Draft Section 6, Rev A
API 625, Aug 3, 2007
Draft for API Refrigerated Tank Task Group
SECTION 6 DESIGN CONSIDERATIONS
Draft No. A
Date: August 3, 2007
Section 6 Leader: Sheng-Chi Wu
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SECTION 6.1 Tank Sizing and Spacing Requirements
(drafted by Anant Thirunarayanan)
6.1.1 General
Design of the refrigerated tank system requires considerations that shall be given while evaluating the available site with reference to the following:
a. Sizing of the tank
b. Spacing of the tank
6.1.2 Sizing Considerations
(included later)
6.1.3 Spacing Considerations
Refrigerated tank system shall be within a secondary dike or impoundment area depending upon the type of the tank system. The spacing shall be provided in such a way that it is within the thermal radiation protection zone beyond the impoundment area. The thermal radiation protection zone shall be large enough so that the thermal radiation flux from a product fire does not exceed the limit specified by corresponding NFPA standard for people and property.
The siting and layout of the tank system shall be evaluated based on relevant NFPA guidelines and local regulations having jurisdiction of the plant. The thermal radiation distances shall be calculated by a suitable analysis. The wind speed, ambient temperature and relative humidity of the site shall be used in the analysis. The analysis shall also address phenomena such as spills, fires, vapor cloud dispersion apart from thermal radiation in relation to lay out and safe distances from the tank system.
SECTION 6.2 Maximum Design Liquid Level (drafted by Anant Thirunarayanan)
The tank shell shall be designed to the maximum design liquid level. This level is higher than the normal maximum operating level of the tank. This will include the sloshing height for seismic conditions as required in Appendix L of API 620 in cases where the tanks are located in seismic zones. The net capacity of the tank for refrigerated product is basically governed by the normal maximum and minimum operating levels.
SECTION 6.3 Performance Criteria (drafted by Jack Blanchard)
Design and erection of tank systems in accordance with this standard shall satisfy the performance criteria of this section.
6.3.1 Normal Operating
The primary liquid container shall contain the liquid and the primary vapor container shall contain the vapor under all normal operating loads and conditions.
6.3.2 Abnormal and Emergency Conditions
Performance criteria for primary and secondary liquid and vapor containment are defined with the individual loading definition below.
6.3.3 Commissioning and decommissioning
The tank system shall be provided with components that allow the criteria in Section 12 to be met. In addition, the tank system must be capable of being decommissioned including purging to a gas to air mixture considered safe for personnel access.
6.3.4 Boil-off
The tank insulation system shall be provided to produce a boil-off rate below the rate required by the plant design or maximum specified by the owner. The boil-off rate, normally specified in percent per day of maximum normal operating capacity assuming a pure product, shall be based on climatic conditions as specified for the project.
Climatic conditions that are normally considered in the design include:
Maximum daily mean temperature (over 24 hour period)
No wind
Solar radiation effects
6.3.5 Cool Down
The tank system shall include a separate fill line specifically for cool down of the tank. The system shall have a means for control of the flow to meet the cool down rates defined in Section 12. For products stored at temperatures below -600 F (-510 C), the cool down line shall incorporate spray nozzles and shall introduce liquid near the top center of the primary liquid containment tank.
6.3.6 Roll-Over
The tank system shall include a means for measuring changes in density over the full liquid height as a means of detecting potential roll-over conditions. A means of mitigating a detected stratification, by recirculation or other means shall be included. If the tank system can receive product at more than one density, a top and bottom fill shall also be provided.
6.3.7 Design Metal Temperature
Primary and secondary liquid containment tanks and process lines carrying liquid or gas:
Generally, design metal temperatures shall be selected no higher than the pure product boiling temperature at one atmosphere (See Appendix A). However, design conditions such as introduction of sub-cooled product or a cool down procedure which eliminates the warm product purge may require a lower design metal temperature.
Primary vapor containers subjected primarily to ambient temperatures.
Design metal temperatures shall be equal to the lowest one day mean ambient temperature for the locality involved.
Local areas of the primary vapor container exposed to lower than ambient conditions:
Areas such as process nozzle thermal distance piece connections to the vapor container may be subjected to temperatures below ambient conditions. The design metal temperature for these locations shall take this local cooling effect into consideration.
