A Maxi Tanker At Mini Cost
In the past we have discussed selecting a tanker for water supply and writing specifications to suit your department’s needs. The problem that many fire departments face is that they can’t afford to buy what they have decided they need.
The trend toward combination units has aggravated this problem. Many departments have been buying apparatus that can not only provide water supply but also be used for fire attack. The use of a pumper-tanker for initial attack can make possible rapid containment of the fire and minimize the need for a sustained water supply operation If the attack is not successful, a basic conflict comes into play: The two functions—fire attack anil water supply —are not compatible. To attack the fire, the apparatus must remain on the foreground in a position to apply water to the fire; to haul water, the vehicle has to travel from the fire scene to the water source and return on a frequent basis. A decision has to be made as to where the pumper-tanker is needed. In most cases, where the initial attack was made by the pumper-tanker, it will probably remain in this capacity and be unable to haul water. Water supply then will depend on other tankers operated by the fire department or be delayed until mutual-aid units arrive.
Another consideration is the weight involved in building a combination unit. Since each gallon of water added to a tanker increases the overall weight by approximately 10 pounds, the Gross Vehicle Weight Rating (GVWR) of the chassis is usually the limiting factor in its carrying capacity. As a pump, a hose body, and cabinets and the equipment to fill them are added to the vehicle, the water-carrying capacity decreases proportionately. Some departments try to overcome this by increasing the GVWR of the chassis. This is self-defeating to a certain extent. In the process of upgrading the engine, springs, brakes, and other chassis components from 30,000 to 35,000 lbs., the weight of the chassis itself probably will increase by 2,000 to 3,000 lbs. for a net gain of only 2,000 lbs. What actually is happening is that the chassis is being beefed up to carry itself, and the net payload doesn’t increase as much as would be expected.
C ost also increases rapidly. In building a pumper-tanker, it is not uncommon to spend as much or more money than the cost of two separate vehicles.
DESIGN CRITERIA
I designed a maxi-mini tanker to deal with these problems. My first objective was to build a water supply tanker that would do the best possible job of moving water. Trying to build a vehicle that will do everything often results in a vehicle that does a lot of things poorly and none really well. 1 designed this tanker solely to move water as efficiently as possible.
My other objective was to minimize the cost To make it feasible for a department to purchase the number of tankers necessary to sustain a reliable water supply, my goal was to put a tanker on the road at a cost of no more than S30,000.
CHASSIS SELECTION
In order to provide a dependable water supply over distances of more than one mile while retaining the maneuverability and speed of a relatively small vehicle, I set a minimum capacity of 2,000 gallons as design criteria. Considering the price of new trucks, this tanker obviously would have to be constructed on a used chassis. While this concept is not common in the fire service, it makes a lot of sense. The average fire truck in rural areas will not travel much more than 30,000 miles in a lifetime. Considering modern diesel engines with their expected life of 250,000 miles before overhaul, the engine is barely broken in wfflen the truck is junked. An ovcr-the-road truck that has been properly maintained should provide reliable service even up to 200,000 miles or more.
In selecting a used truck chassis, age was a concern. While many fire trucks have very low mileage, maintenance still becomes difficult. Certain parts such as brake lines, hoses, and electrical wiring deteriorate w ith age. It is also difficult to get replacement parts for vehicles that have become obsolete due to age. I selected a 1980 International SI900 cab and chassis equipped with an International DT466 diesel engine for this purpose. The GVWR can be as low as 30,000 lbs. because the cab and chassis w eigh less than 10,000 lbs. with a full load of fuel. This permits a payload of 20,000 lbs. without exceeding the rating of the chassis. If the GVWR of the chassis is exceeded, braking and suspension will be inadequate and operation as an emergency vehicle will be unsafe. In addition to the practical concerns with operating an overloaded vehicle, there is a certain amount of legal liability that may be incurred if it is involved in an accident. In some areas, the insurance evaluators have refused to recognize a tanker for rating purposes if it weighs more than the chassis is rated to carry.
While the rule of thumb is that each gallon of w ater represents a load of 10 lbs., this can be reduced by constructing the tank and as many parts as possible out of aluminum. As constructed. the tank is made of Vs” aluminum. as arc the rear step, compartments, fenders, and nearly all of the metal parts. The completed weight of the tank, pump, piping, and body is only 2,350 lbs., which results in a fully loaded weight of 28,757 lbs. This allows the vehicle to carry a folding tank, low -level strainer. 100 feet of 3” hose for filling, and a few tools without exceeding the GVWR of the chassis. One benefit of the way this tanker is built is that it carries 2,000 gallons of usable water on a vehicle that weighs less than 30,000 lbs. when fully loaded—a distinct advantage when operating in a rural area. When there is a load limit on bridges or when road conditions are difficult, this tanker can provide a reliable water supply in places that a larger unit couldn’t go. Its relatively short wheelbase gives it added maneuverability and the power supplied by the diesel engine contributes to a quick turnaround time and maximum water supply over short-to-medium distances.
TANK CONSTRUCTION
The tank is designed to have a long life, provide minimum water movement when the vehicle is in motion, and allow for rapid loading and unloading with a maximum amount of usable water.
