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The Dry Hydrant Concept

Diagram of the Dry Hydrant Concept

Dry Hydrant Advantages

Having water available in area streams, ponds and cisterns helps a fire department only if the water is readily accessible. The needed water may be located so far away from where it is needed that a fire department's ability to control the fire is impaired. Mobile water supply vehicles can move water from distant sources, but the critical factor is whether or not the fire department can maintain an uninterrupted supply of a predictable rate of water at the fire scene.

Installation of dry hydrants into nearby and developed water supplies eliminates the inefficiency and complexity of long-distance water shuttle operations. This arrangement also allows access to water sources from a roadway instead of having to work on soft ground immediately adjacent to the pond or stream.

In any area without water mains and domestic fire hydrants, the dry hydrant concept can provide a simple, cost-effective solution to the need for rapid access to water sources. A dry hydrant consists of an arrangement of piping with one end in the water and the other end extending to dry land and available for connection to a pumper. Dry hydrants have the following features:

  • Use a non-pressurized pipe system.
  • Use relatively inexpensive piping materials and other supplies.
  • Are permanently installed in existing lakes, ponds, streams and cisterns.
  • Provide a means of access whenever needed, regardless of weather.
  • Allow years of simple operation with a minimum of maintenance.

The time savings are many. Multiple lengths of hard suction hose extending to the water are not needed; usually one section to the dry hydrant is enough. The strainer is also permanently attached, saving more time. Fewer people are needed to make a hookup compared to make a conventional direct drafting hookup.

When a strategically placed dry hydrant with all-weather road access allows more water to be distributed in less time, and the water can be applied effectively on the fire, fire fighter effectiveness and safety is improved.

Savings can be financial as well. Fire departments save money by reducing fuel and equipment costs through shorter transportation distances and lower operational demands. Communities can preserve more of their treated water supplies, since dry hydrants use untreated water.

Photo of Hydrant

Planning

Planning for dry hydrants involves several considerations and should involve all affected agencies and individuals so a coordinated effort can take place. Some factors to consider are:

  • Current and future population and building trends.
  • Property values.
  • Potential for loss.
  • Fire history of the area.
  • Current water supply systems.
  • Other potential water sources.
  • Cost of project.
  • Equipment and personnel of the local fire department.
  • Training needs of the fire department.
  • Other specific factors of local concern.

Dry Hydrant Location

The location of individual dry hydrants is influenced by several factors. After conducting a water source survey of the area, use a county road map to mark potentially good sites. Mark them in priority order, since most fire departments will not have sufficient funding for all the dry hydrants that may be preferred. Consider these factors:

  • Maximum distance of travel between dry hydrants. This can vary for several reasons, but a target distance could be one dry hydrant every 3 square miles. This would produce a travel time of about 6 minutes between the water and the fire, assuming an average safe constant speed for a loaded truck of 35 mph.
  • Ownership of the land. The authority should contact the legal property owner to secure written permission (in conjunction with the town or county attorney) to use the water source. If a possible dry hydrant site is along a road right-of-way, you will need town, county or state approval. In some cases US Army Corps of Engineers approval may also be needed. Obtaining written permission is an important requirement that may take some time.
  • Depth of water at the source. Careful note should be made about the useful depth of a lake or pond, measured from the minimum foreseeable low-water surface level to the top of the suction strainer (not the bottom of the lake). The low-water mark considers drought, freezing and other effects, such as where the water level is lowered to generate power. The absolute lowest level must be not less than 2 feet, to prevent a vortex or whirlpool which could allow air to enter the pump and cause loss of pump prime. You may need a minimum of 4 to 5 feet of water over the suction screen and pipe during low water to prevent a freeze-up of the screen.
  • Composition of the bottom material. For long-term useful hydrant operation, the best composition for the bottom of a lake, stream or pond is sand, gravel or rock or a combination of these. Decaying vegetative matter could clog the suction screen.
  • Ease of digging. A backhoe will need to get close enough to the water's edge to reach out and dig at least 5 feet below the surface of the water to start the trench.
  • Protection of the connection. A location that is conveniently accessible to fire apparatus may also be exposed to accidents from other passing vehicles. An impact barrier constructed of partially buried posts may be needed to prevent a vehicle from destroying a dry hydrant in a heavily traveled area. Special markings may be necessary to avoid damage from snow plows.
  • Costs. The expense of a dry hydrant installation depends on local practices and the length of pipe needed. The Colorado State Forest Service has estimated the cost there to be between $500 and $1200, including the cost for contractor labor and machines.
  • Beware of other utilities in the digging area. You must carefully check for the presence of buried lines and pipes and notify utility companies before you start digging.

Design Features Checklist

Design factors are affected by:

  • Desired flow from the hydrant in gallons per minute
  • Suitability of pipe materials
  • Size and type of fire apparatus pumper that is available

The following design features are suggested for dry hydrants using PVC pipes:

  • Use minimum 6-inch-diameter pipes, schedule 40 pipe.
  • Prime and paint all exposed pipe.
  • Use a minimum of elbows.
  • properly joint and cement all connections.
  • Purchase or construct a suction screen with adequate hole openings. The total area of strainer holes must exceed 4 times the area of the diameter of the pipe.
  • Installation depth must be below the frost-free depth for the area. (Consult local university extension service or Larimer County Building Department for frost depth.
  • Install dry hydrant as close as practical to the water source.
  • A flow of 600 to 1000 gpm from the hydrant is desirable.
  • Avoid designs with lifts in excess of 12 feet. (Above this height vapor pressure will begin to exceed atmospheric pressure and cavitation will occur, making pumping virtually impossible.)
  • Place the pump at a higher elevation than the hydrant connection. (This will eliminate air bubbles which will limit flow, and also prevent the operator from getting wet.)
  • When rock is encountered, installation must be adjusted to fit the rock profile. (Additional bends may be necessary.)

