By Zarl Bedalov, P.Eng., and Colin Hull
The electrical room for a typical metallurgical concentrator plant must be well positioned as a power distribution point with easy access to all plant areas. The decision to have one or more electrical rooms in the plant is determined by economics, plant operational/maintenance policy, and cable and equipment access. This paper demonstrates the cost savings realized by having a secondary electrical room in addition to the main electrical room. This estimate is based on an actual recently completed gold processing project .
Gold Processing Plant
A typical gold processing plant comprises four principal areas as shown on Fig.1, as follows:
(1)Grinding Bay. This bay is adjacent to ore handling and transportation equipment. It usually has a laydown area and a raised roof to allow for crane operation around the mills.
(2)Flotation Bay. This bay contains rows of flotation cells and piping. In the absence of flotation cells, this area may contain the gravity/ spirals circuit.
(3)Reagents & Utilities Bay. This area contains numerous small motors/loads for the plant additives, tailing circuits and concentrate pressure filters.
(4)Leach/CIL Area. This area is generally outside and adjacent to the reagents bay.
Electrical Room Location
The need for an additional electrical room greatly depends on the relative location of the main electrical room. The following are three typical locations within a plant for the main electrical room:
In the centre of the plant (Loc.A on Fig.1) at a raised elevation to allow for cable access from below. This centralized location lends itself to convenient cable distribution to all parts of the plant. While there are a number of plants built this way, the approach is often not practicable due to constraints in laying out the mechanical equipment, a priority in the plant layout design.
The most commonly used location (Loc.B) is along the long side of the plant building in the area of the flotation bay. This arrangement offers the convenience of cable access to the grinding bay on one side and to the rest of the plant on the other, thus achieving cable segregation by voltage since the medium voltage cables generally go to the grinding bay only.
A third location (Loc.C) also used is locatedcat the short side of the building, along the grinding bay. Cables in this layout generally access the plant at one side only thus necessitating a large concentrated area for cable marshalling. This concept requires a large space for cable passage at an elevation over the other areas of the plant to bypass the laydown area and the crane operating space. Due to longer distances involved, cables are uprated to meet the voltage drop criteria. Cable trays may have to be enlarged to allow for additional cable spacing to maintain cable ampacity.
One or More Electrical Rooms
Many plant operators have defined policies regarding both the layout and the number and location of electrical rooms within the plant. While some operators stress the beneifts of a single electrical room, others feel that there are definite cost advantages to multiple electrical rooms. Those who support the former insist that a layout with one electrical room is better from a maintenance and safety point of view. These proponents are willing to sacrifice some cost for the benefit of having all electrical equipment in one place and under one key.
Other operators consider that maintenance and safety are actually enhanced with an extra electrical room located closer to the plant loads. A fire in the main electrical room will certainly lead to a major setback for the plant. Also, the maintenance personnel may be tempted to make dangerous "shortcuts" to avoid long travel to the electrical room. They also feel that a reduced number of cables in the plant, with fewer layers of tray runs, enhances fire safety.
Modern electrical power distribution equipment can be safely placed around the plant closer to the area loads. As shown herein, a substantial cost can be realized by adding an "area electrical room" (Loc.D) in addition to the principal electrical room. Each area room may contain one or two unit substations with associated MCC's/ PLC's.
The MCC control is facilitated from the main control room via a data highway that connects all the plant PLC's into one system. Based on the estimate given here, cost savings in the following categories can be achieved by adding an area room for feeding the reagents/ tailings preparation and Leach/CIL loads.
Civil/HVAC
The main electrical room becomes smaller as part of the equipment is transferred to the second electrical room. Also, the area electrical room is generally less elaborately designed. The overall cost for both rooms is expected to be somewhat higher from that of a single room.
Cables and Cable Trays
This is an area of major cost savings. Cable tray design is considerably less complex since the amount of cables is reduced. Fewer cable tray and elbows are required as there are fewer corners and elevation transitions. Since the cable runs are shorter, the associated voltage drops are lower, it enables the designer to select cable sizes without deratings.
VFD Considerations
Variable frequency operated motors may be subjected to significant voltage spikes caused by fast VFD switching in combination with long cable leads. It is believed in the industry that motors with cables in excess of 50 meters (150ft) will experience severe voltage stresses resulting in rapid motor burnout.
VFD's with long leads to motors require harmonic filters to ensure voltage limits are not exceeded. Plants with multiple electrical rooms generally do not have this problem.
Fire Protection
Fire prevention measures are enhanced when there is less flammable material in the plant. Of particular interest are the fewer layers of cable trays in the main plant routes and in the electrical rooms. Insurance companies maintain that a bank of vertical layers of horizontal cable trays is a major fire hazard in the plant.
Engineering
Cable tray design for a multiple electrical room layout requires less engineering input and coordination with the other disciplines. Cable tray design begins early in the project by defining major cable tray arteries. When all area piping and ducting is completed, the design is adjusted and finalized .
Plants with a secondary electrical room will not require a major cable route along the side of the plant, thus significantly reducing interference with the piping and reducing the engineering effort in the design
Zark Bedalov and Colin Hull are with Fluor Daniel Wright Inc. located in Vancouver, B.C. Tel: (604) 488-2000.