Field Fabricated Cryogenic Tanks


Reference Documents
28-3CS02008 Foundation Design for Elevated, Cryogenic, Field-Erected, Flat-Bottom Storage Tanks
28-3EQ15001 Flat Bottom Cryogenic Storage Tanks
28-4WCB50001 Perlite for Cold Boxes


Field-Erected, Flat-Bottom, LOX and LIN Storage Tanks (Pressure Vessel Design)
28-4WEQ1526 Internal Shutoff Valves for Field-Erected, Flat-Bottom, LOX and LIN Storage Tanks
28-4WEQ6001 Cellular Glass Insulation for Field-Erected, Flat-Bottom, Cryogenic, LOX and LIN Tanks
28-4WEQ6804 Painting and Corrosion Protection for Operating Temperatures to 427C (800F)
28-600277 Perlite Loading of Equipment Jackets and Field Erected Storage Tanks



  1. Preparation
    1. The field must have the latest revisions of the approved tank drawings, specifications, and the schedule.
    2. If the vendor or contractors’ procedures are being used, Air Products design must approve them prior to contractor usage. Particularly for welding, testing, cleaning, and perliting.
    3. All weld procedure specifications (WPS) and Procedure Qualification Records (PQR) must be on hand and approved by Air Products Specifying Engineer prior to any welding.
    4. Welder qualifications shall be submitted to the Air Products representative for each type of weld being performed by each welder.
    5. Before starting construction, a kick-off meeting should be held with the tank contractor’s superintendent so that all requirements are completely understood. This shall include:
      1. Site Setup – Assign lay down and work areas for materials, equipment, and roof fabrication, as well as determining at what stage in construction the tank will be classified as a confined space.
      2. Quality Control – Go through the specifications and parts of this audit with the contractor supervisor so that Air Product’s inspections and sign-offs are known.
  • Schedule – Develop and agree to schedule and manpower loading. Discuss any unusual job specifics that may cause schedule problems.
  1. In addition to the typical weekly meeting topics, make sure to include such items as:
    1. The inspections that will occur within the next week, and what coordination or preparation will be required.
    2. Discuss the strategies for items such as external scaffold, internal scaffold, roof fabrication, cleaning, and other upcoming work phase issues. Keep in mind safety, schedule, other contractor’s space requirements and coordination.
  2. Make sure the actual properties for all certified materials are as specified in the specification, design codes, or the purchase order. A PMI test may be needed to confirm materials supplied.
  3. Examine all the stainless plate. Check certificates against identification marking on materials.  Make sure there are no defects and that the plate thickness is according to the design requirements.
  1. Civil Work Concerns
    1. Please review Audit #04 for general concrete quality audit requirements.
    2. Most tanks will have three concrete pours: The base foundation, the cellular glass leveling slab, and the inner tank support slab.
    3. Make sure the concrete contractor understands concrete specification requirements, mixing procedure, installation method, levelness tolerances, and curing.
    4. Usually in the base foundation there are a number of embeds and piping penetrations. Make sure all of these are in place before the contractor begins placing concrete.
    5. If the base foundation is on piers or piles, spiral type ties may be required.
    6. In each concrete pour the relative dimension tolerance for slab elevations under the tank is 6.5mm (0.25 inches) over 6.1 meters (20 feet). Take several readings from a common datum point.
    7. The inner tank anchor straps or supports may need to be boxed out for the cellular glass leveling slab and the inner tank support slab pours.
    8. Obtain written acceptance of the tank foundation dimensions, levelness, anchor strap and bolt locations, and other civil concerns from the tank erector before they start work.
  2. General Tank Shell and Roof Welding Work
    1. Check the tank bottom warpage after all welding is complete. Measure the amount of warpage and report to the home office if it is more than 25mm (1 inch).
    2. Witness vacuum box testing on the tank bottom and roof plates, including shell to bottom corner joint.
    3. Examine shell and roof plates for proper fit up or damage.
    4. Inspect shell thickness of various courses to ensure they are installed per the drawings.
    5. Make sure that the proper weld procedure specifications are being used.
    6. Make sure the correct weld seam setups are in place before welding, such as edge preparation and alignment.
    7. Perform visual inspections of all welding: vertical and horizontal seams, roof seams, piping penetration seams, piping nozzle or attached fitting seams, and so forth.
    8. Check size, locations, and welding of stiffener rings, if applicable. Gaps in the stiffener, known as “mouse holes” are required around vertical seam welds.
    9. Verify that the shell-to-roof weld and compression ring welding are per specifications.
    10. Verify that the top man way is installed as designed or as detailed in the specifications.
    11. Before pressure testing, confirm that all radiography and Non-Destructive Tests have been performed, reviewed, and accepted.
    12. All radiography needs to be identified for future reference by a weld map or a similar drawing.
  3. Special Concerns for Outer Tank Shell Welding
    1. Filler plates need to be installed to seal cutouts for anchor bolt sleeves and straps under bottom plate.
    2. Verify that the shell-to-bottom corner weld is per the specifications. Full fillet weld outside and full fillet weld inside.
    3. Before the inner tank construction begins make sure that the outer tank bottom has been vacuum tested.
    4. Inspect welding of the exterior anchor bolt chairs.
    5. A mastic seal is required around the outside bottom of the outer shell.
    6. Make sure that the inner tank support structures are as designed and the parts touching the inner tank are of the same material to avoid contamination.
    7. For stairways and handrails, make sure that the orientation of the stairway is correct, that the treads are uniformly installed and fully welded to tank shell, and the first step elevation is correct. Additional information on stairways and handrails is in Audit #08.
  4. Special Concerns for Inner Tank Shell Welding
    1. Verify shell-to-bottom corner weld, both inside and outside is per the design. It should be a full penetration weld with a fillet on either side.
