Water Supply Fluctuations
Anyone with experience in the sprinkler industry has come across at least one situation where a sprinkler system is installed or tested, then does not perform as expected. Looking into the root cause of the substandard performance, another water flow test is sometimes performed, and the numbers are vastly different.
Usually when the system is not meeting design specifications, the water supply is worse. Final acceptance testing is a bad time to find out the water supply does not meet system demands, but it can be just as devastating prior to installation. The typical proposed solution is to conduct another flow test; does that truly indicate that the system can perform during the worst case scenario?
Most fire protection engineers are faced with water supply challenges when designing sprinkler systems. Fire protection systems are required to perform under worst case scenarios including times of above-average domestic water demand. How does the system designer compare the test data obtained through a flow test to possible worst case conditions? How does the designer know if the test results even demonstrate the water supply during normal conditions? For these reasons, the idea of having an equation for adjustments to water flow data has become an increasingly hot topic. Although significant research has been performed on topics that affect water supplies, including forecast (predictive) modeling, water demands, and hydraulic modeling, insufficient research has been conducted with the focus on developing a single comprehensive adjustment equation for use in designing sprinkler systems.
NFPA standards have typically identified the need to account for variations in water supplies but have not provided guidance for adjustments to water supplies or provided safety factors. Fire protection engineers and sprinkler contractors have historically looked to local amendments or policies to the fire codes for adjustment factors to account for variations in water supply conditions in many jurisdictions.
Sometimes the adjustments are known before starting a design. Sometimes they are found during the review of drawings or during testing of the systems. The problem is that the methodologies for making these adjustments vary widely and there may be little consistency from one jurisdiction to the next. Additionally, the justification for these adjustments may not even be based on fluctuations in water supplies, but account for variations between the calculated design drawings and the actual field installation.
To understand why water supplies have fluctuations, we need to understand the different industry practices when designing water distribution systems.
INDUSTRY PRACTICE: NFPA VS. AWWA
Sprinkler systems that are not supplied by a dedicated gravity tank, or tank and pump must rely on the water authority for the pressure and flow to supply the sprinkler system. A fire pump can be used to boost the pressure from the water distribution system, but it cannot create more water if the water supply system cannot provide the necessary amount. For this reason, it is important to understand the design difference between sprinkler systems designed to National Fire Protection Association (NFPA) standards and water distribution systems in accordance with American Water Works Association (AWWA).
When designing sprinkler systems, the designer has NFPA codes and standards regulating the minimum performance of the system. Design discharge densities over sprinkler coverage areas are clearly identified through requirements in the reference standards. Hydraulic calculations are preformed from the hydraulically most remote area to the connection to the water supply. Minimum water supply demand requirements for the system are identified. Minimum water demand requirements for the system do not vary over time unless the hazard being protected changes or the system is modified. In such cases, a new analysis of the system needs to be performed.
When water authorities are designing water distribution systems, they do not have AWWA codes or standards regulating minimum system pressures at specific flows. Local regulations may require certain minimum system performance, but these regulations are specific only for their jurisdictions and are not universally adopted. Neighboring jurisdictions that have different water authorities may have vastly different regulations.
Additionally, the consumers of water and the usage of water vary between different communities. This variation in the usage of water creates variable demands that are difficult to quantify. Because the demands are variable, water authorities typically have operational ranges within which the systems are maintained. Through the use of elevated storage tanks, operation of additional pumps, pressure regulating valves, and other system components, the water authorities try to maintain their system pressures while the demand varies. The methods these water authorities use to maintain pressure also varies. One jurisdiction may have manual pumps while the neighboring jurisdiction uses variable speed drive pumps or a gravity supply system.
AWWA Manual on Distribution System Requirements for Fire Protection acknowledges the variation in water supplies and states “the design of sprinkler systems requires knowledge of the water pressure in the street. However, there is really no such thing as a single, constant water pressure in the street that should be used for design. The pressure in water mains varies over time due to a large number of factors. With all of the sources for variations in pressure, it’s clear that there is no single water pressure in the street. Instead, pressure fluctuates over time, and the sprinkler system designer must select a single value as the basis for design from a reasonable worst-case condition.”
If the manual used to design the water distribution system indicates that the water supply fluctuates, why are sprinkler designers only using one set of data to design the system? An example of this variation in water supplies is discussed next from an evaluation of a campus water supply.
CAMPUS WATER SUPPLY STUDY
Variations in water supplies can sometimes reveal bigger issues. The authors completed the evaluation of a campus water supply to address what the client thought was an aging water distribution system at its facility. In this case, water was supplied to the campus through two separate connections to a privately operated water distribution system. Each water supply connection was equipped with a water meter and backflow preventer.
