The Importance of Fire Sprinkler System Main Drain Testing
Impairment to a Sprinkler System Can Have Catastrophic Consequences
By Neil P. Wu, FPE, CBO
Introduction: Degradation of the water supply or impairment to a sprinkler system can have catastrophic consequences. Periodic inspection, testing, and maintenance (ITM) of the sprinkler system is an essential, often mandated, and expected preventive measure to promote proper operation of a sprinkler system. Performing improper ITM can result in undue risk to the contractor if improper testing and/or maintenance are found to contribute to fire damages caused by an underperforming sprinkler system. Good engineering practices are discussed for the evaluation of water supplies through main drain testing for automatic, wet-pipe, water-based fire sprinkler systems. Background Much of a sprinkler system’s overall performance relative to controlling the fire is dictated by the available water supply, assuming that the proper design was applied to the hazard being protected. The basic principle of a hydraulically designed waterbased fire sprinkler system is that the peak flow and pressure demand is no greater than the available supply. If an inadequate flow and/or system pressure is delivered, the sprinkler system is likely to operate improperly, possibly allowing the fire to spread out of control.
The hydraulic demand at a system reference point, such as the base of riser (BOR), can be graphically compared to the available supply to determine the adequacy of the water supply. A hypothetical system demand is shown in Figure 1 where the system demand point is less than the supply curve (plotted as a line on semi-log graph).
The design margin or “buffer” is the pressure difference in the available supply curve and the system hydraulic demand point, in psi. The model codes governing the design of automatic sprinkler systems are silent regarding the minimum design buffer, leaving the size of the margin to the discretion of the designer. The buffer should account for foreseeable variations in water system strength such as seasonal effects and peak water consumption hours.
Water Supply Evaluation
Accurately characterizing the flow and pressure of the avail- able water supply is paramount not only during the initial design and installation of a fire sprinkler system, but also for the continued protection of the building. The following actions are provided for consideration for the ITM of water-based wet pipe fire sprinkler systems:
1. Perform Periodic Testing of the Fire Sprinkler System
— NFPA 25, Standard for the Inspection, Testing, and Mainte- nance of Water-Based Fire Protection Systems, is a reference commonly adopted for enforcement and offers a schedule of preventive ITM activities. In particular, with respect to water supply, Section 13.2.5 of NFPA 25 states that a main drain test shall be conducted annually at each water-based fire protection system riser to determine whether there has been a change in the condition of the water supply, supply piping, and control valves. Although it is customary to consider the main drain test as an “annual” event, more frequent tests are required depending on the water supply configuration. The frequency of the main drain test is increased to a quarterly basis where the sole source of the water supply is through a backflow preventer and/or pressure reducing valves. Verify these tests are being conducted on a timely basis in accordance with NFPA 25 or other applicable requirements. Written records should be kept for future refer- ence and troubleshooting.
2. Conduct an Initial Water Distribution System Analysis
— A key fundamental to using the main drain test data to moni- tor changes in the water supply assumes that the water supply was adequate at the time of original installation of the system. Performing a water distribution system flow test (i.e., hydrant flow test) at the initial acceptance testing for comparison to the sprinkler system design is often overlooked and not validated during the building construction inspection process. Original data obtained well in advance of construction may not accurately describe the water supply strength when construction is complete, and more importantly, at the time of building occupancy.
Although a main drain test may repeatedly produce similar residual pressures at regular inspections, the system supply may have been inadequate from inception. Without calibration of the actual water distribution supply curve to the sprinkler system design criteria, an inadequate water supply can remain undetected despite performing the required main drain tests.
NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants, is a recommended practice that addresses hydrant flow tests. For new building construction, a hydrant flow test should be performed at the time of initial acceptance testing for the sprinkler system, just prior to building occupancy. For an existing system in service, a water supply flow test should be performed at the next inspection interval, if the baseline test was not performed and recorded at the completion of construc- tion. The water supply curve can be used to validate the design flow data used by the design professional and/or installing fire sprinkler contractor and for calibration of the main drain test residual pressures. If the as-built data does not agree with the design flow test figures and/or the supply is inadequate to satisfy the sprinkler system demand, investigation of the water supply should occur immediately.
