Rethinking chemical storage

All too often, chemical storage is still treated as a peripheral design task.

A tank is typically specified late, material compatibility checked, wall thickness calculated, and attention moves on.

That mindset may once have been adequate when duty cycles were stable and regulatory scrutiny lighter, but it is increasingly misaligned with how modern plants today actually operate.

Aggressive chemicals such as hydrochloric acid, sodium hypochlorite, caustic soda and oxidising blends are rarely stored under steady-state conditions. Concentrations fluctuate, temperatures cycle, aeration varies, and cleaning regimes introduce additional exposure modes. Under these conditions, traditional carbon steel and lined steel tanks tend to fail in predictable but costly ways: under-film corrosion, localised attack at nozzles and welds, lining disbondment, and a gradual drift from “known condition” into managed uncertainty.

Regulatory expectations have moved on. Frameworks such as COSHH, DSEAR and COMAH now place considerable weight on demonstrable containment integrity over the asset life, not just initial compliance. For the process engineer, this raises an uncomfortable question: why are storage vessels still being specified as if long-term material behaviour were someone else’s problem?

Compatibility is not the same as durability

The availability of advanced thermoplastics and fibre-reinforced composites has widened the design space for chemical storage, but it has also exposed weaknesses in how materials are often selected. Compatibility charts remain useful, but they are blunt instruments. They rarely account for permeation, concentration, oxidising potential, dissolved gases, or the mechanical consequences of sustained load at elevated temperature.

Treating these materials as interchangeable 'plastic tanks' ignores the fact that their failure mechanisms are governed by strain, creep and laminate architecture and weld strength rather than corrosion rate

Materials such as PP, HDPE, uPVC, cPVC, PVDF and ECTFE behave very differently under stress and time. The same is true of GRP laminates based on vinyl ester, isophthalic or bisphenolic resins. Treating these materials as interchangeable 'plastic tanks' ignores the fact that their failure mechanisms are governed by strain, creep and laminate architecture and weld strength rather than corrosion rate.

In practice, many robust designs now use composite construction deliberately: a thermoplastic liner to manage chemical exposure, backed by a GRP structure to carry load and control deformation. Resin systems are selected to suit oxidising or reducing environments, while fibre orientation and local reinforcement are used to manage nozzle loads, thermal gradients and cyclic stresses. This is materials engineering in the proper sense, not procurement-driven substitution.

Designing for failure

One of the persistent errors in chemical storage design is the assumption that minimum wall thickness equates to safety margin. For polymers and composites, this is rarely true. The dominant risks are vacuum collapse during pump-out, creep under sustained hydrostatic head, local strain concentration at penetrations, and buckling under wind or seismic load.

This shifts the design question from “does it meet code?” to “does it behave predictably over 20 years of service?”

Modern design approaches increasingly address these failure modes directly. Standards such as BS EN 13121-3 provide a framework, but credible designs go further: finite element assessment of supports and nozzles, explicit vacuum and sloshing checks, and defined strain limits based on long-term material behaviour rather than short-term strength. This shifts the design question from “does it meet code?” to “does it behave predictably over 20 years of service?”

Chlorine-bearing systems provide a useful illustration of why this matters. Elemental chlorine and hypochlorite solutions combine strong oxidising chemistry with toxicity and, in some cases, gas–liquid equilibrium behaviour that challenges conventional metallic containment.

In one of our recent installations, a two-stage chlorine scrubbing system was engineered to reduce inlet concentrations from over 534,000 mg/m³ to below 1 mg/m³. Separate 1% and 18% NaOH stages were used to balance reaction driving force, mass transfer efficiency and control of by-product formation. Downstream of the scrubbers, a catalytic Hydecat reactor achieved greater than 99.99% destruction of sodium hypochlorite at feed strengths exceeding 56,000 ppm.

What is notable here is not the headline performance figures, but the integration of materials selection, process chemistry and containment design. Thermoplastic linings were chosen to manage oxidising exposure and permeation risk, while GRP structures carried mechanical loads and accommodated complex nozzle arrangements. The containment strategy was designed as part of the process solution, not added afterwards. That distinction is often the difference between a system that merely complies and one that remains stable over time.

A challenge to the profession

For chemical and process engineers, the uncomfortable conclusion is that storage vessels can no longer be treated as passive items. Advanced plastics and composites do not simplify engineering judgement; they demand more of it. The relevant questions are no longer “will it corrode?” but “what strain will this material see over its life, how will it deform, and how will we know if it starts to behave differently?”

Plants that address these questions early tend to gain more than just improved containment. They reduce inspection burden, avoid unplanned outages, and gain confidence in the integrity of systems that sit uncomfortably close to the boundary between process and environmental risk. That outcome is not driven by materials alone, but by treating chemical storage as a first-order engineering problem rather than an afterthought.

Indy Singh (pictured) is group sales engineer at ERG Hygrade