The evaluation was prompted as the result of an identified water supply inadequacy found for a single building where two tests had been conducted with differing results. The first test, conducted by a design engineer, demonstrated that the water supply was sufficient to meet sprinkler system demand. The second test was conducted by the sprinkler contractor more than a year later as part of the sprinkler system design. This test identified an inadequacy in the water supply and raised questions because the water supply at this location had been considered sufficient for designing sprinkler systems prior to that test.
The objective of the evaluation was to identify areas of improvement at the campus. Hydrant flow testing was performed to facilitate evaluation of the adequacy of the existing underground fire main piping to supply the required flow and pressures for manual and automatic fire fighting needs and to develop an effective friction coefficient for the underground pipe network.
Testing the system demonstrated worse performance than originally anticipated. Comparison of the water supply results to fire protection system demands throughout the campus indicated that the water supply was inadequate for a number of the buildings. Static pressure readings measured throughout the facility were lower than values historically recorded. Additionally, residual pressure readings under fixed flow conditions (measured along isolated distribution system paths) yielded abnormal results. The data was used to compare measured results to similar conditions with known Hazen-Williams coefficient of roughness values. However, the results of the evaluations did not allow for isolated repairs or replacements on campus as hoped. As a result, a second series of tests was conducted when the water supply was believed to have the lowest usage and its best performance. That was not the case; the results of this testing revealed worse performance than previously observed.
The water authority was contacted. It was determined that the water distribution system was controlled through observation of water pressure by an operator at the supply pumping station with a normal variation of approximately 5 psi. Supply pressures of 45 to 50 psi were normally maintained at the pumping station. However, the differences in the measured static pressures at the campus during the two tests were determined to be approximately 7 psi. It was determined that the water authority reduced the system operating pressure over a number years and system demands increased in the surrounding communities due to the construction of several new facilities. As a result, the available water supply (pressure and flow) at the entrance to the campus deteriorated over time.
WHAT IS BEING DONE TO HELP SYSTEM DESIGNERS?
NFPA technical committees are trying to quantify daily, seasonal, geographical, and other fluctuations to water supplies that are encountered during water flow tests to determine the typical pressure and flow during peak demands when tests are performed during non-peak times. Limited attempts have been made to provide an adjustment factor(s), which have been proposed to several NFPA technical committees, but could not be accepted due to insufficient technical support for these adjustments.
A Fire Protection Research Foundation (FPRF) literature review, titled “Quantification of Water Flow Data Adjustments for Sprinkler System Design,” found several things. First, daily, seasonal, geographical, and other fluctuations are real and can impact the data collected during water flow tests; secondly, there is relatively little data on how these fluctuations affect pressure. The majority of research on water supplies has been conducted for determining the quantity of water end users consume during certain periods of time. In most cases, this research is focused on domestic users because they are easily categorized by type of residence and number of individuals. Commercial and industrial users are harder to categorize due to the many uses of water other than domestic.
It was determined that there was insufficient data to provide recommendations at this time regarding water supply adjustments. The reasons: 1) a lack of data associating flow rates and available pressure, 2) insufficient data to provide meaningful comparisons between regions and within specific regions, 3) a lack of data for all identified variables, and 4) data was not limited to a single variable or discrete number of variables that would allow for development of adjustment factors.
WHAT SHOULD DESIGNERS DO NOW…
Without an adopted adjustment factor, the system designer and plans reviewer should contact water authorities when performing flow tests to determine the best time to conduct the test during normal or high demands. For tests that have already been conducted, designers can ask how the flow conditions observed during the test compare to normal or high system flow conditions. This would meet the current recommendations of NFPA 291 for conducting tests during a period of ordinary demand.
When water authorities provide modeled hydrant flow for use in sprinkler system designs, they should be asked how well the hydraulic models are calibrated (how the modeled hydrant flow tests compare to actual hydrant flow tests) and reduce the modeled hydrant flow test by the percentage difference they observed in their calibration (e.g., 10%, 20%, 25%, etc.).
When designers are unable to contact the water authority, they should perform several flow tests at different times of the day to determine when the period of normal or high demand occurs, or the designers can design the sprinkler system with a margin of safety with the intent to account for variation in the water supply. Design practices such as reducing pipe sizes that increase demand pressures should be minimized in areas where the variation in the water supply is not known.
Ultimately, it is the designer’s responsibility to understand the characteristics of the water supply they are using to supply their fire protection system. Recent research has identified major factors that contribute to these variations; however, insufficient data is currently available to quantify these factors for the development of an overall adjustment factor.
Joseph E. Kurry and Mark Hopkins are with JENSEN HUGHES.