Chlorine? No problem

A flexible and long-lasting tank liner with a high resistance to chlorine has been developed by Hereford-based tank manufacturer Franklin Hodge

A corrosive agent in water, chlorine's highly aggressive ability to corrode can cause significant damage to tanks, especially in warm climates that exacerbate the problem. A 10_ rise in temperature doubles the harmful effects of chlorine.

EPDM and Butyl liners have a proven track record of long service life but as organic materials - chlorine will react with them - as well as with the chemicals in the water. Chlorine, when correctly dosed in water, will always react first to the materials in contact with the water, such as a liner. The excess volume of chlorine will evaporate from the water into the air as a poisonous gas.

At an ideal dosing level, chlorine is completely consumed or neutralised by sterilising the micro-organisms and bacteria in the water. If the chlorine is not fully consumed by the water, the remaining chlorine will continue reacting with the liners, reducing their life, until it is fully consumed or evaporated into the air.

Folds and tensions in an installed liner will also reduce lifetime as folds are where the chlorine will attack most.

All flexible liners will be attacked by chlorine but those produced from PTFE, Fluorocarbon rubbers or Fluor silicones will be more resistant, though significantly more costly than Butyl or EPDM.

Trials
To reduce attack on the liner, dosing should be made outside of the tank. Also, adequate and constant measuring of chlorine content is vital.

Normally a small part of the chlorine in the water will remain active, to prevent the growth of any bacteria or micro-organisms. Both the total amount of chlorine and active chlorine should be carefully measured and controlled to safe, recommended levels.

Chlorine causes depolymerisation, an oxidation that takes place on the liner surface and makes the sheet surface of Butyl become soft and tacky. With EPDM the surface becomes much harder and more brittle. Chlorine concentrations constantly higher than 0.2mg/l will also have an effect on the lifetime of liners.

Working with a new PVC-based material, Hereford-based tank manufacturer Franklin Hodge has completed a series of highly successful trials in Africa, where its new liner has proved exceptionally resistant to chlorine.

Nigel Snee, managing director at Franklin Hodge, explains: "We've developed a new liner that has almost the same flexibility for installation as Butyl, but has been designed specifically to have a very high resistance to chlorine. The new liner has coped very well with the rigorous testing we've given it in Africa's high temperatures, where typically, the local water supply is from a borehole.

"This inevitably sees an over cautious modus operandi, especially in rural villages, with chlorine often dosed in simple tablet form - all too enthusiastically - with a detrimental effect on the tank's liner."

Downtime
Though sudden, catastrophic tank failures are not caused by the gradual build up of chlorine, the heat transfer from the tank's wall to its interior that results in the build up of gas has been a long standing problem for tank manufacturers. Liners currently last from three to four years, but according to Franklin Hodge, its new liner will dramatically improve this lifetime to close on ten years.

"A liner with this greatly prolonged lifetime means a substantial reduction in maintenance costs, parts and downtime," adds Snee.

"This longevity also has a very positive knock-on effect for the local water companies who don't want to waste resources, especially those in developing countries."

He continues: "Water shortages are becoming more and more common, even much closer to home. In Spain for example, there is an increasing need to save, store and source a water supply, which will in turn raise questions about the quality of water taken from the ground - and then we are back to chlorine and the need for a good tank liner, which is hopefully where our company comes in!"

www.franklinhodge.com

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