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Des joined Forbes following the retirement of Colin Wills in 2009.
Des will be happy to assist you with all aspects of Forbes products & services.
Seasoned estimator with Forbes since 1988.
You will always find Garry friendly and helpful.
Experienced in all contract aspects from £500 to £500,000
Joined Forbes in 1999 following 7 years experience with another major GRP chemical plant fabricator as design and production engineer. Promoted to the Board of Directors in September 2008.
Joined Forbes in 1969, huge experience in all aspects of thermoplastics and GRP production techniques.
The contract was won against stiff competition from the Chinese who are near neighbours to Kazakhstan. The consignment was shipped via Berwick-on-Tweed docks to St Petersburg and then overland south to the site. The tanks are used for chemical extraction & precipitation of various minerals.
Kemira Water, the largest suppliers of water treatment coagulants in the world, commissioned Forbes Technologies Ltd to design, build & install 16 large capacity storage and processing tanks for their new PAX (polyaluminium chloride) plant near Chester.
The process involves the production and storage of concentrated corrosive solutions.
The system is designed to reduce the CO2 content from borehole water at volumes up to 1500 m³/hour.
The installation comprises 3 off degasser towers which are elevated above a manifold assembly feeding into 3 off 20 m³ capacity Floculation vessels.
All items including the tower internals being approved products as listed in the latest edition of “List of Products and Processes Approved Under Regulations 31 For Use In Connection With The Supply of Water For Drinking, Washing, Cooking and Food Production Purposes” – (December 2001) as issued by the Committee on Products and Processes for Use in Public Water Supply.
As well as the design & manufacture, Forbes also delivered to site and installed utilising one of our own experienced installation teams.
They are also used 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, many 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 50s, namely pressed unplasticised polyvinyl chloride sheet (UPVC) reinforced with glass fibre reinforced polyester resin (GRP). Due to the low cost and ease of manufacture during the 60s, 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 60s European interests followed the low cost all-thermoplastic tank developments with medium & 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 fared better than others. The length of service, before leakage, was seen to vary from 3 years to in excess of 9 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 period of the 70s through to the 90s, polyethylene was specified and used, with some manufacturers giving guarantees of up to 10 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.
Concern is expressed by the polymer manufacturers such as Basell and by the German national standards authority, TUV. The technical manager of the world’s leading polymer manufacturer – Basell has recently stated. Parts made of Hostalen PE grades can be applied in contact with Sodium Hypochlorite at 30° C for 1 year and at 20° C for 4 years. After this time period we recommend to exchange the containers.
With the ever increasing regulations designed to protect the environment and peoples safety, the majority of 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 long term there is a very 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 60s the Americans 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 10 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 (approximately 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 degrees Centigrade for 10 hours.
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. Whilst 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 & the manufacturer.
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