Feature Articles
Design Considerations for Fire Pumps in High Rise Buildings, Part 2
by Robert H. Thompson, CIPE
October 5, 2006
To conclude this article, the author discusses design points on the fire pump curve—and provides tips for reducing shut-off pressures and system layout.
Discharge Head
The required discharge head, whether expressed in feet of head (ft.) or pounds per square inch (psi), is the pressure that is required to produce the sum of:
- The required residual pressure at the highest, most remote fire department valve outlet at the required flow rate.
- The frictional losses of fittings, valves and lengths of piping in the flow path, adding the friction of subsequent flowing standpipes from the most remote outlet back to the source fire pump.
- The static elevation pressure of the most remote outlet’s location (2.31 ft. = 0.434 psi).
The required residual pressure varies with the codes in question. For example, the requirements are 25 psi in New York, 65 psi in Chicago and 100 psi for the International Building Code and International Fire Code. The 100 psi requirement appears in NFPA 14, the Standpipe Installation Standard.
The piping design layout influences the frictional losses. The Building Code may dictate requirements for the piping layout, especially as the building’s overall height increases, resulting in the need for multiple zones.
For example:
- Chicago allows a single standpipe zone height of up to 275 feet (see Figure 1). Above that height, separate additional zones of a maximum 20 stories each are required. The additional zone must be supplied by a minimum of two express risers per zone.
- New York requires a roof tank on buildings higher than 300 feet and an additional manually controlled fire pump (see Figure 2 on page 80). Upper levels of the standpipe system must be supplied from the roof tank. The low zone of the system must have the pressure reduced to 160 psi maximum.
- NFPA 14 requires 4" minimum diameter standpipes and 6" minimum diameter combined sprinkler/standpipes be connected by a common bulk main from the fire pump at the lowest level (see Figure 3 on page 82). If a roof tank is a part of the design, the standpipes may be connected at the top level, with check valves to prevent circulation. Pressure regulating valves are required to reduce pressures if the pressure is greater than 175 psi.
Design Points on the Fire Pump Curve
Figure 2. New York requires a roof tank on buildings higher than 300 feet and an additional manually controlled fire pump.
This is the design point on the pump curve that is normally used to select and specify the fire pump model and the horsepower.
NFPA 20 prescribes two additional points that must be met on the fire pump curves.
- The “Secondary Rating Point” of 150% Q at 65% P expresses the conditions at the normal fire pump test flow point.
- The “Shut-Off” condition of 0% Q at 120 to 140% P expresses the pump’s discharge pressure at no flow.
All fire pumps listed for fire protection service must have performance curves that meet these three curve conditions. Fire pumps are generally selected within a range of 90% to 130% of the primary rating point of a given pump capacity. As an example, a 750-gpm pump might be selected and utilized for a demand of between 675 gpm and 975 gpm, but once the primary rating point of the next size pump is reached, the larger pump should be selected.
Once a fire pump is selected from a specific manufacturer’s curve, other important system design points can be evaluated. Plotting the fire pump’s performance curve on a graph of the project’s water flow test data allows for the prediction of the system’s maximum churn pressure.
According to NFPA 20, the “Shut-Off”condition pressure, plus the water supply static pressure, should not exceed 175 psi for single-zone sprinkler/standpipe systems and 350 psi in multiple-zone systems.
Tips for Reducing Shut-Off Pressures in System Designs
Figure 3. NFPA 14 requires 4" minimum diameter standpipes and 6" minimum diameter combined sprinkler/standpipes be connected by a common bulk main from the fire pump at the lowest level.
- 1. Upgrade the sprinklers and piping to a higher listing pressure if only a few floors exceed the maximum.
2. Select a fire pump model that has 20% or less rise to shut-off, not 40%.
3. Consult the manufacturer’s representative to determine if the model with the lowest rise to shut-off is being specified. Trimming the impellers may reduce the rise to shut-off further. Only certain manufacturers and models may benefit from trimming, with some having a rise to shut-off as low as 5% above the system’s required duty point.
4. If two fire pumps are piped in series for the high zone, determine whether the high zone pump can meet the zone requirement by taking suction directly from the residual city water main in parallel. This avoids adding two rise to shut-off pressures to the water supply static pressure.
5. Increase the system pipe sizes to reduce the friction loss in the system.
6. Consult the fire pump controller manufacturer regarding the use of a “variable speed controller” to electronically reduce the pump’s rise to shut-off. This method produces a similar result to trimming the impeller and can be used with any pump model or manufacturer.
