5.3 Bulk Storage Tank Design, Overfill Protection & Bunding

Bulk Storage Tank Architecture (API 650 / BS EN 14015)

Bulk liquid storage tanks in the chemical and petroleum refining industries store millions of litres of flammable, combustible, or toxic liquids. Tank construction is governed by international standards such as API 650 (Welded Tanks for Oil Storage) and BS EN 14015.

              Storage Tank Architectural Comparison
              
     Fixed-Roof Tank (Frangible Joint)          External Floating-Roof Tank (EFRT)
             /--------\  <- Weak Joint                  ==================== Floating Deck
            /          \                                |                  |
           |  Vapour    |                               |  Liquid Bulk     | <- No Vapour
           |   Space    |                               |                  |    Space
           |------------|                               |------------------|
           |  Liquid    |                               |  Liquid Bulk     |
           +------------+                               +------------------+

1. Fixed-Roof Storage Tanks

Fixed-roof tanks feature a permanent conical or domed roof welded to the cylindrical shell.

• Frangible Roof-to-Shell Joint: API 650 specifies a weak roof-to-shell circumferential weld joint. In an internal deflagration or severe overpressurisation, the frangible joint fails preferentially before the shell-to-bottom joint. This allows the roof to rip open cleanly, directing the blast wave upwards and keeping the liquid containment intact on the ground.

2. Floating-Roof Storage Tanks

Floating-roof tanks feature a steel deck that floats directly on the surface of the liquid mass, rising and falling with liquid inventory levels. They are classified into External Floating Roof Tanks (EFRT) and Internal Floating Roof Tanks (IFRT) (which combine a fixed weather roof with an internal floating pontoon).

• Elimination of Vapour Space: By resting directly on the liquid, floating roofs eliminate the vapour headspace, dramatically reducing volatile organic compound (VOC) emissions and eliminating internal explosive atmospheres.

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Rim Seal Fires & Fire Protection in Floating Roof Tanks

The sliding interface between the floating deck and the stationary vertical tank shell is sealed using a flexible secondary fabric or metallic shoe arrangement known as the rim seal.

         Rim Seal Protection & Fire Suppression
         
         Tank Shell Wall            Floating Roof Deck
               ||                       ||
               ||    +-------------+    ||
               ||    | Rim Seal    |    ||
               ||    +-------------+    ||
               ||           |           ||
         ======||===========v===========||====== Liquid Level
               ||    [Linear Heat Cable]||
               ||           |           ||
               ||   (Foam Dam & Pourer) ||

Causes of Rim Seal Fires

Rim seal fires are the most common major incident on EFRTs, caused by:

1. Lightning Strikes: Direct strikes or induced electrostatic charges igniting flammable vapours escaping through worn or degraded seal fabrics.
2. Worn Seal Membranes: Mechanical abrasion creating gaps that allow hydrocarbon vapour concentration to enter the flammable range.

Rim Seal Fire Mitigation Systems

• Foam Dam & Foam Pourers: A steel baffle plate (foam dam) installed 1.0 to 1.5 m from the shell perimeter on the deck holds a blanket of expanded low/medium-expansion firefighting foam discharged from automated rim-seal foam pourers.
• Automated Detection: Linear heat detection (LHD) sensor cables or optical infrared flame detectors routed along the entire rim seal perimeter to actuate deluge foam valves within 30 seconds of ignition.

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Secondary Containment & Bunding Capacity Sizing

Secondary containment systems (bunds) are engineered retaining walls and basins constructed from reinforced concrete or low-permeability earthwork designed to capture spilled liquid if a primary storage tank fails.

                  Bund Capacity Calculation Rule
                  
   +-------------------------------------------------------------+
   |                        BUND WALL                            |
   |   +-----------------+                 +-----------------+   |
   |   |   Tank A        |                 |   Tank B        |   |
   |   |  (10,000 m³)    |                 |   (5,000 m³)    |   |
   |   |  [LARGEST]      |                 |                 |   |
   |   +-----------------+                 +-----------------+   |
   |                                                             |
   |  Bund Net Capacity MUST BE GREATER OF:                      |
   |  1) 110% of Tank A = 11,000 m³   (GOVERNS!)                 |
   |  2) 25% of (10,000 + 5,000) = 3,750 m³                     |
   +-------------------------------------------------------------+

Sizing Rule (CIRIA C736 / HSE Guidance)

The net capacity of a bunded enclosure containing one or more tanks must equal at least the GREATER of: $\text{Capacity}_{bund} = \max \left( 1.10 \times V_{largest}, \quad 0.25 \times \sum_{i=1}^{N} V_{total, i} \right)$

Where $V_{largest}$ is the gross volume of the single largest tank inside the bund, and $\sum V_{total}$ is the sum of all tank volumes within the same bunded area.