    2. Examine edges of openings in shell and roof plates for nozzles to ensure that these are free from laminations and other defects.
    3. Make sure the proper welding wire is being used, especially for the stainless or 9% nickel walled tanks. Conduct welding rod audits regularly.
    4. Confirm that the proper weld wire is being used to weld all temporary supports and brackets on the shell. Otherwise the base metal will become contaminated.
    5. Confirm that the brackets for temporary supports are the proper metal or are “buttered” to protect the base metal.
    6. The structural integrity of the inner tank critical. Load bearing full penetration welds need to be radiographed.  All fillet welds need to be made with two passes.
    7. Check that the dimensional tolerances are as per the design requirements and as detailed in specification 28-4WEQ1516.
    8. All dye penetrant test materials need to be removed from both inside and outside the tank after dye penetrant testing is complete.
    9. Make sure that all scaffold clips, temporary brackets, waste lumps of weld, and other projections on the shell have been removed before pressure testing or cleaning.
  5. Piping work
    1. More information on piping work can be found with Audit # 07.
    2. No additional welds in the piping are allowed other than those shown on approved drawings. Verify this for both shop fabricated and field erected piping.
    3. All piping that was shop fabricated needs to be pre-cleaned and capped.
    4. Inspect piping orientation and location. Check prior to cutting holes in tank shells.
    5. Clean and inspect cleanliness of all piping prior to installation.
    6. Inspect pipe-to-shell connections to ensure they are per drawings:
      1. Proper pipe thickness to Air Products specifications on shell attachments.
      2. Required reinforcing plates installed on shell connections.
  • Liquid piping wall thickness and radiographed to Air Products specifications.
  1. Flanges for liquid piping DN75 (NPS 3) and smaller shall be 300# rating. Flanges for liquid piping larger than DN75 (NPS 3) shall be 150# rating. Flanges shall have 125 to 250 RMS finish.
  1. Make sure liquid level lines are installed as per the drawing, including their supports.
  2. 100% radiography is required on all internal liquid piping. It is recommended that 100% radiography be continued to the first block valve on the exterior of the tank.
  3. 30% of joints shall be 100% radiographed on all internal vapor piping.
  4. A weld map needs to be made for identification and location of all piping radiographs.
  5. Inspect thermal breaks where inner tank piping penetrates outer shell or roof.
  6. Check to see that all internal valves operate properly prior to completing piping.
  1. Cellular Glass Installation
    1. Make sure the contractor has enough cellular glass, filler powder, and layer foil paper to complete the work.
    2. All materials need to be kept dry at all times. Reject wet materials.
    3. Confirm the cellular glass blocks are staggered appropriately.
    4. Make sure that the interspatial powder fills all the gaps between blocks and on the top.
    5. Make sure that the interlayer paper or fabric is between the cellular glass layers as specified.
    6. Insulation needs to be properly installed around any piping that will be in the insulation.
    7. Verify required insulation thickness has been achieved once the contractor is finished.
    8. A top layer of concrete will be poured over the cellular glass before the tank is lowered onto the insulation. Make sure the proper quality checks and tolerances are followed.
    9. The inner tank needs to be lowered before the hydrostatic test.
  2. Anchoring the Inner Tank
    1. Anchoring of the inner tank needs to be completed before any pneumatic testing can begin.
    2. Pressure testing occurs after the tank has been liquid filled to the 100% level and the foundation and tank settlement is confirmed to be acceptable.
    3. The structural integrity of the inner tank anchoring system is critical. All load carrying full penetration butt weld joints shall be 100% radiographed. Fillet welds shall be made with a minimum of two passes and dye penetrant inspected.
    4. Inspect the welding of anchor bolt brackets, chairs, and other attachments to the shell.
    5. After the tank straps have been welded on and the settlement of the cellular glass is acceptable, the inner tank supports can be removed. Please make sure that the inner tank is protected from sparks and slag so the parent metal does not get contaminated.
  3. Pressure Testing – Preliminary
    1. Make sure that the proper pressure test procedures are available and have been approved by Pressure Vessel Group.
    2. Tanks are a confined space at this point. Proper confined space procedures must be followed.
    3. Make sure that all safety precautions are in place such as roping off the area.
    4. Confirm all expansion joints are installed at the specified dimensions and inspect for damage after welding. Remove their temporary supports and protective wrapping.
    5. Install the inner and outer tank pressure and vacuum devices.
    6. Make sure that all block valves are positioned as per the pressure test procedure.
    7. Connect test equipment, manifold, and a recorder if specified.
    8. A special over pressurization protection device is required for testing to prevent tank rupture. See specification 28-4WEQ1516 for specifics.
    9. Confirm that the test water has been checked for pH and chlorides, and that the appropriate amount of neutralizing chemical has been added to the tank.
    10. Contractor must have a plan for water disposal prior to filling the inner tank. The schedule impact to fill and drain the tank needs to be considered as well.
    11. The permanent safety devices need to be tested under their specified conditions. This is usually best done immediately after the testing of each tank. If possible have the Start Up Representative witness these tests.
    12. After the testing is complete, no additional welding is allowed on either the inner or outer tank, without home office approval.
  4. Pressure Testing – Inner Tank
    1. Ensure that top manhole or other roof fitting is open and has an area as large as water inlet before filling inner tank with water.
    2. During the water filling of the inner tank settlement information must be recorded. Make sure the following information is recorded and forwarded on to Air Products home office:
      1. Tank foundation settlement
        1. Record the elevation of tank foundation at four locations, 90 degrees apart, before starting the water filling.
