Flat-Bottom, Cryogenic Storage Tanks

Table of Contents


Section Title Page
Purpose 2  
Scope 2  
Related Documents 2  
Regulatory Requirements 3  
Safety Requirements 4  
Spill Prevention 7  
Economic Considerations 8  
Change Log 8  
Required Testing and Type Approval of Cellular Glass Material for Use by Air Products for Flat-Bottom Storage Tank Base‑Insulation Systems 9  
Manufacturer and Material Grades, Tested and Approved by Air Products for Use in Flat-Bottom Storage Tank Base-Insulation Systems 11  
Crush-Testing Interleaving Material Arrangement 9  






1.1       This global engineering standard defines Air Products’ minimum requirements for flat-bottom, cryogenic storage tanks.



2.         SCOPE


2.1       This standard applies to all flat-bottom, cryogenic storage tanks for liquid oxygen (LOX), liquid nitrogen (LIN), and liquid argon (LAR) storage.



3.         RELATED DOCUMENTS – Related documents not referenced are located in ESKB  Related Items


3.1       Air Products Engineering Documents


            3ES40201              Safe Design Requirements for Prevention of Cryogenic-Vapor Clouds and Liquid Spills

4WPI-SW70003      Oxygen Clean (Class AA) Inspection and Acceptance Requirements


3.2       American Iron and Steel Institute (AISI)


Document              Design of Plate Structures


3.3       American Petroleum Institute (API)


STD 620                 Design and Construction of Large, Welded, Low-Pressure Storage Tanks, with Annex Q – Low-Pressure Storage Tanks for Liquefied Gases at -325 F or warmer

STD 625                 Tank Systems for Refrigerated Liquefied Gas Storage


3.4       American Society for Testing and Materials (ASTM)


            C 240                     Standard Test Methods of Testing Cellular Glass Block Insulation


3.5       The American Society of Mechanical Engineers (ASME)


BPVC, Section II     Materials

B31.3                     Process Piping


3.6       British Standards Institution (BSI)


BS 7777, Part 4      Flat-bottomed, vertical, cylindrical storage tanks for low temperature service. Specification for the design and construction of single containment tanks for the storage of liquid oxygen, liquid nitrogen and liquid argon

(This standard has been withdrawn, but contains useful reference material)


3.7       British Cryogenic Gas Association (BCGA) Code of Practice


CP20                      Bulk liquid oxygen storage at production sites

3.8       European Committee for Standardization (CEN)

EN 826                   Thermal insulating products for building applications–Determination of compression behaviour


3.9       European Industrial Gases Association (EIGA)


IGC Doc 127           Bulk liquid Oxygen, Nitrogen and Argon Storage Systems at Production Sites



3.10    Compressed Gas Association (CGA)


CGA  P-8.9               Bulk liquid Oxygen, Nitrogen and Argon Storage Systems at Production Sites





4.1       The tanks shall be designed and constructed according to a recognized design code such as API STD 620, Appendix Q. This is to establish minimum design and construction requirements for the tanks and cover design features, testing, and NDT requirements without the need to spell this out in detail in the equipment specification.


4.1.1   BSI BS 7777 Part 4 has been withdrawn and shall not be used as the design code, however it contains useful reference material that may used to supplement the design code used.


4.2       API STD 620 covers the design and construction of large, welded, low-pressure storage tanks, and Annex Q covers the storage of liquefied gas at cryogenic temperatures down to -198°C (‑325°F). This is applicable for LOX and LIN service.


[Note:  Before Addendum 2 of Edition 11 the scope was limited to temperatures down to -168°C (‑270°F). This was not truly applicable to LOX and LIN service, however, the design methods and construction features were considered to be a good fit. Edition 11 Addendum 2 extended the temperature range down to -198°C (‑325°F).]


4.3       API STD 625 which covers tank systems is intended for hydrocarbon liquefied gas facilities and the industrial gas associations (EIGA and CGA) do not consider it to be applicable to bulk liquid oxygen, nitrogen and argon storage systems at production sites. However, it does provide some useful guidance.


