FPEeXTRA Issue 76

A Look at Fire Pump Operations & Maintenance: The “High Pressure” Lifestyle of a Fire Pump after Commissioning

By: Jodi Balido, PE

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If you recently commissioned a fire pump, a number of important steps have probably taken place, but what is next? After the initial installation, inspection, testing, commissioning, and overall service placement, who guides the next steps in operating and managing this critical system? 

Answer: The owner. The owner is the responsible party that has to deal with the “high pressure” lifestyle of operating and maintaining a fire pump, after its initial commissioning.

It is High Pressure for All

A fire pump provides higher pressure and waterflow capacity for a water-based fire suppression system, but owners are also under high pressure to keep this fire protection at the ready—with everyone’s safety at stake. Proper maintenance of a fire pump (or set of fire pumps) is needed to satisfy fire protection and building codes, comply with insurance requirements, and above all, ensure safety of lives and protection of assets.

Some buildings projects will require only one fire pump. There are some instances, driven by code or by owner preference, to have 100% backup fire pump capacity, which usually means one primary electric driven fire pump and one secondary diesel driven. For large site water supply systems, either for an entire facility campus or industrial plants, one might even see three or more fire pumps to meet the high system demands. Regardless of the number of fire pumps on a fire protection system, owners need to look at the National Fire Protection Association (NFPA) standards for guidance on how to properly inspect, test, and maintain their fire pump systems. This article will explore some of those requirements and provide some recommendations for complying with those requirements in further detail.

NFPA 20 & NFPA 25:  The Fire Pump Standards

NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, is the law of the land when it comes to designing, installing, inspecting, testing, commissioning, and placing a fire pump, or set of fire pumps, into service.  Once in service, the owner will need to become very intimate with NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, in order to keep their fire pump(s) running in top condition.

Keep in mind that NFPA 25 is the inspection, testing, and maintenance (ITM) standard for all water-based fire protection systems. It’s easy to be overwhelmed and confused with all the different sections, various testing, and inspection intervals for the many types of wet suppression systems out there. When it comes to fire pumps, owners need to focus on NFPA 25’s Chapter 8, which is simply titled “Fire Pumps”. In this chapter, owners will find a lot of what they need to know for ITM of their fire pumps, including a fire pump’s electrical components, drive system, piping, valves, gauges, controllers, batteries, pump body, filters, oil levels, pump room enclosure, and so forth. Some of these ITM frequencies are often as weekly while some are often as yearly.

For example, NFPA 25, Section requires a weekly no-flow test be conducted for a diesel driven fire pump. The suggested run time for this no-flow test is 10 minutes. During this test, maintenance personnel can check the diesel fire pump for signs of unusual activity, such as irregular movement of piping, excess vibration, or suspicious engine noises.  Another item to check for would be the proper operation of the fire pump’s circulation relief valve (also known as the casing relief valve).  It is common to see the relief valve discharge water in a no-flow (churn) condition as water pressure builds and water temperature rises. The water discharge is the pump’s way of taking care of itself. If there is no discharge from that relief valve, the fire pump and the associated piping system might be in trouble with over-heating and over-pressurizing.

Another example, NFPA 25, Section 8.3.3 requires annual flow testing of all fire pumps in a facility. This can be a huge undertaking depending on the number of fire pumps and means for discharging the water flow downstream of the fire pump discharge outlet. Some fire pump installations might have an in-line water meter, but these have a tendency to lose calibration and provide inaccurate results. For the best results, the fire pump test header needs to be used. Annual flow testing of a fire pump is an orchestrated effort. Also, it is important to consider where the flowed fire pump water will be discharging. Owners may need to coordinate with their site environmental and safety personnel to control water flow and prevent disruption of nearby landscape and wildlife habitat. Depending on environmental and safety conditions, special waterflow diffusers may be needed.

Performing the Annual Flow Test

Performing the annual flow test for a facility’s fire pump(s) is a crucial part of checking the health of a fire pump system. NFPA 25, Section requires the flow test to be performed at three flow conditions:

  1. No-flow (churn or 0%) - Accomplished by flowing no water through the pump test header. The pump will be running in a “dead-head” condition.
  2. 100% flow - Accomplished by flowing 100% the fire pump rated flow through the pump test header and the adequate number of test valves.
  3. 150% flow - Accomplished by flowing 150% the fire pump rated flow through the pump test header and an even larger number of test valves. For example, if the fire pump is rated at 1,000 gpm, then the 150% rating is 1,500 gpm.

