|Maintaining Sprinkler Systems As They Age|
Issue 93: Maintaining Sprinkler Systems As They Age
By William E. Koffel, P.E., FSFPE, President, Koffel Associates, Inc.
Requirements based upon the age of the system
The most commonly cited requirement that is a function of the age of the system is the need to test a random sample of sprinklers. The purpose of this test is to determine if the sprinkler is still operational. Historically, the requirement to test one percent of the sprinklers from a system applied to sprinklers that have been in service for 50 years. Once the sprinklers have been in service for 50 years, additional testing of one percent of the sprinklers is required every 10 years. For sprinklers that have been in service for 75 years, the test frequency increases to every 5 years.
The procedure used for the test is referred to as a plunge test. The sprinkler is placed into the test apparatus, exposing the sprinkler to an airflow that has a controlled velocity and temperature. The temperature in the plenum of the test apparatus is considerably higher than the operating temperature of the sprinkler. An air pressure of 0.5 psi (0.4 bar) is placed on the inside of the sprinkler cap. The time for the fusible element or glass bulb to activate is recorded. If the sprinkler fails to operate in the specified amount of time, the sprinkler fails the test and all sprinklers represented by the test sprinkler are to be replaced.
The frequency of the plunge test requirement for sprinklers using a fast response element is different. The initial test is required when the sprinkler has been in service for 20 years instead of 50 years. The 20 year criteria were established for two reasons. First, fast response elements are relatively new in the marketplace. Without having the history associated with standard response sprinklers, the Committee selected a more frequent test to evaluate the performance of the newly introduced sprinklers. Secondly, in many cases the fast response element is used for a specific purpose (life safety, suppression, etc.). Note that the need to ensure that the sprinkler continues to operate within an acceptable response time may be more critical than standard response sprinklers which by definition do not have a maximum response time index ("RTI”). As data is collected regarding the performance of the sprinklers using fast response elements is collected, the Committee could consider revising the threshold at when the plunge test is required to be performed.
There are also special thresholds for the plunge test for sprinklers that are installed in what might be considered more harsh environments. Sprinklers included in this category include:
As one might expect, an alternative to the periodic testing of older sprinklers is to replace all of the sprinklers once the functional test is required for that particular application and sprinkler.
Whereas corrosion is the gradual destruction of a material, typically metal, the impact is clearly time dependent. NFPA 25 addresses the issue of corrosion in various ways. With respect to external corrosion, sprinklers, sprinkler pipe, and other components of a sprinkler system are required to be inspected at intervals not to exceed an annual inspection. While NFPA 25 indicates that sprinklers shall be "free of corrosion”, Annex paragraph A.220.127.116.11 refers to sprinklers that are severely corroded and lightly corroded. When one has a concern regarding the amount of corrosion on a sprinkler, the sprinkler should be tested to verify performance. However, if a sprinkler is severely corroded it should be replaced. Tests such as the plunge test do not address all of the concerns one might have regarding a severely corroded sprinkler. While the sprinkler may still operate, severe corrosion could impact sprinkler performance even once it operates (i.e., distribution pattern). During recent meetings of the Technical Committee there has been discussion regarding the need for research to be performed to assist one in determining when a sprinkler, with some corrosion, needs to be tested or replaced. However, an argument has also been made that the actual impact of corrosion cannot be assessed by a visual inspection and therefore, when corrosion is observed a representative sample should be tested or the sprinklers replaced.
Internal corrosion is addressed in a slightly different manner. An internal assessment of metallic sprinkler pipe is required every five years unless a risk assessment has identified a different interval as being acceptable. One of the items to be evaluated during the internal assessment is the presence of corrosion. The impact of corrosion on the inside of a pipe can be highly dependent on the type of sprinkler system. When a metal is immersed in water, the rate of corrosion is not a linear function. In this instance, the rate of corrosion will decrease as a function of time. Routine testing of the sprinkler system involving water flow and the introduction of new water into the pipe will also impact, and could increase, the rate of corrosion. The rate of corrosion in other types of sprinklers systems, in which water is introduced into the pipe and then removed, will be different than a wet pipe system if oxygen is present in the piping system when the water is removed.2 Internal assessments also provide an opportunity to identify other concerns such as tubercles or slime (potential indicators of microbiologically influenced corrosion, also referred to as MIC) and the presence of foreign organic or inorganic material (such as zebra mussels) which could obstruct pipe or sprinklers.
Lacking much data regarding the reliability of sprinkler systems as a function of age, the primary requirement in NFPA 25 that is age dependent is the testing of sprinklers. Test data has shown that the performance of some sprinklers is definitely impacted as a function of the time the sprinkler has been in service. Another age related concern is that of corrosion which is addressed by periodic visual inspections of sprinkler system components and a periodic internal assessment of the piping system. With respect to other performance concerns, NFPA 25 relies on periodic inspections, testing, and maintenance to evaluate the ongoing performance of the components.
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4th Quarter 2010 – Lessons Learned From Unsatisfactory
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from the investigating engineer’s perspective – R. Thomas Long, P.E., Neil P. Wu, P.E., Andrew F. Blum
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