DRAFTING HYDRANTS FOR HIGH-CAPACITY WATER SUPPLY
Areas not served by public water systems can be equipped with drafting hydrants to provide a reliable water supply. If the incident is close enough to the water source, a pumper can be used to bring water directly to the scene by means of a hoseline or a relay. When farther distances are involved, a pumper can draft from a hydrant to fill tankers that will shuttle water to the scene. In either case, the hydrant should be capable of supplying enough water for the pumper to operate at its maximum rated capacity-
The maximum flow rate of a dry’ hydrant can be affected by staticlosses resulting from natural causes or by the piping used with the drafting hydrant. Static losses—such as the lift from the water to the fire pump, atmospheric pressure, and the temperature of the water—result from natural causes; they remain the same regardless of the hydrant’s flow rate. A dynamic type of pressure loss occurs as the water moves through the drafting hydrant and associated piping; the loss is proportional to the velocity of the water as it travels through the waterway.
Most dry hydrants use six-inch pipe to get the water from the static source to the pumper. Since atmosphericpressure is used to force the water into the pump, the flow can increase only until the losses in the system arcequal to the atmospheric pressure at the water’s surface. As a result, a typical dry’ hydrant generally supplies between 600 and 1,000 gpm. This may not be enough to sustain the flow needed for a fire. Larger pipe will increase the flow and enable a pumper to realize its full potential.
Lift and distance determine the size of the pipe indicated for a dry hydrant that reliably will supply more than 1 ,000 gpm. Piping larger than six inches is needed when the lift is more than 10 feet, because too much of the available atmospheric pressure is used to overcome the head pressure and the maximum flow is limited by the remaining pressure that can be used to overcome the friction loss in the pipe. Larger piping also is indicated when the distance from the proposed drafting location to the water source is greater than 50 feet, because in most instances the friction loss in the pipe limits the maximum flow to less than 1,000 gpm.
EXCESSIVE LIFT, CALVERT COUNTY
In a water supply master plan that I developed for Calvert County, Maryland, I recommended that a number of dry hydrants be installed to address the water supply needs of outlying areas. The hydrants were installed at three locations, two of which had a lift of more than 10 feet. Eight-inch PVC pipe reduced to a standard dry hydrant head with six-inch fittings on the line side and six-inch threads on the pumper connection was used. (The suction side of the hydrant later was changed to 4’/2-inch threads.)
Initial tests of these hydrants revealed a 1,043-gpm flow for the location with a 13-foot lift, a 1,087-gpm flow for the hydrant with a 16-foot lift, and a 1,122-gpm flow for the hydrant with an eight-foot lift.
Since one of the goals was to attain a flow of at least 1,000 gpm for loading tankers, the hydrants initially were installed with six-inch fittings with National Standard Thread. The strainer on the end of the pipe was positioned four feet below the water’s surface. Doing this provided a margin of safety by enabling the dry hydrant to remain usable and able to maintain the expected flow’ rate should the water level in the pond from which water was being drafted drop during a dry season. Schedule 40 PVC pipe can become brittle and lose its resiliency when exposed to the ultraviolet rays in sunlight; painting the portion of the hydrant that extends above the ground helps reduce these adverse effects.
CONNECTING SUCTION HOSE
Positioning the outlet on the hydrant about two feet above the ground reduces the stress placed on the hydrant by the suction hose and makes the hose easier to connect. This method eliminates the air pocket at the high end of the dry hydrant because the hydrant end of the hose is lower than the steamer connection of the pumper. The weight of the water tends to strain the PVC pipe, and some ty pe of support in the middle of the suction hose minimizes this stress and protects the dry hydrant from damage.
The new ultralight, flexible type of suction hose makes it easier to connect the pumper’s steamer and the dry hydrant. The pumper need only get close to the hydrant, and only one individual in addition to the driver can do it—an important consideration when a fill site is needed to load tankers during the early stages of a fire when manpower is at a premium. At least three people in addition to the driver are needed to connect conventional hard-suction hose, and it requires very precise maneuvering of the pumper.
‘rite dry hydrants initially were installed with six-inch outlets to ensure maximum flow, and the original plan was to purchase dual six-inch double female adapters for each company in Calvert County. Under this plan, the pumper had to be equipped with a double female adapter—with a sixinch coupling on one end and the other end sized to fit the pumper’s suction hose. A switch to 4’/2-inch fittings—the same size used for a standard pressurized fire hydrant— later was made so that most pumpers (including mutual-aid units from outside the county) called to the scene would be able to connect to standard and dry hydrants, set up a draft, and fill tankers using the equipment they normally carried.
Tests made at the same location using the same pumper, suction hose, and equipment to determine the maximum flow with the 4’/2-inch and the six-inch fittings showed that the flow actually increased when the smaller threads were used: 1,087 (4’/i-inch) vs. 1,043 gpm (six-inch) at one location and 1,043 vs. 1,021 gpm at the other location. While there may have been a difference in atmospheric pressure between the two tests, it is difficult to account for the increase in flow using the smaller threads.