A common issue with these tests is when the contractor shows up on test day without enough test hoses and flow measuring devices to flow the 150% flow rating for fire pumps with high gpm flow rates.  A recommendation would be to have a discussion with the contractor before test day to see how many hoses and flow devices will be available. Depending on hose size, a flow measuring device is only good from about 250 gpm to 750 gpm, or 500 gpm on average. For a 1,000 gpm rated fire pump, a safe bet is to bring at least three (3) hoses and flow devices to adequately flow the 1,500 gpm needed to achieve 150% rated flow. Another piece to consider is hose diameter and hose length. Hose diameters should be 2.5-inch to 4-inch diameter, and hose lengths should not exceed 50-feet. Any smaller than 2.5-inch and any longer than 50-ft may create too much friction loss between the pump test header outlet and the flow device, yielding inaccurate results.     

Once the proper flow rate is achieved for the particular flow condition, the following values must be recorded as they relate to a fire pump’s performance curve:

  1. Pump Suction Pressure (PSI) - Read the pressure gauge located on the suction (inlet) side of the fire pump casing.
  2. Pump Discharge Pressure (PSI) - Read the pressure gauge located on the discharge (outlet) side of the fire pump casing.
  3. Pump Driver Speed (RPM) - Use a digital tachometer to flash the pump driver shaft and record the pump speed. Some newer model fire pump controllers will have the RPM values on the display. However, it is still good practice to use the handheld tachometer to cross check.

Analyzing the Results

Once all the flow and pressure values have been collected for the three pump data points, the values need to corrected and plotted onto a curve. An owner can’t just look at discharge pressure at the measured RPM’s. Data that has not been corrected is misleading. Pump affinity laws will need to be used to correct the data so it can be properly compared against the original factory pump curve.

Pump Affinity Laws (Equations):

  1. Flow Correction

Q2 = Q1 x (N2/N1)

  1. Pressure Correction

H2 = H1 x (N2/N1)2

Q1 = Tested Flow                       Q2 = Corrected Flow

H1 = Tested Pressure                 H2 = Corrected Pressure

N1 = Tested Speed                     N2 = Rated Speed

When analyzing fire pump data, the values and corrected figures should be arranged similar to the table below:

Further, the values can be plotted similar to the graph below:

[Note: The above table and graph serves as an example.  Values shown were made up solely for the purpose of discussion and do not reflect an actual fire pump installation.]

NFPA 25, Section requires three conditions be met in order to consider a fire pump’s current performance curve as acceptable:

  1. Fire pump meets the most demanding fire suppression system flow and pressure requirements
  2. Fire pump supplies 100% of the rated flow
  3. Net pressure is at least 95% of manufacturer’s pump curve, original field test (commissioned) curve, nameplate pump curve.

In the example table and graph above, the tested and corrected pump curves are compared to the rated pump curve, which is taken directly from nameplate pump data. At Churn and 100% flow, the example fire pump appears to be right on curve. However, there is an obvious difference between the rated curve and the tested curves at 150% flow. The test curve is showing that the fire pump is only performing at 81% the rated pressure. This example fire pump is not in compliance with NFPA 25 requirements.   

If an actual fire pump is showing noncompliance or is trending year-after-year towards becoming noncompliant, steps need to be taken by the owner to perform maintenance or replace the fire pump in order to be within compliance. 

Keeping Up the ‘High Pressure’ Lifestyle

Owners should budget and plan for regular maintenance in order to keep up with the ‘high pressure’ lifestyle of their fire pump. Following the manufacturer’s recommended maintenance guidelines is an excellent start. When downward trends are starting to be noticed on the fire pump performance curve, steps can be taken to possibly bring that fire pump back onto the curve without resorting to an all-out replacement project for the fire pump. 

The below list reflects some items that need to be checked regularly as part of a fire pump inspection and maintenance routine that can help keep its performance:

  1. Packing glands - The fire pump packing is the seal where the pump shaft penetrates the pump casing. To keep the fire pump cool and lubricated, the fire pump packing is designed to stay within a certain range of leakage. When leakage is too much or too little, the packing needs to be brought back into a suitable range. Adjusting the tightness of the packing gland nuts will achieve that proper leakage rate and help to extend of service life of the fire pump. Care needs to be taken as to how much the packing glands are tightened.  Overtightening of the packing can prevent any leakage, which remove the pump’s ability to self-lubricate and self-cool.  The packing will ultimately dry out and cause damage to the pump shaft. 
  2. Wearing rings - Over time, erosion due to liquid leaking will occur where the impeller and pump casing are near contact. The wearing rings are meant to take this erosion as opposed to the actual impeller or pump casing. These rings are designed to be replaced periodically during the lifetime of the fire pump.
  3. Bearings - Fire pump bearings are a crucial part in supporting the fire pump shaft in the correct alignment and allowing the shaft to rotate as intended. Bearings should be checked for overheating and excess vibration as some of the common causes of bearing failure are shaft misalignment, inadequate lubrication, and presence of contamination. Bearings can be replaced to ensure safe and proper operation of the fire pump.

