. The larger buildup
of corrosion products on the bottom side of the pipes is also consistent with
gravity flow of leaking water.
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