3. Routinely check for plugging of vents on tanks in fouling service.
Always keep in mind the following points about low-pressure tank layout and design:
• Tank spacing and layout are critical. Various property insurance publications and pamphlets from the National Fire Protection Association (NFPA) offer some guidance about the proper spacing of storage tanks, especially those that contain flammable or toxic liquids.
• Tanks containing incompatible chemicals shouldn't be allowed within the same diking systems.
• Fire-protection features, including static electricity dissipation, vapor space inerting, protective foam generators, water spray and dike designs, demand professional handling.
• Venting systems not only must be well designed but also must be inspected and maintained during the life of the equipment. Tamper-proof vent designs are ideal.
• Tank bottoms should be sloped and associated piping should be laid out to facilitate complete drainage. Tanks should be checked for internal and external corrosion.
• Local conditions, such as the possibility of flooding or hurricanes, which can affect low-pressure storage tanks should be considered.
• Adequate ongoing corrosion monitoring is essential. The BP booklet  contains a number of photos and brief descriptions of tank failures from corrosion.
• Many helpful references are available. These include API-650, API-620, API-510 "Pressure Vessel Inspection Code — Maintenance, Inspection, Rating and Alteration," API-653 "Tank Inspection, Repair, Alteration, and Reconstruction," NFPA codes and Reference 3.
To quote from the BP booklet , "Most tanks are made of carbon steel, which can corrode when exposed to air and water. Over time, uncontrolled rusting can weaken or destroy the components of a tank, resulting in holes or possible structural failures, and release of stored products into the environment."
Effective timely inspections can drastically reduce failures from corrosion.
Three different approaches to tank inspections are widely used.
A periodic visual inspection by operators is the first line of defense. This type of routine monitoring focuses on evidence of seepage or leakage, tank settling, bulging or significant corrosion.
In-service inspections generally are less frequent than operator reviews and typically are performed by certified inspectors. Such checks often start five years after commissioning, with frequency adjusted according to tank history, the risk involved and the corrosion rate. These most often involve taking ultrasonic thickness readings at key locations.
Periodic internal inspections after the tank is drained and washed are a must (Figure 2). These can identify components that have shifted, localized pitting, etc., that may not be apparent from an external inspection. Typically internal inspections take place at a frequency between annually and once every ten years. The exact frequency is best determined by the corrosive nature of the fluid, including its trace components, and the past history of similar equipment on the site.
DON'T TAKE TANKS FOR GRANTED
Tanks can and do hold large inventories of a wide variety of raw materials, intermediates and finished products safely for decades. However, if a tank and its accessories are poorly designed, abused by operations or deprived of effective inspection and basic maintenance, bad things can happen.
ROY E. SANDERS is a chemical process safety consultant based in Lake Charles, La. E-mail him at Sanders.Roy@Suddenlink.Net.
1. Kletz, Trevor A., "What Went Wrong? Case Histories of Process Plant Disasters," p. 97, 5th ed., Gulf Publishing, Burlington, Mass. (2009). Similar details are also found in all earlier editions.
2. Beacon, a free single-page monthly publication of the CCPS comes in many different languages. To subscribe, go to: http://www.aiche.org/CCPS/Publications/Beacon/index.aspx.
3. "Safe Tank Farms and (Un)Loading Operations," BP Process Safety Series, BP Safety Group, Sunbury-on-Thames, U.K. (2008).
4. Sanders, Roy E., "Chemical Process Safety: Learning from Case Histories," p. 108, 3rd ed., Elsevier Butterworth-Heinemann, Burlington, Mass. (2005).
5. Sanders, Roy, "Human Factors: Case Histories of Improperly Managed Changes in Chemical Plants," p. 150, Process Safety Progress (Fall 1996).