Writing Specs for Mobile Water Supply Apparatus

Writing Specs for Mobile Water Supply Apparatus

APPARATUS & EQUIPMENT

It’s not just a rural concern. For many departments, mobile water supply vehicles are the answer to their fire flow problems.

It is rare for a fire department to purchase a new pumper with a pump capacity of less than 1,000 gpm; many are rated at 1,250 or 1,500 gpm. A lot of planning goes into the purchase of a new pumper, and specifications generally ensure that the pump is large enough to furnish the fire flow needed for the protection area. But many departments aren’t nearly as concerned with providing the water that the new pumper needs to operate effectively.

Fire pumps don’t manufacture water; they only move it from one place to another. The most common size for a pumper’s water tank is 500 gallons. Simple mathematics indicate that a 1,500-gpm pumper only can operate for 20 seconds at its maximum capacity from a 500-gallon tank. After the tank on the apparatus has been emptied, the fire pump serves no useful purpose until additional water is available.

Traditionally the fire service has depended on fire hydrants for water supply. Some fire departments virtually ignore the fire problem where fire hydrants aren’t available. They take the attitude that where there are no hydrants they shouldn’t be expected to extinguish fires in a timely manner. There are two basic problems with this approach.

FIRE FLOW PROBLEMS

A large percentage of the structures in the United States are not located within 1,000 feet of a fire hydrant. Residential subdivisions with large lots in a rural setting are popular, especially near larger metropolitan areas. Many commercial and industrial installations, as well as multifamily residential buildings, also are being constructed in areas that aren’t served by public water systems. Public water systems have been slow to expand enough to meet the needs of the community because of the high cost of construction and the elimination of most of the government grants that were used to subsidize this work.

Another difficulty is that many hydrants are not able to supply enough water to provide the fire flow that is required for the area they serve. Many of the water systems, especially in older cities and towns, were constructed years ago when usage was much lower. The pipes in the water distribution system are not large enough to handle the demands that the growing community has placed on them. Most older systems used cast-iron pipe for water mains. Many of them have corroded and deteriorated to the point that the original water flow capability of the system is greatly reduced.

Even in those areas where a good water system exists, some hydrants are not accessible to fire department pumpers. Interstate highways are a good example of this problem. Even though there may be hydrants in the vicinity of an emergency, the barriers that limit access to the highway also make it difficult if not impossible to establish an adequate water supply in a timely manner.

Long bridges also present a problem. The Francis Scott Key Bridge over the Baltimore, Maryland harbor is a typical example. It’s nearly two miles long and, other than a standpipe connection that fire boats could supply, there is no water supply on it. During rush hour this bridge may have traffic backed up and confined for as long as a mile. A transportation emergency on this bridge, such as a fire in a tank truck carrying flammable liquids, could result in considerable loss of life since it would be difficult to evacuate people out of the traffic jam.

Even in the midst of an effective hydrant system, construction activities can set the scene for a catastrophe. A number of lives were lost in Kansas City when a construction trailer exploded. Hazardous materials are often present on a construction site before the fire hydrants and distribution system are ready to provide an adequate water supply to handle them.

Many buildings have been lost to fire because of defective hydrants or other problems in the water system that limited available fire flow or that caused a delay in getting the water supply set up. While other hydrants may be within reach, it takes time to move the hoselines to connect to them.

The Francis Scott Key Bridge over the Baltimore, Maryland harbor has no water supply other than a standpipe connection that fire boats could supply. This is one example of where mobile water supply may be effective.

(Photos by author.)

THE NEED FOR MOBILE WATER SUPPLY

Rural fire departments have been conscious of the need for mobile water supply apparatus for some time. Most of the earliest tankers that fire departments used were conversions from used commercial vehicles. When a petroleum product distributor replaced a delivery truck, it was donated to the local fire department and used to haul water. Milk trucks, bottled gas delivery trucks, and military surplus vehicles have been modified and used for this purpose.

Many of these conversion units weren’t safe. Some were involved in serious accidents. Since water weighs appreciably more than the product that the vehicle was originally designed to haul, the truck probably will be overloaded for fire department use. Inadequate brakes, weak springs, tires that are too small, baffling that doesn’t limit movement of water sufficiently inside the tank, and engines that don’t have enough power all contribute to the instability and limit the roadability of some conversion units.

Unless the conversion has been done properly, the tanker won’t transport enough water to be of much help. Using the pump that was originally installed to deliver fuel can limit the transfer rate to less than 100 gpm in some cases. This would not provide enough water to sustain more than one 1 1/2-inch line and would not support an effective initial attack. Even when modifications are made, the fill rate and dump rate may be too slow to provide a dependable water supply.

