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Jul
07

Top Ten Limitations – Turndown

Plant turndown determines how effectively a production unit can minimize inventory and energy costs. As utility rates change with supply and demand, energy efficiency changes with weather, and product demand changes with customer shipments and downstream requirements, the need to be able to run at lower and lower rates becomes increasingly important. Temporarily shutting down a plant is not a good option because startup poses safety concerns and losses in efficiency.

The turndown of plants largely depends upon the turndown of flow meters and the rangeability of control valves. Unfortunately, the stated turndown and rangeability is rarely achieved in actual operation because of noise and stiction and the mismatch between meter, valve, and process maximum flows.

Oscillations from noise at low velocities and from friction at low valve positions can cause unacceptable increases in variability. The fact that the meter and valve maximum flows are much higher than the process maximum aggravates the problem. Most meters don’t ever reach the maximum velocity that corresponds to the upper end of their flow range. Most valves are sized to be 50 to 80% open at the maximum process flow.

Meters with greater upstream straight run requirements and valves with higher seating and sealing friction (tight shutoff) tend to exhibit more noise and stiction, respectively at low flow. For maximum turndown consider magnetic flow meters and Coriolis meters and sliding stem valves with digital positioners. For large line sizes consider smart multivariable insertion flow meters and Vee-BallTM (contoured segmented V-notch) throttling valves with digital positioners. As with any application thoroughly check out suitability for process operating conditions and requirements. While these meters and valves with greater turndown may cost more up front, they can pay for themselves by enabling flexible manufacturing.