Sprinkler Sensitivity Requirements - April 2019

by Grant Lobdell

According to the current, 2017 edition of NFPA 25 Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, sprinklers are to be sent in for field service testing on a periodical basis according to 5.3.1.  This field service testing is further defined in the annex of the standard under section A.5.3.1.1 stating that sprinklers should be laboratory tested for sensitivity and functionality, specifically.  While the 2017 edition of NFPA 25 does discuss sensitivity and functionality requirements to some degree, the upcoming 2020 edition of NFPA 25 will have much more defined requirements.*  Note that these requirement changes have already been implemented at Dyne and other sprinkler labs across the country.  For periodical testing purposes, a sprinkler is functional if the waterway clears at 7 PSI.  The periodical sensitivity requirements for a sprinkler will be defined as shown in Table 1.

 

Table 1: Periodical testing sensitivity requirements per the 2020 edition of NFPA 25.

 

What is a RTI?

 A sprinkler’s sensitivity requirement is not defined by response time.  Instead it is given as a response time index (RTI) which can be defined** as shown in Figure 1.

Figure 1: A RTI equation where time is in seconds, temperature is in °C, velocity is in m/s, and RTI is in (m·s)^1/2_.

 Why define a sprinkler’s sensitivity by a RTI?

Response time by itself does not give the whole story about sensitivity.  The conditions it was tested at must also be factored in.  For example, testing a sprinkler starting at 0°C will take a lot longer to go off than one starting at 100°C.  It just has further to go to reach its activation temperature.  The RTI equation above takes these factors into account.  Likewise, differences would be seen in sensitivity if the test oven is set several hundred degrees above a sprinklers activation temperature vs just barely above it.  Therefore, we need to account for the variables in the test to effectively compare results across sprinkler types and test conditions.  The way the sprinkler industry does this is through the RTI equation.

 

*Please consult Dyne’s article from November 2018, titled Sprinkler Requirement Changes, for more details on the differences.

**Other definitions do exist that account for additional variables.

 

How does Dyne determine a sprinkler’s response time requirement as reported?

 The RTI equation above can be solved for response time as shown in Figure 2.

Figure 2: RTI equation solved for response time where time is in seconds, temperature is in °C, velocity is in m/s, and RTI is in (m·s)^1/2_.

In doing so, a sprinkler’s maximum allowable response time can be calculated based on the test conditions and NFPA’s RTI requirements.  For example, what is the response time requirement for a standard response sprinkler rated for 155°F (68.3°C)?  Because the sprinkler is standard response, the RTI must be below 350 (m-s)^1/2.  Assuming the sprinkler is being tested from ambient conditions (24°C) into a 135°C oven with an air velocity of 2.5 m/s as outlined by UL 199 Standard for Automatic Sprinklers for Fire-Protection Service, all of these values can be plugged into the equation above to solve for response time (Figure 3).

Figure 3: Response time calculated for a standard response sprinkler rated for 155°C (68.3°C) and tested from ambient conditions (24°C) in an oven set at 135°C with an air velocity 2.5 m/s.

The sprinkler in this example would need to go off at 112.7 seconds or less.  Changing any condition in the equation will result in a change in the response time requirement but such change should also impact the response time such that a sprinklers RTI remains constant.

Is any degradation allowed for field sprinkler samples?

Both the current, 2017 and upcoming, 2020 editions of NFPA 25 state the following:

Sprinklers that have been in service for a number of years should not be expected to have all the performance qualities of a new sprinkler.

Note the periodical requirements spelled out in the 2020 edition of NFPA 25 have already accounted for some allowable degradation of field sprinkler samples (Table 2) when compared to the new sprinkler requirements as outlined by UL 199 and UL 1767 Standard for Early-Suppression Fast-Response Sprinklers with the exception of standard response sprinklers which already are given a considerable RTI allowance.

 

Table 2: Maximum RTI allowed for new and field sprinklers as defined by various UL standards and NFPA 25.

 

***Some exceptions apply.

 

If you have any questions regarding this article or would just like more information, please contact Dyne Fire Protection Labs at lab@dyneusa.com or (800) 632-2304.

 

©Dyne Fire Protection Labs 2019