Design Options for Overfill Protection for Aboveground Atmospheric Tanks - Best Practices
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Overfilling of a tank is an important safety hazard. It may result in loss of tank fluid and potentially severe consequences if the fluid is flammable or environmentally sensitive. Additionally, it is necessary to preserve the mechanical integrity of a tank. This article first looks briefly at various ways liquid may overfill a tank, and then describes different design options as best practices to take care of situations where overfilling is a possibility. The main paper will contain diagrams and appropriate references.
API Standard 2350 (2012) specifies in fair detail various methods, operating procedures, measurements and record keeping needed to protect a tank from overfilling. This standard applies to aboveground storage tank in petroleum service that receive Class I (flammable) and Class II (combustible) liquids. The standard, however, is not prescriptive and allows flexibility in details of design of overfill protection option according to the risk presented by overfill and risk tolerance of the owner. NFPA 30 (2012) (Sections 21.7.1 and 19.5.6) also discuss overfill protection; however, it is not as complete as the API STD 2350. Moreover, API 2350 has been prepared to be consistent with NFPA 30.
Pressure Relief Protection Used for Overfill Protection
Pressure relief devices may be used under limited circumstances for overfill protection.
1. Relieve liquid through pressure-vacuum breather vent or fire relief vent: Weight loaded vapor breather and fire vents are not designed to relieve liquids. They may not open at the set pressure, may require high overpressure to be fully open and may not have the capacity to allow the full flow of incoming liquid. Additionally, the backpressure may affect the opening.
2. Relief through pilot operated liquid relief valves: Pilot operated breather relief valves are available for liquid relief. For one vendor the minimum set pressure is 0.07 barg (1 psig, 28” WC) which may require a higher tank design pressure resulting in a more expensive tank. Importantly, the tank must be designed so that the maximum liquid level is at the relief valve to maintain the integrity of the tank. The disposal must be appropriate for the fluid. Some codes (e.g. Australian) may not allow pilot operated valves in liquid service for tanks.
3. Relief through frangible roof: Frangible roof tank with a weak seem would lift as the liquid level rises in the tank beyond the seam. However, the roof may not open at the design point. More importantly, case histories demonstrate that cases exist where the roof has lifted and landed by the side of the tank. Liquid would overfill to grade uncontrollably, creating a hazard if the liquid is flammable.
Overfill Protection Using Overfill Lines
Overfill is typically mitigated by an overfill line or instrumentation, depending on the situation. Following are some guidance on overfill lines.
Overfill Lines for Tanks without Gas Blanket: The first, and the simplest, option is that of providing a discharge line that is adequately sized for the incoming fluid flow rate and located at an elevation commensurate with the maximum allowable capacity of the tank. The outlet of this drain pipe should be directed to a sump to prevent the discharging fluid from splashing on the ground and exposure to the personnel in the vicinity. The tank content for such simple overfill protection must be safe for direct environmental discharge and have no exposure concerns to humans or animals. Water and water based dilute mixtures of chemicals, generally recognized as safe (GRASS) fall in this category. In these tanks, there is no requirement for the tank contents to be protected from long term air or oxygen contact; therefore, such tanks do not require any blanketing by inert gas.
Overfill Lines for Tanks Blanketed with Gas – Inverted U-Loop: The second and third types of overfill protections are for fluids that require blanketing with inert gas such as nitrogen because they are sensitive to or reactive to oxygen or air. The liquid itself may degrade in quality with prolonged contact with air, such as amine or caustic solutions, or the fluid may vaporize into the vapor space so that a flammable or combustible environment may be created in the vapor space inside the tank. Replacing air/oxygen with an inert gas mitigates such a possibility. The tank is operated under a pressure of blanket inert gas higher than the atmospheric pressure to ensure that the inert blanket is maintained inside the tank.
The overfill line from the tank cannot be discharge directly into atmosphere to prevent the blanket gas from escaping and must be kept sealed by a liquid seal leg. A liquid seal can be maintained by two methods. In one, an inverted seal, the discharge line is connected near the bottom side of the tank at or below the low-low liquid level, and forms an inverted U, the top of which is at an elevation that corresponds to the maximum allowable liquid level in the tank. A siphon break is provided at the top of the seal. In this design, the seal is inherently maintained by a liquid level in the tank, and only if the tank is essentially empty, is there a chance for the blanket gas to blow-by to the atmosphere. However, the location and elevation of the final discharge point of the siphon break must still meet the ‘discharge at a safe location’ requirements. In a slight variation of this design, the discharge pipe may be located physically inside the tank, i.e. extending from near the bottom of the tank, and exiting at the side of the tank, with the pipe ultimately draining to a safe disposal location. Here too a siphon break should be provided at the top and outside of the tank.
Tanks that may have slight oil sheen on the top but not blanketed may also use this inverted U-pipe design, to prevent the oil from reaching the drain, since the discharge is from the bottom water layer. The oil layer may be vacuumed out at a suitable time and disposed of safely. While calculating the height of the inverted U-loop , it is important to take into account the tank operating pressure because the height of the liquid in the discharge leg will be higher than the liquid level inside the tank by the hydrostatic head equivalent of the tank pressure. Also, if two liquid layers are formed inside the tank, the height of the liquid inside the tank at which overfill will happen will further change, because even though the tank contains the two fluids of different densities, the discharge leg will contain the fluid from the tank bottom till this layer is all drained.
Overfill Lines for Tanks Blanketed with Gas – Maximum Fill Line. In this design, the take-off point for the overfill drain is at the location of the maximum allowable fill level for the tank. The liquid seal is placed at the end of the discharge line located at or below grade. The disposal location is carefully selected in the event that the seal is lost and nitrogen blows through the seal leg. A level gage is installed at the leg to visibly monitor the presence of liquid seal and a low-level alarm is also provided to alert the operator in case of loss of liquid seal. The seal leg is designed to match the tank design pressure. Appropriate consideration must be taken into account while designing the liquid seal height if liquid layers of two different densities exist inside the tank, because the exiting liquid will be the top layer and, therefore, the fluid that will create the liquid seal.
Overfill Protection Using Instrumentation
In case of hazardous, toxic, or flammable liquid, it may not be acceptable to discharge the fluid to the atmosphere for reasons of fire, danger to personnel safety and health or its environmental impact. Tanks that contain Class I (flammable) or Class II (combustible) organic fluids fall into this category. These tanks are characterized by the presence of inert gas blanketing and absence of any overfill line. The incoming flow is stopped at the appropriate tank level or pressure by high integrity protection system (HIPS) instrumentation. Risk analysis is performed to determine the safety integrity level (SIL) required to ensure that the tank is adequately protected from overfilling. Generally a SIL of 1 or 2 is required. Most commonly, the stopping of flow is achieved by tripping the pump or closing the valve on the inlet line if the pump is protected from low flow conditions through a minimum recirculation line or instrumentation. API 2350 Figure C-1 is a sound basis to start designing such a protective system.