NFPA 25 provides great insight on how to perform routine ITM on a fire pump to prevent future fire pump failure, but here is a list of additional considerations that can have impact a fire pump’s susceptibility to degradation and eventual failure:

  1. Pump & Pipe Cavitation – During fire pump operation (especially when trying to achieve 150% flow rating when performing the annual flow test), the owner must monitor pump suction pressure. Too low of a suction pressure causes cavitation in the pump and the pump’s supply pipe, which is the introduction of unwanted air bubbles in the system that erodes away at the pump’s internals and the supply pipe’s interior. Cavitation can accelerate wear on a pumping system, essentially reducing the usable life of a fire pump and the associated pipe, which would warrant early replacement of the fire pump system components or the fire pump system itself. Flow testing should be limited to suction pressure of no less than 10 psi, at which point the fire pump flow and pressure readings need to be taken even if the full 150% flow rating is not achieved.
  2. Impeller Damage –Damage to the impeller is critical to a pump’s ability to transfer mechanical energy into the fluid energy for creating the pressure and waterflow needed to satisfy the fire suppression hydraulic demand. Sources of impeller damage include pump cavitation, internal erosion (due to small foreign particles in the water like tiny rocks or sand), and corrosion (due to an acidic water source such as raw water like a river or presence of microbial growth also from a raw water source). It can be difficult to detect impeller wear with the naked eye, but a trend in degrading fire pump performance (from analyzing annual flow test results year after year) could be an indicator of impeller damage.  When replacing an impeller due to damage from erosion or corrosion, it may be beneficial for an owner to invest in a higher quality material or a protective coating for the replacement impeller. 
  3. Improper or Inadequate Maintenance - It is important to inspect and test per the requirements of NFPA 25, but it is equally, if not more important, to perform the required maintenance in accordance with the manufacturer’s guidelines. There are multiple fire pump manufacturers, and owners need to ensure their fire pump service contractor is familiar with their particular brand of fire pump. Hiring a service contractor unfamiliar with the manufacturer’s guidelines run a risk for owners of having their fire pump being maintained out of spec, which in turn could cause a fire pump to slowly run itself to self-destruction and lose its hydraulic performance capabilities.
  4. Design Flaws - An issue that is often discovered with older fire pump systems is that the fire pump system was incorrectly designed in the first place, which in turn resulted in improper installation. This may be more the fault of lack of industry knowledge built into the design standards at the time as opposed to the design engineer’s technical abilities. In comparison to recent changes in design standards, here are a couple of examples of common design flaws that could be found in older fire pump installations:
    1. Size and length pump suction piping: NFPA 25, Sections and 4.28 require a proper pipe diameter to be installed on the suction side of the pump for at least a distance equivalent to 10 pipe diameters, which is measured along the piping. Failure to have this proper pipe size for the proper pipe length can result in a turbulent flow into the pump. Turbulent flow can introduce unwanted forces on the pump that can damage the pump and result in further inability to meet its required performance.
    2. Pressure drop settings: Automatic fire pump operation is designed to activate upon a fire pump controller sensing the proper amount of pressure drop within the piping system. NFPA 25, Annex A.14.2.6 provides recommendations on how to set the pressure drop settings, taking into account system pressure (pressure held within the piping downstream of pump discharge), jockey pump (pressure maintenance pump) activation pressure, and fire pump activation pressure. It is recommended that the jockey pump activate upon 10 psi drop from the system pressure, the fire pump activate upon 5 psi drop from the jockey pump activation pressure, and any subsequent fire pumps activate upon additional 10 psi drops. With too small of pressure increments, the owner risks having the jockey pump and fire pump(s) kick on too soon and run unnecessarily. With too large of pressure increments, there is a risk of a pressure surge acting on the downstream piping system upon fire pump activation to fill this large pressure gap. This pressure surge can force the pipe to sway violently, rip out pipe hangers, and break the piping. If this were to happen, not only would the entire fire suppression system be out of commission, but the facility could sustain significant mechanical damage.

Planning for Replacement

Owners should realize that a fire pump will need replacement eventually.  A good rule of thumb is 25 years of usable life, and trending the fire pump’s performance curve year after year can help an Owner predict when the actual replacement project needs to occur.

A fire pump replacement project is a big deal with multiple aspects to consider, including but not limited to:

  1. Insurance requirements to maintain a working fire suppression system
  2. Personnel and equipment safety concerns for an out of service fire suppression system
  3. Reasonable method for removing old pump and bringing in the new pump
  4. Considering the need for a temporary fire pump that can be connected elsewhere in the system
  5. Costs for fire pump package, installation materials and equipment, and labor
  6. Need for upgrading other systems (such as fire alarm and detection, electrical power, fuel tanks, or water supply) to comply with current fire protection standards or other risk management best practices
  7. Preparation of necessary project specifications and design documents
  8. Finding qualified fire protection contractors to perform the work
  9. Obtaining approvals from authorities having jurisdiction
  10. Methods for commissioning the new fire pump after installation

Jodi Balido, PE is with Mason & Hanger