Writing Specs For Mobile Water Supply Apparatus
APPARATUS & EQUIPMENT
When writing tanker specifications, you must include details about the construction of the tank, the size and location of the vents, and the arrangement of the piping, among other requirements.
COMPARTMENTATION
Antiswash partitions, also known as baffles, limit the movement of water inside the tank. Water is a dynamic load and tends to resist the change when a vehicle switches direction or speed. Water moving inside the tank makes a tanker difficult to handle and greatly increases the stopping distance in an emergency. The amount of force exerted depends on how much water is moving and the distance it moves before coming in contact with an obstruction.
‘Hie baffles should be close together to limit the unrestricted movement of the water as much as possible. On the other hand, if the baffles are too restrictive, the water won’t be able to move between compartments in the tank toward the outlet rapidly enough to sustain a high dump rate. In this case the pump may go into cavitation, or the flow from the dump outlet may decrease to the point where it isn’t worthwhile to continue dumping even though there is still a substantial amount of water in some compartments of the tank. This water left in the tank after the discharge is completed is known as ballast. It serves no useful purpose and needlessly increases the weight of the vehicle as it moves back and forth in a water shuttle.
Specify baffles that will limit the unrestricted horizontal movement of water inside the tank to no more than 36 inches in any direction. Also, require openings in the baffles, both top and bottom, at least twice as large as the largest outlet from the tank. As a performance standard, require that at least 95 percent of the water be removed from the tank at its maximum flow rate before the pump goes into cavitation or the discharge from the dump outlet decreases to less than 25 percent of the size of the opening.
VENTING
The vents on a tanker serve two primary purposes. When the tank is unloaded, air has to enter the tank to replace the water being discharged to prevent a vacuum from developing inside the tank. When the tank is filled with water from an outside source, the air inside the tank has to escape as quickly as the water enters to prevent pressure from building up inside the tank. With the exception of a few specialized applications, the water tank is not a pressure vessel and is designed to work with a minimal pressure differential between atmospheric pressure outside and water pressure inside. Either a vacuum or pressure buildup inside the tank will result in damage to the tank.
One pumper tanker recently constructed was specified so that 1,500 gallons of water could be loaded within two minutes. The tank on the apparatus has been replaced twice by the manufacturer at its cost but still has a problem meeting the required fill rate without damaging the tank. When 1,500 gallons of water enter a tank, 200 cubic feet of air have to escape. As much as 150 psi may be applied to the fill opening in the tank to force the water in, but the objective is to allow the air to escape without any appreciable pressure differential across the vent opening. To enable this to happen, the vents should be as large as possible. As a minimum, the unrestricted area of the vent must be at least as large as the area of the total fill openings, and a ratio of three to one is best.
The vent size is also critical for rapid dumping of the water. Damage to the tank when dumping is not as likely, since any vacuum that develops inside the tank subtracts from the head pressure that determines the flow’ rate through a gravity dump. This causes the water to discharge slower and prevents a dangerous vacuum from developing but increases the dump time. When a large dump outlet is used, the area of the vent should be at least as large as the size of the dump and a ratio of two or three to one will increase the performance of the apparatus. Inadequate venting can decrease the discharge rate from a large dump to 50 percent of the maximum attainable with vents that don’t restrict the flow of air into the tank.
The vent also serves as an overflow from the tank. Many tankers leave a trail of spilled water as they respond to a fire. Not only does this water subtract from the amount of water that is usable at the scene of the emergency, but it also can create a hazard if it spills on the road in front of the rear wheels of the vehicle. Spillage can be minimized by the location and construction of the vents. Experience has shown that the best location for a vent is in the exact center of the tank. This is the area of least movement, no matter what the change in direction or speed of the vehicle. When there is a hosebed on top of the tank, this may not be a practical location for the vent, but the shape of the vent is not critical -it may be possible to split the hosebed and install a long narrow vent in the center between the two sections.
It is not necessary to have a lid on a vent. If there is a way to close it, there must be some means of automatically opening it either electrically, hydraulically, or by air pressure any time that a fill line or dump valve is opened. Offsetting baffle plates and a grille covering the opening will prevent debris or foreign objects from falling into the tank without obstructing the movement of air significantly and without causing potential damage to the tank by having it closed at the wrong time.
Contain in your specifications a provision that any discharge from the vents or overflow connection be discharged behind the rear wheels of the vehicle. You also can include a requirement that no more than 1 percent of the total capacity of the tank be lost when traveling over the road for a given distance under specified conditions.
HANDLING TIME
Two of the factors in the formula- fill time and dump time-are dependent on the construction of the tank and its associated plumbing. Specifications often list a minimum dump rate and fill rate in gallons per minute. It has been our experience that the actual flow rates are not nearly as important as the time it takes to complete filling or dumping a tanker.
