Consider a common mining application where water flowing into an underground mine is allowed to collect in sumps from which it can be pumped to the surface. This system generally pumps dirty water. There is no consideration here of restricting flow into the mine, so the dirty water needs to be pumped continuously, barring a certain surge capacity in the system of sumps. This approach precludes having dry areas near the bottom of the mine, thus influencing efficiency and mining procedure. Following are five factors to be considered in selecting such a system.
1. Percent Solid of the Fluid to be Dewatered
Ten percent solid is on the high side. Abrasive wear influences maintenance costs of dewatering. As fluid moves through the pump, abrasive wear is proportional to the velocity cubed. This is a consideration in using centrifugal dewatering pumps. Positive displacement pumps allow slurries to travel relatively slowly through the pump.
2. Depth of the Underground Sump
A crucial factor while focusing on the depth of the underground sump is the pressure the pump needs to develop. Up to a few hundred feet, centrifugal pumps may offer the more economical solution. Above that, centrifugal pumps will need to be staged to develop required pressure. This will influence abrasive wear, complexity of piping and maintenance. If the vertical pumping distance is more than 1,000 feet, several stages will be required and other factors come into play.
3. Mechanical Efficiency of the Pumping System
Centrifugal pumps operate along a curve of head vs. flow. A best efficiency point (BEP) occurs at a certain flow. As pressure changes occur in the system due to wear or other factors, the operation may move away from the designed operating point. For this discussion, consider a functional efficiency of a certain system with centrifugal pumps at near 55%. A positive displacement piston diaphragm pump operates with efficiencies near 90%. Additionally, the slurry path through a piston pump does not include tight constrictions. Elastomeric diaphragms are used to propel the slurry. The diaphragms are relatively passive when interacting with the slurry. Thus, fatigue limits are determinate of diaphragm life, not abrasive wear.
4. Need for Gland Seal Water
Centrifugal pumps need gland seal water. Water needs to be added to the system, which is counterintuitive since the objective is to dispose of water. Gland seal water can be costly, and the gland seal pumps consume massive power to operate. Consider an example of six centrifugal pumps in series, each using some 30 gallons per minute (gpm) of gland seal water at 750 pounds per square inch (psi). Water costs about US $25 per 100 gallons, and power usage is near 18 kilowatts (kW) per gland seal water pump. Operation is 8,300 hours per year. Annual gland seal water costs for six pumps can top US $300,000. Since positive displacement (PD) pumps do not use gland seal water, these costs are saved.
5. Power Costs
If capacity of each centrifugal pump is 910 gpm and pressure is a combined 1,022 psi, then at 55 percent efficiency, power usage at 8,300 hours annually is about 6.1 million kW. At 90 percent efficiency, PD pumps use 3.7 million kW. Using $0.10 per kWh for power, savings using the PD pump can be close to US $240,000 per year. Thus, savings in this example due to power and gland seal water combined can exceed US $500,000 annually. The capital costs of the PD pump still far exceed the centrifugal series of pumps, but the payback period can become attractive.