Sump Impact on Pump Reliability

June 2, 2015
One of the most critical parts of a system involving pumps is the suction inlet, whether in the form of piping, an open pit or a lift station sump. A centrifugal pump that lacks proper pressure and/or flow patterns at its inlet will not respond properly or perform to its maximum capability. As such, the design of the sump can have a large impact on the performance and reliability of the pump(s), with the most important issues being the achievement of an entrained, air-free uniform flow, with sufficient NPSH available to the impeller eye.


By Allan R. Budris

One of the most critical parts of a system involving pumps is the suction inlet, whether in the form of piping, an open pit or a lift station sump. A centrifugal pump that lacks proper pressure and/or flow patterns at its inlet will not respond properly or perform to its maximum capability, which could adversely impact the pump and pump system reliability.

Intakes and Suction Piping

Uniformity of flow and flow control, up to the point of impeller contact, are most important. Part of this may be controlled by proper pump design, but the pit designer and suction piping designer have definite responsibilities to ensure good pump operation and optimum lifecycle cost. Disturbed inflow can cause deterioration of pump performance and may shorten pump life due to vibration and cavitation. Transitions resulting in flow deceleration at the pump inlet should not be used. A good design ensures that the following adverse flow phenomena are within the limits outlined in the Hydraulic Institute Pump Intake Standard1:

  • Free-surface and/or submerged vortices (these can draw entrained air or gas into the sump)
  • Entrained air or gas bubbles entering the pump
  • Excessive pre-swirl of flow entering the pump
  • Non-uniform spatial distribution of velocity at the impeller eye
  • Excessive variations in velocity and swirl with time

The negative impact of each of these phenomena on pump performance depends on the pump suction energy level (see http://www.waterworld.com/articles/print/volume-28/issue-12/departments/pump-tips-techniques/key-consider-help-deter-opti-npsh-margin-centri-pump-apps.html Key Considerations to Help Determine Optimum NPSH Margin for Centrifugal Pump Applications," WW, December 2012), specific speed and size, as well as other specific design features. In general, high and very high suction energy pumps, large pumps and axial flow pumps are more sensitive to adverse flow phenomena than low suction energy, small or radial flow pumps. Typical symptoms of adverse hydraulic conditions are reduced flow rate, reduced head, changes in input power, and increased vibration and noise. A model pump test may be required to ensure maximum mean time between failure (MTBF)/design performance.

Pump Sump

There are many variations in sump arrangements that are acceptable; however, best results are obtained when the sumps or sump bays are oriented parallel to the line of flow. Flow approaching from an angle creates dead spots and high local velocities, which can result in the formation of vortices, non-uniform entrance velocities and increased entrance losses.

The flow to any pump should not pass another before reaching the intended pump. When sumps or sump bays are normal to the direction of flow, such as in sewer systems, the distance between the sump (or sump bay entrance) and the pump must be sufficient for the flow to straighten itself out before reaching the pump.

Sump Volume

It is important to know the required active sump volume. This volume is defined by the highest start level and lowest stop level in the pump sump. The minimum required sump volume depends on the maximum inflow to the pump station, the pump capacities, and the number of starts per hour allowed for the pump drive and starters, as applicable. The number of starts per hour that a pump and motor system can sustain is determined by the selection of starting equipment, the load and inertia characteristics of the pump, and the motor design. An active sump volume that is too small reduces motor, pump and electrical equipment life due to the excessive starts and stops.

Wet wells for variable-speed pumping stations designed to match outflow with inflow need not be designed for storage but rather only to accommodate the inlets and geometry required for velocity limitations and cleaning. This compares with constant-speed pumps where the water level must fluctuate, rising when pumps are off and falling when they are running, thus increasing the required active volume.

Another way to reduce the required active sump volume is with multiple pump installations. The sequence with which the pumps are brought on- and off-line plays an import role, as does the total number of pumps. By designing the control system for alternating pump starts, twice as many starts per hour are possible for a station with two operational pumps, reducing the required active sump volume by 50 percent. This also distributes the operating time evenly between pumps. Larger pump numbers could allow even smaller sump volumes.

Control Sediments

Means should be provided for controlling the accumulation of sediments in a sump that handles solids, with features such as:

  • Designing the wet well to provide currents swift enough to carry settleable solids to the pump intake. For domestic sewage, increased storage time promotes septicity during periods of low flow.
  • Periodically cleaning the well of solids during pump shutdown.
  • Creating violent mixing to suspend sediments while the mixture is being removed by the main pumps.
  • Allowing the pump sump level to fluctuate, which will create differences in flow patterns that may minimize solids sedimentation and particle buildup on the intake surfaces.

Organic solid accumulations that are not removed may become septic, causing odors, increasing corrosion and releasing hazardous gases.

Specific Sump Recommendations

In addition to the general sump recommendations outlined above, more specific suggestions for improved sump design to reduce the probability of strong submerged vortices or excessive pre-swirl are presented below:

  • The inlet structure may take several forms, such as rectangular, trench-type, formed suction, or suction tanks (unconfined or circular). The primary requirement of any structure is that it prevents cross-flows in the vicinity of the intake structure or creates asymmetric flow patterns approaching any of the pumps.
  • It is important to position the inflow pipe(s) radially and normal to the pumps.
  • For the last five pipe diameters before entering the sump, the inflow pipe should be straight and have no valves or fittings.
  • All of the head losses incurred from the free liquid surface to the pump inlet must be considered when calculating the NPSH available to pump.
  • If the pump submergence is not sufficient, it can cause pump noise, vibration and high loads on the impeller, and hence should be limited to brief, infrequent periods. Pumps should be stopped as soon as they lose prime.
  • The required minimum submergence to prevent detrimental free-surface vortices is also related to the inlet bell diameter (inlet velocity) and the flow rate. The smaller the bell diameter and higher the flow rate, the greater the required minimum submergence, as shown in Table 1.
  • For applications where suspension of bottom debris may be a problem, a 5D minimum clearance is suggested.
  • The minimum liquid level must be high enough to avoid a free fall of the liquid entering the wet well. Even a short free fall entrains air bubbles and drives them deep into the pool where they may be drawn into the pumps, which can reduce the pump flow rate, head and efficiency.

Conclusion

As discussed, the design of the sump can have a large impact on the performance and reliability of the pump(s), with the most important issues being the achievement of an entrained, air-free uniform flow, with sufficient NPSH available to the impeller eye.

About the Author: Allan R. Budris, P.E., is an independent consulting engineer who specializes in training, failure analysis, troubleshooting, reliability, efficiency audits, and litigation support on pumps and pumping systems. He can be contacted via email at [email protected].

References

1. Pump Intake Design, ANSI/HI 9.8-1998, Hydraulic Institute, 2000.
2. Heinz P. Bloch & Allan R. Budris. Pump User's Handbook: Life Extension, 4th Edition, The Fairmont Press, 2014.
3. Igor J. Karassik, William C. Krutzsch, Warren H. Fraser & Joseph P. Messina. Pump Handbook, McGraw Hill Book Co., 1986..

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