In recent years, many conversion units have been replaced by specially constructed tankers purchased for that purpose, but they pose the same problems as conversion units. Because of a lack of generally accepted standards in the industry, some tankers are much better than others. Some of the most expensive apparatus is the least effective, while some less expensive tankers have the ability to move large amounts of water very efficiently.

The only way to ensure a tanker will suit your department’s needs is to develop a detailed list of written specifications that will help the manufacturer construct the vehicle. Nearly every Class A pumper satisfies the requirements of NFPA 1901. Every pumper is tested at the factory before delivery by a representative of Underwriters Laboratories who will certify that it meets these requirements. Unless a tanker is equipped with a Class A fire pump, it will not be certified by a UL representative and does not have to meet any standards. Even when the mobile water supply apparatus includes a major fire pump and is certified by UL, only the pump itself is tested. Many of the characteristics that determine how much water it can supply are ignored. While the NEPA is presently in the process of revising Standard 1901 and developing a separate standard for mobile water supply apparatus, it is not likely that fire departments will be able to rely solely on the new standard—they still need a set of detailed specifications.

Departments frequently used tankers that were converted from used commercial vehicles. However, many conversion units were unsafe because they had been designed to haul lighter loads than those used in water supply operations, and their brakes, springs, tires, and engines could not handle the water weight.

WATER SHUTTLE OPERATIONS

Many rural fire departments have increased the size of their attack pumper’s water tank to provide more water for the initial attack. The 1,000gpm pumper with a 1,000-gallon water tank is gaining popularity in some parts of the country. While it is a big improvement over the standard 500gallon tank, 1,000 gallons would still only supply the full capacity of the pump for one minute.

Other departments use semitrailer units to deliver as much as 6,000 gallons on the initial response. This should be enough water to handle any single-family dwelling and mount an effective attack on some larger fires, but it would only provide six minutes of water for a 1,000-gpm pumper operating at its maximum capacity. It is obvious that no fire department can expect to handle a major fire with the amount of water that the initially responding apparatus can carry. The ability to shuttle water for some distance between the water source and the fire scene is the only practical way to maintain the needed fire flow when the water supply is too far away to lay direct hoselines.

In Standard 1231, Water Supplies for Suburban and Rural Fire Fighting, the NFPA uses a formula for calculating water supply potential for a particular tanker operating at a specific location. The recommended formula is:

Q =V / A +2T + B -10 percent,

where Q = continuous flow capability in gpm, V = capacity of the tank in gallons, A = time required to dump the full load in minutes, B = time required to fill the tank in minutes, and T = time required to travel one way between the water source and the fire scene in minutes. (Note: The NFPA considers only 90 percent of the total capacity of a tanker to be usable in a water shuttle and subtracts 10 percent to reflect this limitation.) All of the components of this formula are dependent on the design and construction of the mobile water supply apparatus that is being evaluated. The specifications for construction of a new tanker should be written to minimize the time required for each round trip in a water shuttle, in order to provide the maximum water supply capability.

TANKER CLASSIFICATIONS

Many different terms have been used to describe mobile water supply apparatus. The most practical method is to categorize tankers according to their intended use. There are four basic types of tankers:

Pumper tanker—a Class A pumper that meets all the requirements of NFPA 1901 and carries all required engine company equipment; carries at least 1,200 feet of supply line; and has a water tank that holds at least 1,000 gallons.

Tanker pumper— a unit that is designed as a water hauler, that is equipped with a Class A fire pump with a capacity of at least 500 gpm, and that carries a minimum amount of fire hose and equipment.

General purpose tanker—a tanker that carries some engine company equipment including attack lines, fire hose, and breathing apparatus. It has a booster-type fire pump, a capacity of up to 500-gpm, and a water tank large enough to carry 1,000 gallons or more.

This water shuttle, flowing 2,750 gpm, hauls water for 3/4 of a mile in Sussex County, New Jersey. The ability to shuttle water between its source and the fireground helps firefighters maintain needed water flow when the water supply is too far away to lay direct hoselines.

Water supply tanker—a specialized vehicle that is designed soley as a water supply tanker. If it has a pump, it will be designed for maximum transfer of water rather than for supplying attack lines, but it may depend on a large dump outlet to unload quickly. It carries only that equipment which is needed for water supply operations.

APPARATUS SELECTION

The first step in developing specifications is deciding what you expect the apparatus to do. Three of the tankers I mentioned above are combination units, which can be used for more than one purpose. While their versatility is appealing, keep in mind that apparatus designed for a number of purposes rarely does any of them as well as a specialized unit can.