4.4       The tanks and tank systems shall comply with the international harmonized standard bulk liquid oxygen, nitrogen and argon storage systems at production sites. There are regional editions IGC Doc 127 or CGA, P-8.9.


4.5       Annular space piping shall be designed and constructed according to a recognized design code such as ASME B31.3.


4.6       Oxygen tanks for the United Kingdom shall also comply with the applicable sections of BCGA Code of Practice CP20. This document requires 100% radiography of all butt welds below the liquid level, and compliance is viewed favorably by the regulating authority when considering requirements for periodic inspection/validation of tanks.

4.7       Inspection is required to ensure that the tank construction is of the required standard. When permitted by local requirements, the inspection may be performed by one of the following:


  • The tank supplier
  • The Air Products construction supervisor
  • A third-party inspection organization or competent person


In all cases the responsible person shall have appropriate experience.





5.1       Pressure Relief Considerations


5.1.1   The inner vessel of each storage tank shall be fitted with three full-size pressure relief devices. This exceeds that required by IGC Doc 127 and/or CGA, P-8.9 which requires a minimum of two independent in service pressure relief devices with set pressure no higher than the max allowable working pressure (MAWP). It is not practicable to have a relief valve and bursting disc both with set pressure at or below the MAWP and for the relief valve set pressure to be sufficiently low to ensure that bursting disc does not rupture before the relief valve functions. As such the three devices shall be two pressure relief valves and a rupture disc, this applies to Air Products owned-and-operated tanks and for sale-of-equipment tanks. No more than two of the pressure relief devices may be connected to the same pipe or nozzle to avoid rendering more than two of these devices inoperative by the blockage of one inlet pipe or nozzle.


5.1.2   The set pressure of the two pressure relief valves shall be no higher than the design pressure of the inner vessel. The bursting disc shall have a burst pressure set point no greater than 1.25 times the design pressure of the inner vessel at 21°C (70F). A maximum tolerance +/- 10% on the burst pressure set point is permissible. The pressure safety valves and bursting disc shall be mounted so that it is/they are readily and safely accessible.


5.1.3   The relief valves shall be pilot operated and shall be of a type that has dual pressure/vacuum operation. Vacuum protection is required to admit atmospheric air into the inner tank if the pressure drops below the design vacuum value.


5.1.4   A manual valve shall be installed between each relief device and the storage tank to enable a single device to be temporarily isolated for maintenance or replacement purposes. A captive key interlock system shall be used such that only one pressure relief device can be isolated at a time. The interlock key must be used to unlock the mechanism to close the manual valve; the key is retained in the locking mechanism. The key shall be such that it can be removed only if the manual isolation valve is in the wide-open position.


5.1.5   Instrument connections are permitted on the relief device nozzles. There shall be no other devices or piping connections on the relief device nozzles. This is in case the connected devices fail in any way, admit ambient air or moisture into the tank, or cause pressure fluctuations that might interfere with the operation of the relief devices.


5.2       Nozzles and Piping


5.2.1   Wall thickness of liquid piping shall be Schedule 40 minimum, the stub penetrating the outer tank thermal distance pieces shall be schedule 40 for both liquid and vapor piping. These requirements are to ensure that the piping is basically robust, especially at the outer-tank junction, to minimize the possibility of failure of the internal piping from imposed external loadings. (Reference 3ES40201, Safe Design Requirements for Prevention of Cryogenic-Vapor Clouds and Liquid Spills, Hazard 40201.10.)


5.2.2   Transition joints are not permitted in the annular space because of the possibility of leakage and the lack of access to these joints for inspection purposes. This requirement applies to aluminum tanks for which a supplier might consider a transition to stainless steel piping.


