|Emerging Trends in Water Supplies|
Issue 4: Emerging Trends in Water Supplies
By John Frank, PE
There are many ways to examine trends in water supplies. This article
will focus on pressure control for fire pumps that boost public water
pressure, referred to in this article by the common term "booster pump".
Changes made to the 2003 edition of National Fire Protection
Association (NFPA) 20 Standard for the Installation of Stationary Pumps
for Fire Protection provide additional opportunities for the fire
protection engineer to add value to a project involving booster pumps.
The selection of a booster pump requires significant engineering
analysis to meet the sprinkler and standpipe flow and pressure demand
without overpressurizing the system. Overpressurization can cause a
system failure when it is needed most. This could have catastrophic
results. The fire protection engineer is well suited to perform this
engineering analysis. The obvious difficulty is selecting a pump that
will meet the system demand while considering normal and long-term
variations in suction pressure.
It is difficult to predict the future performance of the public
supply. In rapidly growing areas, the supply pressure could first
decline with increased system use and then increase beyond the original
pressure as improvements are made to compensate for the growth. In
mature areas, the public main's C factor could decrease with time with a
resulting increase in friction loss. Without fire protection
engineering involvement in annual tests, the problems associated with
long-term increases or decreases in suction pressure may go unnoticed.
Too often, the results are simply recorded, and as long as the pump
itself meets its performance curve, nothing is said to the owner about
the overall effect on the system.
In situations where there is a high static pressure and where the
residual pressure drops significantly at the required flow, a booster
pump can develop significant pressure under churn (no flow) conditions
or when just a few sprinklers are flowing. This is especially common in
warehouse conditions when ESFR sprinklers are used. It is not uncommon
to see pump churn pressures approaching or even exceeding 13.8 bar (200
psi). Components in warehouse and other non-high-rise installations are
typically rated for only 12.1 bar (175 psi).
Historically, a common way to prevent system overpressurization was to install a main relief valve. This valve would open before the system reached dangerous pressures. Using a relief valve for routine pressure control is now specifically prohibited by NFPA 20 Standard for the Installation of Stationary Pumps for Fire Protection.1 The prohibition was added in 2003. NFPA 20 states:
A note in the annex of NFPA 20 states:
New technology offers an additional solution to the overpressure
problem. Controls that vary the pump driver speed are available to
manage overpressure problems. These are referred to in NFPA 20 as
"variable-speed pressure-limiting control[s]." Since pump discharge
pressure is proportional to pump driver speed, overpressure can be
managed by reducing the engine speed when the pump is operating at churn
or low flow conditions. Note that a relief valve is required when a
variable-speed pressure-limiting control is used; however, the relief
valve is a safety measure – it is not used as the primary means of
overpressure control. Manufacturer's instructions should be consulted
for specific application details.
Other more traditional approaches are:
It may be possible to avoid the need for any special feature to manage excess pressure. This is done through a coordinated engineering approach among design of the public water supply, the pump, the sprinkler and standpipe system, and the structural support needed for the system piping. This is where the fire protection engineer could add significant value. Often these components are engineered separately with the goal of minimizing component cost, without considering the overall cost and reliability of the system.
Smaller sprinkler piping makes the sprinkler system less expensive
and possibly the structural support system less expensive, but it can
require a higher-pressure pump with the associated problems of
overpressure control. One solution is to use a pump with a "flatter"
curve (see Figure 1), which will reduce the churn pressure. Pump/driver
units with flatter curves may cost more, but savings in pipe may offset
this. Conversely, it may be found that a one-pipe-size increase reduces
the overall system cost by reducing pressure requirements.
The fire protection engineer should also consider larger orifice
sprinklers that reduce pressure requirements, possibly to the point
where a pump is not needed at all. It is important to be sure that the
larger-orifice sprinkler (ESFR sprinklers with nominal K factors of 17,
22, or 25 versus 14, for example) is listed for the application in mind.
Newer sprinklers may not have been tested for as many applications as
sprinklers that have been around for a while.
It may be found that there is no alternative to an allowable form of
pressure control. By using a fire protection engineer to coordinate the
design process, the most reliable and cost-effective solution can be
Another consideration is when two booster pumps are planned for
redundancy at high-value installations. In this author's experience, the
reliability of the public supply is not given adequate attention as
part of the overall system redundancy. If two pumps are desired for
reliability, the public water supply should also have redundant
reliability. This can be done by connecting to two separate mains or
valving the public supply such that the loss of any component will still
result in an adequate supply. Again, this requires careful coordination
of all of the components of the system. This is best done by a fire
protection engineer who understands the interaction of all the
components of the system as well as the impact of different designs on
overall system reliability.
As the use of fire pumps to boost public water supply pressure
(booster pumps) increases, the fire protection engineer has increased
opportunities to add value to a project. More detailed examples can be
found in Chapter 12 of Designer's Guide or Automatic Sprinkler Systems.2
John Frank is with GE Insurance Solutions, Global Asset Protection.
1 NFPA 20, Standard for the Installation of Stationary
Pumps for Fire Protection, National Fire Protection Association, Quincy,
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