On June 6, 2013, surveillance cameras in Lakewood, Wash.,
caught images of two camouflaged men trying to cut the lock at a water tank,
according to the Lakewood Water District. According to a news report from
King5.com, the district put up surveillance cameras after the tank had been hit
by vandals and metal thieves earlier in the year. The camouflaged men were
captured on the cameras trying to cut a padlock at the base of a ladder leading
to the top of the covered, 13.3 million L (3.5 million gal) tank. In the
surveillance video, the men gave up and ran off after their bolt cutters could
not cut the padlock.
What were the men going
to do to the city’s water supply, and what other threats surround water tanks?
Water tanks can be safe
vessels to store healthy drinking water, but they must be maintained securely.
As the backbone of the
entire water system, tanks must remain free from any threat, at all times.
Threats can be presented in the most obvious form as intruders, but tank
deficiencies can be just as threatening and less obvious to the untrained eye.
Water tank intruders
Intruders can fall into
different groups. Some are rebellious or bored individuals looking for
entertainment; these intruders often do not intend harm to others, but could
still present a serious liability. They may climb the tank and/or use it as a
canvas for graffiti. Other intruders do intend harm, and they may contaminate
the water or cause serious physical damage to the tank. Insects, birds, and
other animals are a third kind of intruder, sometimes forgotten. But, these
potential parasite carriers can contaminate tanks and cause serious illness or
Security measures must be
taken to prevent all types of intruders. One way to discourage unauthorized
access is to enclose the tank with an intruder resistant chain-link fence that
is at least 2 m (6 ft) high and topped with three strands of barbed wire. A “No
Trespassing” sign should be posted. Surveillance cameras, lighting systems, and
intruder alarms also should be installed around the tank.
A designated entrance
should be established with proper locks on all gates and hatches, and ladder
guards. Site traffic procedures should be enforced, and all tank openings
should be sealed or properly covered with an appropriate screen and locked.
These steps should help
discourage unauthorized access from people, but other measures must be taken to
prevent other unnoticed threats that could surround a water tank.
Other threats include
structural, code, and sanitary deficiencies that could render the tank and its
contents unsafe. For example, a tank with structural deficiencies could
collapse under specific conditions and lives could be at risk. Foundations with
rebar exposed or settlement problems can jeopardize a tank’s structural
If the floor has settled
and voids between the floor and compacted sand have been created, then a
trained professional should be called to pressure-grout more sand — compacted
to 95% on a proctor scale — under the tank to maintain the cushion and
stabilize the floor.
Ground storage tanks
without self-supporting roofs rely on the center column and roof rafter for
support and stability. If these are damaged or the foundations have settled,
the roof also could be in danger of collapsing. Anchor bolts and chains should
be spaced properly and installed to required depth to meet specific design
requirements needed to maintain the structural integrity of the tank.
The metal thickness of
steel water tanks must be maintained. If the thickness of the steel has
deteriorated to unsafe levels, the tank could spring leaks or rupture. Pits
also can develop in the metal of the tank from deterioration and may need to be
filled with a seam sealer or pit filler. Streamer pits are continuous pits that
have formed in a vertical or horizontal line along the interior of a carbon
steel storage tank. These types of pits need to be filled or overlaid with
In elevated water tanks
with legs, struts, windage rods, and catwalks, the riser pipe should be
monitored closely for structural stability. The majority of water weight
settles where the riser meets the bowl. Often leaks or stress cracking can
appear at this connection. Legs should be anchored properly in the foundation
and should be free from thinning metal. The struts should be welded to the legs
properly and the windage rods should be secured and tightened regularly. The
catwalk is a structural girder and should be securely attached to the shell of
the tank to maintain the overall structural integrity.
