Engineering guide
Change of Service on an Existing Tank: How to Check a Heavier Liquid
Storing a different product in an existing tank is a change of service. Here is what has to be checked, and how FEA can set a safe filling height when the shell is too thin for the full design level.
By SLETEC Engineering · Updated 2026-11-06 · 3 min read
Why a heavier liquid matters
Hoop stress in a tank shell is proportional to liquid density and depth. A liquid with a higher specific gravity than the original design produces higher stress at every course, and the bottom course is the most affected. A change in temperature, corrosivity or vapour pressure also affects allowable stress, corrosion allowance and venting.
Checks for a change of service
- Current shell thickness from the latest inspection, minus the future corrosion allowance.
- Required shell thickness for the new specific gravity and liquid level, using the API 653 evaluation of existing shells.
- Bottom and annular plate condition, and settlement.
- Roof, venting and emergency venting for the new product.
- Material suitability and temperature limits.
- Foundation bearing for the increased load.
- Nozzle and connection loads if piping changes.
Worked example (SLETEC Ref. 79)
An existing vertical tank with a diameter of 6,096 mm and a 5.3 mm shell had to store a fluid with SG 1.83. SLETEC built a static-structural FEA model of the shell under the hydrostatic pressure gradient and reviewed deformation and stress against the target stress level.
| Tank diameter | 6,096 mm |
|---|---|
| Shell thickness | 5.3 mm |
| Specific gravity of new fluid | 1.83 |
| Safe filling height established | 4,450 mm |
| Maximum shell stress at that height | 48.02 MPa |
The owner received a documented operating limit for the tank in its new service.
How shell stress grows with specific gravity
For a vertical cylindrical tank the hoop membrane stress at depth H is proportional to the liquid's specific gravity G, the tank diameter D and the depth: σ ∝ G · D · H / t, where t is the shell thickness. If the new product is 1.5 times as dense, the stress at any given level rises by 1.5 times. To keep the same stress, the maximum liquid level must fall, or the shell must be thicker.
The bottom shell course is usually governing because it carries the highest hydrostatic head, but discontinuities — the shell-to-bottom junction, nozzles and reinforcing pads — must also be checked, which is where FEA is useful.
Documents to update after the change
- Tank data sheet and nameplate data with the new maximum liquid level and specific gravity.
- Operating procedure and high-level alarm and trip settings.
- Inspection plan and corrosion rate assumptions for the new product.
- Venting calculation if vapour pressure or pump rates change.
- The engineering report, kept with the tank's records for future API 653 inspections.
Options when the tank is not adequate
- Limit the maximum liquid level, as in the example above.
- Replace or reinforce the lower shell course under API 653.
- Reduce the design specific gravity by changing the product concentration where the process allows.
- Use a different tank.
Send the tank drawings, latest inspection report and new product data to enquiry@sletec.com. See our tank FEA verification project.
Frequently asked questions
- Can an existing tank store a heavier liquid?
- Sometimes, at a reduced maximum filling height or after shell reinforcement. The shell, bottom, foundation, venting and materials must be checked for the new service.
- Which code covers a change of service?
- API 653 governs evaluation, repair and alteration of existing tanks, including a change of service.
- How did SLETEC set the safe filling height in its example?
- By static-structural FEA of the shell under the hydrostatic gradient of the 1.83 SG fluid, which gave 4,450 mm at a maximum shell stress of 48.02 MPa.
Start a project
Send your scope, drawings, codes and schedule to enquiry@sletec.com or WhatsApp +966 55 545 8298. An engineer usually replies within 24 hours.
Discuss your project with an engineer
Send drawings, datasheets or a short scope – an engineer replies with scope, schedule and price, usually within 24 hours.