A water shuttle depends on a smooth flow of traffic between the fill site and the dump site. If one of the tankers takes appreciably more time to fill than the others, tankers will be tied up waiting at the fill site while the attack pumper runs out of water. The same problem can occur at the dump site where the water supply in the onsite reservoir is exhausted while tankers that require excessive time to unload tie up the dump site and loaded tankers wait to get into position to dump.
A good objective to set in writing specifications for a tanker is that it have the capacity to dump at least 95 percent of its full capacity in two minutes or less. It also should have the ability to be filled in two minutes or less where the water supply source is capable of supplying water at a high enough rate to do so.
PUMP INSTALLATION
Most mobile water supply apparatus is equipped with a pump. There is some debate as to whether a tanker needs a pump. There are ways to set up a water shuttle that will enable a water supply tanker to operate effectively without a pump on the unit, but a properly installed pump provides the apparatus with an extra element of versatility.
A few tankers, primarily conversion units, use a pump with a separate engine drive. While there are some advantages to this arrangement, this type of pump is not very’ efficient. The maximum capacity is limited by the power of the engine, and most are not large enough to unload very fast. Even the smallest tanker would have to be able to unload at a rate above 500 gpm to meet the two-minute objective, and very few of them can do this. There is an even bigger problem when an attempt is made to use one of these pumps to load the tanker. When operating from draft, very few separate engine-driven pumps can deliver more than 200 gpm, and the loading time becomes too great for it to be usable in a water shuttle.
Any of the other types of pumps – power takeoff (PTO), front-mount (FM), or midship transfer (MS)-can be used effectively on a tanker. For maximum efficiency, the pump should be as large as possible, with 1,000 gpm as a good standard. The maximum flow’ rate from the tank should be specified, since the size of the tank-to-pump line or lines can limit the pump’s ability to supply its rated capacity from the tank. The maximum flow from the tank for many 1,000-gpm fire pumps is less than 500 gpm because the tank-topump line is too small. The maximum flow that can be expected from various size tank-to-pump lines would be approximately:
If the tank-to-pump line is too long or includes a number of 90-degree bends or T-fittings, the maximum flow rate will be reduced proportionately. This is frequently a problem with front-mount pumps. Due to the distance between the pump and the tank outlet and the difficulty in finding room to route the lines to the tank located behind the cab, numerous 90degree bends and T-type fittings may be needed. This can result in a greatly reduced maximum flow rate. On the other hand, a very short 3-inch line without any bends can increase the maximum flow rate of a typical 400 gpm FI’O pump to more than 500 gpm. The easiest way to increase the maximum discharge rate from the pump on a tanker is to reduce the friction loss in the pipe between the tank and the pump. In writing the pump’s specifications, set the requirement that a specified flow rate be maintained until at least 95 percent of the maximum capacity of the tank has been discharged.
Even though a large gravity dump valve or a jet dump may result in a shorter discharge time, there are two practical advantages to having a pump on a tanker. In the early stages of the fire, the water tank on the attack engine should be filled immediately after the first tanker arrives, even before you set up the water supply operation. By using the pump on a tanker for direct supply, you can maintain the flow’ to the attack lines w hile setting up the folding reservoir and putting it in operation. If the tanker doesn’t have the ability to do this, flow to the attack lines will probably be interrupted, possibly at a critical time in the fire attack.
During a water shuttle operation, a pump on a tanker can be put to good use. If a nurse tanker is being used for storage on the fire scene, a separate pumper is required for drafting from tankers that are unable to discharge the load under pressure. Even when a portable reservoir is being used to store water at the dump site, a pump on the tanker can be useful.
Tankers that can only dump, either by gravity or with a jet dump, have to get within a few feet of the tank. In a large water shuttle operation, the dumping area w ill be crowded. Tankers that can pump off at a good rate can set up a separate dumping station some distance from the tanks and pump off through supply lines 100 feet or longer. This setup minimizes congestion and can increase the maximum water supply from the shuttle significantly.
Unless cost is a critical consideration, such a pump is a good investment. Some of the newer FTO pumps, which are designed for maximum transfer of water rather than highpressure operation, can supply as many as two 2’/2-inch lines flowing up to 250 gpm each while keeping costs down and not increasing the size and weight of the vehicle excessively.
LARGE DUMP OUTLETS
The use of large dump outlets on tankers and folding tanks as portable reservoirs for them to dump into has made it possible to sustain fire flows in excess of 1,000 gpm by water shuttle. I-arge dump outlets range in size from 4 1/2 inches to 12 inches. The average flow rate from these outlets depends on the size and depth of the tank, the location of the outlet, and the way it is mounted on the apparatus.
large gravity dump outlets use the head pressure of the water in the tank to force the water through the opening. The lower the outlet is positioned on the tanker, the more head pressure available and the greater the flow’ rate through the outlet. The bottom of the dump fitting must be higher than the tank that will be used, which limits the options in deciding how to install it. A typical folding tank is 30 inches high. To allow for variations in terrain, the specifications should establish a minimum clearance between 32 and 36 inches above the ground at the bottom of the dump outlet when the tanker is fully loaded, depending on the type of tank that will be used, liven if the tanker carries a “lowprofile” folding tank, the dump shouldn’t be lower than 32 inches above the ground. Mutual aid is essential for effective water shuttle operations, and a tanker should be able to dump in any standard folding tank.