5.2.3   Butt-welded nozzle connections are standard, except for relief device connection that shall be flanged.


5.2.4   Stainless steel piping may be of the welded type because experience has shown this product form to be of a high quality, essentially equivalent to seamless pipe. Welded pipe is manufactured by an automatic process without the use of filler metal. Air Products has used tens of thousands of feet of electric fusion welded pipe in all types of service. There have been no problems with leaks or failure in the longitudinal welded seams of stainless steel pipe.


5.2.5   Aluminum shells and piping is prohibited.



5.3       Design Issues


5.3.1   The cylindrical height of the inner tank shall include sufficient freeboard that liquid does not impact the tank roof during earthquake sloshing or enter the roof space as a result of liquid expansion. The amount of freeboard shall not be less than the greater of the calculated slosh height or 1% of the cylindrical shell height.


5.3.2   Pad-type attachments below the liquid level in liquid nitrogen or liquid argon service shall be stitch welded to avoid problems with the expansion of trapped liquid. Pad-type attachments below the liquid level in LOX tanks are not permitted, because of the additional hazard of hydrocarbon accumulation (see paragraph 5.4.3).


5.3.3   Liquid outlet lines shall have no upward loops and shall have a slight downhill pitch from the inner tank to the outer-tank penetration. The magnitude of the slope shall not be less than 1 in 40 and preferably not less than 1 in 25 to ensure that any vapor formed flows back into the storage tank rather than being trapped (and thus reducing the flow path) or flowing towards the downstream equipment. Horizontal piping runs should be avoided or minimized if unavoidable.


5.3.4   Because of liquid argon’s higher density compared to that of the test fluid, its applications require special consideration regarding pressure testing.


5.4       Oxygen Service


5.4.1   The use of aluminum alloys is not permitted in the storage of liquid oxygen for safety reasons, with the exception of certain piping components in the relief valve assemblies. This requirement has been extended to prohibit the use of aluminum alloys in liquid nitrogen and liquid argon tanks.


5.4.2   Tanks in oxygen service must be cleaned according to 4WPI-SW70003 to meet oxygen clean (Class AA) acceptance criteria.


5.4.3   To ensure that the required cleanliness is achieved, crevices must be avoided in oxygen service. Therefore, lap-welded roof joints and internal roof support beam joints shall be seal-welded. Any internal attachments below the liquid level shall be welded with full penetration welds, and there shall be no pad-type attachments inside the inner tank below the liquid level.


5.4.4   Backing rings shall not be used on stainless steel pipe in oxygen service. One backing strip for a manway closure is permitted in LOX tanks. If backing strips are used, they shall be self draining, to avoid hydrocarbon accumulation behind them.


5.5       Construction Considerations


5.5.1   The integrity of the inner-tank anchor system is critical. All load-carrying weld joints in the inner-tank anchor system shall be 100% nondestructively tested. This shall be accomplished by radiography when possible. Load-carrying fillet welds shall be made with a minimum of two passes and shall be subject to dye penetrant examination.


5.5.2   When inner-tank anchor straps are used, they shall be provided by the tank supplier and set in the concrete foundation by Air Products. To facilitate installation and to provide clear access to the concrete foundation, the straps shall be spliced above the top of the concrete. This field weld is critical to the integrity of the tank and must be fully radiographed.


5.5.3   It is important to prevent the possibility of frost heave under the tank foundations. For tanks supported on a pilecap with adequate ventilation underneath, no action is necessary. A minimum air gap of 1.0 m (3.3 ft) is required to ensure adequate air circulation. Foundations resting directly on the ground must be equipped with foundation heaters.



5.6       Perlite:  The inner tank shall be designed for a minimum perlite compaction pressure of 0.07 bar g (1 psig). Unless otherwise stated in the applicable design code, the number and size of stiffening rings shall be calculated using the methods given in the AISI document “Design of Plate Structures.” A safety factor of 2 shall be applied in the design of stiffener spacing, and a safety factor of 2 shall be applied in the design of stiffener sizing.


