The question of chlorine
The bulk storage of sodium hypochlorite in polyethylene is banned in Germany. Lee Forbes looks at the medium- to long-term problems that are concerning German material suppliers.Sodium hypochlorite solutions are widely used in water treatment and as a sterile bleach solution from the common domestic applications to the food industries to swimming pools. With the high levels of available chlorine, up to 15% in a "fresh" solution, thousands of tonnes are consumed on a regular basis.
Rubber- or ebonite-lined carbon steel tanks were used for bulk storage tanks until the introduction of the plastics materials in the 1950s, namely pressed un-plasticised polyvinyl chloride sheet (UPVC) reinforced with glassfibre reinforced polyester resin (GRP). Due to the low cost and ease of manufacture during the 1960s, other thermoplastic materials were, and still are used.
Despite these lower cost thermoplastics, such as polyethylene, the recognised materials of construction for long-term storage of hypochlorite solutions has remained UPVC reinforced with GRP, this being proven by case histories of use in excess of 20 years.
During the 1960s, European interests followed the low-cost all-thermoplastic tank developments with medium- and high-density polyethylene (HDPE) being specified for the duty. Following a number of failures caused by environmental stress cracking, the major tank design standard, the German Merkblatt DVS 2205, omitted to list sodium hypochlorite within the Standard as a suitable application for polypropylene and HDPE. The relatively new, far less stringent European Standard BS EN 12573 - 2000 also omits sodium hypochlorite in the listings for the chemical factor.
In the UK and other parts of the world, this advice from the German manufacturers was ignored and many thousands of tanks were produced in polyethylenes of various types and methods of manufacture. It must be said at this stage, some tanks faired better than others.
The length of service, before leakage, was seen to vary from three years to more than nine years. Having looked at specific applications, some tanks were very infrequently filled, thus the active available chlorine levels were considerably reduced due to the relatively unstable nature of hypochlorite solution. This may have given a false impression of the resistance to the concentrated fresh solutions.
During the 1970s through to the 1990s, polyethylene was specified and used, with some manufacturers giving guarantees of up to ten years. In many cases, these guarantees were not followed up, as a number of tanks suffered leakage before this length of service was achieved, particularly on high-usage facilities where fresh hypochlorite solutions were always present.
While some end users still specify HDPE for sodium hypochlorite, the German authorities do not permit the use of HDPE for storage tanks handling 15% sodium hypochlorite. Concern is expressed by the polymer manufacturers such as Basell and by the German national standards authority, TUV.
The technical manager at one of the world's leading polymer manufacturers, Basell, said: "Parts made of Hostalen PE grades can be applied in contact with Sodium Hypochlorite at 30°C for one year and at 20°C for four years. After this time period, we recommend to exchange the containers."
With the increasing regulations designed to protect the environment and peoples' safety, most manufacturers of chemical storage tanks in the UK no longer consider HDPE as a suitable material. Tanks manufactured in HDPE may be considered for short-term use as demonstrated by the statement above. However, in the long term, there is a high liability placed not only on the manufacturer but also the specifier and the end user.
Most installations have a design service life of 15-20 years or more. From in-service experience, this is not being achieved with tanks constructed or moulded in polyethylene.
An alternative material of construction to UPVC / GRP which gives the required service life is a vinylester laminate. During the 1960s, the US developed vinylester resin systems, which proved to be highly resistant to chlorine environments such as 15% available chlorine hypochlorite solutions.
Following the material manufacturers' recommendations and the experiences in the field, it is evident that for long-term storage of high concentrations of sodium hypochlorite either UPVC reinforced with GRP or a full post-cured vinylester GRP laminate are the only safe options to be considered.
Unfortunately, many specifiers are persuaded to opt for the much lower-cost option of an all polyethylene thermoplastic tank, often quite innocently as the manufacturer states an expected service life in excess of ten years. In many instances, these guarantees are not implemented due to being lost in the mists of time.
Many of the drinking water treatment plants now incorporate chlorine injection into the water supply using on-site generated sodium hypochlorite. This solution is produced on a continuous basis from a brine solution, which is passed through an eletrocatalytic cell producing a low-strength solution with a low pH. These solutions are often quoted as 0.8-1% strength sodium hypochlorite.
Due to the low pH (about pH 9 - conventional hypochlorite is stabilised with caustic soda with a minimum pH 13), the chlorine content is highly aggressive. Traditionally chemical-grade pressed UPVC reinforced with GRP has given good service.
This highly aggressive material can accelerate the problems of stress cracking seen in polyethylenes. Even the vinylesters successfully used for the conventional hypochlorite storage tank can be attacked if a specially formulated system is not incorporated and fully post cured in an oven at 90°C for ten hours.
Caution must be exercised when specifying bulk storage tanks for sodium hypochlorite solutions. The liabilities generated by potential leakage or catastrophic failure are now so onerous, that engineers and end users must be confident they are receiving the correct advice.
While cost considerations must play a part in the selection of a hypochlorite storage tank, medium- to long-term safety must be paramount in the mind of the specifier, end user and the manufacturer.
Lee Forbes is managing director of the Forbes Group
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