Other considerations:

  • Each elbow in a dry hydrant installation increases friction loss. Try using 45 elbows and a minimum number of 90-degree elbows.
  • An access road with a minimum width of 12 feet and a maximum grade of 8 percent.
  • Does the site have proper drainage?
  • Avoid vertical lifts of more than 12 feet with other than Class A pumps.
  • PVC piping greater than two full sections will require additional personnel for installation.

Construction of Dry Hydrants

Installation starts with arranging for a large backhoe and at least three people to handle and place the pipe in the trench. An ideal time to start installation is in the late summer when the water is warm and usually at its lowest level. Haul clean fill material to the site. Choose material that will not wash out easily. Follow these installation steps:

  • Dig the trench. Mark the backhoe arm with a ribbon to indicate the desired vertical depth. (This is helpful only when the ground is level.) Start excavating the ditch in the water and complete the entire horizontal section of the trench. Keep the bottom of the trench level all the way to the hydrant. (It is less complicated to maintain a level trench rather than a sloped one which requires figuring correct angles of joints.)
Photo of Backhoe Digging Trench
  • Cut the pipe to the desired lengths and assemble. Check dry fit. As a rule of thumb, 6-inch-diameter pipe will not flow 1,000 gpm at horizontal lengths greater than 100 feet.
  • Prepare the joints. It is better to use two 45-degree elbows for the riser joint instead of a single 90-degree elbow. If your hydrant connection is later broken off accidentally, the wider sweep of the 45-degree elbows would allow you to insert a 2 1/2-inch suction hose into the pipe. (Some may prefer to apply primer at this point, before glueing the joints.)

 

Photo of Men Preparing Joint Photo of Men Gluing Pipe
  • Join the pipe sections with glue. Make sure you understand the technique, because timing is important. Use PVC cement; never use all-purpose cement to join PVC pipe and fittings. Joints must be held tightly together until both surfaces are firmly cemented. Do not disturb the joint until initial set occurs, which varies according to the temperature. Above 60 degrees, the recommended time is at least 30 minutes. Decrease the chance for an air leak by taping the joints with a rubberized, adhesive-backed wide tape. Attach the strainer with a collar or sleeve so that it can be removed if necessary.
  • Pressure test the joint only after adequate curing according to the instructions for the particular cement. Do not take short cuts!
  • Carry the prepared piping to the trench.
Photo of Pipe Being Laid in Trench
  • Force the strainer under water until it fills the pipe. If more than 8 feet of pipe is out in the pond, a support bracket behind the strainer is a good idea. Support can be as simple as stacked concrete blocks. The strainer must be above the bottom of the pond so that the strainer holes will not be clogged with mud or other debris. Proper placement is necessary for successful operation of the dry hydrant. 
  • Backfill around the pipe assembly, starting with the riser, which should be covered during this operation to prevent rock and fill from falling into the pipe. Tamp the dirt for rigid support. Mound the fill material higher for more freeze protection. If extra insulation is needed, install a styrofoam barrier around the pipe 2 to 3 feet under the surface.
  • Cut off the top of the riser after measuring the distance from the bottom of the intake on the pump (positioned as it would be for pumping) to the ground. Cut off the riser so that when you attach the hydrant connection to the riser, the top of the opening of the hydrant connection is lower than the bottom of the pump intake. That is, the pump intake must be above the hydrant connection.
  • Plant grass seed or other vegetation over the disturbed areas to retard erosion. Mulching helps the seed or seedlings to get established.
  • Add any needed suction hose support (especially if using 45-degree elbows) remember that connecting a suction hose and drafting through the dry hydrant connection places a lot of stress on the hydrant connection. 
  • Place a sign to identify the dry hydrant and warn people against parking or obstructing access. Paint the cap a reflective color for improved visibility during emergencies. If the exposed PVC is not sunscreen protected, the pipe must be painted to prevent chemical decomposition from ultraviolet light.
Photo of Cap Painted Red

Maintenance and Training

New installations should be initially flushed to ensure removal of any debris that could be harmful to pumps.

Dry hydrants require quarterly inspection, testing and maintenance. More frequent cleaning may be needed at streams and ponds to make sure that silt and aquatic growth do not clog the water intake. Aquatic growth can be a special problem in ponds and in slow-moving water sources in some parts of the country. In extreme cases it may be necessary to drain a pond to control the growth. This will require careful timing and good communications to assure that other water sources are available for emergencies and that the pond will refill without undue delay.

Photo of Hydrant Being Tested

Hydrants should be tested with a pumper once a year and backflushed as part of training exercises. Pay particular attention to safety-related features, such as warning signs and bumper guards.

Appearance is another consideration. Grass and vegetation will need to be kept trimmed. Repainting will be needed periodically. Maintaining the grounds around the dry hydrant assures better visibility in an emergency, and it will help keep good relations with the landowner.

Records should be kept of all inspections and procedures. Keep the records available with the maps showing the location of all installations. NFPA 1231 presents a sample maintenance record that may be adapted and altered for local use, as well as additional design criteria.

Background Image: Rocky Mountain National Park by Sue Burke. All rights reserved.