Hole Requirements for Tanks
Note: The following information only applies to the
vertical extensions and or connections to the tank, used by the
fire departments ( Suction ports, Fill ports and Vent ports). Other
specifications regarding lifting hooks, installation tie downs,
burial depth, wall thickness, pressure testing etc are not included
in this document. Consult your tank manufacturer for this information.
In order for you to properly specify tank requirements for vertical
connections that will attach to your tank wall, we recommend a "requirements
pre-plan meeting" be held with your local fire officials. Specifications
vary from place to place. The tank itself will require several openings.
These will generally vary to some degree, based on local fire department
specifications and fire codes. The tank manufacturer may also prefer
one type of connector over another. Usually the NPT threaded adapter
or flat face flange adapter are the preferred two standards.
Generally, five holes or openings may be needed in the tops of
tanks:
- Man Way Access to the inside of the tank.
- Cistern Vent Holes for air intake & discharge.
- Dry Hydrant Suction Holes for drafting of the supply.
- Fill Holes for refilling & circulation of water.
- Depth Gauge - probe holes.
Each of these are discussed in more detail below.
1. Man way Access:
Generally, this will be a 2 Ft. square or round opening into the
tank. While not all tanks have these, they do provide a means of
entering the tank should repairs or cleaning be needed. The tank
manufacturer usually has standard configurations on these openings.
Ladders can also be pre-installed inside the tank. Review local
specifications and/or tank manufactures specs to determine how this
feature should be constructed. Openings should be installed high
enough above ground level to prevent surface water from entering
the tank during heavy rains. It is highly recommended,
if a man way is installed, the access be made with a locking mechanism
sufficient to prevent vandalism and accidents by children.
2. Cistern Vent Holes:
Rapid intake and discharge of water (often exceeding 1,250 gpm)
requires free air exchange. The Cistern Vent provides several functions
necessary for the protection and use of buried tanks for fire protection.
- Allows free escape of air during filling, preventing tank stress
& rupture.
- Allows free intake of air during drafting, preventing tank
collapse.
- Provides a sight glass window, which permits a red/green indicator
to be seen from quite some distance. "Green" indicates
the tank is full of water, ready for use, while "red"
indicates the tank has a leak or was not refilled after it’s
last use. Regardless "red" usually indicates a problem.
- Four holes in the lower end of the vent, act as a drain should
the system be overfilled. As water seeks it’s own level,
this drain provision prevents freezing of all pipes extending
to the surface.
The vent itself is made using 8" Schedule 40 PVC pipe having
an outside diameter of 8.73 inches. The tank end of the vent is
designed with a 2 ft. long, lower cage and houses the float ball.
This end of the Vent must pass thru the wall of the tank and extend
down into the water in order for the sight indicator to work properly.
This section has several elongated slots providing free movement
of air and water to the float ball and upper pipe section.
The lower cage assembly is important for several reasons:
- Helps prevent shipping damage to the float ball & rod assy.
- Helps prevent damage from handling during installation.
- Helps prevent damage from water turbulence inside the tank
during filling and drafting operations.
- Provides a lower guide for the float ball & rod assy.
While a Cistern Vent can be made without a cage, the unit can only
be warranted if it is made with the cage. As manufacturer of the
Cistern Vent, no liability will be accepted for any damage, which
may occur to the float or guide rods when it does not have the protection
of the cage.
In order to install the Cistern Vent into a tank (as it is intended
to be installed) certain specifications must be given to the tank
manufacturer to assure proper hole sizes are provided during the
construction of the tank.
Requirement: The lower vent pipe uses 8"
Schedule 40 PVC with an outside diameter of 8.73 inches. This pipe
extends down thru the wall of the tank into the water in order for
the float ball to work. Therefore, the tank wall must
have a hole of 8.75 inches in diameter. Note: This is not a standard
hole size!
Solution: To provide a hole of 8.75 inches, the
fitting made into the tank wall can be designed in one of three
ways:
- Install a 10" Female NPT Coupling into the wall of the
tank. A "special" Male 10" NPT x 8" Flat Face
Adapter can then be used to mate up with the cistern vent. ETT,
L.L.C. has this adapter.
- Install a "standard" 10 Flat Face - 12 bolt hole
flange. A "special" 10"x 8" Flat Face Adapter,
having an outside hole pattern for a 10" an inside bolt hole
pattern of the 8" can then be used to mate up with the cistern
vent. ETT, L.L.C. also has this adapter.
- Install a "special" 8" Flat Face, 8 hole flange,
with the I.D. machined to 8.75" diameter. (Note the tank
mfg. would be required to machine the I.D. of a standard flange
to make this work.) If the tank is made with this flange the cistern
vent will drop in with no adapter required.
The first two adapters are always in stock at ETT, L.L.C. however,
should a tank mfg. need dimensions to build their own adapters,
drawings will be furnished upon request.