Remember, fire pumps are manufactured in a range of capacities and styles, each offering their own benefits and drawbacks in terms of costs, system layout and hydraulic design.
Popular configurations are:
- Horizontal split-case and vertically mounted horizontal-split case,
- In-line and vertically mounted in-line,
- Multi-stage vertical turbine,
- Multi-stage horizontal.
Tips for System Layout
Some suggested tips:
- Low zone fire pumps should always be provided with a full-size bypass from the suction to the discharge. During maintenance procedures, the lower-floor sprinkler systems may be kept in service utilizing the water supply pressure.
- Make sure that there are no abrupt changes of direction at the suction flanges of either horizontal-split case or vertically mounted horizontal-split case pumps. These changes in direction will cause cavitation in the double suction impellers.
- Where vertically mounted pumps are selected, be sure that space and provisions are provided to lift the motor from the pump frame during maintenance procedures.
- When pumps are required to be connected to a second emergency source of power, make sure the fire pump controllers specified have an integral automatic transfer switch.
- If the emergency source of power is a generator set, specify a “reduce voltage starting type” fire pump controller. This reduces the fire pump’s demand for starting current below the level that could stall the diesel drive unit during initial start-up, before the diesel engine has a chance to come to full speed.
- Check NFPA 20 and manufacturer’s literature regarding trim for pump systems to verify that the correct trim is specified for the style and drive unit of the fire pump intended.
Following these design considerations will result in plans and specifications that are easily understood. This will assist in expediting the project during plan review, as well as reduce any possibility of costly design revisions.
SIDEBAR:Why Maximum Pressure Rating is Important
Early sprinkler/standpipe systems had only upright, pendent or horizontal sidewall sprinklers. The piping system was Schedule 40 steel pipe with threaded or flanged 125-pound valves and fittings. These 125-pound fittings were installed in systems that had a maximum pressure of 175 psi. For systems having a pressure in excess of 175 psi, extra heavy 250-pound valves and fittings were installed. Extra heavy fittings were considered acceptable for systems having a maximum pressure of 400 psi.
What is often forgotten is that the125-pound and 250-pound designations were a “nominal class rating.” This indicated that the maximum service pressure of the valves and fittings was established at the temperatures’ saturated steam.
If you review the valve or fitting manufacturer’s rating chart, the pressure rating determined at the water service temperature of 150ºF for a sprinkler/standpipe systems shows a 125-pound class fitting equates to 175 psi maximum pressure. Likewise, a 250-pound class fitting equates to a 400 psi maximum system pressure.
Hence, the industry has maintained the 175 psi as a rating pressure for the testing and listing of sprinkler components.
Does this mean that a failure may occur if the rated pressures are exceeded? Not really. The maximum pressure of listed sprinkler components is used by testing agencies to establish a common point for evaluation.
Sprinkler and standpipe components are further tested to provide an additional safety factor of 2.5 times the listed pressure for leakage and five times before burst.
The degree to which the design exceeds the rated pressure of sprinkler components reduces the safety factor. This becomes the discretion and judgment of the engineer/designer. The engineer should consult with their firm’s risk management officer regarding the advisability of such a design. Of course, the authority having jurisdiction must approve the design. A water damage lawsuit from a single, failed sprinkler component that was exposed to pressures exceeding the rated pressure of the component will be costly to defend.
Fire pump tests normally start with the fire pump running at the churn condition while pressure and motor readings are being recorded. Then the 2-1/2-inch fire department valve outlets are opened to show and document the readings at the primary and secondary rating points. The pump is slowly brought back to churn condition by closing the 2-1/2-inch fire department valve outlets.
If all parts of the test were normal, the system is then switched into automatic mode of operation for its intended service. Remember, any sprinkler/standpipe system designed to churn in excess of the maximum listed pressure of the sprinkler component installed will be exposed to this higher pressure during the operational life of the system. The pressure will only be lowered for testing and maintenance.
Robert H. Thompson, CIPE
mailto:rthompson@mehandeseng.com
Robert H. Thompson, CIPE, has nearly 40 years of plumbing and fire protection design experience and has led design teams of prestigious consulting engineering firms. He is the plumbing and fire protection director at Mehandes Engineering P.C., a MEP/FP engineering firm that specializes in commercial and high-rise residential projects.
Thompson pioneered the design of centralized hot water supply and recirculation systems for high-rise buildings that is the arrangement of choice in the Chicago area. His e-mail address is rthompson@mehandeseng.com.
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