Variable frequency drives have many uses in paper mills. In this discussion, we will focus on their potential use to control the fan pump in the approach section.
The fan pump is the center of the paper mill approach system and serves to mix the pulp stock with white (treated) water. The fan pump delivers this mixture to the headbox, and ultimately, on the wire of the fourdrinier table (see Figure 1).
Figure 1 - Simplified Paper Mill Approach System.
Enhanced Product Quality
The fan pump is typically the largest pump in the paper machine system and the demands made on it are very critical to the paper making process. Flow rate and pressure must be stable, without pulsations or surges. Fan pumps must also be able to vary the flow over the entire range of paper machine operation.
Because of change in product requirements or operating conditions, the flow from this system is constantly varying. This flow change has traditionally been accomplished by throttling valves, recirculating systems or by turning different pumps on and off. These methods can potentially cause flow and/or pressure transients, and will increase maintenance and power consumption.
Today's requirements for paper quality require very close tolerances and extremely tight variation requirements. Increasingly tighter tolerances are expected in the future, as well. The only way to meet these increased requirements is to control each variable that impacts the steady state condition. Precise pressure and water flow is required. As water usage in each mill process increases or decreases, the pressure available for any other mill process may vary. Any pressure variation will change water flow at a given header or valve setting. This variability in flow can impact the paper product tolerance. A fast response adjustable speed driven pump can reduce or eliminate this flow transient. Therefore, fan pumps are well suited for variable frequency drives (VFDs) to regulate flow and operate at peak efficiency.
Medium voltage AC VFDs like the Dura-Bilt5i MV® have become an attractive alternative when focusing on consistent product quality, minimized system and motor maintenance requirements, operational costs, and initial investment. AC drive vector control technology for induction motors in variable torque applications, requiring accurate speed control, have improved to the level where the need for a speed feedback device has been eliminated. This technology, tied in with the excellent performance of AC drive systems in the pump market, makes the Dura-Bilt5i MV a great choice.
Reduced Power Consumption
Historically, constant speed induction motors have been used to drive fan pumps. When
a reduced flow is required, a constant speed pump must either be turned off, throttled or recirculated.
Throttling and recirculation techniques cause additional losses in the system. As seen in Figure 2,
the pump efficiency is severely affected when flow is controlled by throttling.
Figure 2 - Typical head-flow (H-Q) and throttling efficiency curve of specific
pump at rated speed.
Applying a VFD to the fan pump eliminates the need for throttling or recirculation. Significant savings can be achieved by operating fan pumps with variable speed AC drives. A VFD allows the pump to operate on the system characteristic curve as shown in Figure 3 (thus saving throttling energy loss).
Figure 3 - Typical pump
H-Q curve with system curve added. Artificially adding
friction by throttling changes the system curve and reduces pump flow.
Improved Power Quality
Variable speed drives have proven to be superior over all other types of mechanical flow control. Changing the speed of the pump motor, and thus the pump, allows the flow and the head pressure to be controlled. By utilizing pressure sensors, the pump can be controlled to deliver a constant pressure under any conditions. As flow requirements change, the pump will maintain constant pressure. This technique improves system response and reduces energy consumption.
The Dura-Bilt5i MV drive maintains a power factor of 0.95 or greater over the entire motor speed range, resulting in a higher input power factor and less reactive power consumption than found in a typical fixed-speed pump application. Additionally, the Dura-Bilt5i MV variable speed drive is capable of providing the same high starting torque across the full motor speed profile. The Dura-Bilt5i MV drive uses a 24-pulse input diode rectifier to convert the utility power into a DC buss for the inversion to Variable frequency by the inverter. This 24-pulse input design is significant in that it produces less than 3% total harmonic distortion in most applications, meeting or exceeding IEEE 519-1992 standards without filters.
Retrofits and New Installations
VFDs are ideal for retrofit applications of existing constant speed pump motors and can usually be applied to existing motors without modifying the motor or pump foundations. This helps avoid considerable cost and expensive downtime. In retrofits and new installations, the Dura-Bilt5i MV provides a controlled application of voltage and current when the motor is started. Motor heating and system mechanical stress from high inrush currents are also eliminated. The ability to vary pump speed, as plant conditions require,
results in longer motor and pump life. A VFD also limits the need for flow regulating valves. The valves may be needed for isolation, but the wear associated with constant use for flow regulation is eliminated. The benefit is less wear on motor and pump seals and bearings, as well as shorter downtime for maintenance.
Reliability, Service, and Warranty
GE Toshiba has a significant number of Medium Voltage VFDs for pump applications in service today. Our current generation Medium Voltage VFD is the Dura-Bilt5i MV. This product has been designed from the ground up to be a high performing and reliable product. It is also backed by one of the largest field service organizations in the world. GE Toshiba is available 24/7 to meet your engineering, startup, service, and training needs.
You can count on GE Toshiba for unmatched drive and system application knowledge. And every Dura-Bilt5i MV is backed up with GE Toshiba's 5-year parts and labor warranty.*
Rich Hiltz is an application engineer with GE Toshiba Automation Systems, LLC.
*GE Toshiba Automation Systems' 5-year warranty applies only to installations in North America.
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