6.3.8 Differential Movements
The design of the tank system shall consider differential movements between tank components resulting from differential design temperatures and erection vs. operating temperature. Components that are restrained from free differential movement shall be designed to incorporate flexibility as the thermal gradient is run out.
6.3.9 Foundation Settlement
Design of Liquefied Gas storage tank systems must consider how predicted settlements (both short term and long term) can affect components such as:
· the bottom insulation system
· the steel or concrete primary liquid container
· the concrete outer wall of Full Containment LNG tanks
· post-tensioning system
· the various tank attachments
· piles or other structural support systems
Depending on the loads, temperature, time and other design parameters, settlement effects can result in stresses that are either positive or negative for the structure design.
Tank settlement patterns and resultant tank distortions can be very complex and unpredictable. Important factors that can affect how a tank reacts to settlement include heterogeneous soils (both vertically and horizontally), variable as-built distortions, and sensitivity of structural details to movement.
Predicted settlements shall be determined as part of the site specific geotechnical study. Soil Improvement, as determined necessary by design of the tank system may be applied to reduce the predicted settlements.
Settlement can be separated into four specific types: uniform settlement, global tilt, differential center-to-edge settlement and differential circumferential settlement. Values provided below are intended for guidance. Variations from these values are acceptable if accounted for in the design of the tank and foundation system.
i) Uniform settlement: The amount of acceptable uniform edge settlement is dependent upon the connections to the tank.
ii) Global Tilt: Global tilt (also addressed as planar tilt) is associated with rigid body rotation of the superstructure caused by non-uniform soil across the width of the structure. While large tanks may be able to accommodate significant uniform tilt without damage, other components usually require lower values of tilt.
Global tilt, measured in inches, of a flat bottom tank shell should be limited to 5 times the tank diameter divided by the height.
iii) Differential Center-to-Edge Settlement: Liquefied gas tanks are constructed with self supporting roofs. Differential settlement between the center and the edge does not affect the roof. While bottom plate can accommodate significant settlement, tank internals supported by the bottom and the bottom insulation system inherent in liquefied gas tanks cannot. Significant short or long term settlement of the bottom can crack or damage the bottom insulation system which would seriously increase the heat leak of the structure, potentially causing freezing of the soil under the tank.
Differential settlement between the edge of the tank and the center should be limited to the radius of the tank divided by 240.
iv) Differential Circumferential Settlement: Irregular settlement of soils around the periphery of a tank can cause out-of-roundness and localized distortions and buckles in a tank. These can affect the stability or the performance of the tank.
Differential settlement around the periphery of the tank should be limited to 3/8 inch in an arc radius of 30 feet (1:1000).
In monitoring the settlement, an independent datum reference point located beyond the influence of local foundations shall be established. Permanent markers shall be provided to facilitate settlement monitoring around the perimeter of the foundation at a minimum of 8 locations not more than 30 ft apart. In addition, for concrete outer wall tanks and for settlement conditions that are expected to approach the design limits set for the tank, provisions shall be made to measure dishing settlement. (An inclinometer system can be provided to accomplish this requirement.)
6.3.10 Protection from freezing of Soil
Soil located below foundations is subject to thermal heat leak through the bottom insulation system. This can lead to freezing of the soil progressing to frost heave and potential tank system damage. The tank foundation design shall include a means to maintain the soil at a temperature above 32oF (0oC).
Foundation heating systems shall be designed to allow replacement of individual heating elements and to protect against the accumulation of moisture within the cable conduits.
6.3.11 Concrete Steel Liner
Performance criteria for steel liners, forming a liquid or vapor barrier for a concrete component, are defined in ACI 376.
6.3.12 Concrete Reinforcement
Concrete reinforcement requirements for concrete components including concrete bearing rings placed under the primary liquid container shall meet the requirements of ACI 376.
6.3.13 Design Considerations for Concrete Tanks
Requirements for the design, erection, inspection, and testing of concrete primary and/or secondary containment tanks that make up part of the overall tank system are found in ACI 376. Performance criteria specific to the concrete components of a tank system are located in ACI 376 Part 4.
