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Considerations to make when sizing pumps

By |  December 29, 2020
Tech sizing pumps

Weather and subsurface groundwater account for the majority of the pumping that temporary and permanent pipes handle. In the event of a pump system failure, it’s not uncommon to assume that the pump itself is the cause – when in fact, it’s really a matter of a system failure. Photo: Sunbelt Rentals

Having the ability to remove water from a mining site is vital to maintain an efficient operation. 

Weather and subsurface groundwater account for the majority of the pumping that temporary and permanent pipes handle. In the event of a pump system failure, it’s not uncommon to assume that the pump itself is the cause – when, in fact, it’s really a matter of a system failure. 

A pumping system is the combination of elevations, pipe and hose, valves, fittings, power, controls and, lastly, the pump. So, how do systems fail? There could be an issue with the pipe size to and from the pump, the vertical elevation change, how far water is being pushed from the water source, or what’s in the water. For that reason, it’s important to evaluate the whole system first when determining pump size and to follow some key best practices. Careful planning and collaboration between operation, procurement and engineering teams pays dividends. 

First considerations 

Mining sites are diverse, each requiring a unique pump system solution. Operations may have to consider the constraints of a highway, river or other configurations on the acreage being mined. 

Teams must also assess the mine’s elevation, existing pipe, the required source of power, and the type of chemicals or abrasive matter in the water. 

It’s also critical to figure out how much water will need to be pumped from the mine today and in the future. This can be difficult because the decision must often be made based on historical rainfall data in the area – particularly major rain events from many years ago. 

Planning depends on solid technical estimations and designing the pump system to pump more water than anticipated. A larger pipe size allows for proper flow rate so there is less resistance as the water transfers out of the mine. Selecting a larger pipe also reduces ongoing fuel or electric costs.

Planning for today & tomorrow

Determining the appropriate pump system size is as much about the current needs for water removal from a mine as it is about future needs.

The earth is always changing, and weather and other factors affect mine depth over time. Planning for increases in the size of the system is key.

Growth in a demographic area may increase demand for rock to be converted to concrete or asphalt. If growth occurs, the mine will become deeper as it depletes the rock, requiring larger pipes and pumps to remove a greater amount of water from a bigger area. 

The Hazen-Williams friction loss calculation provides a starting point for evaluating the sizing needs of a pump system by generating a predictable model for estimating the amount of energy required to move water from a mine. Proper evaluation takes into consideration the full system. 

The static head and friction loss determine the total dynamic head needed to dewater the mine now and as the mine becomes deeper and larger. For example, a mine may be 200 ft. deep today, but in five years, it could be 300 ft. deep. The total dynamic head of the system needs to be able to accommodate that difference. 

It’s also wise to assess existing piping in relation to future pump system needs. Again, if a team predicts a change of 100 ft. in depth in the next five years, it’s best to install pipe for a higher flow rate that can be added to later. Teams also need to identify minerals in the water. These determine the required pipe material and pump metallurgy to prevent corrosion and erosion.

How pumps are powered is also important. For large mines, electric infrastructure can be expensive to deliver, making diesel-driven power appealing – especially for larger pumps that require more energy. 

Choosing the pump

Pumps with variable-speed control offer an efficient and economical solution for removing water from a mine. 

A variable-speed pumping application matches the water that needs to be pumped by changing the speed of the pump, eliminating the need for frequent starts. This helps extend the life of the rotating assembly, whether it’s electric or diesel-driven.  

A suction lift pump, in particular, relies on atmospheric pressure to push water into the pump. The higher and closer the water is to the pump’s center line, the more available atmospheric pressure there is to push water into the pump. If the pump rpm matches the water flow rate at high water levels, the system is optimized by maximizing atmospheric pressure. 

Adding variable speed utilizes energy better to gain the greater potential of pumping. It also saves on the cost of rotating machinery. Variable-speed control on electric-driven pumps reduces in-rush current or a rush of power to the pumps during startup in mining operations using AC power. Called “demand,” in-rush current is expensive for mine operations because utility providers charge for the extra electric infrastructure. 

Accounting for rate, depth & power

The rate of water removal needed governs the pump system design and size. 

The pump needs to be large enough to match how much water needs to be pumped. If the pump is rotating too slowly, it causes deadheading, in which the pump is pumping zero gallons of water per minute while still operating. This is damaging to the rotating machinery and mechanical seal.

The depth of the mine and the distance water must be transferred also impacts pump system sizing. The deeper the mine, the larger and longer the pipe needs to be and the greater the total dynamic head. 

Depth often creates a need to increase the amount of power to the pump. If a mine plans to bring line power to the pump to use electric pumps, the conductor and the power source must be big enough for future pumping needs.

If a mine plans to use diesel-driven pumps, the team must consider the maximum size available. Presently, engine emissions requirements limit diesel horsepower to about 700 hp. Any horsepower needs above this require the use of multiple pumps, which, in many instances, is actually better.

Using two smaller-sized pumps to transfer water, plus a backup pump, not only addresses power and emissions requirements, but the configuration creates redundancy should a pump fail. The mine can continue to use the functioning pumps to transfer water and avoid the cost of downtime.

When creating a configuration of pumps, match the pump sizes correctly. A bigger pump can overpower a smaller one, leaving the small pump to operate without any water being transferred. When this occurs, the pump can deadhead, overheat and experience mechanical failure. Two pumps with equal-sized impellers operating at the same rpm can work well in parallel pumping conditions, so be sure to review mismatched pumps before deployment.

Maximizing long-term results

Most mines measure cost per hour of production and per ton produced, so any downtime in the operation affects its efficiency and profitability. If a pumping system fails, it can shut down everything. Investing in larger pipe and pumps and backup pumps not only adds insurance, but it can also pay back in power saved. Pumping experts, whether OEMs or rental companies, can help with selecting the right system.

Dwight Evans is product line manager of pump solutions at Sunbelt Rentals.

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