Fire Hydrant

Background

A fire hydrant is an above-ground connection that provides access to a water supply for the purpose of fighting fires. The water supply may be pressurized, as in the case of hydrants connected to water mains buried in the street, or unpressurized, as in the case of hydrants connected to nearby ponds or cisterns. Every hydrant has one or more outlets to which a fire hose may be connected. If the water supply is pressurized, the hydrant will also have one or more valves to regulate the water flow. In order to provide sufficient water for firefighting, hydrants are sized to provide a minimum flowrate of about 250 gallons per minute (945 liters per minute), although most hydrants can provide much more.

The need for fire hydrants developed with the advent of underground water systems. Prior to that time, water was obtained from easily accessible public wells or ponds. During the 1600s, London, England, began installing an underground water system using hollowed-out logs as pipes. When there was a fire, firefighters had to dig up the street and bore a hole in the wooden pipes. Later wooden plugs were inserted into pre-drilled holes at fixed intervals along the log pipes to make it easier for the fire-fighters to get water. This gave rise to the term fire plug, which is still sometimes used to refer to a hydrant.

As cities grew, so did their water systems. Larger systems meant increased pressures, and cast iron pipes were laid to replace the rotting wooden logs. When Philadelphia's new water system commenced operations in 1801, it not only served 63 houses and several breweries, but it also had 37 above-ground hydrants for fire protection. The first fire hydrant in New York City was installed in 1817 by George Smith, who was a fireman. He wisely located it in front of his own house on Frankfort Street.

Following the earthquake and fire that devastated San Francisco in 1906, the city installed an extensive emergency water system that is still in use. In addition to more than 7,500 hydrants connected to standard-pressure water mains, the system includes a reservoir and two tanks located on hills to supply nearly 1,400 high-pressure hydrants throughout the city. There are also two salt-water pumping stations to draw water from San Francisco Bay, plus five additional connections along the waterfront to allow the city's fireboats to pump into the hydrant system. As a final line of defense, the city has over 150 underground cisterns connected to unpressurized hydrants. Fire pumpers can connect a rigid suction hose to these hydrants and pull the water out of the cisterns by creating a vacuum.

Today, the size and location of fire hydrants in an area affect not only the degree of fire protection, but also the fire insurance rates. In many urban areas the lowly fire plug is all that stands between the first spark and a multi-million-dollar fire loss.

Types of Hydrants

There are two types of pressurized fire hydrants: wet-barrel and dry-barrel. In a wet-barrel design, the hydrant is connected directly to the pressurized water source. The upper section, or barrel, of the hydrant is always filled with water, and each outlet has its own valve with a stem that sticks out the side of the barrel. In a dry-barrel design, the hydrant is separated from the pressurized water source by a main valve in the lower section of the hydrant below ground. The upper section remains dry until the main valve is opened by means of a long stem that extends up through the top, or bonnet, of the hydrant. There are no valves on the outlets. Dry-barrel hydrants are usually used where winter temperatures fall below 32° F (0° C) to prevent the hydrant from freezing.

Unpressurized hydrants are always a drybarrel design. The upper section does not fill with water until the fire pumper applies a vacuum.

Raw Materials

The hydrant barrel is usually molded in cast or ductile iron. Some iron wet-barrel hydrants have an epoxy coating on the inner surface to prevent corrosion. Other wet-barrel hydrants are molded in bronze. The hydrant bonnet is usually made from the same material as the barrel. The valve stem in a dry-barrel hydrant design is steel. The valve stems in a wet-barrel hydrant are usually made from silicon bronze.

The hydrant outlets are molded in bronze. If the barrel is cast or ductile iron, the bronze outlets are threaded into the barrel. If the barrel is bronze, the outlets are cast as part of the barrel. The outlet caps may be bronze, cast iron, or plastic.

Valve seats, seals, and gaskets are made from a variety of synthetic rubbers including styrene butadiene, chloroprene, urethane, and butadiene acrylonitrile. Fasteners may be zinc-plated steel or stainless steel.

