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:
- Each installation should be designed with the capability of
flowing a minimum of 1,000 gpm.
- 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.
- 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.
- 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.
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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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