Heavy Cycle Oil (HCO) Storage & Tank Compatibility

Storing Heavy Cycle Oil (HCO)? Start Here

Heavy cycle oil (HCO) is a heavy aromatic side-stream drawn off a refinery fluid catalytic cracking (FCC) unit, typically distilling in the ~343–525°C range. It is not a clean fuel cut: HCO is dominated by condensed polynuclear aromatics — often more than 40%, including four- and five-ring species — together with naphthenes, residual paraffins, and sulfur- and nitrogen-bearing heteroaromatics. Refiners generally regard it as a low-value, hard-to-upgrade stream rich in coke and heavy-polynuclear-aromatic (HPNA) precursors, used as a cutter stock, blended to fuel oil, or recycled. For storage and transfer, the controlling property is its extreme aromaticity. Aromatic hydrocarbons attack polyethylene and polypropylene by absorption-driven swelling and environmental stress cracking, which is why HCO belongs in steel (or aromatic-rated lined) tankage rather than a plastic vessel. Material selection here is a safety and containment decision, not a cost preference.

Is Heavy Cycle Oil Compatible With Polyethylene (HDPE / XLPE) Tanks?

No — polyethylene is not recommended for heavy cycle oil. Although published resistance charts rate HDPE as satisfactory for clean distillate cuts such as diesel, gas oil, and kerosene, those same charts rate aromatic hydrocarbons and naphtha as unsatisfactory. HCO sits firmly on the unsatisfactory side: it is dominated by polynuclear aromatics, the exact constituents that polyethylene cannot hold. Aromatic absorption causes the polymer to swell, soften, lose wall strength, and undergo environmental stress cracking over time — a slow failure that can end in a leak. Cross-linked polyethylene (XLPE) improves stress-crack resistance but does not make the material a barrier to aromatics, so it is likewise unsuitable for sustained HCO service. Store and handle heavy cycle oil in carbon or stainless steel tankage, or in FRP built with an aromatic-rated vinyl ester or novolac liner, and pair it with FKM (Viton) or fluoropolymer seals. Confirm the final selection against the supplier's SDS and your engineering standards.

Material compatibility at a glance

Heavy cycle oil is a combustible, high-aromatic petroleum stream, so steel is the default tank MOC — carbon steel (UL-142 / API designs) for ambient bulk storage and stainless where sulfur or temperature warrant it. Polyethylene and polypropylene are unsuitable: the dominant polynuclear-aromatic fraction swells the polymer and promotes stress cracking. FRP is viable only with an aromatic-rated vinyl ester or novolac system. Use FKM (Viton) or fluoropolymer seals and linings; avoid EPDM and natural rubber.

Ratings: S suitable · C conditional / limited · U unsuitable. Verify against the cited resistance charts and your concentration/temperature before specifying.

The safety that actually matters

• Combustible liquid: heavy aromatic oil with a flash point typically above 60°C; keep away from heat, sparks, open flame, and hot surfaces, and bond/ground during transfer.
• Aspiration hazard (H304): may be fatal if swallowed and drawn into the lungs — do not induce vomiting; seek medical attention.
• Possible carcinogen (representative H350): high-aromatic petroleum streams can carry a carcinogenicity classification; minimize skin contact and inhalation. Verify the specific SDS classification.
• Skin and organ effects: causes skin irritation (H315); prolonged or repeated exposure may damage organs (H373). Use chemical-resistant gloves and avoid mists/vapors.
• Environmental hazard (H411): toxic to aquatic life with long-lasting effects — provide secondary containment and prevent release to soil, drains, and waterways.
• Hydrogen sulfide / sulfur species: sour streams may evolve H₂S in headspace; test atmospheres before entry and ventilate enclosed spaces.

Common questions

Can I store heavy cycle oil in a poly (HDPE or XLPE) tank?

No. HCO is a highly aromatic petroleum oil, and aromatic hydrocarbons swell polyethylene and cause environmental stress cracking. Use steel (carbon or stainless) tankage, or FRP with an aromatic-rated liner. XLPE improves crack resistance but is still not a barrier to aromatics.

Why is HCO unsuitable for poly when diesel and kerosene are usually fine?

Resistance charts rate clean distillates such as diesel, gas oil, and kerosene as satisfactory for HDPE, but they rate aromatic hydrocarbons and naphtha as unsatisfactory. Heavy cycle oil is dominated by polynuclear aromatics (often >40%), which places it in the aromatic category, not the clean-distillate category.

What tank material should I use for heavy cycle oil?

Carbon steel (UL-142 / API designs) is the industry-standard choice for ambient bulk storage; stainless steel is preferred where sulfur species or elevated temperatures are involved. FRP is acceptable only with an aromatic-rated vinyl ester or novolac liner. Pair any of these with FKM (Viton) or fluoropolymer seals.

Is heavy cycle oil flammable and toxic?

It is combustible (flash point typically above 60°C, not a flammable liquid) and carries serious health flags: aspiration hazard, possible carcinogenicity, skin irritation, and aquatic toxicity. Exact NFPA and GHS values are SDS- and stream-dependent, so always work from the supplier's data sheet.