Engineered Software, Inc.’s mission when developing and enhancing PIPE-FLO® and PUMP-FLO™ software is to assist clients in realizing energy efficiencies through improved piping system understanding and superior equipment selection.
Recently a plant engineer working at a nickel mine in Canada reached out to ESI when the mines operators proposed an expansion and wanted to find the most optimal approach. This paper is a short case study about how ESI assisted in the redesign of the existing mine and helped with the expansion goals.
Before we go further into the case study, let’s review the Darcy Equation to gain a better understanding of the underlying issue the plant engineer was facing with the proposed expansion. Using the Darcy equation for calculating the head loss in a pipeline (formula 1), it is easy to see the impact fluid velocity has on head loss.
The head loss formula shows that since the inside pipe diameter is in the denominator and raised to the fifth power, an increase in pipe diameter will greatly reduce the head loss in the pipeline.
The Mine Background
This particular nickel mine had been in operation for over 20 years. The water being supplied for operation is pumped from a river to the mine. The change in elevation from the river to the plant is 320 ft, and the pipeline is approximately 26,000 feet in length. The current system consists of five vertical turbine pumps with fourteen stages. Each stage is at a maximum impeller diameter of 11.69 inches. The system’s normal operating mode is four pumps in operation and one pump in standby. This configuration supplies 12,300 gpm to the mine. See figure 1 for details.
Figure 1. System schematic of mine water supply system. Four pumps running in parallel each providing approximately 3,000 gpm of water.
The mine will be going through an expansion, and the makeup water capacity must be increased to 15,000 gpm.
The first idea was to operate all five pumps in parallel. A test was performed, but the maximum flow rate with all five pumps in operation was 13,400 gpm.
Since the installed pumps were at the maximum power rating specified by the manufacturer, it was not possible to add additional stages to the vertical turbine pumps. The pump manufacturer stated that with three new 14-stage vertical turbine pumps in parallel the new capacity requirement of 15,000 gpm could be achieved.
With all eight pumps in operation, the flow rate through each pump is still within the manufacturer’s allowable operating range while not exceeding the maximum horsepower rating of the vertical turbine pump. Each one of the pumps would consume approximately 670 kW of power. However there was an unforeseen issue, the plant utility manager stated the three additional pumps would cause the plant to exceed its normal electrical demand. The resulting imposed utility demand charges would be cost prohibitive.
A Smart Alternative
Using PIPE-FLO®, the plant engineer built a model of the existing system and it became apparent after adding the pump performance data for the vertical turbine pumps what had happened to the system over time. Four operating pumps of 3,065 gpm with a pump head of 993 feet is needed for the current operating condition with 84.3% efficiency. This results in each pump consuming 724 kW. With the system operating 8,000 hrs. per year and a power cost of $0.05 / kWh, each pump has an annual operating cost of $289,600. With four pumps operating the total annual power cost is $1,158,400. Understanding that the mine water system requirement was originally designed for only 5000 gpm and the new requirement is now approximately 15,000 gpm.
The original design consisted of three 6-stage vertical turbine pumps with two pumps in operation and the third in standby. The supply header was 26,000 feet of 20-inch schedule 40 steel pipe with an inside diameter of 18.81 inches. Over the years, the mine water makeup requirements increased, and additional stages and pumps were added to keep up with the changing demand.
When the flow rate is increased to 15,000 gpm, the fluid velocity in the 20-inch pipeline will be 17.4 ft/sec. This high fluid velocity resulted in a head loss in the pipeline of 987 feet of fluid. A model was created based on the existing system, and a 36-inch pipeline was placed in parallel with the existing pipeline. The resulting fluid velocity, flow rate, and head loss in the two pipelines are displayed in Table 1.
Based on the calculated result the pump manufacturer was able to recommend removing 8 stages from the existing 14-stage vertical turbine pump. The resulting 6-stage vertical turbine pumps were sized to pass 3,047 gpm with a head value of 367 ft. At this operating point, the 6-stage vertical turbine pump has an efficiency of 84.7% and consumes 272 kW. When operating under these conditions and each pump running 8,000 hrs per year at a $0.05/kWh power cost each pump costs $108,500 per year with a reduction in total annual operation cost of $542,500. See Figure 2 for details.
Figure 2. System expansion with a new 36 in diameter pipeline in parallel with the existing 20 in diameter supply header. The existing pumps were de-rated by removing 8 stages from the existing vertical turbine pump.
How Did the Original Inefficiencies Happen?
In this case study, the mine concentrated on increasing the capacity of the makeup water system to keep pace with the mine expansion. When increasing flow rate usually the plant engineer contacts the pump supplier to ensure that all manufacturer specifications are met and if additional capacity or to determine if pumps are needed. The pump supplier will take the opportunity to add additional pumps to keep up with the flow demand and stages to the vertical turbine pumps to increase the head required for the additional flow rate.
Since everyone was concentrating on the pumps, and not looking at the system, specifically the 26,000 feet of pipeline, and the head requirements increased rapidly; this particular system reached capacity without review. This is not an uncommon issue with a pipe that is buried so you could not see the pipe size, and it is easy to think of a pipeline as not having capacity limits. As we saw increasing the flow rate through the pipeline has a major impact on the head loss, and the head loss in the pipelines must be overcome by the head developed by the pump.
Adding the 36-inch pipeline in parallel, met the increased capacity demands without having to purchase and install three new 14-stage vertical turbine pumps saving the plant for the additional power consumption. Also, by increasing the effective pipe size, the plant was able to meet the new process requirements, remove eight stages from each vertical turbine, and save $542,000 in annual operating costs.
It is easy to see that by having easy to use tools to aid in operations simulations, this mine was able to resolve a long-standing problem that normal Excel spreadsheets and day to day troubleshooting simply did not uncover. Once the operating data became apparent in the PIPE-FLO® model management was able to resolve quickly the issues, make system improvements, and reduce exposure to increased maintenance issues caused by the original problem. Lastly, management had the added benefit of effectively improving their bottom line coupled with a short ROI on the cost of the improvements making their investment in PIPE-FLO® a solid choice.