 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.Part 1 of this article (see
Aug. ’06 PM Engineer, pp. 19-21) discussed the design
considerations necessary to establish the correct gallons
per minute (gpm) rating of a fire pump for a system. In Part
2, we need to determine the correct discharge head and meet
design points on the fire pump curve. This will complete the
expression of a fire pump’s performance and help to make the
proper selection from a manufacturer’s catalog of fire pump
product lines.
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
The estimated discharge pressure for the system is
established at the pump’s discharge flange. The estimated
gallons per minute flow rate required for the system’s
supply is determined using the required pressure. The duty
point or “Primary Rating Point” of the fire pump expresses
the overall system’s 100% Q (flow rate) at 100% P (pressure)
requirements.
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
If the maximum churn pressure of the system exceeds the
listing pressure of the sprinklers installed in the system,
the following may reduce the rise to shut-off head pressure:
- Upgrade the sprinklers and piping to a higher
listing pressure if only a few floors exceed the
maximum.
- Select a fire pump model that has 20% or less rise
to shut-off, not 40%.
- 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.
- 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.
- Increase the system pipe sizes to reduce the
friction loss in the system.
- 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
The layout of a multiple pump system should be provided
adequate space for maintenance, with 3 feet of aisle space
around all pumps and controllers, and with system control
valves. This also provides access to fire department
personnel in an emergency.
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
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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