Hard suction hose
Hard suction hose (alternatively, suction hose) is a specific type of fire hose used in drafting operations, when a fire engine uses a vacuum to draw water from a portable water tank, pool, or other static water source. It is built to withstand vacuum, rather than pressure, abrasion, and heat. In the United States, it is standard equipment according to the National Fire Protection Association standards for fire engines. It is used in both structural and wildland firefighting throughout the world, and is made in various diameters and connection types.
Hard suction hose, also known as a suction hose, is a specific type of rigid fire hose used in drafting operations. When using this technique, the fire engine draws water for fire fighting from an unpressurized supply, such as a portable water tank, pool, or other static water source, instead of drawing water from a pressurized water supply, such as a fire hydrant. Under complete vacuum conditions, a pump can lift water 33.9 feet (10.3 m), but "no pump on a fire apparatus is capable of producing a perfect vacuum".:388 Due to this limitation and friction loss, fire services do not recommend attempting to lift water more than 3 metres (9.8 ft).
Hard suction hose may also be used to connect a fire engine to a hydrant. This practice is common in Australia, but woven jacket fire hose is normally used for this purpose in the United States. Using hard suction hose in this manner requires that the fire engine be positioned carefully for the limited flexibility of the hose, and may require multiple firefighters due to the weight and bulk of the hose section. In certain circumstances, it may be possible to use hard suction hose to draft from a hydrant with adequate water supply but a low flow rate.
These hoses are built to withstand vacuum, rather than water pressure, abrasion, and heat. They are, therefore, constructed differently to the standard fire hose and have different end fittings. In the United States, they are standard equipment according to the National Fire Protection Association standards for fire engines. Since hard suction hose cannot be folded, it is typically mounted on the side of a fire engine, often opposite ground ladders. Such hose is used in both structural and wildland firefighting throughout the world.
Hard suction hose can also be used to extend the range of a water tender's dumping capability, if the fittings of the hard suction hose match those of the tender dumping water. This could be used to fill a portable dump tank not immediately adjacent to the water tender, from which another fire engine can draft. Since water movement during dumping relies on gravity, rather than mechanical pressurization, hard suction hose is useful for this task.
Characteristics and construction
Hard suction hose differs from standard fire hose in both its construction and usage:
- It is rigid and generally scalloped. Often, such hose is manufactured using molded synthetic material (such as PVC) shaped into a series of rings. This shape allows the hose to be somewhat flexible, without allowing it to collapse in on itself under suction, as a normal, woven-jacket fire hose would.
- It uses suction gaskets. Gaskets in standard fire hose ("pressure gaskets") are designed to minimize the water leaking out between couplings. The gaskets connecting hard suction hose sections, pump, and strainer must instead prevent air from entering at the coupling, since that would spoil the vacuum and allow air into the pump intake. Large diameter (4-inch (10 cm) or greater) hard suction hose will typically use Storz fittings, which are genderless. 3-inch (7.6 cm) or smaller hard suction hose will typically use threaded fittings. In each case, the hard suction hose connection will match the fittings of the pump intake and supply hose, so that hard suction hose can be used in place of supply hose as appropriate.
- It is short. Typically, hard suction hose comes in 10 feet (3.0 m) lengths, while fire hose comes in 50 and 100 feet (15 and 30 m) lengths. Since a fire engine's pump only produces a partial vacuum, it is only recommended for lifting water 3 metres (9.8 ft) or less. For this reason, and because each junction is an opportunity for a crack or imperfect seal to spoil the vacuum, it is rare to see many sections of hard suction hose connected together.
- It is not designed for use in fire streams. The airtight nature of hard suction hose, necessary for drafting, renders the hose unsuitable for the high pressure water flow needed to spray a pressurized stream of water on a fire. Thus, hard suction hose is tested for the ability to "prevent collapse under vacuum conditions" rather than its ability to function as an attack hoseline.
Hard suction hose predates steam or gas powered fire engines and has been available since at least 1888, sometimes referred to as "spiral suction hose".
Hard suction hose comes in multiple sizes, from 2 to 6 inches (5.1 to 15.2 cm) in diameter. Large diameter hose are carried on full-size fire engines, but smaller diameters of hard suction hose can be carried on apparatus with smaller fire pump ratings, such as wildland fire engines. In the United States, NFPA 1901 requires engines to have hard suction hose that matches the engine's pump rating. For example, an engine with a 1,000-US-gallon (3,800 l) per minute pump is required to carry 5-inch (13 cm) or larger hose,:181 while a wildland fire engine will typically carry 2-to-2.5-inch (5.1 to 6.4 cm) hard suction hose.
The United Kingdom, has a standardised range of metric hose diameters: 7, 9, 12.5 and 15 cm (2.8, 3.5, 4.9 and 5.9 in), with the two smallest diameters sometimes used twined to provide adequate flow rates. Within this range, the 7 centimetres (2.8 in) hoses are fitted with "Instantaneous Couplings"; the 12.5 and 15 centimetres (4.9 and 5.9 in) hoses have Storz couplings; and the 9 centimetres (3.5 in) hose comes in two types, one with "Instantaneous Couplings" and one with Storz fittings. Fire Service Manuals provide tables of maximum volumetric flow rates for a given pump pressure, hose diameter and total hose length. The use of these tables, is to facilitate the selection of suitable hoses, taking into account the frictional losses caused by transporting water through the strainer, hoses, pump and fire fighting nozzles, to draft water from source to the fireground.
When being used in a drafting operation from a pool, portable water tank, or other uncovered water source, the length of hard suction hose farthest from the pump is usually attached to a strainer, to keep foreign objects in the water from being pulled into and damaging the pump. If used in a pond, stream, or other body of water, an appropriate flotation device must be used to keep the strainer below the surface and above the bottom, so that neither mud nor air are sucked into the fire engine's pump. When drafting from a solid-bottomed tank or pool, a strainer can safely rest on the bottom without suctioning mud into the fire engine's pump.
The United Kingdom official guidance is to:
- use ropes to take to the weight of the hose and the strainer, in order to avoid putting sideways loadings on the couplings;
- ensure that the strainer is a minimum of three strainer-diameters below the water surface to prevent air cavitation;
- to support the hose with packing, where necessary, when it crosses a wall or other obstruction to prevent an air pocket being formed.
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- Strainer pressure drop calculator Calculate pressure drop across a strainer.