A flat bottom cryogenic tank is one of the largest buys in a gas plant. The spec drives its cost, its safety, and its schedule. A weak spec invites change orders and long query chains. This checklist helps engineers cover the key points before they issue a tender.
Need a copy for your project team? Download the cryogenic tank project selection checklist (PDF) to use during early project definition. The checklist supports discussion and does not replace project-specific engineering or code review.
Start the flat bottom cryogenic tank spec with site data
Start with the stored product, the daily send-out, and the growth plan. Set the design temperature from the coldest real case, not from the boiling point alone. Site data comes next: wind load, seismic load, soil strength, and ambient air. For this reason, the soil survey starts long before the tank design.
Governing standards and design basis
The spec must name the ruling code: API 620 for many US projects, EN 14620 for European ones. State the edition, the pressure range, and the piping code as well. A flat bottom cryogenic tank spec that leaves code lines vague will draw queries, exceptions, and delays. Therefore, give the design basis its own section in every tender.
Insulation system and annulus
Flat bottom tanks use loose perlite in the gap between the inner and outer walls, plus floor and roof insulation. The spec should cover the perlite density, the fill process, moisture control, and room for settling. Perlite settles over years of thermal cycling. As a result, top-up ports and a check plan belong in the tender, not in later talks. In addition, state who supplies the perlite, who expands it on site, and who checks its quality.
Foundation and frost safety
The concrete base needs heating to stop soil freezing under the tank. Specify the heating system, its controls, and its failure alarms early. Frost heave can break a tank base, so this item never stays optional. In addition, set up base temperature checks during cool-down and the first years of service.
Safety systems and boil-off gas
Overpressure relief and boil-off handling need clear terms. The tank vents must match the boil-off gas system they serve. State the sizing basis and the upset cases you assume. For example, a pump-out at full rate plus a fire case can rule the vent size. Also list the level, pressure, and temperature sensors with their alarm and trip points.
Commissioning and cool-down
Cool-down rates, drying, purging, and insulation dry-out all belong in the flat bottom cryogenic tank spec. A fast cool-down strains the inner vessel, so define the thermal limits. Agree on hold points, checks, and records before the crew moves in. Consequently, start-up becomes a planned sequence, not a live bargaining round.
Vendor data and bid forms
Ask each bidder for tanks of similar size and product, with contactable clients. Also request weld records, heat treatment plans, and the names of key staff. However, even strong vendors need clear bid forms. State the price split by scope, so offers stay comparable line by line. Consequently, the bid review takes days instead of weeks.
Piping, penetrations, and loads
The tank shell carries loads from pipes, ladders, and platforms. Therefore, the spec must set the allow rules for nozzle loads and the pipe stress limits at each tie-in. Next, define the pipe support layout near the tank, because poor support plans strain nozzles at start-up. Also state who owns each interface. Consequently, the tank team and the pipe team share one clear border, not a disputed one.
Conclusion
A complete flat bottom cryogenic tank spec links design, insulation, base, and start-up choices. Treating these as one system avoids costly gaps between suppliers of a flat bottom cryogenic tank. The best tenders fix interfaces and long-term service terms before award.
Engineers who apply this checklist protect the budget and the plan. They also give vendors a clear basis for fair, comparable offers.
Flat bottom cryogenic tank specification: common questions
What information should a project team gather before issuing a tank specification?
Define the stored product, capacity and send-out basis, operating cases, site conditions, governing code and project interfaces. A geotechnical survey and clear battery limits help bidders identify scope and price risk early.
When should the project select the applicable tank design code?
Set the governing code and edition in the design basis before tender. The applicable choice depends on jurisdiction, owner requirements and project scope, so bidders should not be left to assume a code or propose incomparable bases.
Why include perlite filling and settlement in the specification?
Perlite handling, moisture control, installation method and allowance for settlement affect the annular insulation system over time. Define responsibilities for supply, on-site expansion or filling, quality checks and future inspection or top-up provisions.
Which interfaces commonly need clarification?
Clarify nozzle loads, connected piping supports, foundation heating, boil-off handling, instrumentation and commissioning hold points. Assigning ownership for each interface reduces gaps between the tank package and other plant contractors.
For project-specific specification reviews, contact The Cryoexpert with the tank duty, project location and design stage.
References and further reading
- API 650 and API 620 flat bottom tank discussion (Eng-Tips)
- Cryogenic storage suppliers directory (gasworld)
- Cryogenic storage tank inner vessel design (European patent EP4283182B1)
Continue reading on The Cryoexpert
- The perlite annulus: design, filling and long-term performance
- Cryogenic tank design calculator
- Cryogenic insulation technology: controlling heat leak
- Cryogenic piping design: managing contraction and stress
European standards reference
For the European standards framework and published work, see CEN and CENELEC. The applicable edition and national implementation depend on project scope and jurisdiction.