Process and control today | Simple Steps to Successfully Sizing Control Valves

Specifying a replacement control valve can be a challenge. Making the right choice guarantees optimal control and precision of the system, but dimensioning requires time and a certain level of expertise. To help solve these challenges, flow control specialist Bürkert has released a new tool that makes sizing and specifying a control valve for almost any application a quick and easy process.

Kieran Bennett, Industry Manager – Food & Bev at Bürkert, explains how to specify a control valve.

When a replacement control valve is required, the user often does not know the full specification details or application parameters. This can make what should be a simple one-to-one swap a complex and onerous task for the plant engineer. Even if it is a third-party supplier, this can increase the lead time while requiring the provision of detailed specifications. Otherwise, it may mean assumptions made by the supplier in the specification that may not guarantee optimum control valve accuracy.

In many cases, the extent of the end user’s knowledge is that they need a particular size of modulating control valve. However, to ensure it is specified correctly, the valve supplier may ask for details that the engineer may not have easy access to, such as fluid flow rate or viscosity. In reality however, for the vast majority of applications it is quick and easy to specify the correct control valve, based on three simple criteria readily available to the end user.

Three steps to specification

The first question is: what does the valve primarily control? The answer is a choice between flow, temperature or pressure. Second, what fluid does the valve control and what type of application? This provides general clarification on whether a liquid, vapor or gas is involved and confirms aspects such as the aggressiveness of the fluid. Understanding the type of application also confirms aspects such as the specific hygienic requirements of the design, as well as the type of control required, for example, modulating or isolating. Third, the size of the input line must be confirmed.

To quickly specify a complete package with references, including the right control valve with process controller, sensor and cables, Bürkert has developed a valve sizing calculator, based on the three selection steps. In action, the valve sizing calculator provides a specification that ensures control accuracy as well as flexibility.

The selection for flow control, for example, specifies a valve with an extremely wide range, making it suitable for almost any purpose. In the case of a water application and a 1 inch / 25 mm line, the specified device can operate efficiently from a minimum flow rate of 900 liters per hour (l/h) up to 16 070 l/h. Alternatively, to ensure compatibility of valves specified for temperature control and pressure control applications, the sizing tool provides minimum and maximum values ​​for these attributes, along with the Kv value.

Control precision

Sizing the components on an individual basis achieves the best performance. However, for the vast majority of applications, using the Valve Sizing Calculator will not only prove much quicker and easier to specify a replacement control valve, but the difference in performance would be negligible compared to a calculation. tailored.

Taking the case of flow control, 20 years ago control valve technology required the precise range of the valve to be between 60 and 80% open. Although these values ​​still serve as a guide, valve accuracy has improved to the extent that individual sizing is no longer necessary in most cases. Bürkert’s pneumatic valves are accurate to within 0.5% over nearly the entire range of motion and, where required, electromotive valves are accurate to within 0.1%.

Provided 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 values ​​given, control valves specified by the simple sizing tool will meet the requirements. Ultimately, this approach covers almost any application except those with extreme limits and fluctuations, such as steam control in a power plant, where a bespoke specification would be required.

Full Specification of Control Valve

To facilitate provisioning throughout the system, the valve sizing calculator also specifies the type of control technology, ranging from angle seat valves for standard liquid flow control to globe control for modulating the pressure. Valve seal design materials are also specified, such as EPDM for aggressive liquids. For hygienic applications, PTFE seals are supplied, as well as crevice-free valve collar ends instead of threaded variants.

Control valve specification doesn’t have to be difficult. Using the basic steps provided by the Valve Sizing Calculator, it is quick and easy to confidently specify a control valve that not only provides the correct fit and performance, but includes all of the components necessary for a quick and easy replacement.

Try the valve sizing calculator here:

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