        2. Check the elevations when the inner tank fluid level reaches 25%, 50%, 75%, and 100%, and record the changes in elevation.
      2. Cellular glass settlement
        1. Establish starting heights of the inner tank at a minimum of 12 locations equally spaced around the inner tank circumference, prior to putting any water into the inner tank.
        2. Record the change in height at these locations when the inner tank fluid level reaches 25%, 50%, 75%, and 100%.
      3. Check shell and fittings for leaks or signs of distress.
      4. Check tank anchorage for gaps and distress
      5. Perform pneumatic and vacuum test as defined in the pressure test procedure. ONLY AFTER THE INNER TANK ANCHORING SYSTEM HAS BEEN COMPLETED.
      6. Reinspect the inner tank anchoring devices and perform follow up NDT requirements after pneumatic test.
      7. The vapor and liquid piping may have different test pressure requirements. In normal conditions liquid piping experiences higher pressures due to product weight.  Make sure that the testing of liquid piping was handled properly.
      8. The inner tank roof needs a leak test with a snoop solution during the pneumatic test.
      9. When emptying the inner tank ensure that all possible openings are used to allow air in. Dangerous vacuums that will collapse the tank can occur if the pressure inside and outside the tank is not balanced.
      10. Document all test data and log events.
    3. Pressure Testing – Outer Tank
      1. Check to see if the pressure test procedures require the inner tank to be under a certain positive pressure before pressurizing the annular space.
      2. Check to see if the pressure test procedures require removal of inner tank top manhole flange during the outer tank testing to prevent uplift on inner tank.
      3. Perform pneumatic and vacuum testing as defined in the pressure test procedure.
      4. Document all test data and log events.
    4. Tank Cleaning
      1. All interior inner tank surfaces must meet the cleaning standards for the fluid contained.
      2. All safety devices and liquid level gauges connected need to be AA clean.
      3. The cleaning method and materials used need to be approved by Air Products.
      4. At this point the tank shall be classified as a permitted confined space. Make sure all the applicable safety requirements are followed
      5. If cleaning for a 9% nickel shell is by grit blasting. Ensure that all the attached piping is clean, and the pipe nozzles are plugged during the process.
      6. Make sure that access to the inside of the tank after cleaning is minimized and that all personnel have clean coverings.
      7. Verify that relief devices are installed and functioning before covering openings to the inner or outer tank. A small hole should be made in the temporary cover so as to allow pressure balancing between the outside and inside pressures. The transition from daytime to nighttime temperatures can collapse a sealed tank.
    5. Annular Space Preparation for Prelisting
      1. All debris, heavy scale, and dirt need to be removed.
      2. All hydrocarbons visible under bright light to be cleaned off.
      3. All of the erection scaffolding needs to be removed.
      4. Check all piping for damage. Focus on small bore or pipe support damage.
      5. Make sure all strain gauges or other instruments are installed.
      6. Check to see if side manways or roof manways need to be packed with mineral wool.
      7. Confirm that all purge piping is installed per drawings and protected against perlite infiltration. Usually this is done with a glass blanket strapped over the purge holes.
      8. Pipe thermal breaks in the shell wall need to be packed with mineral wool before perliting. Audit #11 has more information on insulating with mineral wool.
      9. Piping sleeves in the bottom need to have perlite poured in and rodded so that there are no voids prior to perliting the rest of the tank. Allowing these sleeves to be filled gravitationally with the rest of the annular space will leave voids.
    6. Perliting
      1. Make sure that the perlite has been tested and meets the specifications. Obtain approval by the home office prior to starting.
      2. The inner tank should be pressurized 0.07 bar (1 psig) with dry, oil free air, when perliting to prevent a tank collapse. Relief devices need to be installed and functioning. Monitor the pressure regularly.
      3. Perliting should occur through the upper manway, to avoid over pressurization. Make sure there are sufficient openings so that air can escape during the filling operation.
      4. The contractor needs to control dusting, but sealing of the annular space valve outlets is prohibited because over pressurization can occur.
      5. During the installation of perlite the contractor needs to perform the perlite specific tasks listed in Audit #11 and the specifications.
    7. Nitrogen Purging
      1. Verify the desired dew point requirements.
      2. During purging operations, make sure that necessary dew points are reached for all systems –especially on bottom liquid piping which may have traps filled with water.
      3. Purge the annular space and perlite to specification.
    8. Painting
      1. Painting is covered in more detail in Audit #15.
      2. The outer tank shell and roof sheets need to be grit blasted clean and prime painted.
      3. Inspect surface preparation, particularly in weld zones where clips or brackets were attached to the under side of the outer tank shell, at seams, under the roof eave and at the stairways and platforms. Make sure all loose paint, slag and rust is removed.
    9. Documentation
      1. Welding Procedure Specifications
      2. Procedure Qualification Records
      3. Welder qualifications
      4. Radiography results and weld map locations for both the tank shell and piping.
      5. Vacuum box test results
      6. Dye penetrant test results. Refer to Appendix A.
      7. QA/ QC of any shop work such as radiography, pressure tests, cleaning logs, etc.
      8. Inner tank anchorage:
        1. For anchor bolts need thread engagement depth and torque value
        2. Strap butt welds need 100% radiographed.
  • A dye penetrant test for the anchor straps fillet welded to the inner tank shell.
  1. Inner and outer tank pressure test documentation
  2. Inner and outer vacuum test documentation.
  3. Snoop test results on the inner tank roof.
  4. Cleaning log for the inner tank and piping.
  5. Perlite samples.
  6. Dew point checks and verification.






