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:

 

5.7.4.1 The net pump shutoff (churn) pressure plus the maximum static suction pressure, adjusted for elevation, shall not exceed the pressure for which the system components are rated. (added in 1999)
5.7.4.2 Pressure relief valves shall not be used as a means to meet the requirements of 5.7.4.1 (added in 2003).

 

 

A note in the annex of NFPA 20 states:

 

A.5.7.4 It is poor design practice to overdesign the fire pump and driver, and then count on the pressure relief valve to open and relieve the excess pressure. A pressure relief valve is not an acceptable method of reducing system pressure under normal operating conditions and should not be used as such (added in 1999).


Main relief valves are still required in certain circumstances, but they cannot be installed simply to prevent overpressure from a pump that, by design, would be expected to over pressurize the system.

 

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:

  • Use of high-pressure-rated fittings, pipe, components, and/or pressure-reducing valves on the system side of the pump discharge valve where they are allowed by other NFPA standards.
  • Use of a suction tank or break tank.

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.


Figure 1 – Acceptable Flow Curves for a 1000-gpm pumpi
(Reprinted with permission Pumps for Fire Protection, Copyright 2002, National Fire Protection Association, Quincy, MA 02169.)

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 found.

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, MA, 2003.
2 Gagnon, R. (Ed.) Designer's Guide or Automatic Sprinkler Systems, National Fire Protection Association, Quincy, MA, 2005.

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