6.3.14 Seismic Ground Motions
Tanks designed and built to this standard shall be designed for three levels of seismic motion. The magnitudes of the seismic ground motions are defined in Section 6.4.2. In addition, the design shall meet the requirements of the applicable local building codes.
a) Operating Basis Earthquake (OBE): The tank system shall be designed to continue to operate during and after OBE event. The OBE is also referred to as Operating Level Earthquake (OLE) in API 620, App L.
b) Safe Shutdown Earthquake (SSE): The tank system shall be designed to provide for no loss of containment capability of the primary container and it shall be possible to isolate and maintain the tank system during and after SSE event. The SSE is also referred to as Contingency Level Earthquake (CLE) in API 620, App. L.
c) Aftershock Level Earthquake (ALE): The tank system, while subjected to ALE, shall provide for no loss of containment from the secondary container while containing the primary container volume at the maximum operating liquid volume.
SECTION 6.4 Design Loads and Load Combinations
(drafted by Rama Challa)
6.4.1 Design Loads
The following types of design loads shall be considered in the design of the liquefied gas containers and foundations. EN 14620-1.2006 Section 7.3 and ACI 376, Chapter 3 provide guidance on the types of the design loads and load combinations to be used. They include, but not limited to the following:
1. Dead loads
2. Product Pressure and Weight
3. Maximum internal pressure
4. Construction-Specific Loads
5. Testing and Commissioning Loads such as test, vacuum and pneumatic tests
6. Thermally-induced loads experienced during tank purging, cooling and filling
7. Shrinkage and Creep-Induced Loads
8. Pre-stressed loads for concrete container
9. Live Loads
10. Environmental Loads such as Snow, Wind and ice loading
11. Loads induced by differential settlement
12. Seismic Loads (OBE, SSE & ALE, defined below)
13. Liquid spill condition
14. Loads based on a risk assessment such as fire, blast, external missile etc.
6.4.2 Seismic Loads
Probabilistic seismic hazard studies are required to determine the seismic ground motions for design of tank-fluid-foundation system. Three levels of the seismic ground motions shall be considered:
a) Operating Basis Earthquake (OBE): The OBE is defined as the seismic ground motion having 10% probability of exceedance within 50 year period, i.e. 475 year recurrence interval.
b) Safe Shutdown Earthquake (SSE): The SSE is defined as the seismic ground motion having 2% probability of exceedance within 50 year period, i.e. 2475 year recurrence interval.
c) Aftershock Level Earthquake (ALE): The ALE is defined as half of the SSE.
6.4.3 Load Combinations
The design loads shall be combined to produce load combinations to be used in the analysis and design of the liquefied gas containers. Load combinations are dependent on the material type and governing codes used for the container.
1. Liquefied gas steel containers are designed per the rules of API-620. The basis of the design is the Allowable Stress Design with a direct combination of the design loads. The load factor of unity should be used in the load combinations. The allowable stresses can be increased when the following transient loads are combined with the normal operating loads:
Test loads: 25% increase
Wind or OBE: 33% increase
For load combinations that involve SSE, fire, blast, loads etc: yield stress can be used.
2. Liquefied gas concrete containers are designed per the rules of ACI-376. The basis of the design is ultimate strength design with factored load combination of the design loads. Chapter 3 of ACI 376 provides guidance on the load combinations to be used.
SECTION 6.5 Seismic Analysis (drafted by Jack Blanchard and Sheng Wu)
6.5.1 The tank system shall be designed for three levels of seismic ground motions as defined in Section 6.3.14 and 6.4.2 above. The rules in API 620 Appendix L for LNG tanks shall be applied to all steel tanks designed to this standard. The rules of ACI 376 shall be applied to all concrete tanks designed to this standard.
6.5.2 The site-specific horizontal and vertical acceleration response spectra shall be developed for both OBE & SSE for different damping values of up to 20%.
6.5.3 The ALE earthquake shall be considered only for the seismic design of secondary containment with full liquid condition, assuming that the primary container is damaged after the SSE event.
6.5.4 When the tank foundation is supported on rock-like site (defined as the site class A & B in IBC or ASCE 7), the fixed base condition is considered. In this case, the structural damping values shall be used for determining the seismic responses