How To Interpret A Fire Water Pump Annual Flow Test Result

ANSWERS:

It is typically around the same time of the year (mid-May to mid-June) that most companies' maintenance team starts planning to perform annual testing and inspection on fire pumps in their various clients’ sites. A question that is quite often being raised during this time is what are the minimum code requirements that need to be met during these annual tests and how the results of flow tests should be interpreted.

Fire water pumps inspection, testing, and maintenance requirements are governed by?NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems. As per 2017 Edition, section 8.3.3 of this standard, the annual flow testing of fire pumps should be performed as a minimum at three points across the pump curve; no flow (churn), 100% of the pump rated capacity and 150% of the pump rated capacity by controlling discharged water through approved discharge devices. If the pump can not be run at 150% of the rated capacity, the maximum flow rate under the rated speed of the pump should be achieved during the flow test.

For both electric and diesel pumps, the pump suction and discharge pressure and pump RPM should be recorded at any given test point. For electric pumps, pump controller voltage and amperage readings should also be captured at all test points. The voltage and amperage requirements for electric pumps will be covered in the next blog post

As an example, for a pump with a rated capacity of 2000 GPM, test points are 0 GPM, 2000 GPM, and 3000 GPM (150% of rated capacity). If 3000 GPM can not be achieved during the flow test, then the maximum flow rate should be targeted, and the maximum flow rate along with corresponding suction and discharge pressures should be recorded.

Once the flow test is completed, results should be interpreted using the pump nameplate data and/or manufacturer curve (ideally those two should match but it is not always the case!). If the pump differential pressure at any of the test points is less than 95% of the pressures recorded on the nameplate/manufacturer pump curve for the same test point, then it can be concluded that the pump is failing to meet NFPA 25 requirement for the annual testing or simply put it is under-performing.

For the example discussed above, If pump nameplate data indicates that the pump should deliver the rated capacity of 2000 GPM at 160 psi differential pressure, any differential pressure of less than 152 psi recorded at the same flow rate, indicates pump failure in meeting NFPA 25 flow test requirements. To have a visual presentation of the pump performance versus the manufacturer curve or nameplate data, you can plot both in the same. since pump curves are not linear but second order, choosing the smooth line option for chart type in Excel software rather than the straight line will give you a more accurate graph for performance comparison with the manufacturer curve.

One common mistake in conducting flow tests, specifically when the expected flow rates are not achieved, is to increase pump speed beyond the pump-rated RPM to achieve the desired flow rate. this is NOT permitted per NFPA 25 as a means to meet the capacity requirements of the system. If results are obtained at a pump RPM less than the rated RPM, all flow rate and pressure values should be adjusted to reflect the impact of reduced RPM using formulas derived from affinity laws (to be discussed in another post).

Another common mistake is to compare the recorded discharge pressure in the gauge with a differential pressure indicated on the pump nameplate or manufacturer pump curve. To avoid that, ensure that both pump suction and discharge pressure readings are captured while testing and differential pressure (discharge pressure minus suction pressure) value is used to evaluate pump performance versus with nameplate or pump curve data.

Once all test data are analyzed, the flow test report should be submitted to the owner for the record. If results indicate pump failure in meeting test criteria, the following steps should be taken per NFPA 25, 2017, 8.3.7.2.4:

1. The owner shall be notified.
2. An investigation to find the root cause of the pump's degraded performance should be conducted.
3. The deficiency shall be corrected.