Water Tank Maintenance

A water storage tank can cost hundreds of thousands of dollars to construct, and, although it is initially expensive, it is — over the life of the structure — the most cost-effective means of keeping drinking water clean, safe and in adequate supply. The trouble is, local governments often are remiss in maintaining the tanks, thereby shortening the structures’ lives and increasing ownership costs.

With regular evaluation and maintenance of water tanks, a city or county can ensure that its tanks are viable distribution components for a virtually limitless period of time. (In the United States, there are more than three dozen steel water tanks that have been in service more than 100 years.) Public personnel can inspect the tanks and perform some maintenance tasks with little or no extra cost to the local government. However, other tasks should be handled by specialized tank engineers and contractors.

Monitoring the foundation

Tank owners can and should evaluate their tanks on a regular basis. That process begins on the ground with the foundation, as deterioration of the foundation can lead to major problems elsewhere.

Diligence is key to inspecting a foundation. Utility personnel do not need special training, just a keen eye for detail to spot signs of settlement, cracking, spalling or exposed reinforcing steel. The presence of any of those factors signals the need for immediate repairs. Some repairs can be accomplished in house, while other, more severe, deterioration may require professional assistance.

Vegetation, which can trap moisture against concrete and steel, should be cleared from the base of the tank. Similarly, tree limbs and bushes should be trimmed from the tank shell to prevent scratches in the steel coating. (Scratches are not only unsightly but precursors to premature coating failure and corrosion.)

In addition to examining the foundation and the surrounding area, utility personnel should survey the site for signs of unauthorized access or vandalism. Unauthorized access is a potential liability for the tank owner, and a possible threat to water system security and the tank itself. Manholes and access doors to the tank interior should be checked frequently to ensure that they are secure.

Examining the tank

Assessment of the foundation and tank site should be followed by examination of the tank itself. Inspectors typically look for signs of corrosion and leaking, and they assess the condition of connections and screens.

Under adverse conditions, such as those involving prolonged, unchecked exposure to moisture, the bottom plate projecting from a tank (called the chime or chine) can corrode. If not prevented or arrested, the corrosion can worsen and eventually cause leaking in the chime and/or the corner weld.

Leaks may not be readily visible but instead may be detected through the presence of rust streaks. If a leak is discovered, a professional structural engineer familiar with water tank maintenance should inspect the tank as quickly as possible.

In addition to monitoring the tank for corrosion and leaking, inspectors need to examine the condition of sanitary items such as the overflow discharge screening and vent screening. They must be sure that the screens are clear of debris and free of holes or gaps. Vent screens also are present on the tank roof, and, if staff members are trained in accessing heights and are equipped with the proper safety gear, they can be dispatched to monitor their condition. (Worker training — whether for accessing heights or for confined-space entry — is available from a variety of sources, including the Occupational Safety and Health Administration, www.osha.gov.)

If the tank is equipped with a vacuum/pressure-relief style of vent, the proper positioning and operation of the pallets before and after freezing weather should be checked. (Rapid discharge of water from the tank could result in roof or structure damage if the vent is clogged or frosted over.)

Finally, as part of owner-performed maintenance, utilities must ensure that potable water stored in the tank is not connected with water in the storm or sanitary sewer. Most state health departments require that overflow pipes have a visible air break above grade. If an overflow pipe discharge end is allowed to come in contact with stormwater or water ponded in a catch basin, a siphon action can pull the unsanitary water into the tank and contaminate the potable water stored in it.

Regular tank evaluations help ensure early detection of needed repairs, and they can save the tank owner a significant amount of money. For example, in one community, the tank owner detected eroded soil at the base of the community’s ground storage tank, which signaled a possible leak. The leak was confirmed by a tank engineering specialist and repaired fairly simply. Left unchecked, it would have caused further erosion and eventually might have caused the tank to shift. Monitoring and early response to a leak saved the local government thousands of dollars in repairs.

