We'll Never Replace the Control Valve, or Will We?

Dick Caro reminded control systems engineers that the electronic variable frequency drive (VFD) was an alternative to the control valve as the final control element in liquid flow control applications. Not only is this application less complicated than the more conventional use of a control valve, but it is also widely known to save energy. Some control engineers have known this for many years, but have been reluctant to specify use of the VFD chiefly because they're unfamiliar with it.

Read the full story at "Pursuing Sustainability With VFDs."

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  • <p>The following is a comment from one of our readers submitted via e-mail.</p> <p>I enjoyed reading "Pursuing Sustainability With VFDs" but I think Dick Caro could have done a better job of explaining why flow control with a VSD on a pump is for the most part linear.  </p> <p>He wrote, "A few correspondents argued that since the head changes at different flow rates due to pipe friction, a real installation could not be exactly linear. However, the controlled variable—flow rate—is linear."  </p> <p>This sentence implies that flow rate is independent of pressure drop and it isn't. That's how a flow control valve works with a constant speed pump, changing the pressure drop to modify the flow.  However, the Pump Affinity Laws state that the head a pump develops is proportional to the speed squared, and as luck would have it, per the D'Arcy-Weisbach Equation, the friction loss in piping with turbulent flow is proportional to the velocity squared.  Therefore, if you have a piping system where the pressure drop is proportional to the velocity squared (no fixed pressure drops, turbulent flow, etc) you are in luck. Your control is perfectly linear. Rarely is the flow laminar but frequently a portion of the pressure drop is unrelated to flow.  </p> <p>So Dick's statement that the VSD imparts linear flow control is correct but only for very specific installations where most of the pressure drop is due to turbulent flow in piping, this is true of most common chemical industrial piping installations.  Due consideration should be given to any nonlinearity if you want the best control.  But even having said that, there are changes in pressure drop that are totally unrelated to flow such as changing tank levels.  The flow control loop must be able to deal with any such irregularities.</p> <p>Best Regards,<br />John Chatfield</p> <p>PS:  As an example, say you are feeding a distillation column operating at 2 bar gauge from an atmospheric tank.  At 100 m3/h, the pressure drop in the piping is 1 bar and there is 1 bar of pressure drop due to the change in elevation.  So your pump is supplying 100 m3/h at 4 bar of head.  If you increase the pump speed by 10%, the Pump Affinity Laws say that the pump output should go to 110 m3/h and 4.84 bar of head.  So now there is 1.84 bar for pressure loss in the piping where only 1.21 is needed for the 110 m3/h.  The flow will therefore move out on the pump curve, and the pump will supply more flow until the pump curve intersects with the system curve. Depending on the specific pump curve, this might mean supplying 130 m3/h at 4.69 bar of head.  So you might get 30% more flow with only 10% more speed!</p>

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