For durability, the interior of the tank is coated with a bitumastic solution. This should limit the amount of corrosion and deterioration of the aluminum from electrolysis. It is constructed on aluminum structural members 8″ high and cushioned byoak boards between the tank and the apparatus frame. It is fastened to the frame with four sets of threaded rods and brackets to hold it securely in place. As a safety measure, a piece of angle iron is welded to the frame in front of the tank and side braces are welded to the frame to minimize sideways and forward motions in the event of an accident.
BAFFLING
Three lateral baffles and one longitudinal baffle limit the free travel of the water to three feet in any direction. This creates eight compartments inside the tank, each containing approximately 250 gallons of water.
Openings in each of the baffles are large enough to permit rapid loading and unloading rates without damage to the tank. A design objective was the ability to transfer water freely between compartments at a minimum rate of 2,000 gpm and to allow the air to escape through the vent at a rate of 300 cubic feet per minute with a pressure differential of no more than 0.5 psi buildup. This was accomplished by cutting an opening approximately 10″ square in each outside corner of the lateral baffles. I recorded an average discharge rate through the quick dump opening of 1,773 gpm and an average fill rate of 900 gpm through the 2 ½” rear-fill opening during testing and evaluation, indicating that these objectives were met.
In order to attain a maximum flow rate when dumping through the 10″ square opening at the rear of the tank, the bottom of the tank was left open from front to back approximately 12″ wide. A trough allows the free transfer of water from each of the compartments inside the tank to the gravity dump on the rear and to the sump in the front of the trough. At the rear of the tank the trough is tapered for the gravity dump fitting below the bottom of the tank. This serves two purposes. Since the entire opening is below the bottom of the tank, a full stream of water continues until the tank is empty and the only water left in the tanker is that which is trapped in the trough. T his ensures that there will be a minimum of w ater left in the tank after dumping, and ballast will be negligible. My tests indicate that this unusable water represents no more than 10 gallons in most cases. This type of mounting also increases the average depth of water in the tank throughout the dumping operation by a minimum of 10 inches. The increased depth provides an additional 0.36 psi head pressure, which increases the average flow by at least several hundred gpm.
The trough also improves the pump-off capability of the tanker The sump on the front of the trough is deep enough so that the tank-to-pump line is less that 12” long. This allows the 400-gpm PTO-driven pump to develop peak flows in excess of 500 gpm. Due to the depth of the sump and the antiswirl plate, which has been installed over the tank outlet, the pump won’t begin to cavitate until the tank has less than 10 gallons left on board. Another opening goes from the other side of the sump to a 4″ pipe equipped with a butterfly valve and 4½” NST threads to fit a standard suction hose. This provides two additional options. If it is necessary to use this tanker as a nurse tanker in water shuttle operations, a pumper can hook up with standard hydrant fittings and hard suction hose to draft from the tank. In spite of the reduced waterway, the short length of this pipe along with the head pressure in the tank enables a pumper to supply nearly 1,000 gpm in this manner. It also provides a means of very rapid filling where high-capacity hydrants are available. By connecting a standard sort sleeve from the hydrant to this 4½” connection, fill rates up to 2,500 gpm are possible.
Another means of filling the tanker—and the one used in most cases— is through a 2½” opening directly into the tank with an NST female coupling mounted on it and a petroleum-type quick-connect adapter normally in place on it. There is no external shutoff valve on this inlet, but there is a gravity-operated check valve on the inside of the tank. When the tank is full, the w eight of the water keeps the valve shut; when it is empty, it remains closed due to the weight of the valve. When pressure is applied from the fill line, the valve opens; when the supply is shut off. the valve closes automatically. This arrangement prevents the valve on the tanker inlet from being closed before the one at the source. When this happens, the fill line may be pressurized when an attempt is made to disconnect it, and personal injury may result when the connection is broken. With this type of valve, any time there is pressure on the line the valve remains open, and it is impossible to leave the supply line pressurized when shutting down.
VENTING
Due to the extremely high fill and dump rates that this tanker is designed for, venting is very important. Considering the fact that the large gravity dump area approximates 100 square inches, it would seem that the area of the vent should approach 300 square inches. This amount of venting should prevent damage to the tank by preventing a pressure buildup when pumping into it, even with a relatively high pressure applied to the fill line. It also allows for a maximum dump rate by allowing the air to enter the tank freely to replace the water as it discharges without developing a vacuum. It is constructed to minimize spillage through the vent or overflow^ pipe as the vehicle moves over the road.
A tower approximately 8 inches high and open at the top is in the exact center of the tank. This opening is approximately 24″ by 30″ for a total area of more than 700 square inches. To minimize slosh and spillage, offsetting baffle plates are located at the top of the vent tower and at the bottom where it mounts to the top of the tank.
Mounting the vent in the center of the tank minimizes overflow, regardless of the change in vehicle direction. This is as close to the neutral area of the surface of the water as possible. What tendency there is for the water to splash out is further reduced by the baffle plates. Experience has proven that it is possible to fill this tank to overflowing and drive 10 miles without losing more than 15 gallons.