5.7       Cellular Glass—Base Insulation


5.7.1   Permissible cellular glass insulation loading is a function of interleaving material, but shall provide a minimum safety factor of 2.5 against compressive collapse under normal design conditions, and a safety factor of 2.0 against compressive collapse under seismic loading.


5.7.2   Cellular glass insulation shall have been subject to standard, single-block compressive load testing during manufacture. These standard tests employ bitumen capping on the compression faces of the test blocks to ensure even load distribution and to minimize cell crushing. The use of bitumen is prohibited in the annular space of LOX tanks. Air Products uses interleaving materials such as glass cloth and inorganic powder to aid even load distribution and to minimize cell crushing. The standard tests do not address the mode of failure, failure displacements, and post-failure behavior, which are of critical importance to the intended use of the product.


5.7.3   The cellular glass products used shall have been tested under conditions representative of actual use and type approved by Air Products for use in the base insulation system of flat‑bottom storage tanks. The required testing and acceptance criteria are presented in Appendix A. The suppliers and their grades of material that currently meet this requirement are listed in Appendix B.


5.7.4   Tests have shown that approximately 75% of the published (by the cellular glass manufacturer) average compressive strength value can be achieved by using a combination of dry, inorganic powder to fill the open-cell structure on the top and bottom of cellular glass block and using an oxygen-compatible glass cloth to separate layers of cellular glass blocks. This reduction factor incorporates a stacking factor that accounts for the fact that the base insulation can be up to ten blocks high.


5.7.5   When the above system of powder and cloth is used, 75% of the published average compressive strength value of the approved cellular glass may be used to determine the allowable design loading.


5.7.6   When tests are performed using two blocks as detailed in Appendix A, the average compressive strength values do not always decrease if the inorganic powder and/or glass cloth is omitted. The top and bottom surfaces of the cellular glass blocks crush down to produce even load distribution. A negative effect of their omission is increased displacements as a result of continued “crush down” of the blocks surfaces. Acceptable displacements of a 10 mm/meter (0.12 in/ft) maximum for operational loadings are considered practical for tank design. For this reason the glass cloth and powder are normally both required.


5.7.7   Alternative interlayer systems and strength reduction/stacking factors may be used if test data is available to justify the design.


5.7.8   The inner tank wall shall not rest directly on the cellular glass insulation but rather on a concrete footing on top of the cellular glass blocks to spread the load. The edge of the tank tends to lift up and down as the tank pressure fluctuates, and the high local loads under the tank wall will otherwise degrade the cellular glass over time.

5.7.9   The thickness of the cellular glass insulation between the inner and outer tank bases affects the temperature of the foundation pile cap. Consideration must be given to the allowable temperature of the foundation pile cap and the acceptability of ice formation on the underside of the foundation pile cap when accepting a minimum thickness of cellular glass. If insufficient thickness is used, the foundation pile cap reinforcing bar might need to be stainless steel because of the low temperature. The temperature of the underside of the foundation shall be no less than 10°C (18°F) below the ambient air temperature.





6.1       It is intended to limit the number of liquid connections in the bottom of the tank for safety reasons. Failure of these connections can result in release of the tank contents.


6.1.1   Liquid outlet line emergency shutoff valves are not required inside the inner vessel when the tank is elevated and external emergency shutoff valves are located beneath a raised foundation. When the external emergency shutoff valves are not located beneath a raised foundation, each liquid outlet line shall have an emergency shutoff valve located inside the inner vessel. (Normally, external emergency shutoff valves are not included in the tank supplier’s scope.)


6.1.2   Liquid instrument lines are limited to a maximum size of DN15 (NPS 1/2) and the lines may exit via the outer-tank sidewall. Internal and external emergency shutoff valves are not required on these lines.


6.1.3   The liquid inlet line shall enter the inner tank above the maximum liquid level and shall extend downward to near the bottom of the tank. A method of breaking the siphon shall be provided to prevent spillage of tank contents if there is an external piping failure. This both minimizes connections through the tank floor and prevents direct discharge into the vapor space, which would collapse the vapor pressure and produce a vacuum condition in the top of the tank.