Appendix A

Summary of Nondestructive Testing

Inner Tank
Anchor strap attachments Dye Penetration crack detection before and after hydro-pneumatic test
Bottom annular plate butt welds Radiography as required by code.  Vacuum box before and after hydro-pneumatic test.
Butt welds in stiffening rings, and compression ring welds As required by code.
Compression ring to shell welds Dye penetration crack detection inside and outside
Lap joints in bottom plates Vacuum box before and after hydro-pneumatic test
Lap joints in roof plates Vacuum box.  For LOX tanks, before welding second side
Nozzle-to-shell welds, stiffening rings, and other attachments Dye penetration crack detection
Shell-to-bottom corner joint Dye penetration crack detection inside and outside as required by code
Shell butt welds As required by code
Outer Tank
Lap joints in bottom plates (if fitted) Vacuum Box
Lap joints in roof plates, shell butt welds, shell-to-bottom corner joint Vacuum box, or leak test using soap solution executed during pneumatic test
Penetrations, roof-to-shell joint Leak test using soap solution executed during pneumatic test
Interspace Piping
Circumferential joints on liquid piping 100% radiography
Circumferential joints on vapor piping 30% of joints shall be 100% radiography
Anchor Straps
Any load carrying butt welded joints 100% Radiography
Any load carrying fillet welds 100% Dye penetration crack detection



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