Peristaltic pumps prove to be durable and dependable

Four Watson-Marlow Bredel 520 series dosing pumps are to be installed at United Utilities' Haslingden Grane Water Treatment Works in Rossendale, Lancashire. The new 520 series pumps have been supplied under United Utilities' framework agreement with Watson-Marlow Bredel. The framework agreement relates to the use of peristaltic pumps for the dosing, transfer and metering of liquids and slurries within the water industry. This includes abrasive materials and chemicals such as lime slurry, ferric and sodium hypochlorite, as well as sewage sludge. The framework agreement has been in place since April 2007.

John Hamer, Process Controller at Haslingden Grane WTW, states: "The first Watson-Marlow units we used on site were 504 pumps. They were installed in 1993 when the site was built and have been in continuous use ever since. We have always found them to be incredibly robust and they require very little maintenance. Reliable, accurate dosing is crucial, and the Watson-Marlow pumps give us the peace of mind we need to run the plant efficiently. As a result, when we made the decision to upgrade the pumps, we wanted to continue with Watson-Marlow's technology – it made no sense to change supplier."

Avoiding gas locks

Engineers at Haslingden Grane, which serves a population of 50,000 people, use the 520 series pumps for the dosing of sodium hypochlorite and sodium bisulphite. Sodium hypochlorite is a highly corrosive chemical that has disinfection and bleaching properties. It is used in the purification of wastewater and the disinfection of drinking water. However, dealing with the chemical can be difficult, as it is toxic and emits chlorine when in contact with acids. It naturally releases tiny bubbles of gas, which have a tendency to collect on the small ball valves found in conventional diaphragm dosing pumps. This can cause gas locks in the system, which prevent the pumps from functioning. Because peristaltic pumps retain the fluid completely within the tube and have no valves that can leak or corrode, they can be used for the accurate metering of substances as challenging as sodium hypochlorite, without the associated gas locking and maintenance problems. Peristaltic technology therefore helps to keep costly downtime at the site to a minimum.

The pumps are also used for dosing sodium bisulphite – which de-chlorinates the water after disinfection. The chemical is dosed in very small quantities, which requires a high degree of precision and accuracy from the pump. Often it is necessary to dilute sodium bisulphite in order to control the dosage procedure accurately. This can add an extra degree of complexity to the process. However, this is not an issue for process engineers at the Haslingden site because Watson-Marlow's 520 peristaltic pumps can be calibrated either by weight or volume, allowing very precise dosing and metering. This enables the user to dose small amounts reliably, even at low pressure, without the need for dilution.

Generating a flow rate of between 4ul and 3.5 litre/min, the 520 pumps ensure exceptional accuracy, reliability and control even in harsh operating environments. Each one is rated to IP66, providing washdown protection for arduous conditions. The 520 series delivers user feedback via the pump's own display and have dual analogue inputs to control and scale speed. The 520 series provides users with a 'drop-in' alternative for diaphragm or piston pumps with variable stroke control.