API STD 625
Tank Systems for Refrigerated Liquefied Gas Storage
|Publication Date:||1 August 2010|
This standard covers low pressure, aboveground, vertical, and
cylindrical tank systems storing liquefied gases requiring
refrigeration. This standard provides general requirements on
responsibilities, selection of storage concept, performance
Additional information and recommendations are given in annexes. These general requirements address issues common to all of these tank systems, issues involving coordination of the components of the tank system, and issues of the tank system acting in an integrated way. The detailed requirements applicable to the metallic and concrete containers respectively are contained in the standards named in 1.4 and 1.5.
The annexes of this standard provide additional information that may be used in the selection and design of refrigerated tank systems.
This standard covers tank systems having a storage capacity of 800 cubic meters (5000 bbls) and larger.
Stored product shall be liquids which are in a gaseous state at ambient temperature and pressure and require refrigeration to less than 5 °C (40 °F) to maintain a liquid phase.
Tank systems with a minimum design temperature of -198 °C (-325 °F) (see note), a maximum design internal pressure of 50 kPa (7 psig), and a maximum design uniform external pressure of 1.75 kPa (0.25 psig) are covered.
NOTE Note for concrete containers, that ACI 376 states it "has been developed with the lowest operating temperature of -168 °C (-270 °F). However lower product temperatures could also be used, provided appropriate additional engineering analysis and justification is performed for each proposed application."
The tank system configurations covered are described in Section 5. These configurations consist of a primary liquid and vapor containment constructed of metal, concrete, or a metal/concrete combination and, when required, a secondary liquid containment.
Metallic container materials, design, fabrication, inspection, examination, and testing shall be in accordance with API 620 including either Appendix R or Appendix Q. The applicable appendix of API 620 depends on the design metal temperature and the applicable temperature ranges given in these appendices.
Concrete container materials, design, construction, inspection, examination, and testing shall be in accordance with ACI 376. 1.6
This standard applies to tank system components attached to and located within the liquid, vapor, and any purge gas containers (but excluding dike walls). Piping connected externally to the liquid, vapor, and any purge gas containers within the following limits shall be constructed according to this standard:
a) the face of the first flange in bolted flanged connections;
b) the first threaded joint on the pipe outside the tank wall in threaded pipe connections;
c) the first circumferential joint in welding-end pipe connections that do not have a flange located near the tank.
The boundary of this standard may be extended as agreed between Purchaser and Tank System Contractor to complete, external, pressure containing piping connections (such as relief valves in 7.4 and instrumentation in 7.5) which serve only the tank system.