My tests indicate that this vehicle, through a combination of the type of quick dump, the ability of the baffles to transfer water and air, the construction of the trough, and the type of vent, carries more than 99 percent usable water. At the same time, it has enough baffling so that it handles very well over the road, even when the tank is partially filled.
PUMP AND ACCESSORIES
While this tanker is primarily a quick-dump tanker for use with folding tanks, there are times when it will have to sene other purposes. One example is a relatively small fire where no water shuttle is needed hut the tanker is required to replenish the water tank on the attack pumper. Or it might be used in a water shuttle with a nurse tanker as the water supply. In either case, it would be desirable for it to be able to pump off. A minimum acceptable flow rate might be 400 gpm. This would give it the capability of supplying a single 3″ line at its full capacity and supporting an initial attack of the magnitude that is recommended in NFPA 1410, A Training Standard on Initial l ire Attack. 1988 edition.
lhe maximum discharge pressure is not important since this vehicle is designed as a water supply tanker, and no consideration is given to the ability to supply any type of attack line. The tanker has a Gorman Rupp PTO centrifugal pump model 03H1-GL. Hiis pump is not designed primarily for the fire service but rather for pumping petroleum products. It does have several important advantages for this purpose: It is constructed of lightweight materials and requires a minimum of maintenance; it is designed for extended use in commercial applications and can be adapted easily for moving large volumes of water; and it is rated to supply 400 gpm at a net pressure of 90 psi for use on a tanker.
While this pump is self-priming in normal applications, it requires a separate primer for drafting. The standard method of operation for a water supply tanker involves loading at a fill site with either a pumper or some type of pressurized supply tor loading, but it may be desirable to provide drafting capability as well. To minimize weight as well as cost, I took advantage of the vacuum pump that is used in the braking system of this chassis. It has a Darley vacuum primer and a shut-off valve in the line from the vacuum reservoir for the brake system. At a five-foot lift using 2 ½” suction hose, the tank fills in less than seven minutes—a fill rate in excess of 250 gpm.
One 2 1/2 discharge outlet equipped with a full-flow ball-type valve is mounted on each side of the body directly behind the cab. A 2½” intake line on each side of the vehicle is used for drafting or pumping into the tanker. Neither of these openings is equipped with a valve.
A 2 ½” line is connected from the discharge side of the pump into the front of the sump through a gate valve. This provides a tank fill line for filling from the pump. It also provides a means of cooling for the pump when it is operating with periods of intermittent flow such as may happen when providing a direct supply to a pumper making an attack on a fire. Normal operation is for this valve to be left open one or two turns— enough to provide an adequate flow through the pump without seriously subtracting from the pump’s capacity.
Since the pump does have drafting capability, it has full instrumentation. A compound gauge is connected to the intake as well as the discharge of the pump. This indicates the amount of flow from the tank and gives a warning before the pump goes into cavitation if the maximum flow rate from the tank to the pump is exceeded.
An MC Products five-light tanklevel gauge has two sets of indicator lights. One set is on the pump panel for the convenience of the pump operator. The other set is at the rear of the tank in the vicinity of the dump outlet and the rear fill opening, useful when dumping into a folding tank or filling the tank from the rear. It gives the operator some advance warning when the tank is nearly empty while dumping or when it is nearly full when filling.
BODY AND EQUIPMENT
A bare minimum of compartment space is included, as the primary consideration is weight—both of the body and the equipment. Compartments are behind the rear wheels, one on each side of the vehicle. A long hosebed, 6″ wide and the full length of the tank, is on the left side. A compartment designed to carry a 2.000-gallon folding tank is on the right side. The compartment door has brackets so that when it is opened the tank fits on the door. When the door is closed, the tank is carried inside the compartment upside down to allow more complete draining.
The vehicle has a rear step. This is not for personnel to ride on but for safety when dumping and for access to the top when needed. A handrail is on the rear of the tank so that the operator can get up on the step to avoid getting caught between the vehicle and the tank when it is moving into position to dump into a folding tank. Steps are provided for climbing up on the tank, and a handrail around the top of the tank provides a greater margin of safety if it is necessary to get up on top.
GPM RATING
When evaluating this tanker for its capacity to move water, I used two different methods of operation. For a maximum flow, I tested it by using the dump outlet for off loading and a hydrant that filled at a rate of 900 gpm. I calculated travel time using the ISO standard of 35 mph over a twomile round trip. Tests show that this tanker is capable of supplying a continuous flow of 236 gpm in this mode of operation.
The worst-case use of this tanker would be where it would have to load itself from draft and pump off into another tank. Assuming this worstcase scenario and the same travel time calculations as before, it would provide a continuous flow of 118 gpm.
Variations between these two extremes would occur with different conditions and methods of operation. Longer or shorter distances also would affect the tanker’s water supply capability.
This tanker can provide a maximum of water supply capability at a minimum price. It is difficult to imagine a fire department anywhere that couldn’t afford a tanker of this type and that couldn’t improve its fire attack capability by putting such a tanker to use
