Articles & Case Studies

Purchasing a tank? - be aware of the pitfalls

Posted: Friday 26th May 2006

With the ever increasing Health & Safety & Environmental liabilities being placed on specifiers, purchasers, end-users & manufacturers of tanks containing both hazardous & non hazardous liquids, great care must be taken not only with the design of the basic tank & safety bund, but also with ancillary safety items. It is not uncommon for the safety protection equipment to induce alternative risks.

To ensure the end-user has a totally safe system of chemical storage, even with a simple bunded tank, the designer / manufacturer must make a full HAZOP (hazard assessments) in various “what if” scenarios. Before we discuss the possible problems which can be unwitting induced by the end user or by lack of full “what if” assessments by the supplier, let us examine the tank. options. The majority of ‘one piece’ tanks up to capacities of 200 cubic metres, whether for hazardous, corrosive or non hazardous duty, construction in plastic materials is now very much norm.

Get your materials & design standard correct

There are two common types of plastics materials used for the construction of tanks for chemical storage. For the smaller capacities unreinforced thermoplastic such as polypropylene and HDPE 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 very poor substitute for the well proven DVS 2205. The EN 12573 does not address essential items such as wind loadings, personnel loadings, plus other areas which DVS 2205 includes. For large volumes of liquid &/or any hazardous applications the advice is to insist on calculations conforming EN12573 followed by detailed design parameters not covered in the EN by applying the extensive DVS Merkblatt code. A further major 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, the inference must be drawn that any engineer looking at EN must 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. Therefore 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 reduce safety factors and amazingly stay within the EN standard.

DVS requires the designer to look at the long term strain curves and incorporate an ageing factor. The all thermoplastic tank being strain limited in design rather than stress limited, the chemical resistance of the 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 & medium density linear polyethylene’s the resistance compared to say the chemical grade P100 HD polyethylene; a reduction of over 50:1 to environmental stress cracking has been seen in the laboratory in certain environments.

For GRP composite reinforced laminates BS 4994 – 1987 covers very detailed design of all the required parameters for safe storage of chemicals. Using BS 4994 the design engineer has the total 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 this giving protection against such attack even if problems of damage occur to the corrosion barriers. For highly corrosive conditions sophisticated vinylester resins are the norm.

Omitting a side manway can be an initial saving but very costly for the future

One major consideration for any tank above 2 metres in height is the necessity to include a side mounted manway for safe access during inspections. For hazardous / corrosive liquid storage it is now often a scheduled mandatory requirement that tanks are inspected internally irrespective as to the type of tank or material of construction. Risk assessment into confined space dictates gas levels must be checked & the safe removal of a collapsed person.

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 & DVS 2205 Standards that no connection above 300 mm diameter is permitted in the vertical wall below liquid level.. If a failure is due to ‘stepping’ out side 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 who verifies the German DVS & 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 of Germany.

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.

When is a “bunded” tank not a bunded tank?

Virtually all tanks now have to have a separate bund tank or catchment area to contain 110% of the volume of the tank. The so called ‘double wall’ tank is not a fully bunded system and leaves the end user somewhat exposed should a problem occur. In the true sense a bund is a separate item which should contain the outlet valves, overflows and immediate pipework from the tank. The only exception to this is when there is no side mounted fittings or connections and the contents are pumped out from the top of the tank. If a double wall tank is used in this fashion the outer skin must be designed to contain the entire contents safely for a suitable period..

Common problems associated 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 & pipework connections broken due to floatation, allowing the contents to escape into the bund.

Optional solutions offered on: Forbes MINIBULK® range of tanks:

1) Fit a rain protection shroud together with a TC® (Total Containment) shield.

2) Install a bund alarm to give warning of any liquid on the floor of the bund

3) Fit a separate containment / 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.

4) To prevent overfilling of the tank install a high level alarm switch in the storage tank.

5) Design the base of the tank to withstand the upthrust generated by floatation & ensure holding down detail is adequate.

6) Mount the tank on an elevated platform or plinth within the bund.

· Joints or valves leaking due to corrosion or damage – particularly with highly corrosive contents which fume, such as hydrochloric acid. A shallow dilute solution of corrosive liquor if allowed to build up in the bund can quickly destroy the valve bolting which will lead to the contents escaping into the bund.

Optional solutions offered on: Forbes MINIBULK® range of tanks:

1) Fit bolting which is resistant to the contents – for instance for hydrochloric acid fit a special grade Titanium bolting.

2) Only use plastic valves & union connections in the bund area.

3) Protect the bund area by fitting an overflow bund area within the major bund.

· Leakage occurring caused by a damaged tank “jetting” over the top edge of the bund wall – this scenario is rare but must be risk assessed.

Optional solutions offered on: Forbes MINIBULK® range of tanks

1) Fit a TC® (Total Containment) shield which will prevent “jetting” of liquid over the bund wall should the tank shell be damaged.

2) Increase the height of the bund.

· Tank & bund misalignment due to incorrect assembly causing pipework to be stressed with the potential of pipework failure – this is caused by the installer (often the contractor or end user) not taking care in aligning the pipework & valves between the tank and the bund wall.

Optional solutions offered on: Forbes MINIBULK® range of tanks:

Complete installation service offered by the manufacturer. This places the onus a single source.

Conclusion

It is evident from the above discussion that serious risks of leakage of toxic products into the environment or injury to personnel are a real possibility if the incorrect design, wrong materials of construction are selected, plus failing to carry out an accurate risk assessments in terms of installation, operation and maintenance. To obtain the most cost effective system coupled with safest system of storage, it is extremely important the purchaser & end user selects an experienced tank manufacturer who can fabricate in a number of materials and who can produce verified design calculations. They 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 and “bite you” at a later date.

Lee Forbes is the managing Director of the Forbes Group of companies & has been designing & building tanks & chemical plant in both thermoplastics plastics & GRP composites since the early 60’s

Contact Details

Forbes Technologies Ltd
Denver
Downham Market
Norfolk
PE38 0DR

Tel - 01366 389 640
Fax - 01366 385 274

E-mail - mjupp@forbesgroup.co.uk
Web - www.forbesgroup.co.uk




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