The “High Pressure” Lifestyle of a Fire Pump after Commissioning (Part 2 of 2)
This is a continuation of Part 1 that provided an introduction to fire pump operations and maintenance, discussed the NFPA fire pump standards, and explained the process for performing the annual fire pump flow test.
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)
2. Pressure Correction
H2 = H1 x (N2/N1)2
Where: 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 220.127.116.11 requires three conditions be met in order to consider a fire pump’s current performance curve as acceptable:
- Fire pump meets the most demanding fire suppression system flow and pressure requirements
- Fire pump supplies 100% of the rated flow
- 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. Doing some quick math, 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:
- 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.
- 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.
- 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.
Maintenance and replacement of the fire pump’s key components must be performed by a qualified fire pump service contractor. Most likely the fire pump contractor who performed the flow test is also qualified to perform the maintenance work as well.
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:
- 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.
- 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.
- 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.
- 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:
- Size and length pump suction piping: NFPA 25, Sections 18.104.22.168 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.
- 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.
Having the qualified fire pump contractor perform the key maintenance tasks and upgrades can help buy a few more years of life for a fire pump, but sometimes these tasks aren’t enough. Sometimes, the fire pump will reach a point where repairs and upgrades can no longer do the trick to keep up this ‘high pressure’ lifestyle.
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:
- Insurance requirements to maintain a working fire suppression system
- Personnel and equipment safety concerns for an out of service fire suppression system
- Reasonable method for removing old pump and bringing in the new pump
- Considering the need for a temporary fire pump that can be connected elsewhere in the system
- Costs for fire pump package, installation materials and equipment, and labor
- 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
- Preparation of necessary project specifications and design documents
- Finding qualified fire protection contractors to perform the work
- Obtaining approvals from authorities having jurisdiction
- Methods for commissioning the new fire pump after installation
Planning for a fire pump replacement is no small task. Budgeting for a fire pump replacement is no short order either. If the fire pump was originally $100,000 to install when the facility was brand new, it is very likely that the replacement cost will be that cost if not more depending on the complexity of the operation. Owners need to also factor in any kind of facility operations or schedules that may impact contractor work hours as well as availability of the fire pump equipment. A standard horizontal split case fire pump may be available off the shelf, but a specialty vertical turbine fire pump with epoxy coating for protection against saltwater on an offshore platform may take months for custom fabrication.
Owners may be best equipped for this operation by reaching out to a life safety engineer with the specialty fire protection experience and knowledge of how to navigate the intricacies of planning and executing a fire pump replacement project.