Hydrants are given a coat of primer paint before they are shipped. When a hydrant is installed, the outer surface is coated with an exterior-grade paint.

Design

The basic design and construction of pressurized fire hydrants in the United States are defined by the American Water Works Association (AWWA), which sets general standards for hydrant size, operating pressure, number of outlets, and other requirements. Unpressurized hydrants may be the same design as the pressurized hydrants within a city or fire district in order to maintain commonality, or they may be a simple capped pipe design with no valves.

The main body of the hydrant is called the barrel or upper standpipe. It may consist of a single piece or it may be made in two pieces. If it is made in two pieces, the upper portion with the outlets is called the head and the lower portion is called the spool. This terminology is not exact and varies from one manufacturer to another, as well as from one city to another.

The hydrant outlets usually have male National Standard Threads (NST) to mate with fire hose couplings. The smaller outlets, sometimes called the hose nozzles or connections, are 2.5-inch NST. The larger out-lets, sometimes called the steamer nozzles or connections, are 4-inch or 4.5-inch NST. The outlet caps are secured to the hydrant body with short lengths of chain. The terms hose connection and steamer connection date back to the 1800s. Before the advent of modern fire apparatus, minor fires were often fought by connecting a single hose line directly to the smaller outlet on a pressurized hydrant. If the fire was larger, a steam-powered pumper, called a steamer, took water from the larger hydrant outlet and pumped it into several hose lines.

The hydrant valves are actuated by turning metal stems. The portion of each stem that protrudes from the exterior of the hydrant is pentagonal shaped and is called the operating nut. This five-sided nut requires a special wrench to turn and helps prevent unauthorized use. On some hydrants the operating nut is a separate piece that slips over the stem. This allows the nut to be replaced if it becomes worn from use.

Some dry-barrel hydrants include a break-away feature to allow easy repair if the hydrant is struck by a vehicle. This design includes a breaker ring on the barrel of the hydrant near the ground and a breakable coupling on the valve stem inside the hydrant. When struck, the upper barrel and stem snap free without disturbing the under-ground piping or valve.

Although the basic components of all fire hydrants are similar, the shape of hydrants

Fire hydrants are made through a process of metal casting. Once manufactured, each hydrant is filled with water and pressurized to twice the rated pressure to check for leaks.

The Manufacturing
Process

Making a fire hydrant is primarily a metal-casting process, and most hydrant companies are metal foundries that specialize in manufacturing a variety of municipal water works components.

Here is a typical sequence of operations for manufacturing a wet-barrel fire hydrant.

Forming the molds

• 1 The outer surface of a mold is formed by a piece called the pattern. To make a hydrant pattern, the hydrant's outer shape is generated in three dimensions on a computer. This data is fed into a stereo lithography machine, which uses laser beams to harden liquid plastic into the shape of the hydrant. This hardened plastic piece is used to make multiple copies of left and right pattern halves out of rigid polyurethane.
• 2 The inner surface of a mold is formed by a piece called the core. To make a hydrant core, the hydrant's inner shape is machined into two halves of a block of aluminum or cast iron to form a cavity. The two halves are clamped together, and the cavity is filled with a mixture of sand and a plastic polymer. When the block of aluminum or cast iron is heated gently, the polymer hardens the sand to form the core. The block is then opened, and the core is removed. This process is repeated to make multiple cores.

Casting the barrel

• 3 When a production run of hydrants is O ready to start, the patterns and cores are brought to the mold-making machine. The left and right patterns are pressed into the two halves of a mold filled with sand to form impressions in the shape of the outer surface of the hydrant. Molding sand is a special mixture that holds its shape without crumbling. The hardened sand core is then carefully laid on its side and held with short spacers to form a cavity between the core and the impression in one of the mold halves. The other half of the mold is put in place over the core and the mold is clamped together. This process is repeated for each hydrant.
• 4 Molten metal is poured into each mold through an inlet passage called a gate. Pouring continues until the metal starts to rise through outlet on the opposite side called a riser. As the molten metal hardens, it cooks the polymer in the core sand. This raises the temperature of the polymer far beyond its initial setting point and causes it to break down and allow the sand to become loose again.
  