Code and sanitary deficiencies
Code and sanitary
deficiencies can threaten the safety of a water tank. For example, tanks that
do not maintain proper ventilation can rupture. Therefore, vents, overflow
pipes, drain valves, and vortex plates should be monitored carefully for
obstructions or defects that could lead to a blockage or pressure buildup. A
vacuum-pressure vent is needed to prevent pressure buildup. A defective vent
can cause pressure buildup, and a vent without the proper screen could allow
insects, birds, or other animals into the tank. Several waterborne diseases
could result from the contaminants and could cause obstruction in the pipes.
Overflow pipes also are
designed to protect the tank from overpressure and overload. If the pumps or
altitude valve fail to shut off during tank filling, the extra water must be
evacuated from the tank before too much pressure is built up. An overflow pipe
should not be connected directly to a sewer or a storm drain because the
discharge would not be visible. The 2011 book, Welded Carbon Steel Tank for
Water Storage, D100-11,from the American Water Works Association
(AWWA; Denver) states, “A properly operated tank should not overflow during
normal operation. An overflowing tank is considered an emergency condition, and
the malfunction causing the overflow should be determined and corrected as soon
Overflow pipes should be
properly sized and extend to ground level, complete with a weir box. If an
underground drain is present, it should be sealed and an air break installed. A
screen and flapper valve in accordance with AWWA, NFPA 22, Occupational Safety
and Health Administration (OSHA), and U.S. Environmental Protection Agency
requirements needs to be installed over the opening to prevent the ingress of
contaminants into the water supply, and a splash pad to direct the water away
from the tank’s foundation.
Two manways, one being
at 750 mm (30 in.) in diameter, should be in the bottom ring of ground storage
tanks. These allow access for tank maintenance and must be properly sized to
remove an injured worker. A roof access with a lockable lid also should be
installed with a ladder that extends to the floor.
Safety climb devices are
needed on all interior and exterior ladders to prevent injuries associated with
falls. All ladders, handrails, and landings must conform to OSHA standards, and
the tank should be equipped with lightning protection, confined space entry
signs, and no trespassing signs.
An inspection can be
performed to determine a tank’s state of structural stability, and the seismic
site class in which the tank is located. An inspection can show how the roof is
welded, how it is designed, how many rafters are in place, and how far apart
they are to determine the loads it can withstand. The baseplate thickness,
diameter, height, center-column design, and capacity of the shell also may be
obtained to determine the proper anchorage, freeboard, and weight loads.
Cleanings often are done
during inspections because water tanks must be cleaned out regularly to remove
solids and debris that could cause damage or great risk. Pipes that become
clogged could cause malfunctions and the fire protection system could be
jeopardized. The 2013 book, Steel Water-Storage Tanks-Manual of Water Supply
Practices, M42, from AWWA states, “Tanks shall be washed out and inspected
at least once every 3 years, and where water supplies have sediment problems,
annual washouts are recommended.”
Many water tank owners
do not realize a complete tank inspection and cleaning can be conducted
concurrently and without draining the tank. A remotely operated vehicle (ROV)
can be used to inspect the tank while another robot cleans the tank.
Lockout/tag-out procedures and confined space permits are not needed, because
no one enters the tank.
Cameras on the
inspection ROV are adjustable to enable the operator to zoom in and view
surface features of the tank wall and floor. Front and rear propellers allow it
to move forward or backward, and the vertical and horizontal thrusters allow it
to stop or turn easily. Robots are a safe and effective way to limit liability,
save water reserves, and collect necessary information promptly.
A recording of the
inspection should be provided with the written inspection report. The written
inspection report should include the results of an ultrasonic thickness test,
mil thickness test, and crosshatch test. The ultrasonic thickness test measures
the thickness of the steel to determine its integrity and a mil thickness test
determines the thickness of the existing coating and should be used with the
crosshatch test to determine the adhesion and overall rating of the coating
system. A detailed evaluation with photographs, recommendations of needed
repairs, and code updates should be included.
The inspection alone is
not enough to maintain a safe water tank, but it does provide crucial
information needed to do so. After the inspection has been performed and the
condition of the tank has been determined, the deficiencies must be corrected
to limit the threats that surround a water tank.