The LPS systems that I am familiar with incorporate a semi-positive displacement pump.
The semi-positive displacement pump in the grinder pump station has a nearly vertical H-Q curve. This is the best type of pump for successful parallel operation of many pumps into a system of common low pressure mains. Since each pump will be located at a different point along common low pressure mains and at various elevations, each pump should operate in an efficient and predictable manner, whether one pump or numerous pumps are operating at a given moment; the pumps in such a system do not have a single fixed “operating point,” but must operate consistently over a wide range of heads that are
continually, and often rapidly, changing.
The E/One grinder pump has the capability of operating above the LPS system design criteria of 80 psig, or 185 feet (Figure 1). Based on the maximum daily number of pumps operating simultaneously (Table 3; see PDF) versus the number of pumps connected to the system at the design pressure of 185 feet, the capability to operate significantly above the system’s design pressure is mandatory in order for the system to operate properly during the approximately bimonthly peaks when the “absolute maximum” numbers of pumps are operating. This feature also ensures that pumping will continue under those conditions when higher–than–normal pressure occurs in the pipeline.
System designs with calculated heads approaching the upper limits of recommended heads should be reviewed by E/One application specialists. Contact your local E/One Regional Sales Office or authorized distributor for a no-cost, computerized review of your design. Occasionally during “normal” operation, there will be short periods when higher-than-design pressures will be experienced. These can result from a variety of causes including solids buildup (obstructions) or air bubbles.
Deposits of solids or air accumulation will be purged from the line since the pump continues to produce an essentially constant flow, even though the cross section of the pipeline has temporarily been reduced. Higher velocities through the reduced cross section will provide the scouring action needed to correct such conditions as soon as they start to appear.
These higher-than-expected pressure conditions are transitory occurrences. The only requirement is that no damage be done to the pumping equipment, pipelines or appurtenances during these occasional short periods. E/One grinder pumps are driven by motors rated for continuous operation at 104 F/40 C above ambient temperature. They can operate at 50 percent above rated pressure for at least 5 minutes without excessive temperature rise. Based on the Albany, New York, demonstration project, for this type of overload to last even as long as one minute would be rare.
