Process, Controls & Plant Focus on: Pumps & Valves 26 | Plant & Works Engineering www.pwemag.co.uk Octobert/November 2023 Greg Wainhouse, Industry Account Manager for Water Applications at Bürkert, explains the importance of reviewing input data for control valve specification. Optimising control valve specification When specifying control valves, ensuring the right input data is provided offers the best chance the resulting performance of the application will be optimised. To show the effects, a recent example based on deionised water production highlights how accurate specification is essential whenever control valves are used. Full consideration of the application’s key criteria, as well as accurate calculation of input application data, are vital to optimise control valve specification. Recently, a system integrator approached Bürkert to supply a valve for a water conductivity blending application, with the objective of achieving water quality with a conductivity level of 10μS (microsiemens). Their process design schematic included a ball valve that would isolate the flow of softened water, while a control valve would modulate the flow of deionized water. By controlling this media combination, once blended, the customer’s expected outcome was water production at the desired conductivity level. With this plan, the system integrator requested a control valve that would regulate the deionized water flow. To calculate the size of control valve required, it is important to understand the required flow rate and pressure. The integrator suggested that upstream pressure was 4 bar, with 3 bar pressure downstream. Moreover, they suggested a total flow rate of 1.5 m3//h. Even at this stage, Bürkert suggested a review of this input data because the suggested flow rate appeared to be inaccurate. Furthermore, using the pressure and flow data, the calculation generated a requirement for a 1” diameter valve, which would likely be too large for this application. However, the integrator was satisfied with the validity of their initial input data, and they confirmed the resulting valve specification, supported by the fact that they had used this size of valve on a similar project with the same input data. Oversized valve specification The Bürkert team remained unconvinced and they sought wider input data to help confirm the specification. The integrator reported that when installed, the previous valve project would operate at 10 to 20% of its total capacity. However, the reverse situation should ideally apply, sizing a valve to operate at 70%-90% of its capacity would enable the use of a smaller, more cost-effective valve, that could also achieve more efficient operation. Burkert decided to recalculate the valve size, based on optimising 80% capacity. This time, to ensure a comprehensive approach, the input data was reconfirmed. With a reading from the system’s pump, the upstream pressure was confirmed. Thanks to an installed flow meter, the flow reading was also verified. Using these input data, it was possible to work backwards and determine the actual downstream pressure. This highlighted that the downstream pressure was significantly less than the figure initially provided. Instead of the suggested 3 bar, the new calculation was just 0.1 bar. Based on this new data, it was realised that the customer’s desired, repeatable, and controllable flow range could not be achieved. Optimising control accuracy On further investigation, the customer revealed that an open tank was positioned downstream of this system, which both explained and confirmed the almost 0 bar downstream pressure reading. As a result, the customer agreed to resize the valve, sufficient to handle a difference from 4 bar upstream pressure down to 0 bar downstream. By this stage, the system integrator was working more closely with Burkert, and as a result, were also able to challenge a fundamental assumption about the system’s design. Although the conductivity value of the Achieving the optimum valve selection depends on accurate process data and expert valve knowledge.
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