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Improper Installation of Iron Piping in a Fire Sprinkler System

ENVIRONMENT:

Office Building

EQUIPMENT:

Fire Sprinkler System

MATERIAL:

Iron Pipe

FAILURE MODE:

Improper Installation
 

Summary

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.

 

Background

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

 

Description of Material

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.

 

Figure 1.  Piping Overhead.

Figure 2.  Corrosion on T-Section.

 

Findings

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.

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 36įC for 48 hours.  Examination of the slide revealed approximately 100 bacterial colonies, corresponding to a bacterial count of roughly 1ī105 bacteria per milliliter of water.  This qualitative test indicated significant microbiological activity inside the pipe.  The ideal microbial limit would be 1x103.

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. 

 

Figure 3. Bottom view of the T-section pipe.

 

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. 

An elemental analysis of corrosion products and plumberís putty was performed using energy dispersive x-ray spectroscopy.  The results were tabulated in Table 1.

 

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.

 

Table 1.

Analysis of Deposit and Plumberís Putty

Quantitative EDS Analysis (wt.%)

Element

Deposit

Plumberís Putty

Aluminum

11.0

15.3

Calcium

24.4

43.5

Iron

34.2

0.9

Magnesium

1.3

1.2

Oxygen

14.3

18.0

Silicon

13.8

19.4

Sulfur

0.4

--

Figure 6.  Elbow C showing the relative condition of the threads on the right and those on the left.

Titanium

0.5

0.9

 
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.
 

Discussion

The OD corrosion of the fire sprinkler system in the building was isolated to selected fittings, elbows, etc.

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 plumberís 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. 

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.

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.

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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