Pumper tanker. Many departments have replaced a standard pumper with a pumper tanker, which enables them to operate a mobile water supply unit without adding another vehicle to their fleet. A pumper tanker brings enough water to the scene for a quick attack on the fire before a tanker arrives. This is a significant advantage when manpower is limited or when it is not possible to put the second unit in service. When additional water does arrive, a well-constructed pumper tanker can serve as a reservoir and a nurse tanker. One piece of apparatus can be an attack pumper, supply pumper, and nurse tanker all at the same time. This can simplify setting up the water supply operation—it eliminates the need to set up a portable tank.

There are some cost savings when two units are combined into one —for example, it’s easier to maintain and store one unit. However, you might be overestimating your savings.

A typical fully equipped class A pumper with a 500-gallon water tank weighs nearly 30,000 pounds. A larger tank adds approximately 10 pounds for each gallon over 500. A pumper tanker that carries more than 1,000 gallons of water requires tandem axles on the rear. Many pumper tankers that are being built carry 2,000 galIons of water or more and weigh more than 50,000 pounds. Providing a chassis that is heavy enough to handle this much weight and a drive train powerfill enough to maintain an acceptable level of performance is very expensive. A vehicle that is large enough to carry this load is difficult to drive and may have trouble accessing the places where you need it most. Maintenance costs are much higher for the powerful engines, transmissions, and other drive train components that you need to handle the extra weight. When you take all associated costs into consideration, you may find it cheaper to buy two separate vehicles—a pumper and a tanker—than to combine both functions into one.

Tanker pumper. The tanker pumper is basically a tanker with a major fire pump added. It provides additional pumping capacity and can be used as the source pumper for a relay. It also can be connected to a hydrant in place of a fully equipped pumper. A problem arises, however, when the fire department decides to use a tanker pumper as an initial attack unit. Since it has been designed as a water hauler, a tanker pumper usually has a large water tank to the exclusion of compartment space and hose beds. If too many equipment compartments are then added, the extra weight may well overload the chassis, causing maintenance problems and making it dangerous to drive. But without extra compartments, you won’t have enough equipment and hoselines for an effective fire attack.

Tanker pumpers generally carry at least 1,500 gallons of water and may carry as much as 4,000 gallons. If you keep compartment space and equipment to a minimum, it is possible to carry as much as 2,500 gallons of water on a two-axle chassis, but most units that have a capacity of more than 2,000 gallons have tandem rear axles. These high-capacity tankers are also large, heavy, and expensive. Many of the same advantages and disadvantages of pumper tankers apply.

The first step in selecting the right apparatus for your department is determining your protection area needs and operational needs. A pumper tanker can be used as a quick attack pumper, supply pumper, and nurse tanker.A tanker pumper can be used as a source pumper for a relay.A water supply tanker transports water between a water supply and the emergency scene.

General purpose tanker. Many rural departments find a general purpose tanker to be the best solution to water supply problems. It usually is built on a two-axle chassis and the gross weight can be kept below 35,000 pounds with careful planning. A water tank with 1,200 to 1,800 gallons permits a limited amount of the kind of equipment that an engine company needs to make an initial attack. The booster-type PTO-driven pump provides enough water to make an initial attack and supplies up to 500 gpm directly to an attack pumper.

This type of tanker is especially popular with small volunteer fire departments that can man only one piece of apparatus at slack periods of the day. It allows two or three firefighters to make a quick, although limited, attack on a fire and either knock it down or control it until mutual-aid units arrive. As a mobile water supply, the pump can be used to supply a jet dump arrangement for rapid discharge. As an alternative, a large gravity dump can discharge as much as 2,000 gpm. Even though it is a multipurpose vehicle, good planning and proper design can make it reasonably efficient however you use it.

Water supply tanker. A water supply tanker is designed for one purpose only: to transport water between a water supply point and the emergency scene. Very little compartment space is needed, since it only carries equipment necessary to function as a water supply unit. The pump, where one is included, is set up for maximum transfer of water, not to develop the higher pressure needed to supply attack lines. A primary objective when designing a water supply tanker is to attain the shortest possible loading and unloading time. The shortest unloading time can be achieved with a large gravity dump valve or a jet-assisted dump. Direct fill lines into the tank provide the best loading time and multiple fill inlets provide the needed flexibility for functioning effectively in any type of water shuttle operation, either as a nurse tanker serving as a reservoir on the dump site or as a water hauler.

DETERMINING YOUR NEEDS

A detailed analysis of the needs of your protection area and the types of operations you’ll perform will help you decide which type of tanker to build. Combination units are useful for quick attack. Also, when the fire risks require a large fire flow for initial attack, a pumper tanker or tanker pumper can provide the high initial flow rate to quickly bring an incipient fire under control. These large combination units also can provide an emergency water supply for interim use when there is a delay in establishing the planned water supply. The larger amount of water that they carry can be used to supplement the hydrant system for initial attack when it is inadequate for the risks involved. When the apparatus that initially responds to the emergency is used this way, the second alarm assignment— water supply tankers or mutual-aid units—must set up a water supply to sustain the needed fire flow over a period of time. The initial response equipment is committed to the fire scene and is not available to shuttle water or set up relay.