6.2       All weld joints in piping between the inner and outer tanks and connected to the inner tank below the liquid level shall be subjected to 100% radiographic inspection. (Reference 3ES40201, Safe Design Requirements for Prevention of Cryogenic-Vapor Clouds and Liquid Spills, Hazard 40201.10.)


6.3       There shall be no flanged joints, transition joints, bellows, or flexible metal hoses used in liquid piping between the inner and outer tanks.


6.4       All piping between the inner and outer tanks shall be provided with sufficient flexibility to permit movement resulting from thermal contraction. When stress analyzing the piping, they shall be considered fixed in the translational and torsional directions at both the inner and outer tanks. It is permissible to account for the tank wall bending flexibility in two planes at the connection with the shell. The piping shall be designed so that the maximum combined stress resulting from pressure, dead load, and thermal movements does not exceed the maximum allowable stress value indicated in ASME BPVC, Section II for liquid piping and ASME B31.3 for gas-phase piping. In addition, all lines shall be adequately supported so that vertical deflections resulting from dead weight do not exceed 13 mm (1/2 in).


6.5       Branch lines may be connected to liquid outlet lines outside the tank, as part of the tank fabricator’s supply, upstream of the manual shutoff, if the branch is made with a butt‑welded, forged fitting. Fabricated tees, sockolets, or weldolets are not permitted. This requirement ensures robust, high-integrity construction.


6.6       All flat bottom tanks shall be provided with an overfill protection system. (Reference 3ES40201, Safe Design Requirements for Prevention of Cryogenic-Vapor Clouds and Liquid Spills, Hazard 40201.9.)


6.6.1   Filling a flat-bottomed tank to levels above the top of the cylindrical shell can result in an uplifting, hydrostatic force on the inner tank dome that may cause the failure of a number of components including the inner tank anchor/hold down straps, the inner tank shell to floor joint or the inner tank shell to roof joint. The relief valves do not protect the tank against the overfilling hazard.


6.6.2   It is imperative that the liquid level does not enter the dome of the inner tank. The countermeasures required by 3ES40201 to halt liquid flow into the tank upon overfill detection may require that an increased freeboard in the tank be provided to accommodate the flow of  liquid inventory from upstream and/or downstream process or storage systems. This is to cover situations where the inlet shut off valves fail to close on demand and the liquid production equipment has to be tripped.




7.1       When determining the most cost-effective tank design or supplier offering, the following factors shall be considered:


7.1.1   A low tank design pressure results in low tank cost but greater relief valve cost and greater flashing losses. An economical design pressure has been shown to be 0.14 bar g (2 psig).


7.1.2   The volume of concrete in the tank foundation, which is a function of tank diameter.


7.1.3   Product loss is always a consideration on an Air Products tank and might or might not be for a third-party sale. The equipment specification should state the capital equivalent of lost product to enable the tank supplier to select the optimum insulation thickness for minimum total cost. The evaluation should account for the value of lost product.



Appendix A                                                              


Required Testing and Type Approval of Cellular Glass Material for Use by

Air Products for Flat-Bottom Storage Tank Base-Insulation Systems


A1.       Required Testing


A1.1    The test shall be performed by the party proposing the use of that cellular glass product in an Air Products storage tank. Probably, this will be the cellular glass manufacturer, but may be the tank supplier or Air Products.


A1.2    The test materials shall be provided by the cellular glass manufacturer.


A1.3    The interleaving materials shall be provided by the cellular glass tester. Fresh material shall be used for each test.


A1.4    The tests shall be subject to third-party inspection at the tester’s cost.


A1.5    Air Products shall witness the tests.


A1.6    Test blocks shall as a minimum be 200 x 200 mm and be 100 mm depth (8 x 8 x 4 in).