On any given tanker, the larger the size of the dump, the quicker the tank can be emptied. Some typical average flow rates for various size dump valves are:
These are the maximum rates that can be expected. In estimating the potential flow rates, the assumption is made that the baffles and venting are adequate to permit the water to move freely through the tank to the outlet. One mistake that is frequently made is to position the dump valve on the end of the tank. This reduces the head pressure of the water stored in the tank by approximately .5 psi -as much as 25 percent in many cases- and the outlet no longer runs a “full pipe” once the water level in the tank gets below’ the top of the dump opening.
It is better to mount the dump valve below’ the bottom of the tank. To do this, a section of pipe connected to an opening in the bottom of the tank can be used, but the friction loss in the pipe will limit the maximum flow’ rate from the dump and may nullify the advantage gained by the increased head pressure. The flow rate can be increased and the amount of ballast reduced by constructing a trough in the bottom of the tank with the dump valve mounted on the rear end of it. If the trough is added to the bottom of the tank, the baffles, which end at the bottom of the tank., w ill not interfere with the transfer of water to the dump valve through the waterway mounted beneath them.
With this type of construction, a sump can be added to the forward end of the trough and the tank-topump line connected there to minimize the friction loss to the pump and take advantage of its full capacity. A baffle plate over the opening in the bottom of the sump or some other type of antiswirl device will prevent the pump from drawing a whirlpool and going into cavitation before the tank is empty.
Most dump valves are manually operated with the handle mounted on the valve housing. This is dangerous. One of the people on the dump site has to get close enough to the handle to open the valve before dumping. This means that, in most cases, someone will have to get between the tank and the tanker to reach it, which is very dangerous and should be avoided at all costs.
(All photos by author.)
One way to avoid this is to specify that the dump valve control be operated remotely. Different manufacturers have various options available that will do this. Air pressure or electricity can be used to control the valve remotely either at the side of the body or from inside the cab. One manufacturer offers an option for a 10-inch square dump valve to mechanically extend the handle on the valve to the edge of the body so that it can be operated safely. As an alternative to a remote control, the addition of a back step to a tanker will provide a safe working position for dumping and a certain amount of convenience in working around the unit. The specifications should provide for a safe location to operate the dump valve.
JET DUMP
The average rate of flow from a round dump valve can be increased by using a portion of the pump capacity to speed the flow of water through the dump outlet. The addition of a jet assist to dump valves up to 8 inches in diameter can nearly double the flow rate and cut the dump time in half. The amount of improvement depends primarily on the installation of the jet and the arrangement of the piping to supply it. Some manufacturers have delivered tankers with a jet assist that improved the flow rate less than 25 percent over the gravity dump rate. Specifications for a jet dump should require that operation of the jet increase the flow rate by at least 75 percent.
One advantage of the jet dump over the large gravity dump is the ability to use extension pipes connected to the dump outlet to reduce maneuvering time and enable the tanker to dump into a portable reservoir from a distance of as much as eight feet from the tank. By using extensions with 90degree bends, you can vary the position of the tanker, which allows more flexibility in a high-capacity water shuttle. With a properly designed and installed jet dump, extension tubes have little or no effect on the dump rate and greatly reduce the overall round-trip time by simplifying maneuvering and positioning the apparatus at the dump site.
The big disadvantage of a jet dump is operational. To use the jet assist, it is necessary to put the pump in gear, set it to the proper pressure, and open the valve to the jet assist line. It has been our experience that many pump operators have difficulty doing this, not to mention the time involved. Some manufacturers use a complicated valve arrangement to put the jet in service. We have found that the operator thinks the jet is operating when it isn’t. The gravity dump involves nothing more than operating a handle once the tanker is in position to dump, and there is no need for the driver to leave the cab. The simplicity of operation and keeping the driver in the cab reduce the handling time enough so that the time saved in getting into position to dump is not significant in the overall round-trip time.
FILL LINES
The ability to fill the tanker rapidly is just as important as the ability to dump it quickly. Very large dump valves are required because only the head pressure of the tank, or the atmospheric pressure plus the head pressure when a jet assist is used, is available to move the water. Smaller inlet openings can be used for filling, because the pumper can supply up to 150 psi for loading. Considering the objective of loading the tanker within two minutes, the larger the tanker, the larger the fill lines need to be.
