A throttling valve is usually employed when the process flow requirement is less than the flow at the pumping system’s natural operating point. Throttling valves control flow by increasing the system’s backpressure or resistance to flow. This increase in pressure or head requirements shifts the pump’s operating point to the left along its performance curve, and, typically, away from its best efficiency point. The result is a loss in efficiency.
Adjustable Speed drives (ASDs) provide an efficient flow control alternative by varying a pump’s rotational speed. These drives are broadly classified as mechanical (fluid or eddy current) drives and variable frequency drives (VFDs).
- VFD Application Checklist
- Case Studies
- Wiki Article on VFD
- VFD Savings - Example
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Today, the VFD is the most frequently specified type of ASD, and pulse-width-modulated VFDs are the most commonly used. In centrifugal applications with no static lift, system power requirements vary with the cube of the pump speed. Small decreases in speed or flow can significantly reduce energy use. For example, reducing the speed (flow) by 20% can reduce input power requirements by approximately 50%.
In addition to energy savings, VFDs offer precise speed control and a soft-starting capability. Soft-starting reduces thermal and mechanical stresses on windings, couplings, and belts. Also, VFDs reduce voltage fluctuations that can occur in starting up large motors. Induction motors with across-the-line starting draw as much as six times the full-load current during start-up. During acceleration, a VFD-controlled motor’s locked rotor current is limited to one and one-half times the full load current. Operating at reduced speeds results in other benefit, as well, such as lower bearing loads, reduced shaft deflection, and lower maintenance costs. Estimating Performance We can use the affinity laws to predict the performance of a centrifugal pump with little or no static head at any speed, if we know the pump’s performance at its normal operating point. The affinity law equations are as follows:
The affinity laws show that the pump head decreases significantly when the pump speed is reduced to match system flow requirements (see figure). Pump shaft horsepower requirements vary as the product of head and flow or as the cube of the pump’s speed ratio. Note, however, that the affinity laws will not provide accurate results for systems with static head. In that case, constructing a system curve to calculate new duty points is essential.
We operate a pump with VFD, the significant drop in flow rate comes with significant drop in head also whereas in actual operation sometimes system curves are changing which results in lesser drop in head compared to calculated from affinity laws. In such cases, the VFD alone can not provide the desired head & flow combination becasue speed variation alone can not change system curve.
So Be Careful while selecting or recommending VFD. DO NOT CONSIDER IT without the knowledge of system curve. In such case, we need to consider other alternate also e.g. combination of conventional throttling with VFD.
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