Dry Hydrant Sizing and Standardization

The question often asked, How much flow can be obtained from a dry hydrant? The answer however is not simple and involves several factors: Altitude, Lift, Temperature, Pipe length and diameter, Suction Hose length and diameter, Velocity, etc. To simplify this process, ETT has developed a computerized program which will determine the maximum Flow (MF) in seconds.

This service is offered free of charge to our customers, and we feel will be of value in planning a specific installation, or in developing a "State Standard" for dry hydrant intake connections.

Over the past year numerous states have standardized on the 6" dry hydrant; Alabama, Arkansas, Mississippi, and Texas to name a few. Georgia seems to be using mostly 5" although some 6" have also been installed. A few states, which have been using Dry Hydrants purchased by individual Fire Departments have a mixed variety of 4½", 5" and 6". States which haven't established a state-wide standard, should.

NFPA 1231 (Chapter 8, Appendix B, Water Supply) makes several references to use of the 6" size. You may also consult NFPA 1231 for valuable installation and maintenance information.

ETT offers the 4½", 5" and 6" NST Dry Hydrants, but highly recommends selection of the 6". This selection assures maximum utilization and future growth potential as pump size is increased. In developing a standard, one must look at what will be needed 5-10 years from now, and not what will meet the need of your current equipment.

In making your decision it is our recommendation the following guidelines be used:

1. Each installation should be designed with the capability of flowing a minimum of 1,000 gpm.
2. A successful Dry Hydrant program will be designed with the future growth of your fire department in mind, i.e., 5-10 years and 1,000 - 1,500 gpm pumper trucks.
3. As water supply will usually be the responsibility of the responding mutual aid department, it is very important common thread sizes be used to avoid delays. A state-wide standard eliminates this problem.
4. In calculating friction of flow loss, the intake hose diameter is one of the most critical factors in obtaining needed fire flows. Remember this is a suction supply situation. Low pressure with high volume is needed for filling tankers, and increasing intake hose diameter can usually result in pump capacity improvements of 20-25%.
   
   This means an older (750gpm) pumper can often times be used as a supply pumper, and still pump the 1,000gpm flow desirable for tanker filling. As the 750 pumper usually comes with a 4½" intake, increasing the suction hose to a 5" or 6" diameter greatly increases its pumping capabilities.

By looking at the following charts one can quickly spot the effects of intake hose diameter and horizontal length of pipe, as they relate to maximum flow for a given Dry Hydrant with a 6 ft. lift.

As a rule of thumb, 100 ft. is about the limit one can expect to achieve the 1,000 gpm flow rate without increasing the pipe diameter. An average increase of about 70 gpm can be gained by changing from 90 degree elbows to 45 degree elbows. Of course, as vertical lift increases, adjustments in pipe diameter or reduction in horizontal length must be made to maintain a 1,000 gpm minimum flow rate.

As noted in examples C, D and E, length is offset by increasing the diameter of the pipe. Installations of 580 ft. in length with 13.5 ft. vertical lift have been quiet successful. It can also be noted use of 5" or 6" suction hose greatly improves the overall flow rate of a dry hydrant installation.

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