If the valve at the tank fill end of the hoseline is closed before the valve at the supply end, the fill line still will be pressurized. If this happens, injury may result when the connections are broken to allow the tanker to leave the scene. Alleviate potential injury by including in the specifications a requirement that all fill lines be equipped with a check valve, which automatically closes when pressure is released on the fill line, rather than a gate valve, which is manually controlled. Not only does this provide a greater degree of safety, it cuts down on the time required to fill the tanker. You also can save time by installing quick-connect or quarter-turn fittings on all inlets to the tank.
Direct fill lines at the rear of the tank are not only useful for fill-site operation when shuttling water but can make the operation much smoother when functioning as a nurse tanker. Multiple fill lines allow more than one tanker to discharge its load at a time. They also can reduce handling time for the tankers in the shuttle by providing a way for the next tanker to make its connection and get ready while the preceding unit is still unloading. When the fill lines are located at the rear of the vehicle, an additional indicator for the tank level gauge also should be extended to the rear of the apparatus to allow the dump site operating personnel to function more effectively. Most tanks have the vent on one of the ends, rather than in the middle where it is most effective. If the fill line discharges into the end of the tank where the vent is located, air can be trapped inside the tank at the other end, creating an air pocket that will cause water to overflow before the tank is filled. If the vent can’t be changed, the only solution is to extend the fill line to the other end of the tank and drive the air out of the vent as the tank fills. Include a performance specification that the tank can be filled to at least 99 percent of its total capacity at the maximum fill rate.
Types of Fill Inlets Required to Load Tanker Within Two Minutes
A combination tanker also should have a tank fill line that can be controlled from the pump panel. By having incoming tankers pump directly into the tank, the fire pump is isolated from changes in the incoming pressure as tankers come and go in the shuttle, and a more consistent pressure to attack lines can be maintained. By closely observing the tank level gauge and using the valve on the fill line to keep the tank full, the pump operator can maintain a consistent discharge pressure without interrupting the fire flow. This direct tank fill line can be used the same way when receiving water from a relay to keep excessive pressure from the relay pumpers from causing pressure surges in the attack lines.
CHASSIS AND DRIVE TRAIN
The primary function of a tanker is to haul water. The vehicle must be heavy enough to do this safely and efficiently. The first consideration is the Gross Vehicle Weight Rating (GVWR) of the chassis as it was constructed. From a safety standpoint, the brakes, tires, and suspension are of primary importance. Since it is an emergency vehicle, it often will be operated at relatively high speed and may be required to maneuver suddenly. It is a good idea to choose a chassis that has a GVWR of at least 10 percent more than the maximum load anticipated. This will provide the margin of safety that can make the difference between arriving on the fire scene in a timely manner and getting involved in an accident that may result in extensive property damage or even loss of life.
The balance of the vehicle is also important. If the front wheels carry too much of the weight or too little, the steering won’t be responsive and it may be difficult to control. A bad front-to-rear weight ratio also can cause one of the axles to be overloaded while the gross vehicle weight is well within the specified limit. The specifications should require that the front axle carry no more than 36 percent of the weight or less than 30 percent when the vehicle is fully loaded. If it is equipped with tandem axles on the rear, the front axle should carry between 22 and 28 percent of the total weight.
Emergency vehicles generally have enough horsepower and the accompanying drive train to ensure a high level of performance and a speedy response in an emergency. The need for a quick response is not nearly as great for a tanker, since the water that the initial engine company carries will be enough to last for a while. On the other hand, effective water shuttle operations require that a tanker be able to travel at a reasonable speed and be handled well on the road. To meet this objective, the engine and drive train should be somewhat heavier than an over-the-road vehicle of similar size but not as powerful as the typical pumper. It is important in specifying the chassis to add all heavyduty options such as larger alternators, extra cooling, and heavy duty suspension. Many of these options are available at only minimal extra cost and can pay for themselves many times over during the life of the apparatus.
ACCEPTANCE TESTS
In order to be sure that your vehicle has been constructed in accordance with the specifications that were part of the contract, acceptance tests should be made before the vehicle is put in service. These tests should be done in conjunction with the manufacturer’s representative. They should be observed by representatives of the fire department who are thoroughly familiar with the provisions of the contract and should be thorough enough to demonstrate that all of the specifications have been met. If the fire department doesn’t have the expertise and experience to evaluate the results, it can contract an outside firm to oversee the testing procedures.
There is nothing standard about fire department mobile water supply apparatus. Each unit is constructed to meet the needs of an individual customer and will be built to the specifications given to the manufacturer. Considering the cost involved and the critical nature of the service that will be provided, it makes sense to develop a detailed set of written specifications that can be attached to the contract for construction of the vehicle. Again, if the fire department doesn’t have experience with water supply apparatus or the technical expertise necessary, it can engage an outside consultant. It will be money well spent.
