Optimize control valve specification by challenging the data

Process valve insights

• Validating input data is critical in control valve sizing to avoid errors. Inaccurate data can lead to oversized valves, reducing efficiency and inflating costs.
• Re-evaluating system design assumptions can optimize performance. In the case study, switching the valve’s position achieved desired water conductivity, ensuring operational accuracy.

Ensuring the right input data is provided when specifying control vales provides the best chance of optimizing the application.

When a system integrator approached Bürkert to supply a valve for a water conductivity blending application, it had the objective of achieving water quality with a conductivity level of 10µS. The 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 a control valve that would regulate the deionized water flow was requested. \

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, it suggested a total flow rate of 1.5 m3/hr. Even at this stage, We 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 1in diameter valve, which would likely be too large for the application. However, the integrator was satisfied with the validity of its initial input data, and 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.

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 to 90% of its capacity would enable the use of a smaller, more cost-effective valve, that could also achieve more efficient operation.

The valve size was recalculated, based on optimizing 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 the desired, repeatable, and controllable flow range could not be achieved.

Optimizing control accuracy

On further investigation, it was 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 valve was resized sufficient to handle a difference from 4 bar upstream pressure down to 0 bar downstream.

The system integrator went on to reconsider a fundamental assumption about the system’s design. Although the conductivity value of the deionized water was very low, at 0.5 µS, the value of the softened water was very high, around 600 µS.

Without modulating the softened water flow, the resulting conductivity level after blending the media would be far higher than the objective of 10 µS. the integrator was advised that instead of regulating the flow of deionized water, by controlling the softened water, the desired conductivity level could be reached.

Switching the position of the control valve and resizing it accordingly enabled the system to achieve the required 10 µS water conductivity rating for the application.

Although this example demonstrates the potential implications of incorrect specification, the same principles apply for any application that requires control valve modulation. The fundamental calculations to ensure correct control valve sizing are essential whatever the purpose and industry sector.

A crucial aspect for optimum sizing of control valves is achieving valid application input data. If the input application data is incorrect, or incomplete, then the type, position, and size of the control valve will be sub-optimal. While specifiers can make mistakes in calculations, valve specification errors can also have historic origins.

If the original system wasn’t designed according to the right data, like-for-like replacement will also be sub-optimal. Today’s valves are frequently more accurate, impacting open and close rates, and this means that smaller, less expensive valves can potentially be used instead. So, even when upgrading an existing system, the best practice is to recalculate control valve sizing each time.

– This originally appeared on Control Engineering Europe.

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