Inefficiencies due to archaic or poorly planned control systems can quickly lead to rising costs for many submersible pump applications. Without effective monitoring, these hidden costs will not stay hidden for long. In this month’s SWPA Insight, John A. Evans, president of M. P. Electronics, joins SWPA Executive Director Adam Stolberg to discuss the vital role these components play. Mr. Evans holds BS and MS Degrees in Electrical Engineering from the University of Central Florida, is a Registered Engineer in the State of Florida, and has worked in the wastewater industry for twenty-five years with a focus on business development, design oversight, and production related activities. M.P. Electronics is a leader in the design and manufacturing of industrial controls for the wastewater industry, and has been a SWPA member since 2007, taking an active role in the training that SWPA provides to the wastewater industry.
Given that many pump systems last more than fifteen years, how would you advise submersible pump users best address the excessive costs related to inefficiencies that can accumulate when left unchecked?
Problems occur when pumps are no longer on system curve due to changes in head conditions. One fix is a monitored solution: monitor discharge pressure, run time, and flow calculations to alert end users to possible system changes that may require a fresh look at the station pump curve. Another issue is that pumps become less efficient as the impeller wears and the pump is off BEP. To correct for this, users should monitor flow and run times and look for trends that may signal a time to inspect and possibly change pump impeller or schedule preventative maintenance. It is also possible that the control method may require updating or outright replacement due to it being old technology. Many lift stations don’t have controls that are SCADA-ready, let alone monitoring.
Could you describe some of the common troubleshooting issues submersible pump users should look for when assessing their control systems?
One of the most common is having incorrect settings for soft starts and VFD’s for starting pumps. Soft start ramp time and current limit ranges need to be verified and set properly. Also, users should check if the lead pump is short-cycled. The “Off” and “Pump 1 On” level control settings need to be carefully set to limit short cycling. Care must also be taken not to exceed number of starts per hour per pump manufacturer. All of the Pump On/ Pump Off level control settings need to be carefully set to run pumps to maximize the ratio of “Pump Run” to “Pump Off” times.
It is also important that level measurement method matches the application. Not every level measurement method works for all applications, so users should remember these rules of thumb:
- Float switches may not be a good application with multiple pumps due to the number of floats that could become entangled.
- Very shallow applications are good for ultrasonics.
- Very shallow applications would not be provide enough resolution for a submersible transducer.
- Caustic materials are a good application for ultrasonic measurement.
- Heavy sludge applications are not good for conductance level probes, float switches, or bubblers.
- Bubbler systems may not be good fit in cold weather when the warmer air from the panel may condense and freeze.
- Level probes should not be used in rainwater applications due to a low ion count in stormwater.
- Submersible transducers need to be scaled to the operating range of the wetwell for good control resolution.
What tools are currently available to resolve common issues in existing or new control systems?
Probably the best tool you can have to help resolve control issues is a monitored lift station. Monitoring SCADA software has trending capability that can be used to monitor key points of interest, and provide a notification when the trends move outside of a pre-determined value.
How can submersible pump users optimize their control systems to best achieve operational efficiency for critical processes?
For critical process, pumps users should pay attention to applications that require PID control. Examples of this type of application would be a headworks lift station feeding a treatment plant, or a water pressure booster control system. The control system for these applications require “tuning” the PID loop for tight control of the process variable, whether it be level, pressure, etc. I & I studies are another critical process, and SCADA monitoring of lift station flows can be used to determine these I & I values. The SCADA data could be also used to optimize flow between lift stations for big storm events.
Inefficient pumping systems can be large energy wasters. What solutions can be found in the control systems for submersible pumps?
If the pumps selected for an application are too small, they may need to run much more than a properly sized pump. The solution would be to size the pumps properly so the pump run time would not exceed the manufacturer’s suggested starts per hour. However, it is also a problem if the pumps are too large. This may happen when planning for future growth. In the meantime, there is unused potential and a waste of capital costs. The controls in an example like this would be more expensive as well.
Larger pumps with Across-the-Line (ACL) starters may need to be changed out to soft starters (SS’s). This may not be feasible depending upon the room in the control panel, and may require a new control panel. A good design rule of thumb, such as “480V Pumps, 50HP and above should use soft starters,” would keep this from occurring.
Lastly, much has been written in the past regarding energy cost savings by using VFD’s to control the pumps. However, VFD control offers little or no savings on applications where the pump curve is relatively flat due to a head condition in which the static component is 50 percent or greater of the head that the pump will see. A good analysis of the pump curves should be done to ensure that VFD’s will provide the intended energy cost savings.◆
For More Information:
The Submersible Wastewater Pump Association (SWPA) is a national trade association representing and serving the manufacturers of submersible pumps for municipal and industrial wastewater applications. Founded in 1976, the association’s primary focus is on industry guidelines, education and promotion. Download the latest edition of the SWPA Handbook at www.swpa.org.
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MODERN PUMPING TODAY, August 2016
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