FPEeXTRA Issue 72

Planning Fire infrastructure? Reliability of Water Supplies in Industrial Applications

By: John Ivison CP, P.Eng FSFPE

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Although standards such as NFPA 13 (Standard for the Installation of Sprinkler Systems)[i] set out the water supply requirements for automatic sprinkler systems, additional guidance is needed when designing these systems for industrial fire protection systems. Much of the input relies on the review of system design guidance provided by FM Global and other insurance agencies. While this service provides valuable oversight of the design process towards achieving acceptance for insurance purposes, more attention to detail may be necessary to fully embrace the complete design of fire protection systems and avoid significant operational and other issues.

This article explores issues that contribute to reliability, focusing on more extensive facilities as these are often less familiar to design consultants such as engineers specializing in site services. There is also a fundamental difficulty in meeting applicable regulations' intent, particularly building codes concerning matters such as interconnected floor spaces and firewalls.

Issues in System Arrangement, Reliability 101

Specific vital issues should be addressed when designing fire safety in industrial applications. In cities, many urban areas had industries within their borders; these included sawmills, tanneries, power plants, grain elevators, distilleries, and other facilities to meet market needs in the locality. While these facilities relied on municipal mains for their water supplies, they often had multiple connections- typically to different streets. This would reduce the likelihood that a single street connection would fail in the event of a disruption to the mains in any one street. Fundamentally, this relied on the ability to isolate breaks through the closure of isolation valves. In addition, many facilities also had secondary supplies that provided reliability to the system through gravity tanks and other means. As many plants relied on non-potable water from rivers, wells, and lakes, they often incorporated fire pumps taking suction from these supplies.

Development Factors

Urban sprawl and the value of property in downtown cores have pushed the development of new factories to the suburbs and beyond. In these areas, the infrastructure for water supplies may not exist or be of limited capacity. This means less dependence on municipal water and more reliance on private supplies to meet demand.

Where municipal water flow is sufficient to meet demand but pressure is inadequate, there is an option to boost the pressure to achieve a good supply. Booster pumps are typically electric-driven and may be less reliable during a power failure. However, for moderate risks, they can constitute a reasonable supply. This is because, in many situations, the failure of the pump can still satisfy demand but at lower pressure. Dependence on this arrangement typically requires that the supply be capable of meeting sprinkler water demand only when the electrical supply fails -- if not both sprinkler demand and inside and outside hose. This arrangement alone is insufficient to satisfy the risks in more extensive facilities. Some of the options for typical factories involving relatively high-risk occupancies are set out below.

1. Municipal or private water supplies with or without booster pumps.
2. Fire pumps taking suction from available natural water sources- generally using vertical turbine pumps due to negative head.
3. Fire pumps that take suction under positive head from above-ground tanks or designated portions of suction supplies such as clear-wells.
4. Fire pumps that take suction from reservoirs using centrifugal pumps under head or vertical turbine-driven pumps.
5. Gravity tanks.

It is less likely that gravity tanks are utilized for new facilities due to cost, limited pressure, and capacity.

Sizing Fire Pumps

While it is tempting to choose pump sizes based on precise estimates of sprinkler demand, in practice, it is recommended to carry out an analysis of the major fire risks and get an overview of water demand over the major hazard areas. For instance, if insurers require a sawmill to have a density of 0.2 gpm/ft² over a design area of 4000 ft² then actual demand may be at least:

0.20 (average density) x 4000 = 800  gpm+ 500 gpm (hose allowance) = 1300.00 gpm

In practice, the engineer may select a 1500 gpm fire pump. In selecting rated pressures, the height of the building needs to be taken into account. Consequently, while a 100 psi fire pump may serve most situations, some preliminary calculations should be conducted to ensure that pressures required are achievable at the top of the riser at the highest point in the building. Limitations as to demand often exist, such as:

• Outside storage: while these may be insured as a separate risk in another insurance market, the engineer should ensure the demands of a significant exterior fire can be met. Increasing the fire pump sizing increases the availability of larger hose streams at an earlier stage of a fire. While the fire service may use municipal mains, they may have insufficient volume in some cases, particularly for outside storage risks, and may need to use the site-wide supply.
• Flammable liquid areas processing areas: with open processing areas, tank storage, and other issues, there may be the potential for extensive fires, which may dictate water demand.
• Storage is often the limiting factor in the design of water supplies. It is not unusual to have demand over 2500 gpm.

Duration of Supplies/Fuel Supplies

Extended fires can occur, and a part of the fire risk assessment process demands a feeling for potential situations where extended fires may occur. This often happens with uncontrolled exterior storage as no automatic suppression is usually available. In this case, an analysis of municipal and natural water supplies may be required. In a recent case where a reasonably good supply from the municipality was available, it was only available for 3 hours. While this might be good for most conditions, this may be the best-case scenario. Gravity supplies in smaller municipalities may have less duration. The advantage of natural supplies that should be good over the four seasons is that they may be available for an extended time. Other water supplies may be able to replenish the water tank and other supplies over the period of a fire.

With an additional water supply duration, it is essential to ensure that fuel supplies for diesel pumps are sized to continue in an extended fire event.

While fire pumps over their performance curve will deliver 150% of their rated flow, the pressure from centrifugal pumps drops off significantly to 65% of rated pressure. Consequently, a conservative approach would dictate sizing pumps on the highest demand, including hose allowance. This gives redundancy in the design that avoids running out of available pressure as demand increases over the fire pump range.

