Departments that protect areas without hydrants often rely on water from ponds, cisterns and other sources. For transfer distances of about 2,500 feet or more, water tender/tanker shuttles are commonly used. For shorter distances, large-diameter hose may be able to provide more water with less personnel.

Before departments strip the old 2½inch supply hose off their engines and replace it with 4- or 5-inch hose, however, there are several factors to consider. Larger hose produces larger flows, but it also costs more, weighs more and takes more hosebed space. In some cases, larger hose takes so long to fill that it can hurt a department’s Insurance Services Office rating, rather than help it.

Here is a short guide to understanding large-diameter hose performance.

Pressure drop. Basic hydraulic theory says that if you double the diameter of the hose, you can quadruple the flow rate with the same pressure drop. In actual practice, the pressure drop of a specific diameter and brand of hose depends on the interior surface, diameter under pressure, coupling size and other factors. Despite these variations, moving up to LDH still can give you a big change in flow for a little change in size.

Cost. LDH cost is proportional to the diameter — double the diameter and you double the cost. As with pressure drop, the actual cost depends on jacket construction, linings, couplings and other factors. Usually, the increase in cost is more than off-set by the significant increase in flow.

Weight. The weight of LDH is also roughly proportional to the diameter — again, if you double the diameter, you double the weight. Specifying LDH with heavy threaded couplings instead of light Storz couplings can significantly increase the weight. Depending on the hose selected, switching to LDH can increase the apparatus rear axle load as much as 500 pounds. Departments that already carry a full amount of equipment need to consider this.

Hosebed volume. Like cost and weight, the required hosebed volume increases directly with the diameter of the hose. This usually isn’t a problem, but apparatus built to minimum standards may not be able to carry a full load of LDH. Resist the temptation to heap the hose loosely on top — it’s not safe. All hose must be fully contained on the sides, front, rear and top to prevent accidental deployment.

Hose fill time. This factor is often overlooked. If the hose runs uphill slightly, the entire hose may have to be filled before any appreciable water is delivered to the fire scene. This may add several minutes to the time required to establish a steady water supply. ISO gives departments five minutes to start flowing water after the first unit arrives on scene and another 10 minutes to increase it to the required flowrate — every minute counts. Smaller-diameter hoses take less time to fill, all other factors being the same.

Hose trapped volume. This is directly related to the hose fill time. Departments operating from static water sources with limited capacities need to understand that a significant amount of the supply water will be trapped inside the hose. In some cases, as much as 1,000 to 2,000 gallons of water remains in the hose and is not available for firefighting. Smaller diameter hoses trap less water.

Personnel. Generally speaking, it takes far less personnel to establish a water supply with a large diameter hose than it does to operate a tanker/tender shuttle. This makes LDH attractive for departments with a limited number of responding personnel where a hydrant or other water source is close by. Granted, it takes a lot of personnel to pick up, re-pack and clean the hose after the fire, but the initial hoselay and hook-up requires only a few people.

Fittings. Switching to LDH also requires changes to the pump intake fittings, as well as additional adapters, manifolds and other devices. Lightweight Storz fittings are commonly used on 5-inch hose, Storz or threaded couplings may be used on 4-inch hose.

Application. ISO assigns one classification for structures within 1,000 feet of a hydrant and another classification for structures beyond that distance. A rough rule of thumb is that for distances up to 1,000 feet, a 4-inch hose can deliver about 500 gpm from a hydrant, and a 5-inch hose can deliver up to 1,000gpm. Actual flowrates vary according to the hose, hydrant pressure when flowing and several other factors. For distances of 1,000 feet to 2,500 feet, most departments use 5-inch hose and place an engine in the middle to boost the pressure.

Charged LDH lines contain a lot of stored energy and represent serious safety hazards. Current NFPA standards require that all pump intakes with 3½-inch or larger connections shall be equipped with pressure relief devices, and all pump discharges larger than 2½ inches shall be located away from the pump operator panel.

When calculating pump discharge pressures for long hoselays, remember that most LDH is rated for a maximum working pressure of about 185 psi — running the pump pressure up further to increase the flow may result in some serious and spectacular hose failures.