On the selection of valves
April 2003
Valves, Actuators & Pump Control
Valve selection in the main is an arbitrary subject. It can be based on many factors such as tradition, operational experience, perceived suitability for process conditions, operational lifespan requirements, initial cost, or a combination of any of these. Due to prevailing market conditions initial cost has tended to become a major factor, particularly in plants such as those for potable water, waste or effluent treatment.
Whereas for particular applications when operational and/or sizing requirements necessitate the use of gate valves for isolation, initial cost considerations have tended towards the selection of quarter-turn valves. There are a variety of quarter-turn valves available; the most commonly used being the rubber-lined wafer butterfly valve. This valve is mainly space/mass saving, inexpensive to purchase and maintain, easy to operate and relatively inexpensive to automate. If selected correctly for the application, rubber-lined butterfly valves of good quality and design can be a good choice.
Other popular quarter-turn designs are ball valves and plug valves. The former is a cost-effective and reliable solution for clean applications and where minimal pressure drops are required, whilst the latter is usually more expensive, but provides a higher standard of shut-off and is better suited to aggressive applications.
Due care should be taken over the selection criteria to ensure that the valve of choice is fit-for-purpose rather than just cost-effective.
This is not to say that initial cost should be discounted, but rather that it should be one of several factors that influence valve selection. In this regard the valve manufacturer, or supplier, should be involved in the selection process as much as possible.
As with most types of equipment, there are varying standards of design within each valve category. A basic description or specification can encompass a wide range of valves but with differing design features, and so providing varying service life spans and degrees of maintainability.
These factors should also be taken into consideration and verified with the supplier during the valve selection process, as they may impact on the overall cost of the valve in terms of the operational life of the plant.
Quarter-turn valves are also used for fluid control purposes, and can prove to be a reliable and cost-effective alternative to traditional and (sometimes) more costly control valve designs, and providing that complete process and control application details are given to the valve supplier.
For more information contact El-o-matic, 011 974 3336.
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