Mains and Fire Pump Arrangement

While smaller facilities may typically be fed by a single or a tree arrangement of main(s), this is not preferred for larger facilities. Looped mains are preferred for the following reasons:

• It enables water to be fed in two directions reducing friction losses.
• Mains sizes may be reduced based on the reduced friction losses, although it is recommended that flow in one direction should enable the water demand to be met in any one direction.
• Access to mains is available at any point in the complex, although the initial layout of mains should take account of the potential for expansion.
• Sectional control valves can enable breaks to be isolated without interrupting the supply.
• It enables hydrant distribution to be improved by locating hydrants around the loop.
• The fire service connection can be a supply at the street rather than a connection at each sprinkler valve around the plant.
• Fire pumps should, wherever possible, be juxtaposed at opposite ends of the mains and located in isolated areas with no significant hazards that could result in an interruption. Where multiple supplies exist, more flexibility is provided in determining the locations of key elements.

Protocol for Priority of Water Supplies

Suppose a plant relies on a potable water from the municipality or private supplier. In that case, any non-potable supply such as those described above should be separated so that there is little or no risk of contamination to the potable supply in the street. This is done by supplying a separate feed for potable water needs near the connection at the street. This potable connection is almost always upstream of the backflow preventer and upstream of any feed to the fire protection system. This is fundamental to reducing the potential for contamination from the fire main into the street. The sizing of the various lines is vital to preventing a restriction to flow.

Many supplies have the potential to introduce particulates into the system. For instance, turbidity may be present in river supplies. This can accumulate over time and can reduce the capacity of the mains and even clog sprinklers. Consequently, it is recommended to run the system on potable water to reduce this potential problem. With extensive mains, the potential for leakage increases. In this instance, a jockey pump can be used to maintain pressure artificially. There may also be cases where non-potable water is used for wash-down purposes to reduce the accumulation of combustible fines on portable equipment or to wash down various areas where combustible material can accumulate. In these cases, the jockey pump should be replaced or supplemented with a service pump that will meet normal leakage and nominal flow from wash-down without starting the fire/booster pumps. As burn-out of fire pumps is very common, especially shortly after plant start-up, this is an important consideration in the system's design. This is not preferred where potable water is at a premium.

For most systems that operate on potable water, a jockey pump or service pump is installed to reduce the unintentional starting of the primary fire pumps. It is recommended that jockey pump or service pump pressure be designed to maintain artificially high pressure on the main, typically around 125 psi. This enables a good separation of starting pressures.

Protocol for Using Fire Pumps/Secondary Supplies

Prevention of unintentional starts is a crucial consideration in reliability. Another consideration is the clarity of secondary water supplies. Suppose the secondary supply is from a suction tank. In that case, there should be relatively little contamination if the tank is filled from the potable supply or if a clarifier or other clean suction source is used. Gravity tanks most often are fed by uncontaminated sources but have limited capacity and should be lower in the priority for use. Refilling of tanks following maintenance or other use should generally be possible within 24 hours. This means that if the secondary supply is a long way from the street, then the feed size for refill should be adequate to achieve this.

Suppose a water supply has the potential for the introduction of fines or fish into the system. In that case, the screening arrangement should meet fishery standards to prevent the introduction of small fish into the system. The introduction of fines and fish can be reduced by flushing the suction well with clean water from another source. This produces an outflow of water through the screens preventing, to a degree, the expected inflow of particulates. This also helps if open reservoirs are utilized, particularly clay-lined ones.

Saltwater fire pumps are often used in marine facilities and are common in recreational piers in the United Kingdom. Suction can be taken from vertical turbine pumps with shafts located in tubular suction wells. In one recent case, fish were observed swimming in a circular motion in the suction well. This means that these fish are being pulled into the piping system on testing. Consequently, the entire fire mains and hydrant system can be obstructed by organic material. Therefore, saltwater pumps should be properly screened and back-flushed continually with fresh water to prevent saltwater fish species from entering the system.

Starting Protocol for Fire Pumps (Example)

Assuming all the available combinations of supplies possible:

• Two or three municipal connections.
• A booster pump that meets the required flow and pressure of the system.
• A service pump and jockey pump are designed to prevent accidental fire and booster pumps.
• A suction tank with main fire pump (s) electric and diesel taking suction under head.
• A gravity tank with limited capacity/duration and head.
• An above-grade reservoir designed to address long-duration fires- say a log pile- with clay-lining on reservoirs.

Notes:

1. The above example is a hypothetical case, and it is unlikely that you will get as many supplies as this. If fewer supplies are provided, spread the start pressures out over the start range.
2. * The jockey is designed to address leakage and maintenance pressure on the main. It is working for all the team.
3. ** The service pump can be deleted if wash-down hoses are not fed from the fire main. This pump only needs to start when wash-down hoses are operated.
4. *** the booster pump may have relatively limited flow but is still helpful over much of the required range of water demand. Main fire pumps meet the actual fulfillment of demand.
5. **** If reservoirs are used to satisfy the main demand, i.e., without other supplies, as shown above, they can be increased in size to provide additional duration. If used as the sole supply, consider duplicating pumps and splitting the reservoir to enable shutdown on one reservoir without interrupting the other. Saltwater pumps are to be last to operate if part of the system is partly due to corrosive effects of salt entering the system.

John Ivison CP P.Eng FSFPE, is with John Ivison and Associates Ltd.

References

[i] NFPA 13, Standard for the Installation of Sprinkler Systems. Quincy, MA. (2022).