Fisher Fisher – Overcoming the difficulty of selecting control valve trims

Create Date: 2024-8-30 12:03:23|Source: Fisher

Rules of thumb for flow characterization selection

Different applications favor different valve flow characteristics. These rules of thumb do not cover all situations, but they can address many of the typical situations encountered in the process industry.

Linear control valves are commonly chosen for pressure control, including near-constant DP, near-constant inlet and critical pressure drops, high inlet and low outlet pressures (e.g., steam pressure reducing valves), positive displacement compressor bypass valves, distillation/fractionation column pressure control applications, and others. Linear valves are also selected for level control applications with a near-constant valve DP, temperature control applications to limit the entry of liquids into the heat exchanger, or two-valve or three-way valves in heat exchanger bypass applications.

The equal percentage feature is commonly used in pressure control applications where the valve DP is inversely proportional to the flow rate, such as maintaining air pressure to the burner or providing a constant flow of steam to the reboiler. They are also used for flow control, where the valve DP is less than one-third of the pressure drop in the line loop – and for temperature, flow or pressure control for steam or other condensing/heat transfer processes.

Fisher Fisher – Overcoming the difficulty of selecting control valve trims

Quick open valves generally do not work well as control valves, but can be used in drain/vent applications where fast flow is required.

Regardless of the adjustment of the flow characteristics of the service and selection, the size of the valve must first be determined. In applications where process conditions have not been thoroughly investigated, valves are often oversized to provide margin of error, but this often results in poor control regardless of trim selection.

conclusion

Selecting the correct intrinsic flow characteristics can determine the success of a control valve installation. The idea of implementing a linear install response is simple, but to achieve this, the end user must have a good understanding of their process. Discussing fluid mechanics with process *** can lead to significant benefits in terms of controllability and process stability.

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