Rural fire departments, as well as those that serve predominantly residential or light commercial areas, might make good use of a general purpose tanker. The pump easily supports a 1 1/2-inch line for initial attack and should be able to supply a backup 2 1/2-inch line as well. Compartment space is adequate to carry all of the equipment necessary for a quick attack of this magnitude. The hose bed can carry enough supply line to support an initial attack on the fire and make the transition to a sustained water supply.

This type of apparatus is light enough and small enough to function in tight quarters, travel over narrow roads, and cross bridges with low weight limits safely. When used as a “first out” unit, it either will have to be replaced by a pumper when one arrives or depend on mutual-aid units for additional water supply as the initial supply is exhausted.

When called on to operate in a water shuttle, the general purpose tanker can be very effective. If it is properly designed, its quick turnaround time and maneuverability make it a good tanker to use in congested areas. Its relatively small size and weight give it flexibility in difficult operating conditions. A high percentage of usable water and quick handling time will result in a good water supply.

Water supply tankers provide the most water for the least money. If you keep it simple, a water supply tanker can haul more water than a combination unit and does so with a smaller chassis and drive train. By carefully constructing the body and tank and by limiting the compartment size and the amount of equipment carried, you can put as much as 2,500 gallons on a two-axle chassis; 4,000 gallons on a tandem axle; and 6,000 gallons on a semitrailer rig. Because of the amount of water that a water supply tanker can haul and its lower price, it generally represents the lowest cost per gallon. You may be able to buy as many as four water supply tankers for the cost of one pumper tanker combination.

In general, for those departments that habitually are short of manpower, have limited storage space for apparatus, or are faced with special fire protection problems, a combination unit can be a good choice. In most situations, a water supply tanker, used in conjunction with well-equipped attack pumpers carry ing 750 to 1,000 gallons of water, represents the most cost-effective way to sustain an adequate fire flow outside of those areas that are served by hydrants or as a supplement to an otherwise inadequate system.

DETERMINING THE SIZE

There are many variables that you must consider in deciding how much water a mobile water supply apparatus should carry. Ask yourself: What type of terrain is involved? What type of roads and bridges will it have to traverse? How large is the area that it will serve? How much fire flow is likely to be needed? What manpower is available to operate it? What can the department afford to spend? Weigh answers to these and other questions carefully before drawing up specifications. The following are some principles that apply in this process.

  • The longer the distance that the tanker will have to travel in a typical water shuttle, the more water it needs to carry. Over a short haul small, maneuverable tankers may be able to deliver as much water as larger ones.
  • As the tank gets larger, the vehicle gets heavier. A pumper tanker with two axles generally weighs about 20,000 pounds plus 10 pounds per gallon of water. If a tandem axle chassis is required for a pumper tanker, the heavier chassis and larger body will add another 10,000 pounds. A tanker pumper weighs approximately 15,000 pounds plus 10 pounds per gallon. A general purpose tanker weighs about the same as a tanker pumper, depending on the amount of equipment it carries. A water supply tanker weighs approximately 10,000 pounds plus 10 pounds per gallon carried. A tandem axle chassis will add aproximately 5,000 pounds to the unit.
  • As the weight increases, the price escalates at a rapid rate. Two small tankers may cost less than one large one.
  • If a tanker is to be used in a rural area with narrow roads and bridges with low weight limits, limit the gross vehicle weight to 30,000 pounds. This allows it to carry from 1,500 to 2,000 gallons of water, which provides an adequate supply for initial attack on single-family residences, small barns, and small commercial buildings.
  • In urban and suburban areas that need larger fire flows, water supply tankers with a capacity of 2,500 gallons or more are needed to make an initial attack and to function effective-
  • ly in a water shuttle to provide a sustained fire flow.

The NFPA formula used in calculating water supply capability is based on the maximum capacity of the tank. It then reduces the end result by 10 percent to allow for the fact that on average, 10 percent of the water a tanker carries is not usable—either the tank doesn’t fill completely or there is water left inside after unloading. After testing hundreds of tankers over the past 10 years. I have found that the actual amount of usable water varies from as low as 60 percent to as high as 99.5 percent. When writing the specifications for a tanker, keep in mind that it is the usable w ater that is most important, not the full load capacity.

Construction of the baffles inside the tank, location and size of the vents, and the location and arrangement of the piping all contribute to the end result and must be included in the specifications. I will discuss these in more detail in the next article.

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