Calling for backup

Routine tank monitoring and inspection can be handled in house, but problem assessment and/or repair often will require professional assistance. In addition to having a tank drained, cleaned and professionally evaluated every three to five years (as recommended by the American Water Works Association), owners should consult professional tank engineers if any of the following circumstances is present:

• The tank or tank appurtenances is damaged structurally;

• The sanitary integrity of the tank is compromised;

• The foundation has cracked deeply or crumbled extensively;

• The concrete foundation has deteriorated excessively or has voids;

• Soil around the base of the tank is saturated or eroded for reasons other than normal precipitation or overflow effluent;

• The foundation is settling differentially, evidenced by foundation tops and base plates that are not level;

• Water is ponding on the tank roof, indicating possible damage to rafters or other support members;

• The base plates or bottom plate exhibit extensive metal loss (corrosion);

• Anchor bolts are bent or otherwise damaged;

• Anchor bolt nuts are not tightened;

• Metal loss exceeds the thread depth on the anchor bolts; or

• Valve vault piping is damaged, leaking or deteriorated.

Evidence of the need for a professional evaluation also can be noted on the tank tower and container. Professional assistance should be called for if there is:

• Any evidence of a leak in the container, including rust streaks on the shell;

• Severe worsening of the exterior coating condition. Voids in the coating allow the water or moisture to come in direct contact with the steel, promoting corrosion;

• Significant deterioration or metal loss — i.e., in excess of a quarter to one-half the original member thickness — on any part of the structure;

• Corrosion and metal loss at structural connections (e.g., nuts, rods, rod pins, turnbuckles and clevises);

• A gap of more than one-sixteenth of an inch — and metal loss greater than one-eighth of an inch or half the original member thickness — at the balcony connection to the container or on platforms;

• A seam that is not intermittently or seal welded;

• Evidence of distorted roof plates, which can signal severe changing of pressure and vacuum conditions;

• Evidence that tubular columns are not airtight, as signaled by dents in the columns, a wet riser or wet riser pipe;

• Rotation in the columns or tower structure;

• Excessive slack in the diagonal bracing and riser rods;

• Kinks, bends or discontinuities in the diagonal bracing and riser rods;

• Deterioration, metal loss or missing components at tower connections;

• Excessive deterioration of rivet heads;

• Distortion in the contour of the container, including the roof plates;

• Pitting and metal loss on the roof support structure or on the tank shell above the high water line (as observed during a cursory evaluation through the roof manhole);

• Metal loss, looseness or cracking of ladder support connections;

• Inadequate attachment of vent and access openings; or

• Missing or improperly aligned cathodic protection hand hole cover plates.

Maintaining for generations

A successful tank maintenance plan defines regular monitoring tasks for in-house staff, scheduled maintenance for staff as well as outside professionals, and circumstances in which further evaluation of a problem is necessary.

If a community needs to consult a specialized engineer, AWWA has established a standard for selection (see the information above). Relying on inside and outside resources, the local government can protect its investment and ensure that the tank serves generations to come.

Chip Stein is a vice president for Tank Industry Consultants, based in Indianapolis.

Foundation and tank site:

• Foundation settling
• Foundation cracking
• Spalling
• Exposed reinforcing steel
• Vegetation at tank base
• Vegetation on tank shell
• Unauthorized access Tank:

Tank:

• Corrosion on the chime/chine
• Rust
• Clogged or damaged screens
• Crossed connections

Ask before you act

Always consult a professional tank engineer prior to:

• disturbing a coating that may contain lead or any other regulated heavy metal;

• adding antennas, microwave dishes or decorative features to a tank container or roof;

• cutting or welding any steel on a tank that might have been constructed of high-strength steel;

• using any tank structural member or appurtenances for rigging or personal access; or

• preparing specifications for repainting and repairs.

Whom do you choose?

“The tank maintenance engineer should have knowledge of the traditional engineering disciplines and have specialized training and practical experience in the design, fabrication, erection, inspection, sanitary integrity, coating and maintenance of steel water-storage facilities.”

The American Water Works Association, Manual M42, “Steel Water-Storage Tanks”