When a replacement control valve is required, it’s often the case that the user isn’t aware of the full specification details or the application’s parameters. This can make what should be a simple like-for-like swap a complex and onerous task for the plant engineer. Even if it involves a third-party supplier, uncertainty can extend procurement time and still require the provision of a detailed specification. Otherwise, assumptions made by the supplier in specification might not ensure optimum control valve accuracy.
In many cases, the extent of the end-user’s knowledge is that it requires a particular size of modulating control valve. However, to ensure that it is correctly specified, the valve supplier might ask for details that the engineer probably doesn’t have easy access to, such as flow rate or the viscosity of the media.
In reality, though, for the vast majority of applications, it’s fast and simple to specify the right control valve, based on three simple criteria, readily available to the end user.
The first question is: what does the valve primarily control? The answer is a choice between flow, temperature, or pressure. Secondly, what media is the valve controlling, and what is the application type? This provides general clarification on whether a liquid, steam or gas is involved, and confirms aspects such as whether the media is aggressive. Understanding the application type also confirms aspects such as the specific hygienic requirements of the design, as well as the control type required, for example, modulating or isolation. Thirdly, inlet line size should be confirmed.
To quickly specify a complete package with part numbers, including the right control valve with process controller, sensor, and cables, Bürkert has developed a Valve Sizing Calculator, based on the above selection steps, available via www.is.gd/jihija.
Control accuracy
Sizing components on an individually tailored basis achieves the highest performance. However, for the vast majority of applications, using the Valve Sizing Calculator will not only prove much faster and easier to specify a replacement control valve, but also the performance difference would be negligible compared to tailored calculation.
Taking the case of flow control, 20 years ago, control valve technology demanded that the accurate valve range was between 60 and 80% open. While these values still serve as a guide, valve accuracy has improved to the extent that individual sizing is no longer required in most cases. Bürkert’s pneumatic valves are accurate to within 0.5% across nearly the entire range of motion.
Providing that the pressure drop across the valve is an absolute maximum of 50% of the incoming pressure, and that the temperature and pressure do not exceed or fall below the given values, the control valves specified by the Simple Sizing Tool will fit the requirements. Ultimately, this approach covers nearly all applications, except for those with extreme limits and fluctuations, such as steam control in a power station, where a tailored specification would be needed.
For ease of procurement across the system, the Valve Sizing Calculator also specifies control technology type, ranging from angle seat valves for standard liquid flow control, to globe control for pressure modulation. Valve seal design materials are also specified, such as EPDM for aggressive liquids. For hygienic applications, PTFE seals are provided, as well as crevice-free valve clamp ends instead of threaded variants.