In today’s market, information technology on storage applications has collided with advanced steel tank designs, advanced coating systems and field construction processes. The outcome includes some of the most cost-efficient, high-quality storage-containment products ever developed for industrial and municipal applications. Under review, high-quality storage products offer significant benefits to the client in the form of extended service life, low maintenance and cost savings generated over its service life.
The problem in comparing alternative types of storage construction is separating high-quality, or “Tier A” storage products from marginal quality, or “Tier D” storage products (see Figure 1). Surprisingly in today’s market, premier-quality dry bulk and liquid storage products can be purchased and installed for about the same price as low-quality storage products.
The real kicker is adding the life-cycle costs to the equation. The life-cycle costs of Tier D products will often calculate at more than twice the cost of Tier A products. Needless to say, it becomes an expensive lesson for the client when marginal-quality tank construction is specified and procured for a project.
The construction of a storage tank/system should always be reviewed as a major infrastructure project. The specifications on tank construction, coating systems and field construction processes should be covered in detail. The minimum service life of a storage tank used to be considered 40 years. In today’s storage industry, there are tank/containment products that are being introduced into the market that will provide less than a 10- to 20-year service life and other products that will provide service in excess of 60 to 80 years. Based on proper maintenance of a storage tank and its coatings, a steel storage tank can be maintained for an indefinite service life.
A variety of storage tank designs are offered depending on the design code specified (see Figure 2). Glass/vitreous enamel coatings, which used to be the leader in liquid applications, offer a service life of 30 to 40 years. Some shortcomings of glass storage tanks are the panel edges, bolt holes and high factory reject rates. Unlike glass tanks, which have to be replaced after their service life, bolted steel tanks can be recoated to provide an indefinite service life. Advancements in coatings technology are the driving force behind the shift from glass to steel tanks.
Dry Bulk Storage
Bolted tank construction. In bolted construction, the bolted rolled, tapered panel (RTP) design is one of the top performance products offered in the industry today worldwide. API 12B (flanged panel tank) is an older bolted tank design that has incurred field leak issues in dry bulk and liquid storage applications and is now being replaced by the RTP design.
A newly introduced storage product is the vertical flat panel (FP) design, which is being promoted as another replacement for API 12B construction. It allows the use of older, existing API 12B field construction equipment (e.g., air tugger and gin pole). In precision bolted fabrication, the RTP panel design must be rolled horizontally, the same as API 650 and API 620 fabrication.
Field-weld tank construction. In field-weld (FW) construction, API 650 and API 620 fabrication has changed little, with the exception that state-of-the-art shop fabrication equipment now is utilized. In the field, new field-weld construction technology has allowed for some significant advances in welding processes.
API 650 and 620 construction remain the premier storage products in welded tank construction. All panels (8 to 10 ft high by 25 to 35 ft long) are rolled horizontally, and leading fabricators will minimize the large piece count in the shop. Field-weld construction that does not minimize the large piece count in the shop or is not subject to radiograph examination (both in the shop and the field) per API 650 should be considered Tier D quality.
Hybrid tank construction. Any tank component combination of bolted, field-weld or concrete construction will produce a storage product classified as a hybrid.
Concrete tank construction. Slip-form concrete construction is the premier concrete storage tank design for dry bulk storage applications. Jump-form concrete routinely is utilized in biomass storage, grain applications and elevated water tank pedestals. In liquid storage containment, wire-wound prestressed designs with a steel diaphragm continue to provide good quality containment for tank sidewall applications. Concrete cover applications for liquids continue to be replaced in the industry by aluminum geodesic dome covers.
Specification & Quotation
The following storage checklist should be covered when specifying or requesting a quotation on a storage system:
- Request the tank vendor specification and make sure every detail is specific.
- Request the tank design plate thickness broken down separately (i.e., sidewall, hopper/bottom and deck) at the quotation stage, not after order placement.
- Review the tank vendor performance guarantee in writing at the quotation stage. In dry bulk storage applications, make sure that the tank vendor will guarantee material flow from the tank.
- Request safety information (i.e., EMR, etc.) on any subcontractors that will be utilized for field construction services if the company does not have its own field erection crew. The reality is that most tank manufacturers subcontract their field construction labor. Always do your homework on the field installation crew prior to order release to minimize headaches when it comes time for field construction.
- Request coatings information and dry film thicknesses applicable for the tank interior and exterior coating systems. Some coatings are applied at less than the recommended minimum mil thickness to save costs. There is no reason for tank coatings to be applied at readings less than 2 to 3 mils.
- Request from the supply vendor the name of the tank manufacturer.
Conduct prequalification to verify a vendor’s shop and field capabilities, financial stability, bonding capacity, safety record, etc.
- Request whether or not your tank quote is subject to price escalation after order placement. Unless steel pricing becomes volatile as it did in late 2008, the pricing you receive should be firm through the agreed-to scheduled date for shipment or field installation.
- Set a firm schedule with the tank manufacturer, and hold their “feet to the fire” to ship the tanks on time and on schedule.
If this checklist of items is followed, product specification and procurement will be made as an informed buyer. Remember, if the storage tank supplier is not responsible for the outcome, then the specifying engineer or client has assumed the responsibility. In all cases, cover your bases and specify a premier-quality storage system that will stand the test of time as a major infrastructure investment.