Article Archive
Storage Tanks
Modifications to meet growing demand

By ERIKA HENDERSON/Pittsburg Tank & Tower

The task of maintaining a water tank is not easy, and there are several things to consider when rehabbing or modifying an existing water tank. One of the first questions engineers ask is “would it be cheaper to build a new one?” To answer this question, an inspection needs to be completed first. Sometimes the structural integrity of the tank and the amount of work that go into it are more than half the price of a new tank. However, this is rare and the price is still less than a new tank. New construction requires engineering studies, soils analysis, new foundations and demolishing the existing tank and foundation if a new tank is to be installed at the same location. This requires much more time and planning than modifications. Any tank can be rehabbed or brought up to code to last an indefinite number of years if maintained properly. Changes can be made to extend the life of a water tank and create a more efficient water system. Modifications are becoming a more desirable choice than new construction because they require a fraction of new construction time and the savings can be significant. Permits and fees associated with new construction are normally not needed for rehab. Modifications can easily change water circulation, structural stability, elevation, capacity and even location to meet growing demands.

Water Circulation

Modifications can be made to the internal piping to promote better water circulation and eliminate stratification. Stratification occurs when water separates into layers. These layers are normally arranged according to density, with the least dense and warmer water sitting above the denser cooler layers of water coming in. The layers are caused by differences in temperature, pressure and pH. These unmixed layers cause water quality to deteriorate and age, increasing bacterial growth and stagnation. Many water tank owners are installing mixing systems to prevent stratified water, and they should be designed according to the tank’s unique dimensions and needs. Water from the bottom of the tank should be pulled to the surface and mixed with the incoming water. This process should increase contact time and prevent the development of biofilms. It should also reduce the level of trihalomethane (TTHM) and halo-acetic acids (HAA) which form easily by chlorination in slightly acidic water, or water with high organic matter content and elevated temperatures. This simple modification could increase water quality and prevent health risks.

Structural Stability

A tank can undergo major renovations to return to its former new state.  The entire roof can be replaced, the floor can have a new overlay installed or structural members can be replaced with little downtime. A tank can be upgraded to meet new seismic requirements, increase wind loads or enhance the overall structural stability. A structural analysis can determine the tank’s current state of structural stability. The structural analysis can provide specific characteristics of the tank such as the seismic site class the tank is in and the (SUG) Seismic Used Group for that type of tank. The SUG became popular among engineers only a few years ago; therefore older tanks may not meet this new standard and should be analyzed. Most new tanks are designed to withstand 100 miles per hour winds blowing in any direction, but some locations require a design load of more than 120 mph. An inspection and structural analysis can show how the roof is welded, its design, rafter number and spacing to determine the loads it can withstand. It lists the baseplate thicknesses, diameter, height, center-column design, and capacity of the shell needed to decide the proper anchorage, freeboard and weight loads. If antennas are attached to provide  additional income for the tank owner, the structural analysis can determine how many can be added without compromising the structural integrity.

Elevation

A change in elevation can be made to accommodate system reconfigurations, pressure and even capacity on standpipes. Many municipalities and industrial customers save millions of dollars over the years by raising or lowering tank elevations, rather than supplying a new tank. The resulting storage tank can perform to the new specifications with modifications taking a fraction of new construction time. If the system provides low-water pressure during high usage periods (early morning or late afternoon), raising the tank may provide greater pressure. Raising or lowering a tank by 20 to 50 feet to get existing tanks to match elevations, pressure zones, and to be more efficient is common.

Capacity

Water systems have grown in some areas while population and water consumption have lowered in others. If the current water capacity is not sufficient for the growing population in the area, then the city’s ability to deliver adequate fire flows during emergencies could be jeopardized and provision of additional storage or a system upgrade may be needed. Capacities can easily be changed by adding or removing shell rings. Measure the metal thickness of the existing steel to determine how many shell rings could be added.

Relocation

Relocating a tank is another way to alter performance without building a new tank. Sometimes communities decide to buy or move older tanks in the system because there can be cost benefits depending on the distance and condition of the tank. Before relocation can take place, a thorough professional engineering evaluation must be performed with great detail to safety. The easiest way to move a large water tank is to dismantle and re-erect in a clear, open, dry, and readily accessible site with adequate space. Any overhead obstructions should be removed before work begins. Likewise, the existing tank should be emptied, cleaned, and made safe for welding and burning. 

Demo

If a tank can no longer serve its purpose through modifications, then it should be dismantled and removed from the site appropriately. An experienced demolition crew capable of safely demolishing, disposing of scrap and cleaning the site should be hired. Otherwise, devastating results could occur. Removing or dismantling a tank properly requires great skill and caution. A sample is necessary for testing to determine whether heavy metals, such as lead, are present before dismantling can take place. If the tank has lead-based paint on it, special safety procedures are required by OSHA to remove a lead base coated tank properly. Dismantling a tank is a specific field of its own and an inexperienced person could cause a significant amount of damage. A tank can be demolished safely several ways, but it is the level of lead present, tank style, surroundings and location of the tank that ultimately determine the safest method for dismantle.

A company that puts safety first while performing these types of services can help eliminate significant problems. The company chosen should have a safety program established where they are subject to periodic safety inspections to assure continued compliance with federal, state and company safety standards. They should have a Total Recordable Injury Rate (TRIR) below the Bureau of Labor Statistics average for their industry, and they should have maintained an Employer Modifier Rate (EMR) below 1.0. A professional company that can work under tight time frames during emergencies and that can help develop cost-effective alternatives is also important.
 
 

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