High-tech water treatment solutions

October 2010 Valves, Actuators & Pump Control

Plug and dose approach to liquid dosing systems.

According to sera DoseTech, the usual way to purchase a metering or dosing pump is to approach the supplier with the chemical details and capacity requirements and expect to be provided with the correct pump for the job. This article puts forward an alternative solution that eliminates the common problems that arise from the general approach.

sera DoseTech SA is marketing a streamlined approach to dosing requirements that ensures that the customer receives a complete and tested dosing system instead of merely a metering pump. The company says the concept is based on many years of practical experience with dosing systems and the fact that although metering pumps are well designed and proven products, dosing systems are prone to failure.

Usually it is lack of attention to detail that causes the problems. A common fault is to find that the installation is inaccurate despite the fact that the pumps are rated with a delivery accuracy of ±2% for example. This is generally because positive displacement metering pumps work best against a constant back pressure so the fitting of a pressure sustaining valve is often mandatory.

However, connection fitting problems are by far the most common fault and in particular a lack of appreciation for the use of O-ring seals on most metering pump designs. An O-ring is designed to provide more sealing capability the greater the pressure applied to it, until the overall construction limits are exceeded. It is therefore possible to tighten by hand most metering pump connections and be leak free up to 10 bar or more. However, when the recommended connections are discarded in preference for the use of pipe sockets and PTFE tape the result is exactly the opposite: the more pressure applied to the joint the greater the chances of a leak.

Other issues include the correct fitting of protection devices such as pressure relief valves and pulsation dampers. The use of pressure keeping valves is self evident. However, calling a pulsation damper a safety device may appear strange at first. Pulsation dampers are generally applied to metering pumps because of their reciprocating action. This causes pulsations in the delivery of the chemical that can be undesirable from a process or flow measurement standpoint. Safety on the other hand is vastly increased by the use of a pulsation damper due to the removal of large hydraulic surges within the pipe system thus reducing the risk of leakage, pipe damage and providing overall system protection.

These are just some of the issues to be considered by the process design engineer when developing a design for a chemical dosing system. sera DoseTech concludes that its intention is to overcome these problems by using its expertise in the field to offer compact complete dosing units that include all the items necessary for optimal installation and operation.

For more information contact Wally Feldon, sera DoseTech SA, +27 (0)11 397 5120, [email protected], www.sera-web.co.za

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