After the water supply curve is characterized and determined to satisfy the sprinkler system demand, future main drain tests results can be checked against the baseline value to confirm water supply adequacy. With this foundation established, the results of the main drain test can be used with confidence.
3. Test and Calibrate Pressure Gauges — The data obtained from sprinkler system testing is only as valid as the instruments used for measurement. Relying on faulty pressure gauges can produce inaccurate test results. Contractors performing ITM services should inspect and test pressure gauges periodically to verify proper operation. Based on NFPA 25, Section 12.2.8, pres- sure gauges are to be inspected on a monthly basis and replaced with a calibrated gauge or tested every five years to be within
3% of the full scale in comparison with a calibrated gauge. A system for identifying components and the date of replacement or calibration (e.g., affixing unique identification numbers and dates) should be incorporated. Written records should identify the specific gauges replaced or tested. Use of properly calibrated gauges can prevent the collection of misleading test data.
4. Investigate Reductions in Main Drain Residual Pres- sures — The 2008 edition of NFPA 25 includes a new provision that states where a 10% reduction in the full flow pressure is observed from previous results, the cause of the reduction shall be identified and corrected. However, good engineering practices dictate that any reduction in the residual pressure observed during a sprinkler system flow test should be investigated. A decrease in residual pressure is a possible indicator of a deteriorated water supply, obstruction, closed valve, or other system impairment.
At a minimum, the investigation of the reduced residual pressure should consist of an impact analysis to determine the effect on the sprinkler system hydraulic demand.
The pressure buffer of a sprinkler system can vary depending on the system designer’s anticipation of nominal consumption, future demand, infrastructure improvements, seasonal pressure changes, etc. Minor changes in the observed main drain test residual pressure can swing the demand/supply relationship into a deficit, resulting in an inadequate water supply. When conducting a main drain test, observation of a drop in residual pressure from a previous result is a qualitative indicator of diminished water supply strength. Investigation of any drop in main drain test residual pressure should occur prior to the
10% threshold, since this prescriptive requirement does not account for systems that have smaller design cushions that can be overcome by slight decreases in residual pressure. The main drain test can be used to detect a weakening water supply, but the impact on the sprinkler system is best determined by conducting a hydrant flow test.
5. Establish a Management of Change Protocol — A pro- tocol should be developed in collaboration with the property owner to identify changes, determine the impact, and if neces- sary, remedy the deficiency. In the event a sprinkler system is impaired, administrative procedures for notification and mitigation are presented in NFPA 25, Chapter 14, and should be included in the protocol.
The protocol should include systematic investigation proce- dures for use when a reduction in the water supply is detected. A hydrant flow test upstream of the sprinkler system may serve as a starting point to determine the cause of the pressure reduction. If the water distribution system is found to be adequate, the cause of the reduction is likely to be found between the point of con- nection (POC) to the water utility and the sprinkler system riser. System components from the POC, including control valves, backflow preventers, and pressure-reducing valves, should be inspected and tested for proper operation. Additional direction is provided by NFPA 25, Chapter 13, for investigating fire protection system piping for possible sources of materials that can cause blockage. After repairs are made, the sprinkler system should be restored and tested to verify hydraulic performance is satisfactory.
The successful operation of a fire sprinkler system relies heavily on the characteristics of the available water supply. An inadequate water supply can cause improper operation during a fire event leading to unnecessary fire damage. Internation- ally recognized fire standards are available as best engineering practices for ITM of fire protection systems. Implementing the features of these standards will reduce the potential for an inad- equate sprinkler system water supply to remain undiscovered.
About the Author:
Mr. Neil P. Wu, P.E., CBO is a licensed Fire Protection En- gineer, a Certified Fire and Explosion Investigator, a Certified Building Official, and a Managing Engineer at Exponent, Inc., a scientific and engineering consulting firm.