Pumps for the 21st Century

Given the resulting variable pressure that occurs in the force main is dependent on how many pump stations are operating at one time, designers face incorporating solutions that are costly from a capital and operational standpoint.

The problem is that the force main pressure can be low when only one pump station is operating or much higher when multiple pump stations operate at the same time. The pump station must be designed to handle the worst condition – when all the pump stations are operating at the same time, resulting in the highest force main pressure to overcome.

However, if this is the only condition they are designed for, the pump will vastly over pump when fewer pump stations are operating. This can result in pumps running out on the pump curve, exceeding the maximum motor amp draw. Yet, if the additional pressure is not incorporated in the design, the pumps will go to shutoff at the high-pressure condition and wet well overflows can occur.

Without accounting for the variable conditions, reduced service life and a multitude of problems can result. When you operate a pump outside of its normal operating range you run the risk of low and high flow cavitation. These phenomena result in impeller and volute erosion, reduced bearing and seal life, excessive pump noise and vibration.

Solutions include:

• Costly communication systems, tying all the pump stations together, which limit the number of pump stations on line at the same time
• Oversizing the pump and/or pump motor
• Orifice plates or partially closed discharge valves to create artificial head and prevent run-out – resulting in increased energy usage and increased chances for choked piping
• Installation of a flow meter with feedback loop to a VFD to maintain a constant flow rate regardless of head. The flowmeter complicates the operation of the station and requires additional infrastructure.

Process Methodology

Smith & Loveless sought a better solution to provide longer pump life, reduced operation and maintenance time and cost, reduced capital expenditure, all designed to fit within one of its offsite built pump stations. By utilising the S&L QUICKSMART™ PLC pump station controller with colour touchscreen HMI, a variable frequency drive and a force main sensor, the PLC can receive a signal from the sensor indicating changes in the force main due to additional pump stations coming on or going off.

From this information, an algorithm embedded in the PLC calculates the correct frequency for the VFD to adjust the pump speed to maintain a constant flow rate no matter how many stations are on line.

Conclusion

Monitoring the force main and pump speed via a patented algorithm, it becomes easy to add a standalone pump station to a common force main with multiple pump stations already in service.

It eliminates the headaches and costs of adding elaborate communication systems, oversized pumps, installing costly flow meters or greatly increasing the possibility of clogging.

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