By: Zark Bedalov, P. Eng. (Electrical), Fluor Daniel Wright Ltd.
Brad Matthews, P. Eng. (Process), Fluor Daniel Wright Ltd.
Variable Frequency Drives have become an integral part of mineral processing plants for the purpose of optimizing plant production and saving energy. A typical mineral processing plant with automated process uses 20 to 40 variable frequency drives (VFD's) to control slurry flows, generally via a number of sets of 2 x 100% pumps. The plant designer is often required to make a choice between using one VFD for each pump and a single VFD with a manual bypass switch to operate both pumps. The decision making process normally involves economics, although other design aspects, such as safety, reliability, pumpbox level control and performance (metal recovery), are considered.
Pumpbox Operation
Slurry pumpboxes, in the mineral processing industry, are generally operated on a level control loop as shown in Figure 1. The pumpbox system typically includes two installed discharge pumps, one operating, the other on standby. A relatively constant slurry level in the pumpbox is maintained by balancing the material flowrate out of the pumpbox with the material flowrate into the pumpbox. Consequently, the pumpbox discharge pumps are equipped with VFD's. The pumpbox level controller adjusts the pump speed to maintain the desired slurry level.
The feed to a slurry pumpbox usually consists of several process streams, including slurries from various upstream and downstream unit operations, process water and reagents. The speed of the slurry discharge pump is the key variable for maintaining a balanced state operation. Because of the VFD's ability to automatically adjust the flow, the pumpboxes are designed with small retaining capacities. Any disturbance in the flows may quickly result in the tank overfilling and/or spillage.
The design of this type of pumpbox level control with variable speed pumping can include the use of two VFD's, one for each pump (Alternative A, Figure 2), or the use of one shared VFD with a manual bypass switch (Alternative B). The Alt. A represents a fully redundant system from the incoming breaker down to the motor. The Alt. B, while obviously not fully redundant, is regarded to be less costly and is perceived to require less floor space in the MCC room.
Comparison
Cost
On a recent project, bids were requested for both alternatives for a number of drives. Alt. B was less costly by about 15 to 20%, depending on the bid. The average cost of VFD's for a set of pumps was $160/HP (Alt. A) and $135/HP (one VFD plus bypass, Alt. B).
Space
Both alternatives were assessed to occupy an equal area of floor space. The bypass switch (Alt. B) comprising the main and two switchover contactors requires a separate housing from that of the main drive to allow for unobstructed and safe maintenance on the VFD drive when the pump is run in the bypass mode.
Performance
Safety
Alt. A, with two separate cabinets for the VFD's allows maintenance personnel working on a VFD or on a motor, full protection against inadvertent contact with energized components. The incoming breakers can be locked out for added safety. This is not feasible with Alt. B; in this case the standby motor/pump cannot be tested or worked on. To allow for motor maintenance, each motor must be equipped with a field mounted disconnect switch at an additional cost, thus reducing the cost advantage of this alternative by about 10%.
Operation
Alt. A offers full flexibility of operation. A change of an operating pump can be executed on the fly from the control room without disturbing plant operation. In the case of Alt. B, the bypass switch is arranged for manual operation. The bypass leg is equipped with a control circuit and a set of overloads which allow the circuit operation while the VFD is taken out. The switchover from one pump to the other is initiated by operating the switch contactors.
VFD Failure
VFD's have become reliable pieces of equipment. They do fail, however. Failure of a VFD in the Alt. A arrangement will not significantly affect the operation. Transfer to the healthy VFD/pump can be arranged following receipt of an alarm. On the other hand, a switchover to bypass mode on Alt. B is done manually following a full shutdown. In the bypass mode, the operating pump operates at full speed only.
Each set of slurry pumps is a part of the operating stream of the plant, of about 10 to 20 sets of pumps, all of which must operate in a preset manner with regulated flows to maintain flow balance and to achieve optimum plant performance and metal recovery. Failure of one pump to regulate properly will cause the whole process to be disturbed.
This irregular situation can happen with the system which uses shared VFD's, when one of the pumps within the process is required to operate at full speed, thereby forcing all other VFD's in the plant to modulate flows to follow the rapid changes in the process. Spillages will occur and the operating staff will be in full force on the plant floor to maintain the operation. Operating under this situation will jeopardize plant control and affect plant performance with respect to metal recovery and/or grade. For instance, if a 5,000 tpd gold plant with a gold head grade of 0.15 oz/ton experiences a 10 per cent loss of recovery due to an upset plant condition resulting from a VFD failure, the loss of revenue would be in excess of $25,000/day, which is in excess of the cost of a 200 HP VFD.
Conclusions
The capital cost of a system with a single (shared) VFD between two slurry pumps is lower. However, the probability and potential for loss of production due to a VFD failure must be considered in the design and selection of slurry pumping operations. The system with individual VFD's offers added safety, reliability and simplicity of operation.
Zark Bedalov is a Sr. Electrical Engineer, with Fluor Daniel Wright in Vancouver, B.C., specializing in power distribution for mining applications.Brad Matthews is a Sr. Process Engineer, with Fluor Daniel Wright in Vancouver, B.C., specializing in mineral processes for precious and base metal processing plants.