EXCESSIVE DISTANCE TO THE SOURCE, SALISBURY
The Bostian Heights Fire Department near Salisbury, North Carolina, had a problem supplying the needed fire flow for two large risks in its district. Two large ponds were available, but their distance from the road made them inaccessible for drafting.
The most critical need involved two large churches located in a rural area approximately two miles apart. A very large pond in the vicinity that could provide a dependable supply of water was nearly 400 feet off the road. Building an access road was not practical, since the ground between the road and the pond was marshy and very unstable. The other site involved a farm with a pond and a number of agricultural-type buildings. The pond was more than 100 feet from the road and also involved a lift of about 12 feet. Dry hydrants were installed in both locations. Ten-inch Schedule 40 PVC pipe was used —to minimize friction loss and provide a flow in excess of 1,000 gpm through the extremely long suction line at the first site and to resolve high-lift and long-distance problems at the other location.
A 1,500-gpm pumper was used to make maximum flow tests on both hydrants. Maximum flow was obtained by using a master stream device along with a flowmeter on another discharge. The maximum flow was 1,380 gpm for the church site and 1,376 gpm for the farm site.
Both of these hydrants were approximately 750 feet above sea level. The ponds were drained at the time the hydrants were installed, but the level in the ponds generally remains relatively stable, and the strainer was located approximately 1.5 feet below the surface of the water. To monitor the level of the water near the strainer, a measuring stick with markings at one-foot intervals was driven into the bottom of the pond.
When the 10-inch pipe was installed, the pond was drained and the pipe put in dry. The cap on die drafting connection was installed tightly. As the pond filled with water, the air couldn’t escape from the pipe, and the pipe began to float; the pipe had to be put back in position and covered with dirt again. Leaving the cap on the hydrant loose so that air can enter or escape from the pipe as the pond’s level changes can prevent the pipe from floating, but it then would be easy for a vandal to remove the cap and put foreign objects into the hydrant. It probably would be better to drill a small vent hole in the cap so that a vacuum or pressure does not build up as the pond’s level changes.
(Photos by author.)
PRIMING THE PUMP
All of the lateral portion of the pipe is located below the water level and normally will be full of water. In a typical dry hydrant installation, all except the List lew feet of the riser are full of water, and priming the pump is much the same as drafting through 20 feet of six-inch suction hose.
Priming the pump is more difficult with dry hydrants. The portion of the pipe that contains air instead of water is increased by the use of 45° bends in the pipe instead of 90° elbows. This air has to be removed by the primer before the pump fills with water. Also, using eightor 10-inch PVC pipe instead of six-inch pipe creates a larger volume of air that must be removed from each foot of pipe.
The practical effect of these factors is that priming the pump will take from 35 to 48 seconds instead of the typical 15 seconds. The pump operator should expect this and not cut off the primer prematurely. On the other hand, if the pump doesn’t prime within 60 seconds, checks for air leaks or other problems should be made before continuing to operate the primer.
Because the suction hose generally sags in the middle when it fills with water, the elbow on the dry hydrant may create an air pocket and cause the pump to lose prime when the water starts to move. If this happens, operating the primer momentarily— while continuing to flow watershould get rid of the air and enable the pump to deliver a continuous stream of water.
A brick enclosure was built around the dry hydrant near the church in North Carolina to provide additional support. A cabinet built into the brick wall and the adapters and fittings needed to draft from this hydrant will be stored there, enabling any pumper assigned to the water supply function to use it.
MAINTENANCE
Dry hydrants accumulate silt or debris in the lateral pipe, which can enter while they are in use or can filter in when the hydrants haven’t been used for some time. For example, while one of the hydrants in Calvert County was waiting for the fire department fitting to be installed, some debris must have built up in the pipe. Flow tests made after the hydrant was installed showed a gpm of only 656, which increased to 1,122 gpm after the hydrant was backflushed with a fire pump.
Dry hydrants should be backflushed on a routine basis to ensure their adequate performance. Quarterly testing and flushing are recommended at the onset; the interval between testing and flushing later can be adjusted according to the amount of deterioration evident over a threemonth period.
TRAINING
Using a dry hydrant for drafting is relatively new to both Calvert County and Salisbury. Training is needed for pump operators in the departments, mutual-aid companies, and first-due companies. These personnel should have the opportunity to connect to and pump from a dry hydrant on a regular basis. Since the job of filling tankers often is assigned to mutual-aid companies, those personnel also should be given the chance to draft from a dry hydrant. At any rate, detailed instructions relative to the locations of the dry hydrants and the procedures for drafting from each of them should be given to all departments that might have occasion to use them
