WATER TANKS FOR APPARATUS
VOLUNTEERS CORNER
Fire departments developing specifications for new fire apparatus that will carry water or those shopping for a replacement tank must do extensive research before making a final decision on which water tank to purchase.
Numerous alternatives are available with regard to construction materials, shape, repair and maintenance, warranty, and cost. The research should include an evaluation of the department’s current and future needs as well as past experience with various kinds of tanks and features.
Begin your study by finding out about the water quality in your community (lor example, which minerals it contains and in what concentrations) and the effect it has on the various construction materials. Among other factors to consider are the materials used for tanks in older apparatus and how they have held up; the incidence of problems associated with corrosion, cracks, welds, leaks around piping, mineral buildup, or other failures or leaks; and the frequency and cost of maintenance and repair and the amount of out-of-service time for apparatus.
SHAPE
Several tank shapes are available, and each has its advantages. Most have specific design opportunities and limitations. Rectangular tanks, for example, may be placed vertically to provide a lower hosebed or to better accommodate a rear engine, or they may be placed horizontally to improve weight distribution. T-shaped tanks lower the center of gravity and the height of the hosebed. V-shaped tanks w ith a sloped bottom have larger capacities and provide efficient unloading. Round tanks facilitate road handling as do elliptical tanks, wTtich also have a lower center of gravity. Rectangular tanks and those w ith Vshaped bottoms usually result in higher hosebeds, but they are ideal for high side compartments. Round and elliptical tanks restrict compartment flexibility and number. The sharper the corners in a tank design, the higher the stresses in the tank. A vertical tank can add flexibility in design to the hosebed, rack storage, or compartmentation.
MATERIALS
The materials most commonly used for tank construction are aluminum, fiberglass, plastic, and steel. Aluminum tanks are lightweight, easily fabricated, and generally corrosion resistant. They can be built with a removable top and are not difficult to repair locally. Problems that may arise include a fatigue factor when the tank flexes, such as when traversing uneven ground, and an electrolysis problem where fittings or piping of dissimilar metals are used. Provide cathodic protection along with protective coatings for this type of tank.
Fiberglass is lighter than metal, is noncorrosive, and does not require much maintenance. The tanks may have a removable top. These tanks can withstand relatively high impact, as do auto and truck parts made of fiberglass, and their manufacturers usually provide extended warranties. The shapes and sizes of the tanks are restricted to the molds available, and there can be a problem with flexing and repair. If fiberglass is used for a foam tank, consider especially the resins used.
Plastic tanks can be polyethylene (molded) or polypropylene (welded). Although low in cost and lightweight, polyethylene is not widely used. This tank has high corrosion resistance, but the partitions and fittings are metal. It is limited in shape and size by available molds and not easy to repair locally. The polypropylene tank is noncorrosive and much lighter than steel. It is not affected by flexing and requires little maintenance. A polypropylene tank can be fabricated, like ferrous tanks, to a variety’ of shapes. Field repair requires trained personnel. The tanks have relatively low impact resistance, require extra mounting supports, and have a fixed top. An extended warranty is common.
Steel tanks may be constructed of mild steel, high-strength steel, or stainless steel. High-strength steel costs about 10 percent more hut is 67 percent stronger than mild steel. Stainless steel has the highest corrosion resistance when protected with a chromium-oxide film, but it is much more expensive and only slightly stronger than mild steel. The advantages of steel tanks are that they are low in cost (except stainless) and can be built in any shape or si/e. They are resistant to impact, can be repaired locally, and have a removable top. Steel tanks weigh more and must be protected against corrosion and electrolysis by hot-dip galvanizing, tank coatings, or cathodic anodes. If anodes are used, they should be accessible from outside the tank and be replaced every five years; they are not needed on stainless steel tanks.
Steel tanks may experience fatigue if flexing occurs, and the water quality can affect the required protective coating. A steel tank may have a shorter life span due to stress cracking. Stainless tanks can crack under stress, be damaged by heavy chlorine exposure, and undergo dissimilar metal corrosion.
Several additional features should be considered. Among them are the following:
- Do the baffles allow adequate air flow at the top and water movement at the hottom for your operations?
- Is there adequate venting at a good position for your filling operation? This is more critical on apparatus with tanks of more than 1,000 gallons (4,000 liters), where loading and unloading rates of 1,000 gallons per minute are required.
- What size, how many, and at what locations do you want inlets or outlets to the tank?
- Do inlets need stream deflectors?
- Does your department require a large-diameter outlet or jet-dumping capability from the tank?
- Do you desire straight runs of pipe to rear discharges through the tank with two extra holes in the tank? You may want to specif) larger pipe when going around the tank.
- A sump and cleanout also arcrequired.
Be sure to check the tank section of NFPA 1901, Pumper Fire Apparatus, or NFPA 1903, Mobile Water Supply Apparatus, as you develop your specifications.
MOUNTING AND CUSHIONING
The mounting method and cushioning of the tank on the apparatus frame also should be carefully considered. The larger the tank, the more important these considerations become. They are critical areas with regard to flexing in off-road operations, handling, and stopping. Remember. a safe apparatus is important, and the tank construction influences the degree of safety.
Before you purchase a new or replacement tank, examine the various possibilities in relation to the needs of your community. You may spend some additional time, but the savings you will gain over the lifetime of the tank will be worth your efforts *