  Sideviews of a dry barrel and wet barrel hydrant.
• 5 After the casting has completely hardened, the mold is split apart and the core sand is dumped out. The casting is placed in a horizontal cylinder filled with small metal pellets and tumbled to remove any small bits of metal or molding sand that may have adhered to the casting.
• 6 The cast gates and risers are cut off with an abrasive cut-off saw, and are returned to the furnace. The cast barrel is ground with a handheld power grinder to remove any rough surfaces.
• 7 If the hydrant has a two-piece barrel, the / head and spool are cast, ground, and finished separately. If the hydrant is made from cast or ductile iron, the outlets are cast, ground, and finished separately in bronze.

Machining the barrel and valves

• 8 The entire hydrant is fixed lengthwise in a lathe, and shallow concentric grooves are cut into the face of the lower flange. This allows the flange to seal against a gasket when the hydrant is mounted. The flange bolt holes may be drilled at this point or they may be drilled just before shipment.
• 9 If the barrel is a two-piece design, the lower portion of the head has National Pipe Taper (NPT) threads cut on the inside and the upper portion of the spool has NPT threads cut on the outside to allow the two pieces to be joined. The head is drilled and tapped on one side in the area of the NPT threads to hold a locking set screw.
• 10 The hydrant—or the head, if it is a two-piece design—is repositioned cross-ways in a lathe along the centerline of the larger outlet. A rotating piece, called a fixture, clamps the hydrant in place and provides a counterbalance as the hydrant is spun. The lathe bevels the inner surface of the barrel around the outlet opening to provide a smooth seating surface for the valve disc. The opening for the valve stem insert is drilled and threaded. Finally the outlet or outlet opening is threaded. This process is repeated for each of the outlets.
• 11 The valve stems, valve stem inserts, and valve disc holders are machined, and threaded separately.

Assembling the hydrant

• 12 Starting with the upper valve, an oring seal is placed over the valve stem, and the stem is threaded into the stem insert. The inside end of the stem is pushed through the stem insert opening, and the disc holder, rubber disc, and locking nuts are reached up inside the barrel, threaded onto the stem, and locked in place with a set screw. The stem insert is then threaded into the barrel, and the replaceable operating nut is slipped over the outside end of the stem and held in place with a nut. This process is repeated for each of the valves.
• 13 If the barrel is a two-piece design, an oring is slipped over the threaded portion of the spool and the assembled head is screwed down to seal against the oring. The threads are locked in place by a set screw.

Testing the hydrant

• 14 The AWWA standards require that bronze hydrants be rated at 150 psi (1,034 kPa), and ductile iron hydrants be rated at 250 psi (1,723 kPa). Each hydrant is filled with water and pressurized to twice the rated pressure to check for leaks.

Preparing for shipment

• 15 After the hydrant is pressure tested, the outlet caps and chains are attached, a plastic protector is slipped over the bottom flange, and the exterior of the hydrant barrel is given a coat of primer paint.

Quality Control

All incoming material is inspected to ensure it meets the required specifications. This includes spectrographic analysis of the raw materials used to make the castings. The moisture content of the molding sand is critical to the casting process, and it is checked before every casting run. When a run of castings is machined, the first piece is checked for proper dimensions before the remainder of the castings is machined.

The Future

It is unlikely that the fire hydrant will disappear from the urban landscape anytime in the near future. Water is still the most cost-effective fire suppressant, and the hydrant is still the most cost-effective way to provide a ready supply of water. If anything, the fire hydrant will gain importance as fire departments and taxpayers alike realize that strategically placed, high-capacity hydrants can significantly reduce fire insurance rates.