December 2013, Vol. 25, No.12

Extra

Storage concerns

What threats surround water tanks?

Erika Henderson

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 death.

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 

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 integrity.

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 steel plate.

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 as possible.”

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.

Inspections 

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.

Taking action 

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.

Erika Henderson is director of research at Pittsburg Tank & Tower Co. Inc.