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Wood Storage Tanks: History, Problems, and Maintenance

07.07.16   R. Charles Ireland, PhD, PE | More by this Author, Hayder H. Al-Hilal, PE, SMSI, CIP-1, CCI-2, Level 1 Thermographer | More by this Author

Wood Storage Tanks: History, Problems, and Maintenance

Wood tanks have been widely used for the storage of food products, including meat, dairy, corn, vinegar, pickles, and cider, as well as in many 20th century buildings as part of the fire suppression system. A number of these tanks are still in service across the US, and many are suffering the effects of aging.

A study conducted by the U.S. Department of Agriculture in 1942 showed that wood tanks were exclusively used for the storage and processing of vinegar due to the following: 

  • Long life expectancy, typically in the range of 30 to 50 years, or more if maintained properly;
  • Good resistance to rot and decay;
  • Acid resistance of wood;    
  • Freedom from contamination of stored vinegar.

 
Wooden storage tanks in service.

Wood vinegar storage tanks are essentially giant wooden barrels, consisting of unlined dimensional lumber staves, steel tension hoops, and a bottom made of dimensional lumber planks supported by steel or wood beams bearing on concrete knee walls, as illustrated in Figure 1.


Figure 1: Model showing typical tank elements.

Redwood, Cypress, and Douglas-fir are the most common wood species used in construction of vinegar storage tanks. Heartwood grade for wood staves is typically specified due to their rot and decay resistance.

Unfortunately, over time vinegar storage tanks may become vulnerable to leakage. If not corrected, the corrosive long-term effects of acetic acid on steel tension hoops and concrete foundations can lead to structural failure.

Leakage is generally a long-term result of volume fluctuations in wood staves. This is due to swelling and shrinkage for extended periods mainly resulting from tanks going in and out of service or when they are partially filled in service. These service fluctuations can result in distress cracks and open, unsealed joints between staves.

Typical structural concerns resulting from vinegar leakage include:

  • Section loss of steel tension hoops - Steel tension hoops serve as a primary structural component that resists hoop stresses that result from horizontal hydrostatic fluid pressures.   
  • Degradation of concrete foundation knee walls - Degradation of concrete can result in loss of bearing support of the tank bottom.

SME has performed structural analyses of hoops and staves for a 70,000-gallon wood storage tank to gain a rational understanding of structural conditions observed in the field because loss of a critical hoop can result in a general catastrophic failure. In the structural analyses, various scenarios were checked with different hoops taken out of service due to failure or excessive corrosion, to assess effects on adjacent hoops. Results showed that if only one hoop is removed (due to complete failure) or excessive deterioration, adjacent hoops will be additionally burdened with redistributed fluid pressure loading. This effect may result in rapid progressive failure of the entire tank with wood staves also exceeding their structural capacity in the process.

Figure 2 shows the effects of this redistribution of pressure. The stresses on adjacent hoops significantly exceeded the allowable stress in the steel, potentially leading to failure in those hoops. This scenario may be representative of conditions where a hoop fails. It might also be roughly comparable to a combination of section losses at multiple neighboring hoops making several hoops no longer fully effective.


Figure 2

An active monitoring and maintenance program is the best way to address budding concerns before they become expensive repair items, or possibly lead to catastrophic tank failure. An active monitoring and maintenance program should include, at minimum:

  • Monitoring vinegar leakage, hoops, and concrete foundation at regular intervals;
  • Replacement of corroded hoops while leaving the corroded ones in place until the new hoops are placed and tightened properly;    
  • Repair of deteriorated concrete foundation walls to maintain proper bearing support of the tank base.

 
Successful monitoring and maintenance programs require engaging qualified and experienced professionals. SME’s team of qualified professional engineers are experienced in assessment and rehabilitation of wood, steel, and concrete structures. We can help tank owners avoid the conditions that can develop into a catastrophic tank failure.

For more information, contact R. Charles Ireland, PhD, PE or Hayder Al-Hilal, PE.


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