Key Engineering Rules for Bund Design

1. Displacement Volume Deduction: The net storage volume of the bund must account for the volume occupied by structural foundations, pipework, and other tank shells within the bund up to the top of the bund wall.
2. Bund Wall Height Limits: Wall heights are typically designed between 1.0 m and 1.5 m. Walls higher than 1.5 m restrict natural ventilation (causing gas accumulation) and severely impede emergency access and fire service hose deployment.
3. Impermeability & Joint Seals: Liners must be chemically resistant to the stored product. Concrete construction joints require continuous elastomeric waterstops to prevent subsurface migration into groundwater.
4. Water Management: Rainwater accumulation must be drained through a dedicated sump fitted with a locked isolation valve, operated manually after inspecting water for hydrocarbon contamination.

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Post-Buncefield Overfill Prevention Systems (PSLG Guidelines)

On December 11, 2005, the Buncefield Oil Storage Terminal in Hertfordshire, UK, suffered a catastrophic explosion and fire. A servo-operated tape gauge stuck on a petrol storage tank while filling, and the independent mechanical high-level switch failed to operate. Over 300 tonnes of unleaded petrol overflowed from the top of the tank, forming a massive ground-hugging vapour cloud that ignited in a 200-tonne TNT equivalent Unconfined Vapour Cloud Explosion (UVCE).

       PSLG High-High Automatic Overfill Prevention System (AOPS)
       
  Inlet Flow ---> [ROSOV Valve] ----------------------> Tank
                     ^ (ESD Signal)                      |
                     |                                   |
              +--------------+                   +---------------+
              | Logic Solver |                   |  Independent  |
              | (risk SIL)  | <-----------------|  High-High    |
              +--------------+                   |  Level Switch |
                                                 +---------------+
                                                    (Separated from
                                                     Process Gauge)

In response, the Process Safety Leadership Group (PSLG) issued mandatory standards for storage of Category 1 & 2 flammable liquids:

Key PSLG Engineering Requirements

• Independent Automatic Overfill Prevention System (AOPS): Every tank must feature an overfill protection system completely independent of the daily operational level gauging system (Tank Inventory System).
• Functional Safety (IEC 61511): AOPS systems must be designed as Safety Instrumented Systems (SIS) achieving a Safety Integrity Level determined by IEC 61511 risk assessment (commonly at least SIL 1, higher where justified).
• Remotely Operated Shut-Off Valves (ROSOVs): Upon reaching the High-High liquid setpoint, the AOPS logic solver must directly trip an automated ROSOV installed on the inlet fill pipeline outside the bund wall, isolating flow within target response times.
• Proof Testing & Inspection: AOPS sensors and trip logic require mandatory periodic proof testing (e.g., annual physical wet-testing) with records maintained under asset integrity management programs.

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Tank Breathing & Vacuum Protection (PVRVs)

Atmospheric storage tanks are fragile structural shells susceptible to destruction from minor overpressure (typically > 20 mbar / 0.3 psi) or minor vacuum (typically > 2.5 mbar / 0.036 psi).

                 PVRV (Conservation Vent) Mechanics
                 
                     [ Overpressure Release ]
                              ^
                       +--------------+
                       | Pressure Disc| (Weights / Spring)
                       +--------------+
                              |
       Tank Shell Vapour Space Headspace
                              ^
                       +--------------+
                       | Vacuum Disc  | (Vacuum Inbreathing)
                       +--------------+
                              v
                     [ Air In-Breathing ]

Mechanisms of Tank Breathing

1. Thermal Out-Breathing: Solar heating during daytime expands internal vapour and evaporates liquid bulk, requiring controlled pressure relief.
2. Thermal In-Breathing: Rapid ambient cooling (e.g., sudden heavy rain thunderstorm) rapidly condenses internal headspace vapours, creating a severe vacuum.
3. Liquid Pump-Out: Pumping liquid out of the tank at high flow rates without makeup gas entry creates an instantaneous structural vacuum, collapsing the tank shell inward like a crushed aluminum soft-drink can!

Pressure Vacuum Relief Valves (PVRVs)

PVRVs (conservation vents) employ weight-loaded or spring-loaded pallets to allow out-breathing at set positive pressure and in-breathing at set vacuum pressure, preventing structural failure while minimising evaporative emissions.