3. Dry Hydrant Pipe Holes:
Dry Hydrant is the term used for a suction device that enables
a fire engine to draft water from a non-pressurized source. In this
case, a buried tank. It works like a huge drinking straw. In a tank
application, the vertical pipe for the Dry Hydrant can be installed
in one of two ways. The pro’s and con's of each design must
be considered before specifying this hole in the tank wall.
- A one-piece vertical pipe. This design uses a single pipe,
which passes thru the wall of the tank and extends down to the
bottom of the tank. It is usually made using PVC pipe. As previously
noted with the cistern vent, this method would require a larger
opening which would allow a 6" pipe, having an outside dimension
of 6.73 inches to pass thru the wall of the tank. Specifications
must be given to the tank manufacturer to assure proper hole sizes
are provided during the construction of the tank for this design.
Requirement: The one-piece PVC pipe uses 6" Schedule 40 or
80 PVC with an outside diameter of 6.73 inches. This pipe extends
down thru the wall of the tank into the water to the bottom of
the tank for maximum use of the stored supply. Therefore, the
tank wall must have a hole of 6.75 inches in diameter. Note: This
is not a standard hole size!
Solution: Install either an 8" F NPT threaded fitting into
the tank wall or an 8" standard 8 bolt flat face flange.
A special (machined 6.75 I.D.) flat face flange can also be installed.
ETT, L.L.C. has adapters that will work with the first two fittings.
No adapter would be required if the 6" flange has been machined
to the 6.75" diameter.
The one piece pipe is the preferred method, as there is no danger
of an air leak and PVC pipe shears at ground level, if struck
by an automobile. Providing a shear point, reduces the danger
of tank damage, commonly seen when steel pipe is used.
For the single pipe method, all of the same tank construction
options apply as was previously noted under "cistern vent
holes". The only difference is, it uses an 8"x 6"
adapter instead of 10"x 8" adapter. This adapter is
available as well. To support the weight of the PVC pipe, a 6"
Schedule 80 one-piece flange, with a smooth slip I.D. (for gluing)
is available. The flange glues to the vertical pipe as positioned
and suspends the pipe above the bottom inside wall of the tank.
The PVC is also excellent for applying a variety of strainers
preventing cavitation as the water level drops near the bottom
during drafting. Schedule 80 or standard Schedule 40 PVC pipe
can be used as both have the same O.D.
- A two-piece vertical pipe using threaded joints.
The lower pipe section, which is factory installed inside the
tank, would thread into a 6" Female NPT coupling made into
the top wall of the tank. An upper pipe section would then thread
into the same coupling from the topside. The upper end of this
pipe would have the Dry Hydrant head attached. This method is
usually selected when steel pipe is used.
Caution: The two-pipe method, using steel pipe
has two dangers:
- As this is a suction system, this design has a joint or
connection, which can potentially leak.
- Steel pipe will not shear at ground level when struck by
an automobile. This usually causes major tank damage, often
times resulting in complete tank replacement.
Requirement: A standard 6" FNPT fitting made
into the tank wall.
4. Fill Line Holes:
While it is possible any size could be used, the two most common
size fill fittings are the 4" Storz and the 2 ½"
F NST. Both normally connect into a 4" vertical fill line.
Some tanks utilize two separate vertical pipes. Two pipes, enables
re-circulation of water by one and re-filling of the tank with the
other.
The number of connections on a fill pipe can also vary. While a
single 4" Storz fitting is used on a 4" pipe, usually
two (2) 2 ½" F NST fittings will be used. Some tanks
may use two 4" fill pipes having a total of four (4) 2 ½"
female connection points. Three can be used for filling and one
for re-circulation.
Requirement: Standard Operating Procedures (SOP’s)
of the responding fire department or local fire codes found at the
fire marshals office will usually determine the number and type
of fill connections needed. ETT, L.L.C. stocks all of these fittings.
The dangers of steel pipe vs. PVC, regarding shear, also apply
to the fill line.
5. Depth Gauge – Probe Holes
Some departments may require a means of measuring the full depth
of water in the tank. As the Cistern Vent only measures the top
two feet or so of water, other means must be used if full measurement
is required. Two methods come to mind.
- Floating rod or stick. This is a simple method
and can easily be done. Simply insert a long rod or pipe with
the ends sealed and see where it floats. One must consider the
depth of water required to float the rod in using this type of
measurement. The rod may be bottomed in the tank with 18"
of water remaining. Markings on the rod should take this into
consideration.
- Clock Gauge. Several manufactures make this
type of gauge. It is a mechanical device that measures the water
depth using a weighted float ball. It resembles a clock, having
hands that go from 1 to 12, hence it’s name "clock
gauge".
Requirement: Usually these devices will require
a 2" to 3" pipe to the surface. A mating female NPT coupling
in the tank wall is all that is needed
Remember; always contact your local fire department
prior to ordering your tank.
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