Designing and specifying storage tanks is fraught with risk. Lee Forbes reports
Health and safety and environmental liabilities are increasingly placed on specifiers, purchasers, end-users and manufacturers of tanks for hazardous or non-hazardous liquids. So, great care must be taken not just with the design of the basic tank and safety bund, but also with ancillary safety items. It is not uncommon for safety protection equipment to induce alternative risks.
For the smaller capacities, unreinforced thermoplastic such as polypropylene and HDPE is designed to DVS 2205 - Merkblatt (German Standard). This standard looks at all parameters and is specific in dictating safety factors based on risk.
The recently issued EN 12573 is a poor substitute for the well proven DVS 2205. The EN 12573 does not address essential items such as wind loadings, personnel loadings, and other areas which DVS 2205 includes. For large volumes of liquid, or any hazardous applications, you should insist on calculations conforming to EN12573 followed by detailed design parameters not covered in the EN by applying the extensive DVS Merkblatt code.
A further weakness of EN 12573 is the allowable range of safety factors from 1.3 to 2.0. Unlike DVS 2205 Merkblatt, there is no guidance or comment as to which safety factor to select. As the EN is based on parts of the well proven DVS 2205 Merkplatt, any engineer looking at EN should refer to DVS 2205 for guidance in areas omitted by the EN standard. The engineer cannot ignore parameters such as wind loadings, roof loading and more importantly correct choice of safety factors.
There is no other verified authority to reference. So, the engineer must choose a safety factor of 2.0 for any large-capacity tank holding hazardous material in a critical situation which could cause harm to personnel or the environment.
To cut costs, suppliers of tanks can reduce safety factors, yet may stay within the EN standard. DVS 2205 requires the designer to look at the long-term strain curves and incorporate an ageing factor.
The chemical resistance of all-thermoplastic can substantially be degraded in many aggressive environments as the strain level increases with time. Depending on the type of thermoplastic, this ageing factor together with chemical resistance is a major consideration. Particularly with low- and medium-density linear polyethylenes, the resistance compared with, say, the chemical grade P100 HD polyethylene; a reduction of more than 50:1 to environmental stress cracking has been seen in the laboratory in a number of common chemical environments. (Visit www.forbesgroup.co.uk and click on For the technically minded).
For GRP composite reinforced laminates, BS 4994 - 1987 covers detailed design of all the required parameters for safe storage of liquids. Using BS 4994, the design engineer has the confidence in safety of final design of GRP structures.
Great care must be taken in selecting the correct materials. For example, weak acidic solutions can cause acid strain corrosion in type-E glassfibre laminates. E-CR glassfibre should be selected to give protection against such attack, even if problems of damage occur to the corrosion barriers. For highly corrosive conditions, sophisticated vinylester resins are the norm.
One major consideration for any tank above 2m in height is the necessity to include a side-mounted manway for safe access during inspections. For hazardous or corrosive liquid storage, it is now often a scheduled mandatory requirement that tanks are inspected internally - irrespective of the type of tank or material of construction.
GRP reinforced tanks offer side-mounted manways as a standard within the BS4994 design code. One major disadvantage of the all-thermoplastic tank is the stipulation in EN 12573 and DVS 2205 standards that no connection above 300mm diameter is permitted in the vertical wall below liquid level. If a failure is due to stepping outside the EN standard, in law the entire liability will fall on that engineer.
If any fabricator presents design calculations for larger nozzle connections, including side manways, it is essential the design is verified by an experienced organisation such as TUV, which verifies the German DVS and other Merkblatt safe design standards.
This high-risk area is sometimes addressed by a fabricator's own design methods. The advice is to ensure any such calculations are verified by an experienced authority such as TUV.
The alternative to side access is to enter the tank via the top manway by calling in a specialist company, with breathing apparatus; winches; internal scaffolding; full PPE and gas monitors, which far out weighs the initial cost of fitting a side access during construction if permitted.
Choosing the incorrect design or the wrong materials for tank construction caries serious risk of toxic products leaking into the environment or causing injury. Failing to carry out an accurate risk-assessment in terms of installation, operation and maintenance increases this risk.
To obtain the most cost-effective system, coupled with the safest system of storage, it is important that the purchaser and end user selects an experienced tank manufacturer which can fabricate in a number of materials, and which can produce verified design calculations. It must be able to offer a full Hazop assessment applicable to the specific application. Often making savings on the initial capital costs can come back to bite you later.
Lee Forbes is the managing director of the Forbes Group of companies and has been designing and building tanks and chemical plant in thermoplastics, plastics and GRP composites since the 1960s
Common problems with incorrectly specified, operated or maintained chemical storage systems
Floatation - One of the most common problems is the tank floating within the bund area due to either rainwater accumulating or overfilling of the tank. Unless specifically designed, most tanks in plastics materials will not withstand the up-thrust created by floatation. Reported instances over the years show damaged tanks and pipework connections broken due to floatation, allowing the contents to escape into the bund.
- Fit a rain protection shroud
- Install a bund alarm to give warning of any liquid on the floor of the bund
- Fit a separate containment or catchment area under the overflow, thus preventing any risk of liquid building up in the bunded area if small overfills occur. This area can have warning alarms when liquid enters this separate small bunded area
- To prevent overfilling of the tank, install a high-level alarm switch in the storage tank
- Design the base of the tank to withstand the up-thrust generated by floatation, and ensure holding down detail is adequate
- Mount the tank on an elevated platform or plinth within the bund
- Fit bolting which is resistant to the contents. For instance, for hydrochloric acid, fit a special grade titanium bolting
- Only use plastic valves and union connections in the bund area
- Protect the bund area by fitting an overflow bund area within the major bund
- Fit a total containment shield, which will prevent jetting of liquid over the bund wall should the tank shell be damaged
- Increase the height of the bund
Complete installation service offered by the manufacturer. This places the onus a single source