A1.7    Each test shall comprise of two test blocks stacked on top of each other between two steel loading plates that are each 220 x 220 x 20 mm thick (8 1/2 x 8 1/2 x 3/4 in). The required combinations of interleaving materials shall be inserted between the two blocks and between each block and steel loading plates. See Figure A1.1.


Figure A1.1


Crush-Testing Interleaving Material Arrangement



Appendix A (continued)


A1.8    As a minimum, the following tests shall be performed:


Test Number of Tests
Cellular glass blocks with glass cloth and inorganic powder 12
Cellular glass blocks with glass cloth and inorganic powder (at cryogenic temperature) 5
Cellular glass blocks with glass cloth only 12
Cellular glass blocks with inorganic powder only 12


The cryogenic temperature tests are only indicative to demonstrate that material retains its load‑bearing properties under those conditions.


A1.9    The compression-testing machine shall be suited to the range of force and displacement involved and have two very rigid, polished, square or circular plane parallel plates of which the length of one side (or the diameter) is at least as large as the test specimen side (or diagonal) to be tested. One plate shall be fixed and the other movable, if appropriate, with a centrally positioned ball joint to ensure that only axial force is applied on the test specimen.


A1.10  A constant loading rate of 2 mm/min (5/64 in/min) ±25% shall be applied. The load shall be increased until the maximum load-bearing capacity of the blocks has been demonstrated. Loading shall be continued until a minimum displacement of 20 mm (3/4 in), which is 10% deformation of assembly, is achieved.


A1.11  A load-displacement graph shall be generated for each test.



A2.       Acceptance Criteria for Tests


A2.1    The load-bearing capacity after initial failure shall not reduce by more than 10% if the initial failure load is to be considered applicable. A typical acceptable graph is shown in Figure A1A. If the load-bearing capacity after initial failure has been reduced by more than 10% of the initial failure load, the plateau value of load or the value at 20% deformation if a plateau is not achieved shall be used. A typical graph showing this latter behavior is shown in Figure A1B.


  Figure A1A        Figure A1B

A2.2    Of the twelve tests performed for each interleaving material combination, two of the tests may be disregarded. The result from the remaining ten tests shall be averaged.


A2.3    The average displacement at initial failure shall be less than 4 mm (5/32 in) [20 mm/m (1/4 in/ft), this twice the maximum allowable displacement of 10 mm/m (1/8 in/ft) for operational loading].


A2.4    The average compressive strength at initial failure shall be not less than 75% the manufacturer’s published average compressive stress for those values to be approved.


Appendix B                                                                 


Manufacturer and Material Grades, Tested and Approved by

Air Products for Use in Flat-Bottom Storage Tank Base-Insulation Systems







    Air Products Cellular Glass Strength Grade
    AP-8.0 AP-10.0 AP-12.0
  Guaranteed Average Compressive Strength (1)

– Bar [psi]

8.0 [116] 10.0 [145] 12.0 [174]
  Pittsburgh Corning Foamglasâ HLB 800, F(2) & S3 HLB 1000 HLB 1200
  Cell-U-Foam Ultra – CUFâ (5) 116 145 174
  JiaXing Zhenshen (3) ZES-1000  (4) N/A N/A





  • To CEN EN 826/ASTM C240.


  • Although Pittsburgh Corning grade F has high initial failure loads, the post failure load-carrying capacity reduces to approximately 50% of the initial failure values. Therefore, allowable strength of Pittsburgh Corning grade F is derated to that of a 8.0 bar published compressive strength cellular glass.


  • JiaXing Zhenshen cellular glass is not subject to rigorous quality assurance testing as standard by the manufacturer. Therefore it shall be purchased against DOC000095511, which defines minimum requirements for in process testing of cellular glass to be used for base insulation in cryogenic storage tanks.


  • JiaXing Zhenshen ZES-1000 has a published compressive strength of 10MPa. Based on approval testing by Air Products, it has been down rated to that of 8.0 bar published compressive strength cellular glass.


  • Cell-U-Foam has been purchased by Pittsburgh Corning. Ultra–CUF material might not be available.





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